Potatoes Postharvest
Potatoes Postharvest
Potatoes Postharvest
by
Bob Pringle
Chris Bishop
Rob Clayton
CABI is a trading name of CAB International
© CAB International 2009. All rights reserved. No part of this publication may be
reproduced in any form or by any means, electronically, mechanically, by photocopy-
ing, recording or otherwise, without the prior permission of the copyright owners.
A catalogue record for this book is available from the British Library, London, UK.
Library of Congress Cataloging-in-Publication Data
Pringle, Robert, 1944-
Potatoes postharvest / by Robert Pringle, Chris Bishop, and Rob Clayton.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-85199-502-1 (alk. paper)
SB211.P8P835 2009
635'.216–dc22
2008021193
ISBN: 978 0 85199 502 1
Preface xi
Acknowledgements xiii
Glossary xv
Chapter 1: Physiology 1
1.1 Introduction 1
1.2 Origin of Potatoes 2
1.3 The Growing Tuber 3
1.4 Factors Affecting Growth 12
1.5 Physiology of Potatoes in Store 17
1.6 Summary 28
v
vi Contents
Potatoes Postharvest is written for potato store managers, packhouse staff, academics
and students wishing to know how potatoes are managed postharvest and what sci-
ence underlies the practice. The text is based on the personal experience of the
authors, their own research, applied research by others and laboratory work car-
ried out to confirm findings in the field and in store. The book concentrates on the
essential principles of storage, grading and dispatch of potatoes.
The focus is the potato tuber and all of the influences that can affect its final
quality when sold. Background information such as the tuber’s physical development,
its metabolic processes, its susceptibility to damage and disease are provided where
this aids understanding as to why stored crops develop the problems they do.
Potatoes are increasingly being grown all over the world, with the largest
increases in the developing countries. The book is therefore written for an interna-
tional audience, and includes the Dutch system of high-rate intermittent ventilation
and the Scandinavian and North American system of low-rate continuous ventila-
tion with humidified air. Refrigerated systems of storage in bags, and low-cost nat-
urally ventilated traditional on-farm systems as used in India, Kenya and other
warm areas, are also included.
While many aspects discussed are similar regardless of climate, such as the
physical development of the crop, packhouse practice and quality control, other
aspects are quite different. Different climates affect the diseases likely to be present
on harvested crops. Storage systems in temperate continental zones are designed
primarily to keep crops from freezing and to minimize desiccation when ventilating
with cold dry air. In contrast, stores in maritime areas are designed primarily to
ensure rapid drying of wet crops entering store, prevention of early sprout growth
and the elimination of condensation on stored tubers due to the leakage of warm
humid air into store. In tropical areas storage in the ground and in low-cost struc-
tures is common.
Formulas have been included to allow the reader to calculate the moisture
content of air, ventilation duct sizes, fan backpressure, etc., but these have been
xi
xii Preface
kept to a minimum. Boxes are used to illustrate problems that occur in practice to
reinforce the topics being discussed.
A certain amount of duplication is inevitable in such a publication as influ-
ences such as tuber respiration, crop temperature, ventilation, disease development,
etc., all interact. Such duplication is kept to a minimum by putting a main descrip-
tion in one chapter and then referring to this section (e.g. Ch3.5) in the other
chapters whenever this same subject matter is mentioned.
For those who want a résumé of the main points discussed in the book, they
should consult the summaries at the end of each chapter.
Chapter 1 describes the developing tuber, explaining aspects such as skin set,
sprouting and the points in a tuber that are vulnerable to ingress by disease.
Chapter 2 describes systems of harvesting, transport to store or packhouse, clean-
ing, sizing and loading into store. Chapter 3 defines optimum conditions to mini-
mize tuber dehydration and prevent sprouting over the period in store and suggests
various strategies to achieve these conditions. Chapter 4 focuses on the disease
contamination on tubers entering store and suggests how store climate can be
manipulated to minimize their development and spread.
Chapter 5 looks at alternative building structures, their sealing and insulation.
Chapter 6 covers how air is distributed through the building to dry and cool the
crop and maintain a uniform crop temperature. Chapter 7 describes the use of
refrigeration, when cool outside air is unavailable. Chapter 8 describes the instru-
mentation for both monitoring and control of store climate, and suggests how
logged data can be analysed to identify why a problem occurred in store.
Chapter 9 describes store management from loading to dispatch. Chapter 10
discusses the particular requirements of seed, how it is stored, graded and dis-
patched, and the various ways of achieving single or multiple sprouts on seed
tubers prior to planting. Chapter 11 describes grading and packhouse design and
operation for pre-pack potatoes.
Quality control and traceability, vital elements in meeting farm assurance
schemes and for providing feedback on problems, are described in Chapter 12.
Lastly, a methodology for evaluating the cost of storage, to ascertain whether or
not it is financially viable, is described in Chapter 13. These costings, combined
with the quality of tubers coming out of store, will determine whether it is better
to store crop near the point of production or to harvest and transport crop from
different climatic areas to give year-round supply.
Bob Pringle
Chris Bishop
Rob Clayton
Acknowledgements
The authors would like to thank all those who have contributed to this book. To
Dick Taylor who undertook the huge job of editing the entire manuscript for stu-
dent comprehension. To Bill Leslie of Farm Electronics Ltd, Mike McLaughlin of
Proctors Ltd and Rod McGovern of SAC-Aberdeen for their valuable comments
on technical aspects of storage and provision of photographs. To Kees Wijngaarden
of Tolsma Techniek Emmeloord BV, The Netherlands who kindly provided
detailed plans of stores, photos and technical information. To Alf Johansson,
Consultant, Sweden for his help and information on low-rate continuous ventila-
tion of potatoes with humidified air. To Roger Balls for his plans of packhouses
and comment on their operation. To Alistair Redpath for information on seed
assessments and the ageing of seed to achieve bold or small daughter tubers. To
Eric Anderson of Scottish Agronomy for his photographs. To Herbert’s and Haith
Tickehill for their photographs and technical information. To Nick Winmill,
Greenvale for his help and comments on packhouse quality assurance. To Colin
Johnston, Structural Engineer, Aberdeenshire, for detailed drawings of store floors.
To Dave Ross and Fraser Milne for information on vision grading and bloom
detection. To Hazel Carnegie for her drawings of potato plants and tubers. To
Mary Jo Frazier, University of Idaho, and Todd Forbush for their comments on
US potato storage. To Andrew Norrie, John Taylor, Tony Bambridge and Duncan
Dixon for access to their farms and for their photographs. To Gavin Lishman of
Martin Lishman Ltd, Frank Pirie ex of FJ Pirie, Andrew Bell of SG Baker, Bill
Tennant of Linde Forklift trucks, Graeme Stroud and Steve Gerrish of BPC for
photos and technical information. To Alex Hilton for providing information on
wash up reports and Jay Whooton of Anderson Midlands for financial information.
To John Vessey, Consultant, previously with United Biscuits, for his long-term sup-
port over a great many years.
On a more general note, we would like to thank our colleagues and ex-
colleagues in SAC, BPC and Writtle College for their long-term support and con-
tribution to potato production, in particular Stuart Wale, Phil Burgess, Claire
xiii
xiv Acknowledgements
Term Definition
Air blending As air mixing
Air friction Drag on air jet as it passes through still air or flows
through ducts or potatoes
Air mixing Mixing/blending of recirculated and ambient air,
controlled by regulation of the duct temperature
Airspace Air movement that flows around boxes in store, but is
ventilation not forced through the actual potatoes themselves
Air-on/air-off Temperature difference between air flowing on to, and
temperature air leaving, a fridge evaporator cooling coil
difference
Ambient air Air external to the building structure
Ambient humidity Relative humidity of the outside air close to the point of
intake
Ambient Temperature of outside air close to the point of intake
temperature
Ambient-air Cooling of store and crop by ventilating with ambient air
cooling when it is cooler than the crop
Anaerobic Where no air is present
conditions
Auto-recirculation Recirculation started by automatic timer, defined by a
duration and an interval, until the next period
Backpressure Pressure experienced by a fan due to the friction of air
flowing through wire mesh, louvres, ducts and potatoes
BASIS Training Training in the use of agrochemicals, required by law
for all UK personnel who either sell or apply such
chemicals
Blemish diseases Diseases which cause marks on the skin of the tuber
xv
xvi Glossary
Blindness in seed Sprouts, usually affected by disease, which will not grow into
shoots
Bloom Reflective shine on tubers
BOD Biological oxygen demand, mg/l. The BOD measures
the polluting strength of dirty water and indicates the
amount of oxygen needed by microorganisms to break
down the organic wastes in a watercourse
Box Container for potatoes, usually 1–2 t in capacity. Also
known as a crate
BPC Formerly British Potato Council, now Potato Council
Brock Damaged or diseased potatoes unfit for human
consumption
Bulk Potatoes kept in one mass, supported on three sides by
walls. Also called a pile
Chitting Process of encouraging seed tubers to sprout prior to
planting, to speed foliage development and increase yield
Chronological age Age of a tuber measured in real time, i.e. days
Cleated-belt Inclined conveyor belt fitted with rubber coated bars, or
cleats, to prevent potatoes rolling back down the incline
COD Chemical oxygen demand, mg/l. The COD measures
the polluting strength of dirty water and indicates the
amount of oxygen needed to chemically break down the
organic wastes in a watercourse
Composite panel Factory-made insulation panel, usually polyurethane
injected between two metal skins. Jointed by gaskets
and/or cam-lock mechanism
Condensation The conversion of vapour in air to water when the air
comes in contact with a surface that is below the
dew-point temperature of the air
Condensation – Condensation which occurs just under the surface of
subsurface potatoes in a pile or box
Controller Or control box. Controls operation of ambient-air cooling,
louvres and refrigeration
Cooling coils Heat exchanger (evaporator) of a refrigerator
Crop condensation Condensation occurring on tubers, which can contribute
to development of disease
Crop set-point Target or desired crop temperature
Crop temperature Average temperature of crop
Crop/ambient air Differential between crop temperature and ambient air
differential temperature
Crop/duct Differential between crop temperature and duct
differential temperature for systems with air blending
Crop/store air Differential between crop temperature and store air
differential temperature
Cull potatoes As brock potatoes
Curing period As wound healing period
Dead band Tolerance in °C either side of a set point
Glossary xvii
1.1 Introduction
©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton) 1
2 Chapter 1
While there are hundreds of species of Solanum around the world, only about 200
produce tubers. Eight of these are cultivated on some scale. The potato, Solanum
tuberosum L., is by far the most commonly grown and its exploitation and movement
around the world is well documented (Hawkes, 1979). Cultivated originally in Peru
it was brought to Spain and Portugal, from where it dispersed to other parts of
Europe in the late 1500s. Over the next two centuries it was exported to North
America, Australia, China and latterly elsewhere. Its universal adoption as a staple
carbohydrate has not always been welcomed and has been affected by national
propagation campaigns, such as in France in the 1760s, royal decrees in Russia,
again in the 1760s, and stiff competition from other carbohydrates like rice and
pasta along the way. None the less, the crop is now fourth in the world ranking of
crops behind rice, maize and wheat, with over 300 million t produced annually
(CIP, 2007). Its distribution across the world is shown in Fig. 1.1. Its value as a
carbohydrate is due to the high amount of energy stored as starch within the tuber.
Long starch molecules allow energy to be stored in a non-soluble form. When
required, conversion of the starch into sugars allows energy transport around the
growing plant.
At low temperatures, the tuber begins to convert insoluble starches into soluble
sugars. This increase in concentration of ions in the form of sugar solution allows
the cellular contents to remain liquid at low temperature (Wright and Diehl, 1927),
reducing the tuber’s freezing point, much as salting of the roads prevents ice form-
ing in winter. Tuber cells can therefore function at lower temperatures, which may
be a factor in helping them survive the Andean winter. This does not guarantee
survival of intact tubers and we will see later how freezing damage can compromise
storage. We will also see how this anti-freeze mechanism can cause fry colour
problems in potatoes destined for processing. This is a major concern for store
managers.
120 40
Production ( 106t)
100 Production 35
30
Yield (t/ha)
80 25
Yield
60 20
40 15
10
20 5
0 0
a
ia
pe
be ca
ia
pe
ric
ic
As
an
ib ri
ro
ro
an
er
Af
ar e
ce
Eu
Eu
C m
Am
O
d nA
n
n
th
er
er
an ati
or
st
t
es
L
N
Ea
Continent
Fig. 1.1. World potato production and average yield by continent. (FAO, 2006.)
Physiology 3
In the UK, prior to planting, stones are sieved from the soil and deposited in the
bottom of furrows, usually 1830 mm apart. The seed tubers are then planted in a
pair of secondary furrows, between 710 and 915 mm apart depending on the vol-
ume of stone in the stone-filled furrows. The secondary furrows are located equidis-
tant between the rows of stones, and covered with a ridge of soil, which will
eventually contain the daughter tubers. Planting date depends on final market, lati-
tude, longitude and seasonal weather, with the earliest planting in January/February
in the south-west and the latest in Ireland and Scotland in May/June. Tuber spac-
ing along the furrow depends on variety, seed size and age, and the desired size of
daughter tubers. If a small number of large ware tubers are required, seed tubers
with apical dominance may be planted. If a large number of seed or salad-sized
tubers are required, seed tubers that will produce multiple sprouts will be planted.
Fertilizer is usually incorporated at planting, with a further application broadcast
later if required. Weed control by sprayer wheels running on the separated stones
may be required until the crop canopy closes. Thereafter the sprayer will be used
to control blight, specific diseases or insects. Harvest of early potatoes will start in
April/May, while main crop and seed crop will span September to November.
Knowledge of how tubers are formed can assist managers in discovering what
went wrong when a crop fails its quality assessments.
Shortly after emergence, the stems of most potato plant varieties will begin to develop
underground lateral stems called stolons. These can be subdivided (Fig. 1.2) into:
● Main stolons, borne directly off a main stem.
● Lateral stolons, arising from the axils between the main stem and the main
stolon.
● Branch stolons, arising as branches from the main stolons.
Soil surface
Mother
tuber Lateral stolon
These subdivisions are important as they reflect the hierarchy of main, lateral or
branch stolon by which stored dry matter in the tubers will be converted back to
sugar and reabsorbed by the growing plant in times of stress. Smaller tubers may
also be ‘reabsorbed’ so that resources can be diverted elsewhere.
Following stolon formation, which occurs usually 14–30 days after the crop is
visible above the soil, stolon tips will begin to swell (Fig. 1.3) as tuber initiation
begins. New cells are created through cell division, starch is deposited after conver-
sion from translocated sugars and the tuber periderm or skin is formed (Fig. 1.4).
This process combines the laying down of stacked periderm cells with the deposi-
tion of suberin within and under these cells to form a protective barrier against dis-
ease and water loss. Dry matter accumulation and tuber expansion will then
continue as the season progresses, although, as mentioned above, on occasions of
stress this process can go into reverse.
The resulting tuber after harvest has a stolon scar at one end, where it was
attached to the stolon, and the ‘bud’ or ‘rose’ end at the other (Fig. 1.5). At the
bud end, the apical eye containing the bud is the last to be formed and contains
the physiologically youngest bud (Rastovski and van Es, 1981). After a period of
dormancy, one or more of the buds starts to sprout. The other buds or eyes are
arranged spirally round the tuber, with the eyes nearest the stem being the first to
be formed during tuber development. In the skin of the tuber are lenticels, tiny
holes in the skin usually too small to be seen, which allow gas exchange between
the cells within the tuber flesh and the surrounding soil atmosphere. There are
about 250 of these in the skin of a 115 g tuber. The stolon or stem end, buds and
Hook forms
Swelling continues
with buds concentrated
on tuber
Cortical cells
filled with starch
grains Fig. 1.4. Periderm cells stacked one
above the other with cortical cells below.
(H. Carnegie, Aberdeen, UK.)
Physiology 5
Lenticel
Lateral buds
Bud or
rose end
Fig. 1.5. Potato tuber with stolon scar to the
left, bud or rose end to the right and apical
Apical and lateral buds in between. (H. Carnegie,
Stolon scar bud Aberdeen, UK.)
The causes of hollow heart (Fig. B.1.1) are not fully understood but many research-
ers have proposed a link with water and temperature stress shortly after tuber initi-
ation. Metabolites normally destined for the cells at the centre may be diverted
elsewhere, causing some cells to die. Thereafter, when rapid growth resumes, due
for example to a return to sufficient soil water status, any new growth will ‘pull apart’
the dead and dying area to leave an irregular-shaped cavity. In some situations,
hollow heart observed early in the season will grow out if conditions allow for slow
and steady growth.
Fig. B.1.1. Hollow heart in a tuber. (Courtesy of Potato Council, Oxford, UK.)
6 Chapter 1
lenticels are all important to the store manager as they are points of weakness in
the tuber skin, where disease organisms can enter and multiply or wait for condi-
tions that suit their development.
Research has shown that many physiological processes in the potato plant follow a
predictable timetable for any given variety (Firman et al., 1999). By repeated meas-
urement of key growth stages in a range of varieties of potato crops, researchers
have been able to state with some degree of confidence that:
● Tuber initiation follows crop emergence by a predictable length of time for
any given variety.
● Dormancy break (or first sprouting) follows tuber initiation by a predictable
period for any given variety.
Increasingly potatoes are being grown for a specific market, be it bakers, process-
ing, pre-pack, salad potatoes or seed. Each has an optimum tuber size. The greater
the proportion of the crop in the optimum size range the greater the potential
profitability of the crop.
Firman et al. (2004) showed that the proportion of tubers within the optimum
size range could be increased by extending or reducing the period between tuber
initiation of the seed crop and the planting date of the resulting seed.
To produce many, but small, tubers, an extended period is required (Fig.
1.6a). In the UK, the seed crop is planted in mid-March, with tuber initiation in
mid-May. The crop is harvested in early September, stored until the following
April and planted late April/early May. The result is a crop with a prolific number
of small tubers.
To produce few, but large, tubers, a short period between tuber initiation of
the seed crop and planting of the resulting seed is required (Fig. 1.6b). The seed
crop is planted in mid-July, with tuber initiation in mid-August. The crop is har-
vested in October, stored over winter and planted in mid-February or whenever
soils are warm enough to allow growth.
This approach brings to the industry a degree of predictability it has not had
before and would indicate that a number of unpredictable occurrences in the past,
in terms of bulking rates, desired yield, etc., can be attributed to a lack of apprecia-
tion of the importance of seed provenance.
Many biochemical processes in plants are a function of heat and time. In potatoes
this is best illustrated by sprout development. Under dark conditions, after dormancy
break, sprouts will elongate much more quickly in crops that are stored warm than
in those that are stored cold (Morris, 1966). Tubers that sprout in warm conditions
will typically produce one dominant sprout, which results in low numbers of large
daughter tubers.
Physiology 7
September
November
December
February
October
January
Augusr
August
March
March
June
June
April
April
May
May
July
July
Fig. 1.6. Achieving a prolific or bold crop by changing the interval between ‘seed crop tuber
initiation’ and ‘main crop planting date’: (a) system to produce many, but small, tubers;
(b) system to produce few, but large, tubers.
The tuber develops from a swollen stem into a fully mature tuber, which then goes
into a natural, or innate, dormancy state for a period of time before it starts to
sprout. The chronological age of the tuber is measured from tuber initiation and at
dormancy break is around 6 months, but this varies for each variety. If the growing
tuber is exposed to warm temperatures during growth or subsequent storage, its
maturity or physiological age increases more rapidly and the tuber becomes physi-
ologically older than it would have done at lower temperatures. Chronological age
is the true age of the tuber in terms of months, while physiological age relates to
its physiological state.
required. The apical dominance that results from this physiological ageing requires
tubers to be planted closer together to establish an optimum stem density.
Managing physiological age allows the grower to produce a known quantity of
stems and hence a relatively predictable sized crop. The crop has to be planted
with extreme care so that sprouts are not broken off, which could result in blank-
ing, or significantly delayed or variable emergence. The probability of sprouts
being lost is greatly reduced by chitting, the process whereby sprouting tubers are
exposed to natural or artificial light in a glasshouse or chitting shed (Ch10.5), to
produce stronger, shorter and better-attached sprouts.
Physiological age is measured in day-degrees above 4°C from the time of first
sprouting. Typically accumulations of 200–400 day-degrees are required to create
a significant benefit for an early market. This can be achieved (Box 1.3) by warm-
ing seed to 10°C and storing it at this temperature for about a month in a well-lit
chitting house prior to planting (Ch10.5).
While yields will seldom be as high as from un-aged seed grown for a full sea-
son, physiological ageing can have various benefits for a particular market:
● Early emergence.
● High early yield.
● Early senescence and bulking.
● Rapid skin set.
The visual appearance of skin, termed skin finish, based on its reflectivity and pres-
ence of disease and defects at harvest can be affected by soils in a number of ways,
due to:
● Soil-borne diseases.
● Period since last crop of potatoes was grown.
● Un-decomposed plant residues.
● Soil type.
Table B.1.3. Time to achieve two physiological ages at a range of chitting store
temperatures.
Time (days)
Temperature in chitting store (°C) 150 day-degrees 300 day-degrees
8 38 76
10 25 50
12 19 38
15 14 28
Physiology 9
Where crops are grown in the same field more frequently than one year in seven,
then disease incidence and severity on the crop increase. Various publications
(Read et al., 1995) recommend an interval of more than 6 years between potato
crops if key diseases such as stem canker and black dot are to be controlled. In
other reports, comparisons are made between crops grown more, or less, frequently
than one year in six. More disease-related skin finish problems (e.g. black scurf and
powdery scab) are reported with short rotations compared with long ones (Nolan
et al., 2000). In Europe storage diseases such as silver scurf, black dot and gangrene
can also be exacerbated by short rotations. The concentration of potato disease
inoculum, in the form of resting spores, sporangia or sclerotia (Ch4.5) depending
on the type of fungal species, will be highest immediately after a potato crop and
will then decline over a number of seasons. The level of the peak infestation will
depend on the amount of trash left in the field after harvest, the size of the root
mass and a range of pathogen specific conditions, the main ones being temperature
and moisture. The subsequent decline in inoculum over the years may be caused
by a number of processes working either alone or in combination. Spores can be
leached away from the top layer of soil following rainfall, where other ‘myco-
phageous’ fungi, bacteria or nematodes may destroy them. Alternatively they may
simply die naturally, there being no opportunity to germinate and re-infect fresh
potato tissue. In addition to these direct influences on decline, other factors such as
tolerant weed hosts or potato volunteers may allow a particular pathogen to remain
viable in the absence of a potato crop.
Time between cropping of potatoes can be reduced in some cases if pesticides
are used to suppress diseases or pests. Such practices are coming under increased
regulation to safeguard the environment, so husbandry measures or integrated pest
management systems should be used wherever possible.
Soils support populations of suppressive organisms that have a role in disease
reduction, as well as potential infective organisms (Elphinstone et al., 2004). Attempts
to replicate these have met with little success although various soil-improvement prod-
ucts that contain pathogen-suppressive organisms are starting to gain in popularity.
Careful management of the growing crop will minimize the inoculum and
reduce the likelihood of producing crops with excessive amounts of disease. The
role of storage is to minimize the further development of disease on the harvested
crop by considering the disease triangle (Ch4.2).
As well as biological properties, the soil’s physical properties have a direct
influence on skin finish. Sandy soils are well known for producing a dull skin finish.
Some would argue that a consistent skin finish within a given stock would be more
important than a dull or bright finish because purchasers prefer a uniform product,
while others would say that brighter skin sells better than duller skin and so ought
to attract a premium. Recent electron-microscopy studies at the Horticultural
Research Institute, Warwick, UK (Wiltshire et al., 2005) have compared the skins
of tubers grown in a range of soil types and have shown that the frequency of
ruptured or collapsed cells within the epidermis is higher when crops are grown on
sandy soils. An epidermis made of intact cells is better able to reflect light than one
that contains damaged cells, so exhibits a brighter appearance.
Most sandy soils currently used for potato production produce processing
crops where skin finish is less important than for washed and bagged crops.
10 Chapter 1
Some varieties produce a better skin finish than others even when grown in similar
conditions. Between varieties there may be subtle differences in the way the epider-
mis and periderm are formed. In some varieties, epidermal cells appear very neatly
stacked (Fig. 1.4). This stacking creates lines of weakness between cells, which can
shear or grow apart if significant pressure is applied. While these shears are for the
most part invisible to the naked eye, they can reflect and refract light in much the
same way as damaged cells described previously. In other words, they can result in
a dull skin finish. It has been proposed that where these lines of weakness are very
deep then the resulting shears may be visible to the naked eye and appear as net-
ting on the tuber surface, although this has been difficult to measure in practice
(Wiltshire et al., 2003).
For bright skins, varieties which have a cellular ‘brick wall type’ of stacking will
contain fewer lines of weakness and therefore are less prone to microscopic cracking.
Another cause of poor bloom is caused by the surface periderm cells collapsing (Fig.
1.7). The reason for this collapse is not fully understood, but appears to be influenced
by maturity at harvest and evaporation during storage (Wiltshire et al., 2005).
Discussion above referred to the process of sugar transport from plant to tubers
and the formation of starch dry matter. Dry matter content can be measured in a
number of ways, such as by comparing the weight of an oven-dried sample with its
original fresh weight, but it is usually measured using a hydrometer, where a
Collapsed periderm
cells
Cortical cells
Fig. 1.7. Loss of bloom due to collapsed surface periderm cells. (H. Carnegie,
Aberdeen, UK.)
Physiology 11
(a)
26.1
25.2 26.1
22.9 24.0
23.9 24.4
25.5 Stolon
21.6 18.7 18.9 19.6 22.4
attachment
17.6
Total = 23.1
(b)
23.7 22.9
24.0
23.4
23.3 22.0
22.8
19.9 Stolon
20.9 18.5 18.5 17.9 18.7 attachment
16.7
Total = 21.2
Fig. 1.8. Dry matter percentage distribution within 40–50-mm tubers of: (a) cv.
Saturna; (b) cv. Russet Burbank. (Abbreviated and redrawn from Gaze et al., 1998.)
12 Chapter 1
The store manager often has to identify the cause of a problem in a batch of pota-
toes. The origin of the problem may have occurred during the growing period.
The phenomenon of skins losing adhesion to the tuber flesh below prior to, or
in the weeks after harvest, has been recognized by many store managers. Causes
are still not clearly understood although rapid changes in periderm dry matter
associated with the problem have been identified.
There are numerous examples where growth problems, often caused by stress on
the growing plant (see Box 1.4), allow disease to develop and subsequently cause
major problems in store. These are summarized in Table 1.1.
Stresses of various kinds can weaken potato plants’ natural defences to such an
extent that disease development becomes easier. For example, extremely low tem-
peratures (Wright, 1942) can lead to cellular membrane damage which can make
bacterial access much more likely and result in soft rotting. Higher temperatures
combined with water stress can lead to premature changes from starch to sugars
within tubers, providing a ready substrate (or nutrient source) for subsequent bac-
terial invasion (Ch1.4).
Physiology 13
Table 1.1. Stress-related growth problems and potential for disease development.
Fig. 1.9. Jelly end rot. (Courtesy of Potato Council, Oxford, UK.)
14 Chapter 1
Suberized
Storage
filling cells
parenchyma
Proliferating tissue
Fig 1.10. Lenticel formation and suberization; proliferation may occur in wet conditions.
(From Adams, 1975. Redrawn by H. Carnegie, Aberdeen, UK.)
carefully in future crops. Over-application of nitrogen can also delay the crop in
achieving the dry matter contents required by French fry and crisp processors.
While phosphates appear to have little effect on tuber quality, potash appears
to have an influence in two areas. First, adequate potash nutrition can help in
reducing susceptibility to bruise damage. While this is often debated, there is suffi-
cient evidence to suggest that crops grown with inadequate potash are more likely
to bruise (Fellows, 2004). Choice of potash fertilizer can also have an influence on
tuber dry matter accumulation, as this tends to be higher where potassium sulfate
is used instead of potassium chloride (Dickens et al., 1962).
1.4.3 Drought
Water stress can affect the potato crop in a number of ways. It is particularly impor-
tant from a quality perspective at tuber initiation and in the days and weeks that fol-
low initiation. It is during this stage of growth that tuber periderm is not fully developed
and common scab can infect tubers. Infection is most likely to occur in susceptible
varieties where the soil zone in which tubers are formed is allowed to dry out during
tuber initiation. Water would otherwise support a flora of bacteria antagonistic to
Strepotomyces scabies, the cause of common scab. When these are absent, common scab
develops readily. Various computer programs are available to calculate optimal irriga-
tion required during tuber initiation and range from simple water balance calculations
based on rainfall, irrigation water evaporation and leaching, to more sophisticated
models that take account of crop canopy and evapotranspiration, through to direct
measurement of soil moisture status using various measuring devices.
Physiology 15
Water status later on in crop development can also have an influence on tuber
bulking rate, accumulation of dry matter (Gaze et al., 1999) and susceptibility to
bruising (Fellows, 2004). Rapid changes in soil moisture status, from very dry to
very wet, towards the end of the growing season can cause splitting or cracking of
tubers (Fig. 1.11). This is often more noticeable in certain varieties (e.g. ‘Marfona’
and ‘Nicola’) and following rapid mechanical destruction of haulm by machine
flailing, where the ability of roots to take up water exceeds the rate of transpiration
resulting in an overload of tubers with water. Providing cracks and splits are
allowed to heal prior to lifting there should be little effect on storability, although
market value is likely to decrease.
Rapid changes in soil water status from drought back to wet due to heavy rain
can result in a number of tuber defects. If the drought is particularly stressful, then
bulking will cease. Tuber growth will resume once the water supply resumes but this
may follow discrete physiological changes in the tuber. This can result in the formation
of ‘dolly’-shaped tubers or chain tuberization (Fig. 1.12), where, in sequence, ‘tuber–
small section of stolon–smaller tuber’ may develop on the same original stolon.
1.4.4 Flooding
Where flooding occurs and tubers encounter a period of time during which they
stand in water, three things happen. First, there is an increased risk of disease.
As described later (Ch4.3), most fungal and bacterial diseases develop rapidly
where water is present. This is particularly so for bacterial diseases such as black-
leg and soft rotting, but is also the case for a range of fungal pathogens. In areas
where water sources are contaminated with brown rot bacteria, flooding may
facilitate the spread of this disease. For Erwinia bacteria, which produce blackleg
in the growing plant, their association with anaerobic conditions in store is well
documented (Lund and Kelman, 1977; Pringle and Robinson, 1996). Similar
anaerobic conditions in the soil occur during periods of flooding, allowing bacteria
to develop and spread rapidly.
Fig. 1.11. Cracking in potatoes due to moisture variations in the soil during tuber growth.
(Courtesy of Potato Council, Oxford, UK.)
16 Chapter 1
Second, oxygen depletion and anaerobic conditions may also cause physiologi-
cal changes within tubers. Blackheart and internal blackening are more commonly
associated with oxygen depletion during storage due to crop respiration in very well-
sealed stores. Low oxygen levels in the store atmosphere prevent oxygen from reach-
ing into the tuber flesh, leaving central cells unable to respire and function properly.
Over time the cells start to break down and go black. Similar effects can on occasion
be observed in the soil following prolonged periods of flooding (Hooker, 1981).
Finally, prolonged exposure to water through flooding can cause outgrowth of
lenticels. This can increase the chances of soft rotting occurring during storage, as
bacterial access to tubers is made easier due to the damaged lenticels.
1.4.5 Frost
Since duration of dormancy depends on the heat input in day-degrees above 4°C
between tuber initiation and when the tuber starts to sprout (Fig. 1.13), warm sum-
mers, which result in higher average soil temperatures, will tend to reduce the
period of dormancy while cold summers will tend to increase it. In extreme cases
in the UK, potatoes have started to sprout while still in the ground. Examples are
‘King Edward’ crops in 2006. Such occasions are referred to as ‘sprouty’ years.
Physiology 17
Box
Ridge
Tuber
on stolon
(a) Tuber initiation (b) Tubers bulking (c) Potatoes into store (d) Sprout growth starts
Fig. 1.13. Natural or innate dormancy is number of days between tuber initiation
and start of sprouting.
When managing crops after a particularly warm summer, the store manager
should take the season into account when planning his cooling or sprout suppres-
sion practice.
Conversely, after a particularly cold summer, seed growers may have to ensure
that the crop has been subjected to sufficient day-degrees of heat prior to planting,
if the crop is to start growing immediately it is planted.
In store, it is common to see one or two tubers over a square metre of crop surface
change from being a normal tuber to a bag of water contained by its skin. The rest
of the crop can be quite sound and grade out well. In the same way, individual
tubers tested for disease can give highly variable results. The likely cause of these
variations is the position of the tubers within the ridge (Fig. 1.14). The likelihood
of blight, soft rot, common scab, crushing due to wheel damage, greening, etc. will
alter depending on their position. Sampling must take this variability into account,
by ensuring large samples are taken and that these are replicated.
Other factors during harvest or in store may add to this variability, so this too
has to be considered.
Potatoes are living organisms, which in store take in oxygen and give out carbon
dioxide and heat. A thorough understanding of their physiology is necessary to
understand the processes that occur in store.
18 Chapter 1
Diablo roller
Wheel damage Greening Frosting
pressure
Fig. 1.14. Location in ridge will affect the likelihood of tubers having a particular
problem.
1.5.1 Respiration
Vacuoles
Mitochondria
– the cell’s combustion
Cell wall chambers
Nucleus
Starch grains
evaporation of water from the tuber skins. The warm air in the voids rises through
the mass of potatoes, evaporating moisture from their skins as it goes. If the surface
tubers are cooler than the tubers below, this warm, moist, upward flowing air will
condense on these cool surface potatoes (Ch3.5). The moisture, so often visible
near the surface of highly respiring potatoes, is therefore not given out by the
tubers themselves as part of the respiration process, but is due to this up-current of
warm moist air condensing on the cooler potatoes above. The resulting wet and
warm potatoes provide ideal conditions for storage diseases to develop.
Oxygen consumption, carbon dioxide emission and heat generation are com-
mon measurements in the study of respiration by potato tubers. The store manager
has to consider these processes as they have consequences on how a crop will store.
The rate of tuber respiration can be high in immature tubers (Fig. 1.16).
Tubers lifted prior to senescence may still be increasing in size, with cell division
proceeding at a rapid rate. If crops are harvested later when they are more mature,
respiration will be considerably reduced. This high rate of respiration heat produc-
tion associated with early harvesting has to be removed immediately after harvest
if condensation on the layer of potatoes near the surface of the stored crop is to be
prevented. Because immediate cooling of potatoes in addition to the removal of
respiration heating (termed ‘field heat’) would require very large fridges and
excessive energy use, high rates of ambient air are normally used to remove the
respiration heat, allowing the crop to stay near ambient temperature. Keeping the
crop warm allows wounds, the primary entry points to tubers by disease, to heal
rapidly and prevent subsequent infection.
Respiration increases after tubers have been handled, transported over rough
tracks by trailer or suffered damage in the form of bruises and cuts. The increase
appears to be a combination of the actual movement and damage caused, together
with the subsequent wound healing process (Meinl, 1972; Burton, 1989). Any handling
operation will therefore be associated with increased respiration.
Fan- or wind-induced ventilation is required to minimize the rise in crop tem-
perature from the respiration heat being generated. Moisture evaporation, which
leads to tuber weight loss, can be high at this stage due to the poorly developed
periderm.
35
30 9 September harvest
Respiration heat (W/t)
25
20
15
20 October
10
harvest
5
0
0 2 4 6 8 10
Days after harvest
Fig. 1.16. Potato respiration in the days after harvest. (From Pringle et al., 1997.)
20 Chapter 1
40
35 20C
Fig. 1.17. Respiration of tubers when stored at 10°C and 20°C over a storage
season. (Redrawn from Burton, 1973, assuming a respiratory quotient of 1.)
Respiration rate drops rapidly after harvesting and becomes relatively stable
from senescence through to sprouting/dormancy break (Figs 1.16 and 1.17). It is
less stable at high storage temperatures of 20°C, but storage temperatures in
temperate climates are kept at 10°C or below, so in practice respiration over
the storage period is relatively constant. Respiration rate with respect to temper-
ature (Fig. 1.18) 1 month after harvest shows higher rates at elevated storage
temperatures and a minimum at 5–6°C. As store temperature reduces below
this temperature, respiration increases as part of the tuber’s response to low
temperatures (Ch1.2).
Respiration varies between varieties. In stores containing more than one vari-
ety, especially seed stores, where varieties are segregated into box rows (Fig. 9.12),
one row may be respiring and generating more heat than another. These rows will
therefore have higher convection airflows than the others, and may be more likely
40
35
Respiration heat (W/t)
30
25
20
15
10
5
0
0 5 10 15 20 25
Storage temperature (°C)
Fig. 1.18. Respiration of samples of potato tubers after about 1 month at various
storage temperatures. (Redrawn from Burton et al., 1955.)
Physiology 21
Damage results from various harvesting and handling processes. Once damage has
occurred the tuber will begin the process of wound healing during which a new
waterproof layer of suberin is developed through the wound to prevent further
moisture loss (Fig. 1.19a and b). Thereafter increased respiration fuels the process
of cellular division in the tissues below and stacked corky cells develop to provide
a thicker barrier against the invasion by pathogens. In time further deposits of
suberin around the corky cells produce a more waterproofed finish. There are vari-
ous factors involved but temperature is the most important and its effect on the
process is summarized from numerous papers by Mitchell (2000) in Table 1.2.
After store loading, potatoes are usually held warm for a period of time to
allow any wounds to heal. This is particularly important where damage levels are
high and where disease organisms that can enter by wounds are known to be
present. If crops have surface moisture on their skins, due to inadequate drying
facilities or poorly designed ventilation, keeping the crop at warm temperatures can
predispose the crop to rapid silver scurf, black dot or soft rot development. Cooling
the crop rapidly would reduce this problem, but is likely to result in condensation
forming on the crop (Ch3.5). The best strategy therefore is to have well-designed
ventilation systems, which rapidly dry the crop and keep it dry, so that wound
healing can be practised without fear of moisture-related diseases developing.
Wounded surface
Suberized layer
Suberized parenchyma
Sub-epidermal cells
Wound periderm
Fig. 1.19. (a) Cross-section of tuber periderm; (b) suberization after wounding.
22 Chapter 1
Ninety-eight per cent of the moisture that leaves a tuber during storage is lost
through its skin by evaporation (Fig. 1.20). Only 2.4% leaves the tuber via the len-
ticels along with the carbon dioxide produced by respiration, it being a wet gas
(Burton, 1989).
So long as the pressure within the cells of the tuber skins and the vapour pres-
sure of the air in the voids surrounding the tubers are the same, no evaporation
will take place. For this balance to occur, the RH of the air in the voids between
the tubers has to be 97.8% (Hunter, 1985). This assumes that the temperature of
the air is the same as that of the tuber.
Hunter determined this figure by blowing air humidified to various levels past
a tuber (Fig. 1.21). The air had to be moving to remove the heat coming from the
tuber, which would otherwise have heated the void air and reduced its RH. The
graph shows that when the RH of the air is below 97.8%, moisture is evaporated
from the tuber, and when the RH is higher than 97.8%, moisture in the form of
vapour moves from the air into the tuber. At 97.8% vapour neither leaves nor
enters the tuber.
If surrounding air
is at same temperature
as tuber and has 97.8%
RH, vapour neither
leaves nor enters tuber
Lenticels
Fig. 1.20. Vapour exchange between tuber and void air. (H. Carnegie, Aberdeen, UK.)
Physiology 23
0.18
90% RH
0.16
0.14
Weight loss (%/week) 0.12 95% RH
0.10
97.8% RH
0.08
99% RH
0.06
0.04
0.02
0.00
0 0.02 0.04 0.06 0.08 0.10 0.12
Air velocity (m/s)
During the storage period, the rate of moisture loss from the crop is proportional
to the difference in water vapour pressure (WVP) within the cells of the potato
skins and the water vapour pressure of the air in the voids. The difference is often
referred to as the water pressure deficit (WVPD). Water vapour pressure is measured
in kilopascals or millibars (0.1 kPa = 1 mbar; see Appendix 1 for metric–US impe-
rial conversion tables), and varies with both air temperature and RH (Ch3.1). The
greater the WVPD, the faster will moisture be lost from tubers.
The lower the RH of the ventilating air, the greater will be the WVPD. This
confirms what we know that the drier the air, the more moisture loss though
evaporation there will be.
But in addition, the colder the ventilating air compared with the tubers, the
greater will be the WVPD. Ventilation of the crop with air cooler than the crop,
no matter how humid, will always result in moisture loss through evaporation.
The cooler and drier the ventilating air, the greater will be the deficit between
the water vapour pressures in the tuber skins and the void air and the more moisture
loss there will be through evaporation.
The rate of loss of moisture through evaporation from a stored crop over a storage
season tends to start high (Fig. 1.22), reduce during the dormant period and then
increase if sprouting starts. Initial moisture loss is so high because:
● Evaporation from wound tissue is much higher than from intact skin. On a
commercial crop, the moisture loss in the first week after harvest was three
times that 2 weeks later (Burton and Hannan, 1957).
24 Chapter 1
● Heat of respiration is highest for the 4 to 6 days after lifting (Peterson et al.,
1981; Pringle et al., 1997), and this heat reduces the RH of the ventilating air.
● Respiration is increased following wounding (Burton et al., 1992), contributing
further to the reduction in RH of the ventilating air.
In intermittently ventilated systems (Ch3.7), extended ventilation is practised after
store loading and during wound healing to dry the skins, remove respiration heat
and to prevent subsurface condensation. For the majority of the time this reduces
the RH in the voids between the tubers and increases moisture loss. With intermittent
systems, once the wound-healing period is passed, extended ventilation stops, the
RH in the voids rises and the rate of moisture loss from the crop decreases.
Fry colour and the uniformity of colour within a batch of potatoes have a large
influence on marketability of processing crops. It can be measured in a number of
0.7
Average weight loss (%/week)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Time (weeks)
Fig. 1.22. Percentage weight loss per week over the first 14 weeks in an ambient-air
cooled store. (From Pringle et al., 1997.)
Physiology 25
ways. Visual assessments on fried crop samples are possible by comparing their
colour with charts published by various organizations such as the United States
Department of Agriculture and the British Potato Council. While cheap and rapid,
they are subjective and rely on specific lighting criteria and keeping colour charts
in good condition.
More sophisticated, accurate and expensive methods are available; the Agtron
system™ (Agtron Inc., Sparks, Nevada, USA) works by projecting a beam of light
on to a French fry or crisp sample and measuring reflected light at a given range
of wavelengths. The Hunter-L system™ (Hunter Associates, Reston, Virginia,
USA) uses carefully controlled light emissions but expresses data as coordinates on
red-to-green and blue-to-yellow axes. In each of the three cases, the ability to
replicate prescribed fry conditions of oil type, heat and cooking duration, either
by a particular purchaser or by other researchers, is critical if meaningful com-
parisons are to be made.
Preservation of fry colours is paramount when supplying the processing indus-
try. Some varieties are better at keeping their fry colour than others and have been
selected for use in processing companies. Fry colours are primarily related to
reducing sugar content, so management of reducing sugars is critical. There are
various influences on reducing sugar content that need to be considered when
planning storage for processing.
Maturity
It is essential that crops are sufficiently mature at harvest if fry colour is to be pre-
served. While academics may argue about the definition of maturity, it is sufficient
to say that fry colours can deteriorate rapidly if crops are harvested prematurely.
Crops that initially produce a light golden fry colour may alter in store to produce
darker fry colours if harvest occurs before senescence is complete.
Temperature
We have already seen that potatoes convert starch into sugars at low temperatures.
Prolonged storage at low temperature will result in an increase in sugar content,
which will result in darker fry colours indicated by low Agtron values (Fig. 1.23).
A similar effect is sometimes seen when late harvested crops experience low tempera-
tures in the field. Harvest should be started once senescence is complete but not so
late that cold temperatures are experienced.
Duration in store
After prolonged storage, darker fry colours are sometimes experienced as the tubers
begin their next yearly cycle and irreversible conversion of starch into sugars takes
place to fuel springtime growth. This phenomenon is known as senescent sweeten-
ing and the degree to which this occurs varies from cultivar to cultivar. Senescent
sweetening can be reduced, to some extent, by reducing the storage temperature
slightly. This allows the total storage duration to be extended by a few weeks.
35
30
Fig. 1.23. Senescent sweetening over storage period at four storage temperatures.
easily avoided through either short bursts (e.g. 5 min/day) of flushing with fresh air
that meets safe condensation-free criteria or the fitment of a fan just large enough
to keep the store carbon dioxide at acceptable levels.
1.5.8 Greening
enhance the flavour of potatoes but at the higher levels associated with greening
they can induce a range of symptoms from stomach ache to dizziness and vomiting.
There are well documented but rare cases where people have died after consum-
ing large quantities of potato sprouts, where glycoalkaloid concentrations are
many times higher than in tubers. Planting seed at sufficient depth and creating
sufficient soil cover for daughter tubers by forming ridges usually prevents green-
ing in the field. In the potato store, the prevention of greening is achieved by stor-
ing the crop in the dark, ensuring gaps in the store fabric and damaged louvres,
which can allow light ingress, are repaired and switching off lights after access to
the store. Greening may be more of a problem in retail polythene pre-packs where
exposure to natural and artificial lighting in supermarket display cabinets can
speed up the process.
Glycoalkaloids have been shown to be highly active against fungal and bacte-
rial diseases (Percival and Bain, 1999). Attempts have been made to capitalize on
this by generating aerial seed but with little commercial success. For the store man-
ager who is storing seed only, with no likely possibility of consumption either by
man or livestock, a small amount of greening is of little significance.
1.5.9 Dormancy
Should sprouts start to develop, both respiration and moisture loss will increase
rapidly (Burton et al., 1955). This can lead to thermal runaway in store (Ch1.5.1)
and a reduction in the value of the crop. The sprouts can, however, be knocked
off if the crop is passed over a grader. The exposed flesh of the broken sprouts is
28 Chapter 1
vulnerable to invasion by disease, so if the crop is for use as seed, wound healing
before dispatch is vital. Extensive sprouting leads to tubers shrivelling and becom-
ing unmarketable. Boxes of such crops can on occasions of extensive sprouting fail
to empty, in spite of being completely inverted, due to the intertwined nature of
the sprouts.
1.6 Summary
This chapter reviewed the aspects of the physiology of the potato tuber that relate
to harvesting, storage, grading and dispatch. This information provides an essential
background for those interested in store management.
● Potato tubers are swollen stolons, which act as storage organs, attached to the
stems of potato plants, with stomata that are sealed off by cellular growth,
called lenticels, to provide gas exchange. Tubers produce sprouts and are
capable of vegetative reproduction.
● Tubers receive food reserves in the form of sugars from the living plant and
accumulate, then store them, as starch.
● Tubers’ natural response to low temperatures is to convert some starch to
sugar, which has considerable impact on fry colours and taste.
● The stages of development of a tuber are partly linked to period of growth, or
chronological age, and partly to temperature-related physiological age.
● Warm growing seasons lead to early sprouting in store while cool growing sea-
sons delay the break of dormancy.
● Seed can be manipulated to produce daughter tubers with multiple or single
sprouts which, when grown on, produce small or large sized tubers,
respectively.
● Soil conditions affect skin finish, with sands producing a dull skin and clays
and silts a brighter, reflective bloom.
● Tuber dry matter content, of importance particularly with crops for process-
ing, is linked to variety, sunshine hours, water availability and fertilizer rates.
● Most disease on tubers comes from either its seed or from infections developed
during the crop’s growth.
● While some diseases enter the tubers via its connection to the stem of the
plant, others enter through the skin where its defences are weaker, such as len-
ticels, buds, pest holes or frost damage.
● As the primary defence of a tuber to disease is its skin, a principal aim in har-
vesting is to minimize tuber damage while the first task of storage is to encour-
age wound healing.
● Newly harvested crops should be dried using ventilation applied uniformly to
the whole crop as it enters store.
● The removal of water by ventilation from wet harvested crops is of primary
importance in preventing moisture-induced disease.
● Tuber heat production due to respiration of tubers in store results in convec-
tion currents, evaporation of moisture from the crop and the potential for con-
densation on potatoes just below the surface of the pile or box.
Physiology 29
● Early harvested crops tend to have less disease than later harvested crops.
● Early harvested crops produce more heat than late harvested crops and require
ventilation to prevent subsurface condensation and associated conditions con-
ducive to infection by disease.
● Tuber dehydration during storage is minimized in intermittent ventilation sys-
tems by only ventilating the crop when necessary or by artificially humidifying
the ventilating air in continuously ventilated stores.
● Keeping potatoes below 3–4°C in store can prolong dormancy of seed and
pre-pack crops. If crops are not for seed, chemical sprout suppressants can be
used instead.
● Greening of potato skins is prevented by storing potatoes in complete darkness
and by minimizing the period under lights in retail display cabinets.
2 Harvesting and Store Loading
Systems
2.1 Harvester
2.1.1 Description
Potato harvesters (Figs 2.1 and 2.2) combine a number of functions in the one
machine. They differ in type and complexity but most have the following attributes.
The harvester:
● Lifts potatoes and soil within the ridge from the ground, together with unwanted
items such as clods, haulm, stones and rotting mother tubers.
● Sieves the lifted material as it travels up the inclined first and second webs,
discharging the soil layer and small stones back to the field.
● Has steel fingers fixed above the webs, which align the haulm so that it is
parallel to the direction of crop flow. This allows ‘pinch’ extractor rollers,
located after the second main web, to grab the plant stems and return them
to the field.
● Crushes clods and removes adhering soil using star wheels or axial rollers (see
Ch2.2.3).
30 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Harvesting and Store Loading Systems 31
Fig. 2.1. Tractor with flail in front and harvester behind. (Courtesy of Grimme UK
Ltd, Perth, UK.)
Crop flow
Elevator
Intake Picking table
First Second Tuber/clod
web web
main web main web separator
● May have picking staff aboard who manually remove any rots, remaining
haulm, clods or stones from the conveyor picking table.
● Elevates the crop into a bulk trailer, or boxes located on a trailer; alternatively
it may return the crop to the ground to form a windrow of potatoes to allow
them to dry prior to being uplifted later in the day.
Where stones have not been separated from the soil in the field prior to planting,
harvesters may be fitted with devices which mechanically separate stones from the
potatoes. These separate out most of the stones from the crop, thereby allowing
higher rates of harvesting than could be achieved when separation relies totally on
picking staff standing on the harvester.
32 Chapter 2
The point where the greatest damage to potatoes commonly occurs during harvest
is at the transfer between harvester and trailer (Maunder et al., 1990). Keeping the
harvester elevator discharge as near as possible to the surface potatoes in the trailer
minimizes damage at this transfer point. When the trailer is empty, the elevator
head is lowered close to the floor of the trailer where it is vulnerable to being hit
by either end of the trailer, if the trailer tractor driver fails to keep station with the
harvester or if the harvester stops suddenly. One collision between the elevator and
the trailer can result in extensive damage to the elevator and in harvesting time
being lost while carrying out what may be an expensive repair. Keeping a constant
distance between trailer and harvester requires great driving skill, particularly when
changing trailers.
To avoid collisions between the harvester unloading elevator and the trailer, the
driver of the harvester may raise the elevator clear of the trailer sides and ends.
Dropping potatoes from this height on to the hard floor of the trailer is then likely
Harvesting and Store Loading Systems 33
to damage the crop. Lining the floor of the trailer with foam material to cushion the
first potatoes to be loaded minimizes this problem. Once potatoes cover the floor,
they act as a cushion for the potatoes following. Improving the visibility through the
front end of the trailer can further reduce damage, as the harvester driver can see
the location of the elevator head in relation to the crop within the trailer.
Careful examination of washed tubers from the harvester and store intake can
identify where damage is occurring (Ch12.11), so that any machine malfunction or
damage points can be rectified before too much of the crop is spoilt.
In the UK, 1-t boxes, with a plan area of 1.83 m × 1.22 m, are often carried on a
trailer to transport potatoes from field to store. Preventing collisions between the
harvester elevator and the boxes is even more difficult than when using a bulk
trailer. The preferred solution is to fit a fall breaker to the top of the box (Fig. 2.3).
The other option is to station an operator behind the box holding a straw-filled
sack or cushion in the box, so that the first potatoes in are cushioned. This arrange-
ment, though widely practised, is not recommended as the operator can be hit by
the elevator, bombarded by stones accompanying the potatoes or can find his
cushion trapped under potatoes if not constantly alert. The insulation material used
for 100-mm pipes, when split to form an inverted ‘U’, can be fitted over the edges
of the boxes to reduce tuber damage when moving from one box to the next.
Fig. 2.3. Loading boxes using a fall breaker to minimize tuber damage. (Courtesy of
Eric Anderson, Scottish Agronomy, UK.)
34 Chapter 2
A number of systems are available for transporting crop from field to store and for
preparing material for storage (Fig. 2.4).
Bulk trailers or lorries (trucks) either tip to empty their contents (Fig. 2.5) or are fit-
ted with a horizontal, rubber belt, unloading conveyor (Fig. 2.6). The former needs
a bulk hopper slightly wider than the trailer to empty into. The latter can discharge
directly on to a belt conveyor or into the boot of a cleated belt elevator.
A common problem is a mismatch between the discharge height of the tipping
trailer and the height of the reception hopper. The drop should be as small as pos-
sible to minimize damage to the crop. Attempting to correct height mismatches by
the use of sloping ramps causes the trailer wheels, but not the trailer draw bar, to
be raised. This causes the tipping angle of the trailer floor to be reduced, which
can result in some crop or soil remaining when the trailer is tipped. Trailers and
hoppers should be matched at purchase.
The bulk hopper usually consists of a wide horizontal rubber belt, with sloping
steel sides (Fig. 2.7); it has a capacity allowing it to take the majority of the trailer
contents. In larger hoppers the belt carries the potatoes horizontally for the first
part of the hopper, rising at its far end where it discharges on to a belt or the boot
of a cleated rubber belt elevator. Some bulk hoppers allow the trailer to be reversed
over the hopper so that it starts to discharge its load near the discharge end of the
hopper and completes emptying as it draws forward. This design allows for complete
emptying of the load in one go and more rapid turnaround time for trailers.
The above form of bulk hopper with unrestricted discharge is preferable to ones
conveying the crop through a hole in the hopper’s rear bulkhead, which can subject
potatoes to churning and associated damage as they are forced through the hole.
Harvester
Soil and
Bulk trailer Boxes on trailer stones
Cleaner Splitter
Splitter
Box filler
Elevator
Store
Fig. 2.5. Trailer tipping into bulk reception hopper. (Courtesy of RJ Herbert
Engineering Ltd, Wisbech, Cambridgeshire, UK.)
Fig. 2.7. Bulk hopper. (Courtesy of Eric Anderson, Scottish Agronomy, UK.)
The flow of material from the trailer should be adjustable to provide an optimum
rate of supply to the sizer or grader. This can be achieved using either a manually con-
trolled variable frequency drive on the bulk hopper conveyor motor or an electronic
proximity detector (Ch11.4, Fig.11.2) that senses the presence or absence of crop at the
start of the grader and switches on the conveyor drive when more crop is required.
Once the potatoes are flowing in a stream, a number of operations can be carried
out (Fig. 2.4).
These include:
● Removal of any remaining soil, clods, haulm and stones from the crop.
● Separation of crop into main crop and out-grades.
● Removal of any potatoes unsuitable for sale.
● Application of a pesticide.
● Conveyance to loading elevator or box filler.
Passing potatoes over cleaning and separation equipment, particularly at the high
rates required to keep up with the harvester, will inevitably cause further damage.
Most growers believe that it is better to do all the damage at one time, followed by
an effective wound healing regime when tubers are warm, rather than cleaning and
sizing later in the storage period, when the lower tuber temperatures will result in
slower or incomplete wound healing.
Harvesting and Store Loading Systems 37
Fig. 2.8. Potato cleaning and inspection prior to storage. (Courtesy of RJ Herbert
Engineering Ltd, Wisbech, Cambridgeshire, UK.)
The importance of keeping falls to a minimum, conveyor belt speeds low and
the use of soft materials to which soil cannot adhere has been known for many
years (Vollbracht and Kuhnke, 1956). Cleaning and separation equipment should
therefore have few and small drops, and have cleaning and separation areas of suf-
ficient size so that the passage of potatoes across the machine is kept to a speed
that causes a minimum of damage.
Cleaning the crop into store requires equipment capable of dealing with pota-
toes harvested from a range of conditions, from dry to wet sticky soils. Soil can build
up inside spiral coils, on the sides of conveyors, the floor of chutes and on soft rub-
ber flaps designed to cushion potato impact. This mud when damp may cushion
tubers as they flow over the machine. When dry, it can turn to a sandpaper-type
consistency, which may result in considerable scuffing of the crop. Excessive build-
up of soil requires regular removal to keep the machine functioning effectively. PVC
material should be used in preference to rubber for cushioning, as soil is less likely
to adhere to it.
The benefits from putting the crop over a cleaning and separation system prior
to storage (Fig. 2.8) include:
● The removal of any remaining soil, haulm, clods and stones ensures good air-
flow through the stored crop.
● Small quantities of oversized and undersized material can be removed from
the main crop, so that they can be sold when prices are most favourable.
38 Chapter 2
● Material unfit for sale is disposed of prior to storage, so storage space and
electricity to run fans or fridges is not wasted storing unsaleable material.
● A bulk hopper and cleaning (and separation) system allows bulk trailers to
be used for conveying potatoes from the field while using boxes purely for
storage.
● Box life is enhanced if they are not used for transport.
The disadvantages of putting the crop over cleaning and separation systems are:
● Additional labour is required to staff the cleaning and separation system at a
busy time of the year.
● The additional handling may cause extra damage and remove some of the
bloom.
● If the soil is sticky, it will not fall off the tubers; it needs to dry first.
● The equipment has a high capital cost.
Packhouse buyers may allow suppliers who have modern, low-damage, into-store
cleaning and separation equipment to clean their crops into store. If growers’ sepa-
ration and cleaning equipment is old and causes damage to crops, these suppliers
will be encouraged to harvest crop directly into boxes and store ‘as dug’.
A wide range of cleaning equipment is now available for inclusion in store cleaning
equipment. They are also to be found on harvesters and graders. They tend to suit
different conditions (Herbert, 2006).
Spiral cleaner/grader
Spiral cleaner/graders (Fig. 2.9a) separate loose soil, clods and small potatoes from
the main crop by the sideways action of the rotating helical coils. By altering the
spacing between the coils, a rough grading up to 45 mm can be achieved. Steel
coils can become scored by impact from hard angular stones and can trap small
stones between the coils, so should be avoided where soils contain flints or granite.
The coils can also become filled with moist soil and therefore in wet conditions
need to be cleaned regularly. The rubbing action can damage the immature skins
of first early potatoes.
(a) (b)
Crop flow Crop flow
(c) (d)
Crop flow Crop flow
Fig. 2.9. Types of cleaning system incorporated in graders and potato harvesters:
(a) spirals; (b) stars; (c) continental web; (d) axial rollers.
rot development in store. Like spirals, the rubbing action can harm immature skins
and flints and granite stones can damage the polymer.
Axial rollers
Axial rollers (Fig. 2.9d) tend to be used mainly on potato harvesters when harvest-
ing in wet, heavy soils; they are however sometimes used on separation equipment.
The rollers work in pairs: one fluted roller rotates in one direction while a plain
rubber roller rotates in the opposite direction. The rubbing action removes the soil
from the potatoes and crushes clods. Different sizes of plain rubber roller can be
fitted. The smaller the potatoes and the thicker the coating of soil, the thinner the
rubber plain roller has to be. Damaged fluted rollers can damage crop and need
replacing frequently. To minimize damage axial rollers should be removed from
the cleaning line if potatoes are clean and dry.
Brush cleaners
Where only a small amount of soil is attached to tubers, brushes can be used to
remove soil. Selecting stiffer brushes can increase the severity of abrasion. Brushes
can work well to remove small quantities of soil but can become totally clogged
with mud if large quantities of wet soil are present.
40 Chapter 2
Continuous
chain grader
Clean potatoes
to be stored
Potatoes,
soil, clods,
stones and
trash
Star cleaner Inpection roller
table to remove
Moving floor trash, rots and green
potatoes
Conveyor discharges
soil and clods
Certain models of brush or star cleaners will allow some of the banks to be
rapidly removed for cleaning while another bank is installed. This allows the
brushes or stars to be cleaned when removed from the machine, thereby minimiz-
ing cleaner downtime.
Due to the large volume of potatoes going over the separator, size separation into
store is not as precise at when grading out of store. Potatoes are likely to shrink in
diameter by 2–3 mm during storage due to water loss by evaporation. However,
sizing into store does allow a crop to be segregated into grades destined for differ-
ent markets.
A typical separation system is shown in Fig. 2.8. The operation of into-store
separation is shown in Fig. 2.10. The tipping trailer discharges the potatoes into
a bulk hopper, fitted with a moving floor. The crop is conveyed on to a star
cleaner, where soil and small potatoes are removed and clods are crushed. The
crop then passes over a continuous chain grader, typically 45 mm in size, where
undersize potatoes are removed. The remaining potatoes are put over an inspec-
tion table to allow staff to remove any remaining rots, haulm, clods, stones and
unmarketable potatoes. The rollers continuously rotate tubers so that the entire
surface of each tuber can be inspected. The resultant cleaned potatoes are then
loaded into store.
Damage during harvesting, cleaning, sizing and store loading should be minimized
if the crop is not to be reduced in value. Damage results in:
Harvesting and Store Loading Systems 41
Causes
Handling from
Type of damage Field Harvester operation harvester to store
Abrasions and crush wounds are more likely to get infected than clean cuts
(Adams and Griffith, 1978). Estimates have been made for the loss of material for
sale. In the USA, 6.3% of the value of the crop was lost through damage (Preston
and Glynn, 1995). A ware potato chain analysis in The Netherlands (Molema et al.,
2000) showed that 78% of the total amount of subcutaneous tissue discoloration
was caused by impacts.
Damage can severely downgrade the value of potatoes. On one occasion in
2004 a sample of pre-pack potatoes was downgraded from a value of £165/t to
£65/t due to damage alone. In a national survey (BPC, 2004a), during the years
2000–2002, 47% of respondents had loads rejected due to bruise damage while
60% had loads downgraded.
The most popular damage assessment system used in the UK is the Damage Index
(Robertson, 1970), which has merit in that it:
● Is simple to carry out in the field or store (Fig. 2.11).
● Takes 25, 50 or 100 tubers at random from a sample and divides them visually
into four categories: severe damage >1.5 mm deep; peeler damage 0–1.5 mm
deep; scuff damage to skin; and undamaged.
● Converts the percentage of tubers placed in the three damage categories into
a single figure or ‘index’ suitable for use in experimental correlations with
other parameters (e.g. disease development versus damage index).
● Gives an index for a sample, which is related to the weight of tissue (pulp) that has
to be removed to obtain potatoes free from any marks when they are peeled.
Details of the system used are described in Ch12.11.
Bruises may not show up under normal conditions after harvest until 3–4 days later
(Melrose and McRae, 1987). It is essential that bruising be identified as soon as
Old newspaper
Sample size = 25 tubers
possible after harvest so that either harvesting can be stopped or the cause of bruis-
ing identified. Methods of speeding up the identification of bruising are discussed
in Ch12.11.
The best pesticide to prevent disease developing is air and plenty of it: air to
remove heat produced by respiration; air to remove moisture on the potato skins;
air to prevent subsurface condensation. Many growers, however, like the added
security of applying a pesticide.
If synthetic pesticides are to be used they should be applied after the crop has
passed over the cleaner of an into-store cleaning and sizing system. Applied liquid
chemicals or dusts will not penetrate soil, so any soil should be removed before
application. This is why application of chemicals on harvesters in the UK is rarely
practised. If all fractions of crop are to be treated it should be applied before sepa-
ration. If only one fraction is to be treated it should be applied following
separation.
To accord with BASIS protocols (BASIS (Registration) Ltd, Ashbourne, UK),
a chemical pesticide should be applied only if there is evidence that a disease is
present. Justification is therefore required.
A very restricted group of chemicals are suitable for applying to potatoes enter-
ing store. These are approved by the Pesticides Safety Directorate and listed in
Table 2.2.
In the UK, 45% of ware and 39% of seed was treated with chemicals, into store,
in store or prior to dispatch (Heywood et al., 2006). The aim should be to maximize
coverage to tubers with a minimum of chemical wastage and any related pollution.
Table 2.2. Chemicals available for treating potatoes into store (UK).
The most popular way of applying liquid chemical to harvested potatoes in the UK
is to use a roller table, fitted with overhead hydraulic spray nozzles, with the spray
assembly enclosed within a canopy (Fig. 2.12). The roller table both conveys the
tubers under the spray, while at the same time rotating tubers as they pass. This
ensures that their entire surface receives the spray. The speed of tuber movement
across the rollers is controlled by the rate at which tubers are fed on to the roller
table. The tubers behind keep the tubers in front moving forward. The canopy
over the spray zone reduces potential spray drift, while a drip tray beneath the
rollers collects any runoff.
2.5.2 Ultra low volume, spinning disc, controlled droplet spray applicators
If liquid chemical is dripped on to a fast rotating spinning disc, the liquid is broken
up into a large number of fine droplets. A small amount of liquid will cover a large
area of potatoes. As less water is applied than with hydraulic sprayers, the time
Fig. 2.12. Enclosed hydraulic sprayer applying chemical to potatoes on a roller table.
(Courtesy of Eric Anderson, Scottish Agronomy, UK.)
Harvesting and Store Loading Systems 45
taken to dry the tubers to prevent infection by disease is less. Application rates as
low as 0.5 l/t can be used. This compares with 2 l/t with hydraulic sprayers. Since
the spinning disc does not require much power, it can be driven from a car battery.
The small droplet size makes low-volume sprayers very vulnerable to drift, so a
canopy is essential. The labels on some chemicals state that they should not be
applied at low volumes.
The use of electrostatic spray applicators for the postharvest application of chemi-
cals to potato tubers was first demonstrated by Cayley et al. (1987). The applicators
break up the liquid chemical into very small droplets in the same way as spinning
disc machines, but then give the droplets an electric charge so that they are
attracted to the tuber surfaces while repelling each other. Tubers pass below the
sprayer on a roller table. The system can lead to very uniform application of
chemical to tubers with reduced chemical runoff. As with the spinning disc technol-
ogy, the application rates are lower than for a hydraulic sprayer.
Dust application involves a vibrating dispenser located above the flow of tubers
passing across a roller table. As with hydraulic application, the rate of application
is susceptible to variations in tuber size, cleanliness, throughput, tuber rotation and
crop flow rate. Additionally, with dust application, the presence of moisture on the
tubers can cause chemical application to be uneven, as the dust will selectively stick
to wet tubers or wounds. A canopy enclosing the applicator can reduce upward
and sideways drift of dust, but dust falling between the rollers tends to be dispersed
into the atmosphere unless a well-designed fan extractor system is used.
changes in spray pattern caused by the reduced number of tubers on the roller
table. Furthermore, it was noted that tubers would often mesh together into a ‘raft’,
which prevented rotation and uniform coverage. It has been recommended
(Rodger-Brown et al., 1999; Rollett et al., 2001) that a buffer hopper be installed
prior to the roller table and sprayer, to even the flow of tubers and so minimize
the variation in application. Some form of agitation is required to prevent the raft-
ing, though the less variation in size of the sample the less of a problem this is.
Tubers need to be rotated 1.5 times for application to be effective (Bishop and
Garlick, 1998). Soil on tuber skins prevents chemical reaching the skin of tubers. If
the efficiency of application could be improved, the amount of chemical applied to
tubers could be reduced (Hide et al., 1994).
Calibration of equipment is always important and should be carried out with each
new chemical as well as at the beginning of season. In the UK, sprayer operator
training and certification is organized by the National Proficiency Test Council
(NPTC, Stoneleigh, UK). All operators have to have passed the appropriate
Professional Accreditation (PA) modules required to operate the pesticide applica-
tion equipment being used.
Developments in application systems are leading to systems that will automati-
cally record the weight of tubers passing through so that spray volume is propor-
tional to throughput. This should greatly assist with traceability requirements,
where rate of chemical application has to be put on the shipment paperwork. An
example calibration calculation for a spray applicator is presented in Box 2.1.
The application of water-based chemical may protect the crop from the target dis-
ease but the added moisture may increase the risk of infection from other diseases.
This risk is minimized if the crop is ventilated to remove surface moisture immedi-
ately after treatment. This can be achieved by standing boxes in a windy location
prior to loading into store, by blowing air through the crop immediately after treat-
ment, or by fitting an air knife or fan over the flow of potatoes to remove moisture
as the crop exits from the spray treatment chamber.
For bulk stores the discharge from the cleaner/separator feeds into the intake of
the loading elevator. The elevator needs a long reach to load the top of the pile
without its wheels damaging potatoes at the base of the sloping face (Fig. 2.13). For
a 4-m-high pile the reach required is 8 m. While the store fills with potatoes, the
elevator is drawn back. As the cleaner/separator feeds the elevator, it too has to be
moved back at regular intervals.
Harvesting and Store Loading Systems 47
Fig. 2.13. Long elevator loader required to reach the top of the pile. (Courtesy of
G. Stroud, Potato Council, Oxford, UK.)
48 Chapter 2
A tractor loader is normally unsuitable for loading a bulk store, since the
length of loader frame required to prevent the front wheels crushing potatoes
makes the tractor difficult to manoeuvre.
When storage is in boxes, a box filler is placed at the discharge from the cleaner/
separator. The cleaner/separator can therefore be in a fixed location. At the start
of loading a box, the end of the discharge conveyor is often lowered to minimize
the drop on to the box floor (Fig. 2.14). A proximity sensor may be used to auto-
matically raise the conveyor as the box fills. Previous designs of box fillers, which
elevated the box to the discharge conveyor, have been discontinued due to the
hazard posed to staff as the box is lowered to the ground.
With bulk storage, there is no alternative but to load potatoes directly into store.
The underfloor ventilation system present in almost all bulk stores provides a posi-
tive flow of air for drying the crop. So long as this is switched on to ventilate the
crop immediately on loading, respiration heat removal and drying will start right
away. Ventilated trailers (Fig. 2.15) can be used if there is likely to be a delay
between harvest and loading into store.
Fig. 2.14. Box filler. (Courtesy of Haith Tickhill Group, Doncaster, South Yorkshire, UK.)
Harvesting and Store Loading Systems 49
Fig. 2.15. Ventilated trailers. (Courtesy of Benny Jensen, BJ-Agro ApS, Hovborg,
Denmark.)
With box storage, the potatoes can be loaded directly into store following har-
vesting or passed over an into-store cleaning and sizing system, if this is practised.
Alternatively they can be pre-dried in batches either by leaving them in a windy
location or placed on fan-ventilated drying systems (Ch6.8).
The simplest of these systems is to stack boxes under a canopy and take advantage
of the wind to ventilate boxes. This is possible only in exposed sites where windless
days are rare.
The advantages of this system are:
● Low cost.
● Simple.
● No running costs.
● In good drying weather crops may dry in 2 days.
The disadvantages are:
● No control of crop temperature if nights are cool.
● If frost is forecast it may be necessary to move boxes rapidly indoors.
● The approach of warm weather fronts may result in condensation forming on
the newly dried crop.
the batch temperature within a range suitable for wound healing and to prevent
humid ventilating air from inducing condensation on the crop (Ch8.6). The drying
system may be portable or fixed. A number of options are available.
Drying tents
Tents (Wedderspoon Processes Ltd, Forfar, UK) made of plastic fabric, which fit over
a stack of 36 1-t boxes, laid out in a pattern of two boxes wide by three deep and
stacked six boxes high, provide a portable box drying system (Fig. 2.16). A fan fitted
to a frame on the top of the stack of boxes sucks air from the base of the stack, up
through the six layers of boxes, a distance of 5.5 m. The fan is sized to cope with the
resistance of six 0.8-m-deep layers of potatoes. Some tent users have fitted the tents
over a fixed or mobile frame, or suspended them from the purlins of a building, to
avoid the risk of staff falling when walking on the top boxes while putting the fan in
place. Two alternative sizes of fan are used: a single-phase axial flow fan giving airflow
rate of 2.4 m3/s at 200 Pa or a three-phase axial flow fan giving 3.8 m3/s at 200 Pa.
The tent will work on all types of boxes regardless of their design, but operators
need to check that health and safety regulations for working at height are adhered to.
Letterbox
A letterbox duct is a large box, with holes in it to match the pallet apertures of boxes
placed against it. It can be wall-mounted or made to be portable so that it can be
moved by forklift to where it is required. For small batch-drying units, the fan or
fans are usually mounted on the top of the duct (Fig. 2.17) and either blow or suck
air through the boxes. To minimize the leakage of air from gaps between boxes,
Fig. 2.17. Letterbox duct drying/ventilation system, for a 4×4 stack of boxes.
(Courtesy of Tolsma Techniek, Emmeloord, The Netherlands.)
Suction wall
A suction wall system can be made with two rows of boxes placed with a space
between them (Fig. 2.19). The top and ends of the space are then sealed with canvas,
and reinforced with timber or plastic battens, to form a plenum. If a fan is now
52 Chapter 2
Extractor fan
installed to extract air out from the plenum, the canvas is drawn inwards to seal itself
against the boxes. Air is sucked horizontally into the sides of the rows of boxes, into
the plenum, through the fan and back out into the air surrounding the boxes.
For best results the boxes should be designed to allow ventilation to flow hori-
zontally through the potatoes in the boxes, rather than through the gaps between
layers of boxes. A minimum airflow rate of 0.04 m3/s/t is required to get the can-
vas to seal against the boxes.
2.9.1 Bulk
Bulk loading almost always uses elevator belts for the reasons given above.
Management of the loading process is discussed in Ch9.5.
2.9.2 Boxes
Boxes are loaded into store using a forklift truck, usually fuelled by diesel or LPG,
with solid tyres made from an aerated rubber. Forklifts with a 3.0–3.5-t lifting
capacity are normally used for lifting 1-t boxes, in order to keep the forklift mast
low enough to pass though doors. The last two boxes are often put up together
(Fig. 2.20). This results in the load being 2.2 t in total as the boxes weigh approxi-
mately 100 kg each. Forklifts of this size are rated at what they can lift 500 mm
from the fork uprights. Since UK boxes are usually 1830 mm long and 1220 mm
wide, their centre of gravity is 610 mm forward of the fork uprights. The safe lifting
capacity is reduced as a result and is further reduced the higher the last two boxes
need to be raised.
Forklifts should be designed to minimize the vibration felt by the driver, in line
with requirements in the UK under the Control of Vibration at Work Regulations
(HSE, 2005a). Forklift exhausts should discharge vertically upwards, not down to
the floor, as the jet stream from the downward discharge exhaust causes dust on
the floor to become airborne.
54 Chapter 2
Forklift trucks fuelled by LPG rather than diesel give a cleaner exhaust but the
fuel is more expensive, with spare bottles having to be stored in a secure compound.
Electric trucks produce no exhaust, but usually only have the capacity to run for
8 h and therefore an extra set of batteries, costing up to £5000, is required to cope
with extended workdays at harvest and grading.
Forklift trucks designed for uses in fields, fitted with agricultural tyres, are diffi-
cult to manoeuvre in the tight confines of a potato store and they also need a great
deal of room to turn if boxes are to be stacked either side of a passageway. Use of
these types of forklifts in store is therefore not recommended.
2.9.3 Sacks
Potatoes are usually only stored in jute or polypropylene sacks in countries with low
labour costs (e.g. India). Sacks are normally loaded into position manually. If
labour costs rise, the use of a sack elevator to reduce some of the burden of carry-
ing sacks up multiple numbers of floors may be justified.
Seed growers who harvest directly into boxes sometimes clean and size a portion
or all of their crop in late autumn, after the potatoes have been in store for 3–6
Harvesting and Store Loading Systems 55
weeks and are thoroughly dry. This requires boxes to be tipped on to a belt, passed
over a cleaner and sizer, and returned to their boxes. Grading is therefore done in
two stages, one in the late autumn and one prior to dispatch.
The benefits from this are:
● Attached soil has dried by then, so readily falls off the tubers.
● Grading can be carried out in a relaxed manner without the pressure of getting
the crop into store.
● Low speed of throughput over the cleaner/sizer minimizes damage.
● Potatoes can be split into different size grades so that the quantities are known
and are available for rapid dispatch.
● Grading time required in the spring pre-planting rush is reduced.
2.11 Summary
Harvesting and store loading are the most critical operations undertaken by growers
if crops are to achieve their maximum sale price and disease development in store
is to be avoided. The main points to be followed are listed below.
● Minimizing damage during harvest is more important than high harvesting
rates.
● Regular sampling and inspection of crop leaving the harvester and entering
store is vital to avoid large tonnages of potatoes being damaged before faulty
equipment, incorrect harvester settings or poor harvesting practices are
discovered.
● Potatoes entering store should be free from soil, clods, stones and haulm
to allow free movement of ventilating air and the expense of storing waste
material.
● Cleaning into store removes most foreign material but can cause damage and
loss of bloom.
● Into-store cleaning equipment should be designed to suit the soil conditions of
the area.
● Air is by far the best pesticide, to remove respiration heat, to dry the crop, to
remove any condensation and maintain the crop dry in store.
● Any pesticide used should be applied to potatoes that are free from soil.
56 Chapter 2
● Any liquid chemical applied to potatoes should be dried off using an air knife
or ventilation by wind or fan.
● Whole store drying ventilation systems allow drying after harvest, and at any
time thereafter, during the storage season.
● Batch-drying systems are cheaper to install but usually require double handling
of boxes if drying is needed mid-storage.
● Seed cleaning 3–6 weeks after the end of harvest can reduce the time taken
for out-of-store grading prior to dispatch, but care has to be taken to avoid
damage and related disease development.
3 Store Climate
There are numerous influences that act on the stored crop. These are addressed in
the following sections.
A single tuber left on its own in the grading shed will become shrivelled and have
a soft, rubbery texture after a month. Had the same tuber been kept in the centre
of a 1-t box, it would still have been firm. This is because potato tubers within a
box, sack or pile create their own, largely beneficial, microclimate.
©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton) 57
58 Chapter 3
Tubers
Void between
tubers
RH=97.8%
If an airtight container is filled with potatoes and its lid closed, moisture will
evaporate from the tubers and humidify the air in the voids between the tubers
(Fig. 3.1). As this air approaches 97.8% RH* (Hunter, 1985), evaporation from the
tubers will slow to almost zero and little weight loss will occur thereafter. A state
of equilibrium is reached between the humid air surrounding the tubers and the
tubers themselves. Storage in a sealed building with just enough air bled in to keep
carbon dioxide and oxygen levels near ambient will minimize weight loss through
evaporation and maintain skin appearance. Were it not necessary to remove respi-
ration heat, supply oxygen and cool the crop after harvest, completely sealed stor-
age would be the optimal storage system.
Potato tubers are living organisms which consume a proportion of their food
reserves to stay alive (Ch1.5). The respiration process results in oxygen being
absorbed into the cells of the tubers, which is used in the oxidation of sugars to
provide the substrates and energy required for cell maintenance. Two of the by-
products of this oxidation process, carbon dioxide and heat energy, are released.
A third by-product, water, remains within the cells (Fig. 1.15).
The respiration heat given off by the tubers warms the void space air, reducing
its RH to below the optimum of 97.8%. Unless this heat is continuously removed,
a steady-state RH of 97.8% can never be achieved, so some evaporation of mois-
ture from the tubers will always take place. The heat generated by respiration is a
maximum at harvest and varies with storage temperature, being lowest at 5°C and
higher above and below this figure.
To avoid store carbon dioxide rising to a level that can affect the fry colour of
the stored processing crop or cause blackening of the flesh, usually referred to as
blackheart (Fig. 3.2), a very small amount of ambient air needs to be bled into
store. If crops are respiring highly, as with early harvested crops, excessive carbon
*Other researchers (Cook and Papendick, 1978) suggest that the equilibrium RH is 99.3%, so
this figure may not be precise.
Store Climate 59
dioxide levels can occur in a closed store within 24 h. In most stores, unless designed
to be completely gas-tight, small gaps in doors and the building fabric provide suf-
ficient air leakage to prevent problems with fry colour. Where stores are completely
sealed, a small fan or control on the ventilation system should allow sufficient air
to enter the store to maintain acceptable oxygen and carbon dioxide levels.
In an unventilated bulk pile of potatoes, the air movement within the pile is often
likened to that occurring in a ‘fire’, where the heat produced in the respiration
process causes air to rise up through the pile, drawing in air surrounding the pile
as it does so (Fig. 3.3). While this natural convection current is much lower than
that which would occur in a real fire where temperature differences would be so
much higher, it is impossible to stop. The warm up-current of air evaporates mois-
ture from tubers as it rises, until it meets the surface potatoes, which are often
colder than the potatoes below. This results in condensation forming just under the
surface of the top layers of potatoes, termed subsurface condensation (Box 3.1).
If a ventilation system is installed that allows air to be circulated though the
crop, the respiration heat can be removed as it is being produced, thereby prevent-
ing this subsurface condensation, as well as minimizing temperature differences
within the crop. When cooling is required, louvres can be opened to allow cool
ambient air to be blown through the crop. Alternatively, a refrigeration system can
be used to cool the air within the store, so it can be circulated to cool the crop.
Rising airflow
Pile of
Condensation
potatoes
Cold layer
Air drawn
in from Warm
sides ‘fire’
‘Sweat’ is the term normally used when humans exude moisture, when hot, to cool
themselves. It is also used to describe the appearance of moisture on building sur-
faces, but the former use is the more common.
As potatoes neither exude moisture (i.e. force moisture out through the skin)
nor try to cool themselves by doing so, the term simply confuses the science and
should therefore not be used.
If moisture does appear on potatoes that were previously dry, it is caused by
moisture in the air condensing on potatoes whose skin temperature is below that
of the dew-point temperature of the surrounding air (Ch3.5).
Where the cool surface of the pile is caused by heat loss through radiation to
a cold headspace above, timely heating of the headspace air with electric heaters
will prevent subsurface condensation (Ch3.5).
While it might be thought that the atmosphere in a completely sealed store would
be immune to changes in ambient air temperature, this is not the case. Just as the
sealed store prevents air entry or escape, it also prevents vapour within the air from
passing through the store fabric. In contrast, heat can enter or leave the store, with
the amount determined by the insulation value of its fabric.
The RH in the headspace of intermittently ventilated, well-sealed potato stores
varies between 90 and 98% (Pringle et al., 1997), averaging about 94%.
On a warm sunny day, heat will enter the store and cause its air temperature to
rise and its RH to fall. The RH in store will therefore fall to below 94% (Box 3.2).
For a mass of air enclosed in a sealed box or empty building, the relationship
between temperature and relative humidity (RH) is like a child’s seesaw (Fig.
B.3.1). When the temperature of the air is raised as at a), due to heat entering the
box, the air’s RH will fall. If the temperature of the air falls as at b), due to heat leav-
ing the box, the RH of the air will rise. This happens because warm air can hold
more moisture than cold air.
Temperature/RH seesaw
(a) Temperature (b) Temperature
rises • • • falls • • •
• • • so RH rises • • • so RH falls
On a cold night, heat will escape from the store, causing the store air to reduce
in temperature and its RH to rise. The RH in store will rise above 94%.
Changes in ambient air temperature will therefore result in oscillations in inter-
nal store temperature and RH; a rapid drop in outside temperature can result in
condensation forming on the roof and on the coldest parts of the crop (Ch3.5).
The voidage, or air spaces between the potatoes, allows free air movement through
the crop. If these voids become blocked for any reason, air will not pass, so drying or
cooling will either not take place or be slowed. Voids can become blocked due to:
● Large quantities of wet soil coming into store with the potatoes.
● Soil falling off potatoes being loaded into a pile and forming a soil cone.
● Small tubers separating out from larger ones as the potatoes fall from the elevator
loading the pile.
● Haulm or weeds present in the crop.
● Rotten mother tubers or rots collapsing under the weight of neighbouring
tubers, resulting in an impenetrable cluster of potatoes.
The smaller the tubers, the smaller the voids will be and the more likely they will
become totally blocked. Seed potatoes, often grown on soils in cool, windy areas that
regularly experience wet harvesting conditions, are most likely to encounter this prob-
lem. Blockage of voids can severely affect natural convection of air through crops and
result in tubers staying moist for weeks, months or the entire storage period.
During very wet harvests, potatoes are sometimes harvested when there is free water
in the soil. Such potatoes should be dried before loading them on to a bulk pile. If
harvested into boxes, potatoes can be blown with a high volume of air to dry them
out. They should not be put into a store un-dried. Damp potatoes and soil act in
the same way as an air humidifier. Air moving through the damp material will
approach a RH of 100%, so that store ceilings and potatoes may drip with moisture.
These are ideal conditions for disease to flourish. The removal of this moisture will
reduce the RH in store below 100% and prevent condensation forming on cold
parts of the crop or roof. This will transform a store from one that drips water to
one containing potatoes with dry skins and a headspace RH of about 94%.
If the potatoes are ventilated immediately after harvest, with air passing between each
and every tuber, the respiration heat that causes subsurface condensation can be
removed and the damp crop and soil dried rapidly. The respiration heat will warm
the air slightly, increasing its water-carrying capacity and therefore its drying potential.
Ventilation immediately after harvest will not only prevent subsurface condensation
occurring but will dry any moisture present on skins and in the adhering soil.
62 Chapter 3
Airflow sufficient to remove the high rate of respiration heat being produced
is required for 4–5 days after harvest until the respiration heat produced has
declined to near dormant levels (Fig. 1.16). Further ventilation is likely to be
required to completely dry the crop, with either reduced rate, or intermittent, ven-
tilation required to remove temperature differentials caused by respiration.
Thereafter ambient-air ventilation or refrigeration will be required to cool the
potatoes to the desired long-term crop storage temperature.
Since the soil pores of wet soil are filled with water, if air is forced through any
remaining free voids, the soil pores will gradually dry out. If the potatoes are then
put over a cleaner or grader, the soil will normally shatter and fall off. The clean
potatoes will then have clear voids through which air can pass. Ventilation will
remove any remaining surface moisture. This approach may not work in potatoes
covered in soils with high clay content, so such soils should be avoided for potato
production.
3.1.9 Temperature
Potatoes, kept together in a pile, box, bag or sack, create their own largely benefi-
cial climate. Sealed stores keep store air RH high, minimize tuber weight loss and
prevent the air from warm weather fronts leaking into store and condensing on the
cool crop within. A small amount of ambient air is required to flush the store to
keep levels of carbon dioxide low. Airflow through the crop, especially after har-
vest, is needed to remove respiration heat to prevent subsurface condensation. Air
recirculation is required in intermittently ventilated systems to keep temperature
differences in the crop to a minimum when no cooling from ambient air is taking
place. For long-term storage and disease control, crops should be kept cool, but
unless the air is humidified, ventilation and recirculation should be kept to a mini-
mum to avoid excessive moisture loss from the crop.
Sealed crop stores minimize crop moisture loss by allowing the RH in the voids
between tubers to approach 97.8%, at which point vapour in the air is at the same
partial pressure as that within the skin of the potatoes. Air leaking into store, if at
the same or lower temperature than the crop, will usually have an RH lower than
this value (Fig. 3.4), so that the more gaps in the building fabric, the lower the store
air RH will be and the greater the weight loss. Buildings that are exposed to wind
will be more prone to such leakage than those on more sheltered sites. If ambient
temperatures are higher than the crop temperature, air leakage may result in con-
densation (Ch3.5).
Cooling of potatoes requires ventilation to remove the heat. Cooling is
achieved partly through evaporative cooling and partly through convective heat
18 100
17
Relative humidity (RH; %)
80
16
Temperature (°C)
15 External RH
60
14
13 40
12
20
11 External temperature
10 0
29-Sep 30-Sep 1-Oct 2-Oct 3-Oct 4-Oct 5-Oct
Time (01.00–24.00 hours, 1996)
transfer (Hylmö et al., 1975a), the ratio of the two forms of heat transfer being
approximately 55:45. As the cooling air warms as it passes through the warmer
crop, its RH falls, causing it to evaporate moisture from the potatoes. Cooling
therefore dehydrates the crop. The more cooling is required, the greater the mois-
ture loss. Poorly sealed stores, in comparison to well-sealed stores, will require
cooling systems to run for longer and will experience greater weight loss as a
result.
Warm potatoes
‘fire’
Uninsulated Cool
load-bearing potatoes
Convection air movement
walls
Fig. 3.5. Condensation may occur on cool potatoes near the sides of a pile.
Store Climate 65
and humidified by the potatoes, moisture in the air may condense on the cooler
potatoes near the walls. This is sometimes visible when unloading is taking
place.
If the stored crop temperature is below the dew-point temperature of the ambient
air, a proportion of the vapour in the air leaking into the building may condense
on the crop, leading to possible disease initiation and multiplication on the moist
potatoes (Boxes 3.3 and 3.4).
With farm assurance schemes becoming increasingly important in the UK, keeping
stored crops free from rats, mice and birds is increasingly important. This is most
easily done if stores are sealed to exclude vermin. Bait is usually left in corners
within the store to kill any vermin that do enter the store, but should be placed so
that it cannot be touched by domestic animals or children.
Ventilation is the term used to move air through the potato crop. It has two
components:
● Recirculation ventilation, where the same air is circulated round the store and
possibly through the potatoes themselves.
● Ambient-air ventilation, where outside air is blown or sucked into store, dis-
placing the air that was previously there.
Recirculation ventilation is used with refrigeration systems, which cool the air
within the store using a heat exchanger but do not replace the store air with fresh
ambient air (Fig. 3.6a).
Ambient-air cooling/ventilation, usually coupled with recirculation, replaces the
warm air within the store with cooler ambient air in order to cool the crop (Fig. 3.6b).
Harvest a dry crop of potatoes from dry soil, especially the earliest lifted crops which
respire the most, and look at the top of the pile or box. If you do not recirculate air
through the newly harvested potatoes within an hour or so after lifting, you will start
to see condensation forming just under the surface of the top layer of tubers.
If the pile or box is ventilated immediately the crop is harvested, condensation
due to respiration will not form. Ventilation to keep the crop free from this con-
densed moisture will minimize the chance of skin blemish disease developing or
tubers starting to rot.
66 Chapter 3
To understand how condensation occurs, imagine three 1-m cubes of air at a tem-
perature of 4, 10 and 20°C, respectively (Fig. B.3.2). These can hold a maximum
of 6.4, 9.5 and 17.5 g of water, in the form of vapour, respectively. These values are
called their saturation moisture content or saturation humidity. The amount of water
air can hold increases as the temperature of the air increases.
If air at 10°C has less than 9.5 g of water, e.g. 8.5 g, in a cubic metre, the relative
humidity (RH) of the air is calculated as the proportion this is of the saturation value:
RH = 8.5 / 9.5 × 100 = 90%.
If the RH is 100%, this is equivalent to saturation; that is, the air can hold no more
water at that temperature.
RH = 8.5/9.5100
Relative humidity (RH) = 90%
8.5 g
Cool Warm
Warming / cooling 6.4 g 8.5 g 8.5 g
90% RH 49% RH
The psychrometric chart (Fig. B.3.3) is a chart that allows the condition of air to be
defined if any two properties out of a possible four are known. These are the dry-
bulb temperature (°C), wet-bulb temperature* (°C), relative humidity (RH, %) and
moisture content (kg moisture/kg) of the air. The chart can be used further to pre-
dict what will happen when the air is heated or cooled. Since moisture content is
given on the vertical y-axis of the chart, if no moisture is added to or removed from
the air, movement from one point to another on the chart will always be parallel to
the horizontal x-axis.
It is instructive to plot the example used in Box 3.3 on the chart. In that exam-
ple, moisture content was quoted in grams per cubic metre of air, whereas on the
chart it is quoted in kilograms per kilogram of dry air. To convert from one to the
other multiply or divide by the density of air at 20°C, which is ~1.23 kg/m3.
The starting point A is where the vertical 10°C dry-bulb temperature line and
the curved 90% RH lines cross. If the air is heated from A using an electric heater
(which does not change the moisture content of the air), the horizontal line AB on
the chart shows this heating. Point B is where this line meets the vertical 20°C dry-
bulb temperature line. At this point the new RH of the air will be found to be 47%.
If the air at A is cooled, the horizontal line AC shows the change in its condi-
tion until it meets the moisture saturation line at C. The temperature at C is the
dew-point temperature 8.7°C, which is read off the dry-bulb temperature scale. On
cooling the air further from C, the condition of the air moves down the saturation
line until it reaches D at 4°C, losing moisture as it does so. The amount of moisture
lost can be found by subtracting the moisture content at D from that at C, i.e.
0.002 kg/kg, on the vertical y-axis.
10 90 70 50
Wet-bulb
temperature (°C) C 0.007
Moisture content (kg/kg dry air)
A B
5 0.006
D
0.005
0 0.004
0.003
0.002
0.001
0.000
0 5 10 15 20
Dry-bulb temperature (°C)
Fig. B.3.3. Psychrometric chart. (Section of chart redrawn with permission of the
Chartered Institution of Building Services Engineers, www.cibse.org.)
* Temperature reading of a thermometer wrapped in a wet wick and rotated rapidly in the air to
be measured to maximize the rate of evaporative cooling (see Box 3.5).
68 Chapter 3
(a)
Recirculating air
Cooling coils
Heat to
Pile of potatoes
ambient
air
Duct
Airflow
(b)
Fig. 3.6. Cooling of potatoes using: (a) recirculation ventilation with a heat
exchanger (fridge) to remove heat from circulating air; and (b) ambient-air ventilation
to remove heat by replacing the air within the building with cooler air from outside.
Potatoes are best harvested from soils that are slightly damp. Such soil will stick
together and form a cushion on the primary web of the harvester, protecting the
tubers from damage from the oscillating web bars.
The potatoes entering store will have soil still adhering to their skins. A third
of the weight of the soil may be water. After significant amounts of rainfall, the
proportion of soil moisture may be much higher. Ventilation immediately after
harvest is therefore used not only to remove respiration heat, but also to dry out
the adhering soil and remove any skin surface moisture.
Drying is usually achieved by blowing ambient air through the crop. This pro-
vides the lowest-cost method of removing the moisture. Drying may take between
3 and 20 days to achieve (Pringle et al., 1997), depending on the rate of air passing
between the tubers and the moisture-carrying capacity of the air.
Alternatively, drying can be achieved in a closed store using the evaporator of
the refrigeration system to remove the moisture from the circulating air. This uses
more electricity than ambient-air drying and will prematurely cool the crop, unless
heat is added to maintain the crop temperature (Box 8.1). The addition of heat
adds cost and produces carbon dioxide, the greenhouse gas. If the fridge is used for
drying and is allowed to cool the crop as well, this may result in condensation
occurring on the cooler potatoes when warmer, newly harvested potatoes are
loaded into store (Ch3.5). During drying of the crop, therefore, every effort should
be made to ensure that the crop already in store and the potatoes being loaded are
at the same temperature.
Store Climate 69
Ventilation is used to cool the crop, either by bringing in cooler air from outside
or by recirculating store air through the cooling coils of a refrigeration unit.
Temperature differences in the crop can occur for a number or reasons. They can
result from:
● Stocks of potatoes entering store at different temperatures.
● Uneven cooling of boxes or piles.
● Lack of insulation in store walls or roof.
● Low headspace temperatures on cold nights due to poor insulation or air leakage.
● Surface cooling of potatoes by air distribution jets.
● Different respiration rates for crops of different varieties or maturities.
● Additional respiration heat output from microorganisms once disease has
become established.
When fans are recirculating the air within the store, this forced air movement tends
to dominate over any movement of air due to natural convection. Once fan-
induced ventilation stops, natural convection currents will re-establish. Natural
convection will take place between warm and cool parts of the crop. The greater
the temperature difference between sections of the crop, the faster will the convec-
tion air movement become. When warm humid air from one part of the crop flows
over cooler potatoes in another section of the crop, condensation on the cooler
potatoes may result. To prevent disease development associated with condensation
on the crop, these convection airflows should be minimized, and this is achieved
by recirculating air to minimize temperature differentials within the crop.
In intermittent ‘start–stop’ ventilation systems, movement of air through the
crop to minimize temperature differences will result in some crop moisture loss.
The period of recirculation should therefore be just sufficient to minimize tempera-
ture differences to prevent condensation but no more.
In very well-sealed stores, carbon dioxide levels from crop respiration can rise to
levels which can result in internal blackening, where the centre of the potato is
asphyxiated. This is most likely to occur in processing stores held at warm tempera-
tures, as crop respiration increases with temperature above 5°C (Ch1.5), or where
newly harvested crops are rapidly loaded into store and the store immediately shut.
In most stores, leakage between door and store, and gaps in the fabric, are usually
sufficient to keep carbon dioxide levels below the 0.5% acceptable value. Where
there is risk of carbon dioxide exceeding this value, a small fan can be fitted into
the store fabric to give a supply of ambient air and so ensure carbon dioxide levels
are kept to low. Carbon dioxide monitoring for well-sealed stores should be part of
the monitoring equipment.
70 Chapter 3
Temperature difference,
base to surface (°C) 0 0.7 1.3 2.8 6.5
Fig. 3.7. Potato pile 3.7 m high showing how crop temperature increases with height
under natural convection and two rates of forced ventilation.
Store Climate 71
surface of the pile and cooling the potatoes. The risk of subsurface condensation is
therefore minimized.
If the fan-ventilated air is humidified to as near saturation as possible, the air
within the voids will be kept close to 97.8% RH and so weight loss due to ventila-
tion is minimized.
Constant ventilation with low rates of humidified air greatly reduces the tempera-
ture gradient within the pile, allowing the crop to be kept close to the desired set-point
temperature. The humidified air delivered to the base of the pile minimizes weight
loss in the base of the pile. As the air moves upwards, the respiration heat from the
potatoes raises its temperature, reducing its RH fractionally as it does so. The rising
air is therefore constantly drying the crop at every level up the pile, ensuring that con-
densation cannot occur and removing any skin moisture that may be present.
A cooling front in a potato pile is formed when cool air is blown upwards through
warmer potatoes (Fig. 3.8). The potatoes at the base of the front are cooled to a
temperature that is between the dry-bulb temperature of the cooling air and its
wet-bulb temperature (Box 3.5), while those above the front remain at the same
temperature as when cooling began. As cooling continues, this cooling front slowly
moves up the pile of potatoes.
When ambient air is used intermittently for cooling a bulk pile or a box of
potatoes, two cooling fronts form, the ‘gradient cooling front’ and the ‘horizontal
cooling front’ (Hylmö et al., 1975b).
B 73 h
5.0
Temperature
gradient 60 h
4.0
after 73 h Change of angle of ‘gradient
Height
(m) 3.0 cooling front’ as ventilation
proceeds over time
2.0 Ventilating 30 h
air wet-bulb ‘Gradient cooling front’
temperature at start of ventilation
1.0 = 5.7°C 10 h
Horizontal cooling
front
A
Temperature (°C) 5.0 6.0 7.0 8.0 9.0
Fig. 3.8. An example of the progress of vertical and horizontal cooling fronts in an
intermittently ventilated pile of potatoes.
72 Chapter 3
If unsaturated ventilating air is blown over a wet wick covering a thermometer (Fig. B.3.4),
the thermometer will be cooled by evaporation to the wet-bulb temperature of the air. Since
potatoes are about 80% moisture, they lose moisture in the same way as the wet wick, and
are cooled by evaporation in a similar manner. Because the skins of potatoes prevent the
release of moisture to the same extent as the wet wick, potatoes cool to somewhere between
the air’s dry-bulb temperature and its wet-bulb temperature.
15
10
Wet-bulb temperature
Water
bottle
inclined gradient that occurs when natural ventilation alone is taking place (Fig.
3.8). When the fans are switched on to cool the crop, the temperature gradient
starts to move back to the near-vertical gradient associated with a high ventilation
rate (Fig. 3.8). This cooling occurs simultaneously in all layers up the pile and
serves not only to cool the crop, but also to reduce the base to top temperature
differential.
The slope of the ‘gradient cooling front’ is determined by:
● The time since forced ventilation last occurred.
● The respiration heat output.
● The rate of ventilation.
front starts to rise vertically up through the pile. This causes a temperature differ-
ence within the layer being cooled, which is illustrated in graph form in Fig. 3.8.
In Hylmö’s theoretical example, where the cooling air is a constant temperature
and RH, the crop was at 7°C when cooling started, while the ventilating air was
set at 6°C, 95% RH. As ventilation proceeds, the horizontal cooling front rises up
the pile, while the angle of the gradient front becomes more vertical. After 73 h the
horizontal cooling front reaches the surface of the pile and the gradient front lies
along the line AB. At the end of cooling, therefore, there is still a temperature dif-
ference B – A°C between the base and surface of the pile, due to the respiration
heat coming from the crop.
(a) (b)
Temperature
Warm gradient Warm
Load-bearing
region region
wall
Cooling
zone
Cooled Cooled
region region
Fig. 3.9. Depth of the cooling front in a pile is related to ventilation rate: (a) shallow
cooling zone associated with low airflow rates; (b) deep cooling zone associated with
high airflow rates.
74 Chapter 3
ahead of it. The potatoes are cooled in part by evaporation of moisture from the
tubers and in part by heat being transferred from the warm potatoes to the
cooler air (Hylmö et al., 1975a). Approximately 55% of the cooling is through
removal of latent heat from the crop (Box 3.6) and 45% by convection and con-
duction. The air receives heat from the potatoes and leaves the cooling zone at
the temperature of the potatoes at the top of this zone. As the air rises through
the warm zone, its temperature rises slightly due to the heat being released by
tuber respiration. This increase in temperature reduces as the crop becomes
more dormant.
In storage systems where low-rate ventilation with humidified air is continu-
ous, the cooling front is virtually eliminated (Fig. 3.7).
When water changes phase, from a liquid to the higher-energy state of a gas, heat energy
is required. The heat absorbed in the change of phase is hidden, termed latent, as it does
not change the temperature of the water, only its state.
When water at 20°C evaporates to a vapour, it requires 2450 kJ of heat energy per kilo-
gram of water. In contrast, only 4.18 kJ of heat energy is required to heat 1 kg of water by
1°C (Fig. B.3.5).
When cooling potatoes, the cooling airflow evaporates some moisture from the skin.
Although the amount is small, as the latent heat of evaporation is so high, evaporation is
estimated to account for over half the cooling effect of the air (Hylmö et al., 1975a).
1 kg 1 kg 1 kg
Vapour at 20°C
1 kg 1 kg 1kg
Fig. B.3.5. Heat required to change water from ice to liquid to gas.
Store Climate 75
Refrigeration systems cool the air within the sealed store using a heat exchanger,
more commonly called cooling coils or an evaporator. During fridge operation,
all louvres and doors in the store are tightly shut. In the direct expansion (DX)
refrigeration systems used in most intermittent, ‘stop–start’ cooling systems, the
fridge’s evaporator cools the recirculating air by 2.5°C when the warm crop is
first loaded into store, reducing to a 1.5°C temperature reduction when the crop
approaches its storage temperature (Fig. 3.10). With DX refrigeration systems,
therefore, the cooling air will never be cooler than the crop by more than
2.5°C.
Refrigeration systems used for continuous cooling using low rates of humidi-
fied air provide cooling proportional to the temperature reduction required.
Since ventilation is continuous, cooling of only a fraction of a degree may be
required.
Unlike cooling with refrigeration, ambient-air cooling uses air from outside the
store to cool the crop. The temperature of this cooling air is dependent on the
weather. Not only will the temperature difference between crop and ambient air
vary from day to day, it may vary over the duration of the cooling period. The
temperature difference between crop and incoming air could be as much as
10–15°C, if no limits are put on a minimum acceptable air temperature. This is
much greater than the 2.5°C differential in a refrigerated store, and will, if allowed
to occur, cause an excessive crop temperature difference (i.e. step in temperature)
across the cooling front.
If ventilation is vertically upwards, as in a ventilated pile, warm potatoes will
overlie the cooled potatoes so that when natural convection ventilation restarts
after a period of cooling, cold air will meet only warmer potatoes, so condensa-
tion will not occur. If ventilation is downward, however, as in over-the-top
18.0°C
Temperature differential
2.5°C initially,1.5°C Evaporator
as crop approaches
3–4°C set-point 18°C
temperature
Pile of potatoes
Condenser
15.5°C
Fig. 3.10. Temperature differential between air entering the evaporator (air-on) and
air leaving the evaporator (air-off).
76 Chapter 3
4°C
A B C D
Time
Fig. 3.11. Use of blending to prevent the inlet air temperature becoming too low.
ventilated box stores or in many letterbox ventilation systems, cool potatoes will
overlie warm ones. When forced ventilation stops and natural ventilation re-
establishes, warm air rising from the potatoes yet to be cooled may result in
potential condensation on the cooler potatoes above. For ambient-air cooling,
therefore, control software must be more sophisticated than for fridge units,
if crop temperature differentials are to be kept within limits to prevent such
condensation.
The controller software is usually set to allow cooling if the ambient air tem-
perature is 1.0°C below the temperature of the crop (Fig. 3.11). Ventilation with
the louvres full open will then take place, unless the ambient air becomes too cold
(usually 4°C below the crop temperature). At this point a sensor in the supply duct
will cause the inlet louvre to close and the recirculation louvre to open, so that the
incoming air is never colder than the crop by more than the 4°C (Box 3.7). As the
ambient air temperature rises again, blending will stop at the 4°C temperature dif-
ferential and ventilation will stop when the potatoes are within 1°C of the ambient
air temperature.
Condensation on potatoes can occur if their skin surface temperature is below the
dew-point temperature of the surrounding air.
Store Climate 77
Subsurface condensation occurs on potatoes when the temperature of the tubers on the
surface is below the dew-point temperature of air rising from warmer potatoes below (Ch3.5).
If the RH of the rising air is assumed to be 96% and the crop temperature is 3°C, then the
surface potatoes only need to be 0.6°C lower than the temperature of the potatoes below
for condensation to occur. Temperature differentials in the top layers of potatoes in boxes or
a potato pile should be kept to less than 0.5°C to provide a slight margin of safety.
Cool overhead ventilating air will reduce the temperature of the top layers of potatoes. In
studies carried out (Pringle et al., 1997), the temperature of the surface tubers never
approached the temperature of the cooling air. The cooling air had to be very much cooler
than the crop for condensation to occur. An estimated figure of 4°C temperature difference
between cooling air and crop has therefore been chosen to prevent condensation occurring
in the subsurface layer of the crop.
The recommended maximum temperature differences between stacks of boxes at
store loading and during cool storage are to minimize convective ventilation, which may
result in condensation. The differentials for bulk storage are taken from standard recom-
mendations (Stark and Love, 2003).
This requirement can only arise if the surrounding air is warmer than the crop
(Box 3.8).
The RH of the air due to moisture evaporation from the stored crop is so near
100% that even small temperature differences between groups of potatoes can
cause condensation to occur.
When potatoes are dormant, the respiration heat given out is small, less than
10 W/t (Burton, 1989). Just after harvest the heat produced can be many times this
value (Ch1.5.1). This heat production causes natural convection to occur. The heat
warms the tubers, which in turn heats the air within the voids, raising its tempera-
ture. This warm air is more buoyant than the rest of the air within the store and,
78
Box 3.8. Criteria for condensation to occur
Condensation on potatoes or a building surface will only occur if the temperature of the tuber or surface is below the dew-point temperature
of the surrounding air. As the dew-point temperature of air at 100% relative humidity (RH) is the same as its dry-bulb temperature, it follows
that the surrounding air must also be warmer than the tuber or surface. Table B.3.2 gives dew-point temperatures of air based on its tempera-
ture and RH (CIBSE, 2006).
Example 1
If the air’s dry-bulb temperature is 17°C and its RH is 70%, the dew-point temperature is 11.6°C.
Example 2
To determine the risk as to whether ambient air would condense on potatoes in store when the store door is opened:
● Measure dry-bulb temperature of air and its RH, e.g. 14°C and 70% respectively.
● Get dew-point temperature of air from the table, i.e. 8.7°C.
Chapter 3
● Check the temperature of the potatoes, e.g. 6.0°C.
Since crop temperature is below the dew-point temperature of the ambient air, condensation will occur on the stored crop if ambient air
enters the store.
Store Climate
Table B.3.2. Dew-point temperature of air based on its temperature and relative humidity (RH).
Dry-bulb
RH (%)
temperature
(°C) 60 62 64 66 68 70 72 76 80 84 88 92 96 100
20 12.1 12.6 13.1 13.6 14.0 14.5 14.9 15.7 16.5 17.3 18.0 18.7 19.4 20.0
19 11.2 11.7 12.2 12.6 13.1 13.5 13.9 14.8 15.5 16.3 17.0 17.7 18.4 19.0
18 10.3 10.7 11.2 11.7 12.1 12.5 13.0 13.8 14.6 15.3 16.0 16.7 17.4 18.0
17 9.3 9.8 10.3 10.7 11.2 11.6 12.0 12.8 13.6 14.3 15.0 15.7 16.4 17.0
16 8.4 8.8 9.3 9.7 10.2 10.6 11.0 11.8 12.6 13.3 14.0 14.7 15.4 16.0
15 7.4 7.9 8.3 8.8 9.2 9.7 10.1 10.9 11.6 12.4 13.1 13.7 14.4 15.0
14 6.5 6.9 7.4 7.8 8.3 8.7 9.1 9.9 10.7 11.4 12.1 12.7 13.4 14.0
13 5.5 6.0 6.4 6.9 7.3 7.7 8.1 8.9 9.7 10.4 11.1 11.7 12.4 13.0
12 4.6 5.0 5.5 5.9 6.3 6.8 7.2 8.0 8.7 9.4 10.1 10.8 11.4 12.0
11 3.6 4.1 4.5 5.0 5.4 5.8 6.2 7.0 7.7 8.4 9.1 9.8 10.4 11.0
10 2.7 3.1 3.6 4.0 4.4 4.8 5.2 6.0 6.7 7.5 8.1 8.8 9.4 10.0
9 1.7 2.2 2.6 3.0 3.5 3.9 4.3 5.0 5.8 6.5 7.1 7.8 8.4 9.0
8 0.8 1.2 1.7 2.1 2.5 2.9 3.3 4.1 4.8 5.5 6.2 6.8 7.4 8.0
7 −0.1 0.3 0.7 1.1 1.5 2.0 2.3 3.1 3.8 4.5 5.2 5.8 6.4 7.0
6 −1.0 −0.6 −0.2 0.2 0.6 1.0 1.4 2.1 2.8 3.5 4.2 4.8 5.4 6.0
5 −1.8 −1.4 −1.0 −0.7 −0.3 0.0 0.4 1.2 1.9 2.5 3.2 3.8 4.4 5.0
4 −2.7 −2.3 −1.9 −1.5 −1.2 −0.8 −0.5 0.2 0.9 1.6 2.2 2.8 3.4 4.0
3 −3.5 −3.1 −2.7 −2.4 −2.0 −1.7 −1.3 −0.7 −0.1 0.6 1.2 1.8 2.4 3.0
2 −4.3 −4.0 −3.6 −3.2 −2.9 −2.5 −2.2 −1.5 −0.9 −0.3 0.2 0.8 1.4 2.0
1 −5.2 −4.8 −4.4 −4.1 −3.7 −3.4 −3.0 −2.4 −1.8 −1.2 −0.7 −0.1 0.4 1.0
79
80 Chapter 3
being warmer, it can hold more moisture. It rises up the crop through the voids,
evaporating moisture from potatoes in the process. If the skins of the crop above
are colder than those in the layers below, the water in this warm humid air may
condense on the cooler subsurface potatoes. This is similar to the situation that
normally occurs in an unventilated potato pile (Fig. 3.3), but the high respiration
associated with early harvesting and initial wound healing makes it more likely to
occur. This condensed moisture becomes available to any microorganisms present.
These can be on the skin, on sprout buds or within lenticels. The moisture can
allow spores such as silver scurf to germinate, gangrene or skin spot to sporulate
and re-infect, or anaerobic conditions to occur within the tuber. A complete film
of condensed moisture surrounding the tuber can cause tubers at 10°C to become
anaerobic within 6 h (Burton and Wiggington, 1970), providing conditions that
favour the development of soft rot and pectolytic Clostridium species.
Normally the heat comes from tuber respiration, but increases in the host respira-
tion and contributions from the pathogen’s own respiration in infected plant tissues
are common. Subsurface condensation in parts of the pile or in specific boxes may
therefore be due to localized rotting occurring just below these areas.
It is the difference in temperature between the potatoes lower down and those on
the surface that results in subsurface condensation; thus condensation results from
the combination of respiration heating and top-surface cooling. If air significantly
colder than the crop is jetted across the top surface of the potatoes, some of this
air penetrates the potatoes and cools the upper layers of the pile or box. When
ventilation stops, the warmer air rising from below condenses on the cold potatoes
near the surface (Fig. 3.12). This problem was traditionally minimized by the use
of straw (Box 3.9) but its use in sealed stores is now rare.
The greater the temperature difference between the potatoes below the subsurface
layer and those within the cooled layer, the more likely that condensation will
occur. Highly respiring, newly loaded crops are therefore more likely to experience
such condensation than crops that have become dormant after a period in store.
(a)
Cold ventilation air
penetrates and cools
Cold ambient Cold airflow upper layers of crop
air (e.g. 3°C)
Warm Warm dT
Temperature crop
e.g. 10°C,
dT = 1 to 3°C
(b)
Louvre
closes Natural convection Condensation forms
on subsurface potatoes
Warm Warm dT
Fig. 3.12. Subsurface condensation due to over-the-top ventilation with air that is
too cold: (a) ventilation with cold ambient air; (b) ventilation stops so natural
convection dominates.
Box 3.9. Use of straw to keep potatoes near the crop surface warm
Traditionally, potato piles were covered with 0.3−0.4-m-deep loose straw or 0.36 m
× 0.4 m × 0.8 m straw bales to keep the top surface of the pile warm and so mini-
mize condensation or frost damage in the crop subsurface layer. While this is quite
effective with bulk stores, in box stores only the top layer of boxes can be protected
in this way. It is not practical to cover each box with straw as the stack is being built.
As loose straw can clog up the sizing equipment when potatoes are being graded,
it is preferable to keep the potato surface warm by use of sealed stores and sophis-
ticated ventilation control.
when stored together in a large mass, will lag air temperature changes by a
number of days. This lag will occur with potatoes stored outside. It will also
occur if crops are stored inside with the doors left open, or if air leakage rates
are high.
Should a warm front pass over and ambient air come into contact with crop
with a temperature lower than the dew-point temperature of the ambient air, con-
densation will occur. This is a particular problem when stores are kept open for
long periods for filling and is made worse if ventilating fans are switched to manual
operation to encourage respiration heat removal and drying.
Warm crops are less likely to experience such condensation than cooler crops,
as they are less likely to be below the dew-point temperature of the air. This
82 Chapter 3
explains why crops in store should not be cooled until the store is full and the doors
can be closed permanently.
If potatoes being loaded into store are at a different temperature to those already
in store, convection currents will occur, which can lead to possible condensation on
the cooler potatoes.
To illustrate this, suppose that two stacks of potatoes in boxes, one at 16°C
and one at 10°C, are loaded into store, and the store door is shut (Fig. 3.14). The
store air temperature will assume the average of the two stacks, i.e. 13°C. The void
air between the tubers in the 16.0°C stack is warmer than the air in the store, so
it will rise. The void air between the tubers in the 10.0°C stack is cooler than the
store air, so it will move downwards. If the two stacks are close to each other, a
convection-driven circulatory air movement will be established. If the RH of the
store air is high, and the temperature of the cooler potatoes is below the dew-point
temperature of the circulating air, condensation will occur on the top of the cooler
stack.
Potato crops are usually held below ambient temperature in the UK. If stores are
leaky or if doors are left open, warm humid ambient air can enter the store and
condense on the crop. In a box store, the top layers of boxes are most likely to
experience condensation as the warm air leaking into store will tend to rise to the
Store Climate 83
Convection airflow
13.5°C
Possible
16.0°C 13.0°C 10.0°C condensation
12.5°C
Fig. 3.14. Risk of condensation when boxes at two different temperatures are put
into store.
roof space and fill it first (Fig. 3.15). If leakage is sustained, the whole upper part
of the building will fill with warm air, with condensation occurring progressively
down the layers of boxes.
An even worse scenario may occur if ventilation is started during these condi-
tions. This can completely flood the store with warm air, which may result in con-
densation occurring on the entire crop.
Suction due
Warm ambient to wind
air (e.g. 15°C) Air escapes via leaks
in store fabric
Air enters via leaks Warm leaking ambient air
in store fabric collects in headspace
Possible condensation
Wind
pressure
Potatoes stored
at 3°C
Fig. 3.15. Warm ambient air entering stores through leaks in the fabric can
condense on the crop in store.
84 Chapter 3
(a)
Louvre open
Louvre Louvre
Air circulation
closed closed
Warm
(b)
Fig. 3.16. Natural convection: (a) with recirculation louvre open and fan off, natural
convection is anticlockwise; (b) with recirculation louvre closed and fan off, natural
convection is clockwise.
drawn in by the ‘fire’ (Ch3.1) will enter the pile from areas where the heat being
generated by respiration is less. Typically this is on the sloping front face of the pile
or where potatoes are cooler, near the store walls (Burton et al., 1955).
In warm weather, if warm humid air leaks into store, this warm air will be
drawn into the sides and sloping front, to cause condensation (Fig. 3.16).
In freezing weather, if cold air leaks into store, the ‘fire’ is going to pull this
freezing air in the sloping front and cold sides of the pile. This is where frost dam-
age is likely to occur.
In bulk stores, it is common practice to recirculate air through the ventilation sys-
tem to reduce the temperature gradient between the top and base of the pile. This
is to ensure that all parts of the crop are as near as possible to the optimum storage
temperature to ensure fry colours are uniformly light (Ch3.10). However, com-
monly warm humid air is allowed to leak into the headspace of these stores prior
to recirculation taking place. If the potatoes at the base of the pile are below the
dew-point temperature of the headspace air, recirculation ventilation may result in
condensation around the ducts at the base of the pile (Fig. 3.17). At the warm tem-
perature that processing crops are held, between 8 and 12°C, wetting the crop is
likely to initiate rotting. The first signs the store manager sees that this is occurring
are the surface potatoes slumping into hollows and ventilating ducts filling up with
an odorous brown liquid from rotting potatoes.
Store Climate 85
Fig. 3.17. Recirculation of warm headspace air can lead to condensation on crop
above ducts.
Condensation just under the surface of intermittently ventilated bulk stores was
observed (Hylmö et al., 1976) in strips at the surface, half way between the lateral
ducts spaced at 4 m intervals (Fig. 3.18). Hylmö’s evidence suggested that the
longer air path between the duct and the surface where the condensation formed
resulted in less cooling of the crop at this point. When a fall in ambient air tempera-
ture caused the air in the headspace above the potatoes to cool, the surface layer
of the potatoes cooled too, resulting in cool potatoes overlying warmer potatoes
below. Since the warmest potatoes lay between the ducts, condensation formed
mid-way between each duct. Condensation could have been prevented by either
reducing the distance between ducts or by installing roof-space heating. It is likely
that continuous ventilation, which would have reduced the temperature gradient in
the pile, would also have prevented its occurrence.
Modelling using computational fluid dynamics (Xu and Burfoot, 1999) con-
firmed this interpretation and suggested that, had ventilation stopped earlier, con-
densation could have occurred lower down the pile, out of sight of the store
manager. The maximum thickness of condensation film was estimated to be
0.08 mm, well above the 0.03-mm film that caused anaerobic conditions within
tubers to occur (Burton et al., 1992).
Another investigation by the Hylmö team (Grähs et al., 1977) found condensation
in a number of 3000-t ventilated bulk stores, with temperatures varying from 6.5
to 9.0°C, leading to convection airflows and condensation-induced soft rotting
(Erwinia carotovora) with secondary attacks of dry rot (Fusarium). The uneven tempera-
tures were caused by uneven airflow rates varying from 0.0011 to 0.033 m3/s/t.
Better design of the air distribution system or continuous ventilation with humid
air would have prevented this problem. Conversion of old ventilation systems to
continuous ventilation with low-rate humid air results in lower air speeds in exist-
ing ducts, which in itself results in more uniform air distribution.
While sealed stores are best for keeping the store atmosphere at a high RH to mini-
mize dehydration, they are susceptible to condensation on the underside of the roof
and on the coldest parts of the crop if ambient temperature falls rapidly, as often
happens on clear, cloudless nights.
This can be illustrated by imagining a sealed, insulated store with the tempera-
ture of the crop, headspace air and ambient air all at the same 8°C, with a head-
space RH of 96%. A steady-state situation will result where heat neither leaves nor
enters the store (Fig. 3.19).
If the ambient temperature now rises (e.g. 14°C) heat will enter the store, raising
the headspace temperature. If the headspace air temperature rises to 10°C, its RH will
reduce to 80% (Box 3.2). Heat can pass through the insulation, but the vapour in the
headspace air is trapped within the store. The warmed headspace will have little effect
on the crop other than to increase weight loss through evaporation slightly.
If, instead of the ambient air temperature rising, it falls to 2°C, heat will leave the
store and the headspace air temperature will fall. If the headspace temperature falls to
6°C, its RH will not only rise to 100%, but condensation will occur on the coldest sur-
faces within the store, usually the roof, where it can drip on to the potatoes. Condensation
may also occur on the coldest parts of the crop. While this is a transient phenomenon,
the moisture on the crop can allow spores to germinate and rotting to start.
Fig. 3.19. Condensation can occur on underside of roof or on cold potatoes when
ambient temperatures fall rapidly.
Store Climate 87
This form of condensation is not common, but the author did find it in all five box
stores on one farm 2 months after loading. Condensation occurred primarily in the
bottom layer of boxes. Air speeds along the ducts formed by the pallet apertures
between the bottom layer of boxes and the concrete floor were double those in the next
pallet aperture up (Fig. 3.20). This was probably due partly to the resistance to airflow
of the smooth concrete floor being less than that of the pallet apertures between the
layers of boxes and partly because air from the fridge was cold and dropped to the
floor. This differential air speed caused a Venturi effect, with air being drawn down-
wards in the base layer of boxes when the recirculation fans were operating. Once the
fans stopped, natural convection dominated air movement and warm air from the
warmer potatoes below rose up to condense on the cooler potatoes above.
As the criteria for condensation to occur require that ventilating air must be
warmer than the crop and the crop must be below the air’s dew-point temperature,
warming potatoes, for example prior to grading following storage, is very likely to
result in condensation. Only if the dew-point temperature of the warming air is
below that of the crop will warming take place without condensation occurring.
In summary, condensation on the crop will occur if the crop’s temperature is below
the dew-point temperature of any air with which it comes into contact. Air’s dry-bulb
and dew-point temperatures are the same at 100% RH, so by definition air must be
Warm Warm
Warm Warm
Condensation occurs
when forced Cold Cold
ventilation stops
Warm Warm
Air speed = 2 V
Smooth floor
Fig. 3.20. Venturi effect in floor duct causes cooling air to flow downwards, resulting
in cold potatoes overlying warm.
88 Chapter 3
warmer than the crop if condensation is going to form. The lower the RH, the lower
the dew-point temperature, the less likely that condensation on the crop will form. The
likelihood of condensation can be predicted by looking up Table B.3.2 in Box 3.8.
For the reasons explained above, condensation on potatoes will occur when:
● Crop temperature in a store with its doors open is below the ambient air dew-
point temperature when a warm weather front is passing or the crop has been
cooled below the air’s dew-point temperature.
● Excessive temperature differences develop within the stored crop, causing
moisture in convection air currents from warmer potatoes to condense on
cooler crop.
● Sudden reduction in ambient temperature causes headspace air to reach 100%
RH.
● Potatoes are warmed with air having a dew-point temperature above the tem-
perature of the crop.
All systems of storage are vulnerable to condensation when stores are being loaded
and doors are open, or when no RH sensor is used to control ventilation during
wound healing (Ch8.6). Once stores are shut and set to cool automatically, conden-
sation should be prevented by well-designed ventilation controllers.
Temperature differences within the crop can be minimized by installing a
continuous, low-rate, humidified ventilation system. Even these systems are prone
to condensation on roof and crop from sudden reductions in ambient tempera-
tures, so should be fitted with roof-space heating systems.
In intermittent ventilation systems, the problems of excessive temperature dif-
ferentials and the associated condensation can largely be overcome by well-designed
airflow distribution, coupled with precise, automatically controlled ventilation to
minimize temperature differentials within the crop.
Sample average maximum and minimum temperatures for the coldest and warm-
est months in the year, together with average RH, for different zones of the world
are shown in Table 3.1 (BBC, 2007). More extensive data for the world are shown
in Appendix 2.
When storing crops in different climatic areas, there is greater emphasis on
some aspects than others. In continental climates where winter temperatures can
reach −20°C and below, the prevention of freezing of the stored crop and mini-
mizing crop desiccation due to ventilation with cold, dry air are the major priori-
ties. In the maritime climate of the UK, the drying of crops and excluding warm
humid air from entering the store and causing condensation on the potatoes is of
greater concern. In tropical lowland climates where little air below 10°C is availa-
ble for cooling, growers with simple stores depend on long-dormancy varieties, to
allow a month or so of sprout-free storage (Ch3.8). For longer periods, total depend-
ence has to be placed on refrigeration. In tropical highland climates, the altitude
will determine how much cool night air is available for cooling; this will allow some
ambient-air cooling to take place.
Store Climate
Table 3.1. Average minimum and maximum temperatures, and average relative humidity (RH), for the coldest and warmest months of the
different world climatic zones.
Continental
East Canada −9 (−27)a 0 (14) 76 13 (4) 23(37) 73 Freezing in winter and
weight loss
Maritime
East UK 1 (−14) 6 (15) 89 12 (5) 21(31) 74 Crop condensation from
warm air leaking into store
Continental/maritime
The Netherlands −1 (−25) 4 (13) 86 13 (4) 22 (34) 72 Freezing in winter and crop
condensation from warm
air leaking into store
Mediterranean
Egypt 11 (3) 18 (28) 66 23 (18) 31 (41) 70 Sprouting and desiccation
Tropical lowlands
Bangladesh 12 (7) 25 (31) 46 26 (22) 32 (36) 72 Sprouting and desiccation
Tropical highlands
Peru 5 (−2) 21 (25) 60 8 (−4) 22 (26) 62 Sprouting and desiccation
a
Figures in parentheses are the record minimum and maximum temperatures.
89
90 Chapter 3
3.7.1 Introduction
In cool maritime and continental temperate climates with winters which allow
ambient-air cooling to keep crops near 3–4°C, sprout-free storage for 6–7 months
is usually possible. Storage at this temperature beyond 7 months requires refrigera-
tion or sprout suppressants. Storage for 9 months is routinely achieved. Longer
storage is possible but cumulative weight loss causes quality to deteriorate. Potatoes
for processing are normally held at between 8 and 12°C for up to 9 months, with
suppressants being used for sprout control.
In warm tropical climates, stores without refrigeration or sprout suppressants
can hold potatoes only for the length of their innate dormancy period, a period less
than 1 month (Wustman et al., 1985). With sprout suppressants this can be extended
to 4–5 months at 25°C or 1 month at 30°C. For a longer period of storage, refrig-
eration is required.
Storing potatoes in sealed, insulated, environmentally controlled stores mini-
mizes the dehydration, disease and skin blemishes which can occur in unsealed
stores and clamps. Disease-free, firm potatoes greatly increase their sale value.
Storage ventilation has evolved over time, with the system selected matched to
the country’s climate. These fall into four main groups, which are described below.
They comprise systems of ambient-air cooling that are used worldwide, with refrig-
eration used where insufficient ambient air is available for cooling or in tropical
regions where ambient air for cooling is not available. Simple systems based on
storing potatoes in cool cellars or in open ventilated, timber structures are not
mentioned here but are discussed in Ch5.1.
differences. The traditional airflow rate used is 0.02 m3/s/t (BPC, 2001a). This rate
provided acceptable rates of drying, cooling and air distribution in bulk stores over
many years, so when box storage was introduced the same rate was used. Experience
has shown that so long as this airflow is distributed uniformly over the whole crop,
whether in bulk or box, crops can be kept satisfactorily. However, for positive venti-
lation of potatoes in UK-style boxes (Ch6.8), where between half to three-quarters
of the air (Pringle, 1989) may leak from gaps in and between boxes, this airflow rate
may need to be multiplied by 2–4 times to achieve 0.02 m3/s/t through the potatoes
themselves.
In bulk storage, uniformity of distribution is dependent on the design of the
ducted ventilation system (Ch6.4).
In box storage systems, uniformity of airflow depends on (Ch6.6):
● The design of the air distribution system.
● The box stacking layout.
● The degree of precision with which boxes are stacked.
● Whether the store is airspace or positively ventilated.
● Whether the boxes are designed to suit the ventilation system.
Ventilation on demand has some potential problems:
● Excessive ventilation with non-humidified ambient air can result in up to 10%
evaporative weight loss and associated pressure bruising, especially in the lower
layers of potatoes in bulk stores.
● The cooling air in airspace ventilated box stores, where air is circulated round
rather than through the boxes, may preferentially cool surface layers of pota-
toes, leaving layers below warmer. This can result in subsurface condensation
when convective ventilation is re-established.
● Once forced ventilation stops, the low convective air movement will cause
temperature gradients to develop, leading to a greater range in crop tempera-
ture and increased likelihood of subsurface condensation.
These disadvantages can all be minimized by sophisticated control of ventilation
(Ch8.6). The system also depends on the UK’s natural humidifiers, the Atlantic
Ocean and the North Sea, to supply air that will not dehydrate the crop excessively.
While humidifiers can be put into the ventilation airstream, the high airflows
used in the UK compared with the low-volume humidified air ventilation systems
of the USA and Scandinavia will result in very large media humidifiers being used.
In the UK’s medium-rate airflow systems, humidification with spray jet humidifiers
is the only realistic solution. These need to be fitted with effective droplet arrestors
to stop the crop becoming wet through droplets landing on it.
Companies with a dehydration problem usually solve their bruising problems
by installing more efficient fans, better sealing of stores and better system control.
The Dutch use ventilation on demand, but have pioneered the use of higher airflow
rates of 0.042 m3/s/t (Scheer, 1998) to:
92 Chapter 3
The air approach velocity is the speed of a column of air approaching a mass of
potatoes. This is easier to measure than the air speed between tubers. The air
velocity through the voids between the tubers is inversely related to the ratio of
void, to the total space occupied by the potatoes. The air velocity therefore
increases as it enters the crop.
If potatoes are stored 2.0 m deep and their density is 667 kg/m3, a column
1 m × 1 m in area will hold 1.33 t of potatoes (Fig. B.3.6). An approach velocity of
0.1 m/s will therefore be equivalent to a ventilation airflow rate of 0.075 m3/s/t,
almost four times the rate (0.02 m3/s/t) used in UK stores.
Column of potatoes,
1 m 1 m square, 2 m high
Fig. B.3.6. Relationship between air approach velocity and airflow rate per tonne.
Store Climate 93
1.0
0.8
0.4
0.2
0
0 0.05 0.1 0.15 0.2
Air velocity (m/s)
cooling can be achieved without excessive moisture loss. High airflow rates therefore
help to reduce dehydration.
To achieve these high airflow rates, larger fans than used conventionally in the
UK are required. The success of the Dutch potato industry suggests that the addi-
tional cost of providing these high airflow rates is recouped by the quality obtained.
The system of continuously ventilating potatoes with a low rate of humid air
resulted from a remarkable productive collaboration between mid-west American
and Scandinavian technologists during the early 1970s.
By continuously ventilating bulk potatoes with air humidified to as near 97.8%
RH as possible (Box 3.11), the following results are achieved:
● Crop weight loss is kept to a minimum by keeping the RH of the ventilating
air as near to saturation as possible and by continuously removing the respira-
tion heat.
● The temperature differential between top and base of the pile is kept to a mini-
mum, with the difference declining as air speed rises.
● Subsurface condensation associated with natural convective ventilation is
prevented.
● Sufficient drying of the crop occurs simultaneously at all levels up the pile to
ensure tuber skins remain dry.
● Constant cooling allows the difference in temperature between the cooling air
and the crop to be kept small, thereby minimizing the large step changes that
can occur in intermittently ventilated systems.
● Continuous ventilation dominates natural convection, ensuring that only
colder air meets warmer potatoes; this ensures that condensation on the crop
cannot occur.
94 Chapter 3
Putting an evaporative, media type humidifier supplied by tap water in the main ventilation duct
supplying the crop both humidifies and cools the ventilating air (Fig. B.3.7). Air leaving the fan
at 10°C, 70% relative humidity (RH) will evaporate moisture from the humidifier and thereby
cool the air to its wet-bulb temperature of 7.38°C and have an RH of 97% (Munters, 2007).
Were the 10°C, 70% RH air supplied to the crop directly, the air would still be cooled to
a value approaching the wet-bulb temperature, but the moisture would come from valuable
potatoes rather than tap water. The humidifier therefore reduces moisture being absorbed
by the ventilating air from the crop and so minimizes crop weight loss.
Water
Evaporative
humidifier
Fan
In Scandinavia and the mid-west of the USA this type of storage now predomi-
nates (M.J. Frazier, Idaho, 2005, personal communication). The airflows selected
are 0.0070 m3/s/t for initial storage after lifting and 0.0035 m3/s/t once the potatoes
become dormant, although, increasingly, variable frequency drives are being fitted
to slow the airflow further when crop respiration is low and so save electricity. These
airflows are one-third of the UK airflow rates and so fans and ducts can be smaller
and lower in cost. Low airflow rates for cooling can be used without causing signifi-
cant crop weight loss due to the airflow being humidified.
Attempts to introduce this system for potato storage into the UK have been
unsuccessful to date. This is due to a number of possible reasons:
● The high ambient humidity found in the UK allows potatoes to be stored with
acceptable levels of weight loss using the established systems of ventilation.
● The fear that the low airflow may be insufficient to dry wet crops as rapidly
as the higher airflow systems.
● An investigation (Potter, 2000) indicated that there was inadequate cool air
available in the UK using ambient-air cooling alone to maintain a temperature
of 4°C over the winter storage using this system. Additional backup refrigera-
tion has therefore to be provided as standard.
The same continuous humidified-air ventilation systems have been applied to box
stores. Cooling air is introduced at floor level while outlets are located as high up
Store Climate 95
as possible in the store (Johansson, 1998). The cold air continually floods the store,
rising up from the floor, displacing the more buoyant warm air coming from the
crop as it rises (Fig. 3.22). This airflow movement can be likened to filling the store
with water.
The rising cold air continually spills out the high-level louvre at the top of the
store, like a river flows over a ‘weir’. At the higher rate of 0.0070 m3/s/t, the rate
of air movement is 11 times that of the natural ventilation rate of 0.00064 m3/s/t,
based on the crop emitting 12.8 W/t (Hylmö et al., 1975a).
The difference in sensation when entering a well-sealed potato store using
intermittent ventilation compared with entering one using continuous ventilation
with humid air is almost imperceptible. The potatoes in both have dry skins, as the
heat they generate keeps them dry. This observation is verified by skin resistance
readings (Pringle et al., 1997). However, paper on a clipboard stays dry in the first
but goes limp and moist in the humid store. The humidified ventilation system
therefore keeps the skins of the stored tubers dry while limiting evaporation and
weight loss by its high relative humidity (Box 3.12).
Boxes untreated with preservatives sometimes develop white penicillin moulds
in such high-humidity storage as, unlike the potatoes, they do not generate the heat
required to keep the wood dry (Pringle et al., 1997).
Incoming cold
air displaces
Boxes
warm air above
Humidifier
Continuous ventilation with humidified air reduces weight loss and minimizes con-
densation. Pathologists often say that high humidity encourages disease develop-
ment. If so, continuous ventilation with humid air systems should result in increased
disease. The total acceptance of this system in the USA, Scandinavia and else-
where would not have occurred if this were the case. The likelihood is that in exper-
imental work carried out by pathologists where humidities were high, unseen
condensation was occurring. This free water, rather than high humidity, was there-
fore likely to be the key factor in disease development.
96 Chapter 3
Refrigeration is used as part of the cooling equipment in all the systems discussed
above where sprouting cannot be prevented using ambient-air cooling alone.
However, refrigeration can be used on its own, with no ambient-air ventilation.
The use of refrigeration-only systems allows potatoes to be stored in tropical
areas where cool ambient air is unavailable. This incurs a high energy cost. In
temperate areas, refrigeration in theory uses approximately ten times more energy
(Box 8.3) to cool potatoes compared with cooling using ambient air. The full bene-
fit is not usually seen in practice, probably due to air leakage through the louvres
required for ambient-air cooling. In tropical areas cool ambient air is not available.
Where mountainous regions are nearby, it may be possible to store crops at alti-
tude and transport them to markets when they are required (Ch13).
In temperate regions many growers are attracted to the apparent simplicity
of using refrigeration alone to both dry and cool the crop, even though the energy
costs may be higher. This can be done by using the fridge recirculation fans to
recirculate the air in store as the crop is being loaded, and to rely on wind to
change the air being recirculated. If stores are small, the high capital cost of
installing ambient-air mixing and possibly humidification may make the overall
costs of refrigeration-alone systems less than that of combined ambient-air cooling/
refrigeration systems.
If the fridge is used to act as a dehumidifier, with the doors closed, it will tend
to cool the crop during the drying process. The cooled, dried crop then may suffer
Store Climate 97
condensation when the next load of warmer potatoes is loaded into store. Supplying
heat using an oil heater, fitted with a flue to keep RH, carbon dioxide and ethylene
levels in store down, can offset this cooling, but adds to the cost of drying and pro-
duces additional greenhouse gases compared with the use of refrigeration alone.
The most ‘natural’ system of ventilation for potatoes is continuous low-rate ventila-
tion of humid air, cooled by ambient air when available and refrigeration when
not. It eliminates all the potential types of condensation with the exception of: (i)
condensation on the roof and crop (and possibly boxes) caused by sudden cold
spells of weather; and (ii) the problem of warm air entering the building through
gaps and doors. The first can be prevented by the use of roof-space heating while
ingress of air can be excluded by good sealing of the store and by ensuring that
doors are kept shut.
Both the medium and high airflow UK and Dutch systems are potentially at
risk from all the types of condensation listed above (Ch3.5). In these systems
instrumentation and sophisticated controllers are required to ensure condensation
on the crop will not occur. These systems can be made to work well so long as
they are designed well. The high airflow rates are greatly valued to speed drying
in wet harvests.
Seed stores with grading areas, which are kept open during the day, are appro-
priate where daytime winter temperatures range mostly between freezing and
10°C. Good drying facilities are needed to remove the occasional condensation
that occurs on the crop. There is a danger with such stores that seed kept above
3–4°C may start to sprout prior to dispatch for planting. Varieties that sprout easily
or are destined for later deliveries should therefore be kept within a closed sealed
store at 3–4°C and opened just prior to grading and dispatch.
Stores that rely on refrigeration alone are expensive to run if cooling and drying
are both carried out in the closed store using the fridge. In tropical countries the dif-
ficulty is in loading the stores without allowing ambient air to enter the store. This
warm moist air will not only condense on cool stored potatoes, but also on the fridge
cooling coils to rapidly form ice (Fig. 3.23), which has then to be melted to allow
cooling to continue. Stores should be small enough to be loaded, wound healed and
cooled as a single batch. If stores are large, then batches of potatoes should be loaded
into an intake room, ventilated, wound healed and cooled prior to being loaded into
the main store. In this way temperature differences between batches of potatoes can
be minimized so that convective currents between batches do not occur.
Potatoes are usually kept at harvesting temperature for 10–14 days to allow
wounded skin tissue to heal, then cooled to a lower temperature for storage to
minimize respiration, extend dormancy and slow the multiplication of any disease
present (Ch1.5). Potatoes for seed and pre-pack are cooled to 3–4°C. Potatoes
98 Chapter 3
destined for processing are cooled to no lower than 8–12°C, to prevent the conver-
sion of cell starch to sugar. Due to being stored warmer, dormancy break and
associated sprouting will occur earlier in processing potatoes than pre-pack or seed,
unless chemicals are used to control sprouting.
5 12 8 10 6
10 8–9 4 8 3–4
15 4 3 6–7 2–3
25 3 1–3 4–5
30 2 1 1
Store Climate 99
In warm tropical areas where temperatures average 30°C, storage of only 1 month
is possible. Keeping tubers in trays exposed to bright artificial light or in a green-
house can extend length of storage. This is suitable only for seed as greening of the
skin takes place. Storage life can also be extended by the use of chemical sprout
inhibitors.
Potatoes for processing into crisps or French fries are stored at 8–12°C to minimize
their sugar content so that, when fried, they have a light golden colour (Ch1.5). At
this temperature, dormancy is short, and undesirable sprout growth soon starts. To
extend dormancy, maleic hydrazide can be sprayed on to the growing crop prior to
harvest, or CIPC (chloropropham) can be sprayed on to the crop at store loading.
For longer-term storage in the UK, CIPC is applied as a fog to the crop once loaded
into store (BPC, 2002a; Cunnington and Dowd, 2003).
CIPC
The active ingredient of CIPC is chlorpropham, which in the UK is usually dis-
solved in a solvent such as methanol or dicholoromethane. The chemical acts by
preventing cell division in the tip of the sprout, preventing its growth. CIPC is
injected into a hot airstream, produced by a fan, with petrol (gasoline) fed into the
airstream and ignited using a spark plug (Fig. 3.24). The products of combustion
enter the store along with the CIPC. In the USA, CIPC is usually applied as solid
melted chemical with no solvent present. The store is fogged whenever tubers start
to show signs of sprouts.
Recent research has focused on achieving a more uniform application of CIPC
in box stores than has been achieved in the past, so that the maximum residue level
of 10 ppm, set by the UK Pesticide Safety Directorate (EU, 1995), can be achieved.
CIPC treatment is usually associated with an increase in sugars in the crop, which
Fog
Petrol
(i.e. gasoline)
Swirl
vanes Burner nozzle
adversely affects fry colour. This results from the production of ethylene from
burning the petrol used to heat the hot airstream, which, being a plant hormone,
initiates an increase in respiration and associated change from starch to sugar
(Cunnington and Dowd, 2003). While sugar levels will subsequently reduce, the
recommendation is to shorten the period that the stores are closed to 8 h if allowed
by the label.
Other chemicals
Chemicals other than CIPC are used worldwide (BPC, 2002b). Diisopropylnaph-
thalene (DIPN) is used in combination with CIPC in the USA, while dimethylnaphtha-
lene (DMN) is used in the USA and is undergoing registration trials in Europe. The
naturally occurring caraway seed extract Carvone is sold in The Netherlands and
Switzerland but is not registered for use in the UK at present. It is suitable for
organic crops. Other potential sprout suppressants are clove oil, hydrogen peroxide
and ethylene.
As illustrated by the data from Table 3.2, light is a very good sprout inhibitor.
Potatoes have to be stored in thin layers, two or three tubers deep, usually on trays,
so that the light can penetrate to the tubers’ eyes without requiring the tubers to
be turned. The light may be natural daylight or from fluorescent tubes. Under
Store Climate 101
lights, skins rapidly become green, so extending storage life by this method is only
suitable for seed.
Crops must be mature at lifting as the fry colour of crops harvested when imma-
ture darkens rapidly in store. Skin finish and surface blemishes are relatively unim-
portant as skins are removed prior to processing.
Storage temperatures are commonly reduced to 8–12°C after harvest, the pre-
cise temperature depending on variety, to minimize the conversion of cell starch
into sugar due to low-temperature sweetening (Ch1.5). Maintenance of uniform
temperatures in store is vital to prevent cold areas of crop. This prevents crisps and
French fries becoming too dark in colour when being fried.
At these temperatures rotting and disease development can be rapid and dor-
mancy is reduced significantly. While disease development in pre-pack and seed
potatoes can be slowed by the combination of keeping the crop both dry and cold,
in potatoes for processing, disease prevention relies almost wholly on rapidly drying
the crop and thereafter keeping it free from condensation.
Prolonging dormancy is achieved by the application of sprout suppressants,
mainly CIPC, two or three times over a winter storage period.
Seed potatoes have traditionally been grown in colder, windy areas less attractive
to aphids, which transmit virus diseases. Harvesting conditions in these areas can
be wet, with considerable quantities of wet soil being harvested along with the crop.
In bulk storage this can result in soil cones forming (Ch2.2) and ventilation airflows
102 Chapter 3
‘rat holing’ through the voids surrounding the soil cone. Great care is required to
minimize the amount of soil being lifted and continuous back and forth movement
of store loading elevators is required to prevent soil cones forming.
In box storage systems, the presence of soil together with the small void space
between seed-sized tubers results in reduced convective ventilation and airspace
ventilation systems being less effective. In order to force air through these small
passages, positive ventilation systems, where fans force air through the potatoes in
the boxes, are commonly used.
Seed producers usually grow one to five varieties, of between one and three
generations. However, some producers, especially those producing the highest-
grade seed, will grow many more varieties, both to satisfy demand and to spread
the risk of certain varieties becoming less popular. The requirement to segregate
and store small amounts of these different varieties and generations was the main
reason for the Scottish seed industry storing material in boxes rather than bulk.
In The Netherlands, where large cooperatives and companies handle the pro-
duction of seed, individual growers are allocated a sufficiently large tonnage of one
or two seed varieties that allows bulk storage to be used.
Seed stores in the UK may take 6 weeks or more to fill. An additional 2 weeks
are then required to dry the last crop in and allow wounds to heal. During store
loading the store temperature should track the temperature of the crop in the
ground so that the temperatures of the crop entering store and already in store are
similar (Ch3.5).
Unlike pre-pack or processing stores, which can be filled rapidly, closed and
kept closed for the entire storage period, seed stores are continually being opened
to pre-grade material, send off material early for export and respond to customer
demands for seed to be delivered early for chitting in their own sheds. The risk of
condensation forming on the crop by allowing humid ambient air to enter the
building is very great. Seed stores should be designed with this in mind, and control
and monitoring equipment should be incorporated to alert staff when opening
doors may risk condensation forming on the crop inside.
3.12 Summary
Store climate should maintain potatoes firm, sprout-free, disease-free, and with
uniformly low sugar levels if destined for crisping or use as French fries. The main
factors that influence storage conditions include the following.
● Tubers stored together in a well-sealed building create their own, largely bene-
ficial, microclimate.
● Some bleeding of air into store is required to prevent oxygen starvation and to
ensure carbon dioxide remains below an average level of 0.5%.
● Ventilation immediately after harvest is required to dry damp harvested crops,
and to remove respiration heat that would otherwise warm the crop and cause
subsurface condensation.
● Ambient ventilation air varies both in temperature and RH due to changes in
weather and so can cause wetting of crops, as well as drying during wound
healing and crop drying if ventilation is uncontrolled.
Store Climate 103
● Ventilation is required after wound healing to cool the crop with cool ambient
air when available. If refrigeration is fitted, it can be used when no cool ambi-
ent air is available.
● The temperature of ambient air used for cooling is likely to vary during the
period of ventilation.
● Ambient air used to cool the crop also removes moisture. In non-humidified
storage systems ventilation duration should be kept to the minimum required
for temperature control.
● Cooling of potatoes results in a cooling front developing, which then rises
slowly up through the pile or box.
● Evaporative cooling accounts for approximately 55% of cooling; convection
and some conduction accounts for the remaining 45%.
● The temperature difference between top and base of a pile is a maximum in
naturally ventilated piles and reduces as ventilation rate increases.
● Temperature differences between the base and top of a pile, moisture loss dur-
ing cooling and formation of cooling fronts can all be minimized by the use of
continuous low-volume ventilation of crops with humidified air.
● At the high relative humidities produced by potatoes when stored together,
condensation on the crop is an ever-present risk.
● Condensation can form on potatoes whenever the temperature of the skins of
tubers is below the dew-point temperature of the surrounding air.
● While intact skins of tubers are the main barrier to disease infection, keeping
skins dry is the second.
● Where it is not possible to avoid subsurface condensation by ventilation and
headspace heaters, straw can be applied as an insulating layer to keep the pile
surface warm.
● While low-temperature storage is normally used to prolong dormancy in pota-
toes, chemicals can be used instead where low temperatures would otherwise
cause sugar accumulation and dark fry colours.
4 Disease Control in Store
4.1 Introduction
104 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Disease Control in Store 105
● Sections of the crop that were harvested wet, or have become wet through
condensation, can be dried by directing air towards the wet areas or by venti-
lating the whole stored crop.
● Ventilation of the crop can be taken a stage further, by using prolonged venti-
lation to desiccate (mummify) rots and prevent the pectolytic oozes from
digesting the skins of neighbouring tubers.
In summary, store managers have to select one single compromise temperature for
the store airspace. They have little control over the RH within the crop voids, but
can selectively dry wet sections of crop without necessarily desiccating the main
mass of potatoes.
Host
(the tuber)
Wound
Ruptured periderm
New suberized
periderm
The physical condition of the host can include factors such as damage incurred
during lifting and grading operations, which provides access for certain pathogens.
While such damage could be assigned to the environmental leg of the disease tri-
angle, it is included here to allow us to concentrate primarily on store conditions
when considering environment.
Most potato storage pathogens originate either from the crop’s seed tubers or from
infections that invade the plant during its growing phase. Pathogens, be they bac-
teria or fungi, have a wide range of different life cycles, and infect their host in dif-
ferent ways. Bacteria tend to be limited in the ways they can multiply, move, and
spread and survive. Fungi in contrast may have life cycles that allow over-wintering
and survival in the absence of a host, spore dispersal over long distances, or
sequential spore dispersal. As such, survival, spread and development can be dif-
ferent for a bacterium compared with a fungus. For most pathogens, disease life
cycles (Agrios, 1988; Bissonnette, 1993) have been explored and are well known
(Fig. 4.3). Many pathogens exhibit diversity within the species, so that different
subspecies (or strains) may have different optimal conditions for their development
and react to sprayed chemicals in different ways. The objective for most store
managers is to reduce disease inoculum levels as much as is practicable. Tactics
Disease Control in Store 107
Seed Planting
Ware
Growth
Spores in air at
grading
Storage Sporulation on
seed tuber and
spread to
progeny
Dry curing
Symptoms often develop
prevents
before harvest
disease
Fig. 4.3. Life cycle of silver scurf (Helminthosporium solani). (Source: British Crop
Protection Council, modified from BPC Store Hygiene CD.)
may include selecting seed with low disease levels, employing agronomic techniques
that prevent disease development, and rigorously cleaning stores and equipment
prior to store loading or handling the crop.
Store managers should assess the amount of disease on tubers coming into the
store so that management can be tailored to particular needs. This may include early
sale of crops already starting to rot, rapid drying or omission of wound healing where
silver scurf is the predominant risk. For most diseases it is impossible to improve qual-
ity by storage (an exception may be soft rotting, where rapid and sustained drying can
‘mummify’ rotted tubers so they do not infect neighbouring tubers) and disease levels
normally increase during storage. To evaluate how well the storage environment mini-
mizes subsequent disease development, the crop should be sampled at intake, periodi-
cally during storage and at grading, to assess any decline in quality over the storage
period. A low disease increase may suggest a satisfactory storage regime.
Many pathogens will die, slow their development or enter a static, resting stage if
their environment becomes hostile. Their life cycle therefore fails. Environmental
conditions that induce such failure include some that can be influenced by the store
manager such as temperature and moisture on the tuber surface. A key contribu-
tion to the shape (and size) of a disease triangle comes from the application of
fungicides that kill or prevent further development of a proportion of the patho-
genic organisms present. The application and efficacy of these chemicals are dis-
cussed in more detail in Ch2.5.
108 Chapter 4
In theory, a disease triangle will collapse if any one of the three components meets
criteria where disease development is impossible. For example, where a variety
expresses complete immunity to a given pathogen, then modifying the environment
or removing the pathogen will be irrelevant for disease control. Likewise, if a path-
ogen is completely absent from a growing region then changing variety or storage
conditions will have no influence on development of that disease. More often than
not, however, it is difficult to reach these absolute conditions for a given disease
and so the role of the store manager is to understand that modifying one point of
the triangle will simultaneously influence both of the others.
For example, Polyscytalum pustulans, the causal agent of skin spot, grows most rapidly
on an agar plate in the laboratory at 15–18°C. However, suberin deposition and
wound healing of the potato tuber is most rapid at this temperature range. At 15–18°C,
the tuber’s wound healing ability allows formation of a barrier to the pathogen before
it can access the tuber flesh (Fig. 4.2). In practice skin spot tends to develop most rapidly
on tubers in store at 3–5°C, where the rate of wound healing is slow, while the fungus
can still grow rapidly enough to access the flesh of the tuber (Box 4.1).
Due to the low temperatures at which potatoes are stored, disease multiplication
may take weeks or even months to develop visible lesions. This makes it difficult
to identify what caused the problem and, without a control, there is no certainty
that any particular explanation is correct. In a storage experiment (Pringle et al.,
1997), a quarter of a stock of ‘Desiree’ potatoes was stored in an experimental,
ambient-air cooled, 45-t box store, while the remainder was stored in the farmer’s
ambient-air cooled box store. The farmer noticed condensation on the potatoes in
his store in October and circulated air round the boxes to dry off any condensa-
tion. In January he observed some skin spot, Polyscytalum pustulans, in this
stock, and by April the disease had developed to the extent that the entire stock
had to be sold for stock feed. The 45 t of potatoes stored in the experimental store,
which had skin resistance sensors fitted to monitor condensation, were kept free
from condensation and suffered only mildly from skin spot (Table B.4.1). The 45 t
were sold for seed. The length of time between the likely cause and the subse-
quent expression of the disease, together with the absence of a control, would
normally mean that the two events would not have been linked.
Table B.4.1. Comparative infection on tubers from farm and experimental store
on 7 April 1997.
4.4 Bacteria
4.4.1 Avoidance
Diseases caused by bacteria, such as soft rot, blackleg, brown rot, ring rot, etc., can
cause significant losses during storage if they are not controlled. Methods of access
to the flesh of the tuber can vary. With soft rotting, caused by Erwinia carotovora
subsp. carotovora, access is via damage during lifting, through lenticels where these
have burst or become more prominent and exposed due to the growing tubers
standing in waterlogged soils, or to a combination of fungal pathogens, pest dam-
age and bacteria. Where a fungal pathogen is present, soft rotting typically devel-
ops as a secondary infection.
Store managers can therefore improve the chances of storage without rotting,
even for susceptible varieties, by avoiding crops:
● From waterlogged sites.
● Infected with other primary diseases such as blight.
● With heavy pest infestation such as slugs.
Grading-out defects prior to storage may provide sufficient ‘salvageable’ crop to
pay for the additional work involved.
Where it is impossible to avoid crops that are waterlogged, diseased or dam-
aged, an understanding of the life cycle of the bacteria can help when devising
disease limitation strategies. The bacteria that cause soft rotting develop most
rapidly in susceptible varieties and at high temperatures; the presence of free
110 Chapter 4
surface moisture, which creates anaerobic conditions, increases their rate of devel-
opment. Surface moisture can either be due to wet lifting conditions or generated
through condensation during storage (Ch3.5). In either case careful removal of sur-
face moisture, through either windrowing in the field or ventilating the crop during
wound healing, can ameliorate the disease risk.
The metabolic rate of bacteria, and consequently their ability to multiply, decreases
as temperature decreases. Gray and Robinson (1988) demonstrated that bacterial
multiplication was considerably slower below 10°C than at 20°C, taking over 12
times as long to multiply tenfold (Fig. 4.4). For most potato pathogenic bacteria,
140
Time taken to increase tenfold (h)
120
100
80
60
40
20
0
0 5 10 15 20 25
Temperature (°C)
Fig. 4.4. Time taken to achieve tenfold multiplication of bacteria (Erwinia) at different
temperatures. (After Gray and Robinson, 1988.)
Disease Control in Store 111
their optimum temperature for development is usually higher than normal storage
temperatures. For Erwinia species these optima lie in the low 30s Celsius, with sub-
tle differences between the two subspecies that are prevalent in the UK. For exam-
ple, subsp. carotovora, the cause of soft rotting, has an optimum temperature
marginally higher than subsp. atroseptica, the cause of blackleg, and so will predomi-
nate in hotter growing seasons.
Knowing that the rate of bacterial multiplication decreases with temperature
would suggest that newly harvested crops with a high bacterial count should be
cooled immediately after harvest, rather than being allowed to stay at harvest tem-
peratures to speed wound healing. But not only would such cooling be slow, a
maximum of 0.5°C/day, it would also reduce the rate of periderm formation, the
primary defence to disease entry. In addition, cooling in a store still being loaded
is very likely to result in condensation forming on the crop due to temperature dif-
ferences between the crop in store and the new crop being loaded (Ch3.5). The
order of importance of management actions is therefore to:
● Ensure wounds are healed faster than disease can gain entry.
● Any tuber surface moisture at harvest should be dried and subsequent conden-
sation avoided by good distribution of ventilating air.
● Once wounds are healed, the store can be closed and the crop cooled.
During cooling, temperature differentials within the crop should not exceed
0.5°C or condensation, followed by possible bacterial multiplication, is likely
to occur.
Where bacteria have entered into the flesh of the tubers, through stolon ends for
example, wound healing will have little positive benefit, so cooling to slow bacterial
disease development could be justified. As such crops are very likely to break down
in store whatever action the store manager takes, their immediate sale as ware or
stock feed is probably the best strategy.
The ability of bacteria to survive long term in the absence of the host depends on
a species’ ability to avoid desiccation through the production of polysaccharide
slimes that bind with soil particles and provide protection (e.g. brown rot). Where
this mechanism is absent, as is the case for soft rotting Erwinia, then long-term
survival is rare except in larger clusters of soil and rotting debris. Store managers
can avoid such contamination by ensuring stores are cleaned of debris and that any
remaining soil particles are allowed to dry between seasons to remove any remain-
ing inoculum.
Some store managers choose to use a disinfectant after cleaning to ensure that
the bacterial risk is minimized. This may be good routine practice in areas where
brown rot and ring rot are fully established and is a crucial precautionary step in
very high-health, safe-haven schemes where absence of key pathogens within the
seed multiplication chain is essential. For other crops, however, it has not proved
necessary in the UK to use disinfectants against soft rotting so long as routines for
cleaning stores and grading and handling equipment are in place.
112 Chapter 4
4.5 Fungi
Fungal spores can vary in size from <10 μm (P. pustulans, skin spot) to >50 μm
(Helminthosporium solani, silver scurf). Their structure also differs; in particular, cell
wall thickness can vary from >5 μm for Fusarium dry rot spores down to <2 μm for
P. pustulans (Fig. 4.6). As well as presenting these facts for interest, they are included
to help develop an understanding of how to influence the pathogen component of
the disease triangle. For example, Helminthosporium spores are easily captured by the
filter of vacuum cleaners used for store hygiene, whereas Polyscytalum spores would
pass through most filters. Store hygiene strategies for silver scurf should therefore
be based on vacuuming stores and grading areas to remove spores rather than
attempting to destroy them with disinfectants. Similarly, Polyscytalum spores will be
more prone to desiccation in store than those of Fusarium species. Providing there
is adequate time and conditions to allow spores to dry out, a store manager might
be less concerned with store hygiene following a skin spot problem than following
a dry rot problem.
As well as store hygiene, the obvious way to influence the pathogen component
of the disease triangle is to try to prevent disease development in the growing
crop. While this is not always possible, the store manager can at least develop
an understanding of the factors that contribute to disease risk and use crop his-
tories to anticipate potential problems in store. He can then take the necessary
remedial actions should the market desire low disease levels. Various advisory
tools have been published to guide the store manager using this approach
(BPC, 2006b).
3–10 μm 30–60 μm
Further evidence of these predisposing factors can be found by undertaking the fol-
lowing risk assessments.
Choice of variety is often beyond the remit of the store manager. However, an
understanding of resistance and susceptibility to key diseases will help in undertak-
ing the risk assessments described previously. For some diseases, such as gangrene
and blight, methods for determining host resistance are well documented (e.g. Gray
and Paterson, 1971; Dowley et al., 1999). For others like skin spot and dry rot, vari-
ous assessment methods have been published (Kerr and Parrish, 2005) but not all
varieties have been evaluated. Each country and producer group will have some
relevant data that identify high-risk varieties. For the group of diseases that affect
visual quality, such as silver scurf and black dot, genetic differences in the host have
been studied but as yet they have little practical significance for the store manager
(Hilton et al., 2001).
116 Chapter 4
4.5.4 The role of free surface water, relative humidity and temperature
Like humans, disease organisms can only access water in its liquid state. Water in
the form of vapour in air is inaccessible.
The RH of air within the voids of a mass of potatoes in a well-sealed store
is controlled largely by the potatoes themselves, at or around 94–97%. The
crop alters the condition of any air passing through it. In an experiment by
Grähs et al. (1978), air being blown into the base of a pile, albeit at a low
volume rate of 0.0044 m3/s/t, with a temperature of 7.3°C and RH of 87%,
both cooled and increased in RH as shown in Fig. 4.7. At the ‘turn’, the air is
at its wet-bulb temperature. Above the ‘turn’ the air temperature rises, due to
the heat of respiration emitted from the potatoes, but stays at the air’s wet-bulb
temperature. Even though store managers are able to control when ventilation
takes place, they cannot control the RH of the air within the voids between
the tubers.
As the condition of the ambient air used for ventilation is totally dependent on
the weather, the manager has even less control over the RH of ambient air, unless
a humidifier is used, other than to stop ventilation when weather conditions are
unsuitable.†
Air with high RH is often associated with disease development because, at val-
ues of 94–97%, even a small temperature difference of 1°C within the potatoes will
cause warm humid air to condense on the cooler potatoes above. This condensed
moisture may then become available to disease organisms.
4.0
Surface of pile
3.0
Pile height (m)
2.0
Turn point
0.0
6.0 6.4 6.8 7.2
Temperature (°C)
Fig. 4.7. Condition of ventilating air as it rises up a bulk pile. (Redrawn from
Grähs et al., 1978.)
†
The term ‘dry curing’, used to define the process of ventilating the crop to dry it, is a dubious
term as it suggests the quality of air is controlled.
Disease Control in Store 117
For most fungal diseases water is crucial to some or all stages of growth.
Usually it is moisture in combination with warm temperature that creates condi-
tions that allow disease development.
Various descriptions and illustrations of the life cycle of the silver scurf pathogen
have been published. The one reproduced above (Fig. 4.3) was published by the
British Crop Protection Council and illustrates that heat and moisture in combina-
tion create opportunities for disease development. There is evidence that each of
the key stages in the life cycle – infection, lesion expansion and sporulation – are
influenced by water, temperature or a combination of the two (Box 4.2).
Infection
Clayton et al. (1998) devised a set of four categories for measuring phases in the
infection process (Fig. 4.8):
● Germ tube production to a length two times the diameter of the spore.
● Penetration to a depth equal to or greater than one potato epidermal cell.
● Ramification of germ tubes into new fungal mycelium and curved or prolifer-
ated to surround and invade potato epidermal cells.
● No development observed.
The frequency of spores within each category was measured over time on inoc-
ulated tubers under temperatures of 5, 10 and 15°C and under moisture regimes of
1 h to 4 days. Progress from penetration to ramification (infection) was observed
much more quickly at 15°C than at the lower temperatures, and reached >5% of
all spores present after only 6 h of surface moisture being present (Fig. 4.9). This was
sufficient to cause substantial surface infection, which was confirmed on replicate
samples that were inoculated, treated and stored long term prior to visual assess-
ment. The lesson for store managers is that if silver scurf spores are present on the
This box describes some of the terms used when describing fungal diseases to
help store managers with limited knowledge of the subject.
Germination: term used to describe growth phase where spores produce germ
tubes (which become hyphae) allowing growth and plant invasion to continue.
Usually has a set of specific triggers (temperature and moisture).
Hyphae: individual strands of a fungus that grow in length, often through plant tissue.
Lesion: term used to describe a discrete visible symptom of disease.
Mycelium: collective term used to describe a matrix of hyphae.
Spore: reproductive cell (or cells) that allows a fungus to disperse, survive adverse
conditions and multiply. Usually has a thick protective wall (sporangium).
Scletorium: tightly packed mass of fungal mycelium. Allows over-wintering and
survival.
118 Chapter 4
Tuber periderm
Penetration Ramification
Fig. 4.9. Percentage of silver scurf spores in each development stage after exposure
to different durations of surface moisture at 15°C. (After Clayton et al., 1998.)
newly harvested crop or as fresh spores from in-store sporulation, then surface mois-
ture from lifting from wet soils or through condensation must be removed within a
few hours to prevent infection. Condensation on tubers can occur in minutes rather
than hours if air with a dew-point temperature exceeding the temperature of the
crop enters via a door or through the store ventilation system.
Lesion expansion
Hardy et al. (1997) induced timed condensation on store-infected or inoculated
potatoes for different periods of time at a range of temperatures. After a period of
storage, tubers were transferred to high RH conditions at 15°C to induce sporula-
tion. Spores were collected by washing tubers and filtering the wash water and
were used to indicate the rate of disease development (Fig. 4.10). Similar to results
for infection, they found the combination of high temperatures, common during
store loading and wound healing, together with presence of surface moisture to be
important in speeding up disease development. At lower temperatures, the pres-
ence of free surface moisture allowed an infection to become established, but was
less likely to trigger massive disease expansion.
Disease Control in Store 119
20
18
16
Spores/tuber (millions)
14
12
10
8
6
4
2
0
0 1 2 3
Duration of condensation (h)
Fig. 4.10. Spores harvested from stored tubers infected with silver scurf following
different durations of condensation simulated after store loading. (From Hardy
et al., 1997.)
Sporulation
Most pathologists incubate tubers at relatively high temperatures (15–20°C) and at
high RH, by adding a water-soaked paper wick to their incubation chamber, when
confirming a diagnosis of silver scurf. The conditions created are suitable to allow
sporulation and the spores are used for anatomical confirmation of silver scurf. It
is therefore safe to assume that both temperature and RH have a role to play in
sporulation. The influence of free surface moisture, however, is less well under-
stood. Anecdotally, many store managers and consultants are able to recall the
charcoal-like appearance of silver scurf-infected tubers following condensation in-
store. The charcoal-like appearance is created by the production of fungal sporan-
giophores (Fig. 4.11), which support a fresh ‘crop’ of sporangia of silver scurf and
Sporangium
Sporangiophore
can occur even at low storage temperatures. Indeed, some retailers may be able to
recall a similar appearance after a crop has been washed and bagged and pre-
sented for sale.
The lesson for the store manager is that crops already infected with silver scurf
can deteriorate in quality and provide fresh inoculum to infect otherwise healthy
tubers if surface moisture is allowed to develop. By combining the findings from
various experiments, certain ‘rules of thumb’ can be devised to help the store man-
ager predict quality post-storage.
At 5°C
● 1 h of condensation = twice the disease.
● 2 h of condensation = twice the disease.
● 3 h of condensation = twice the disease.
At 10°C
● 1 h of condensation = twice the disease.
● 2 h of condensation = twice the disease.
● 3 h of condensation = four times the disease.
At 15°C
● 1 h of condensation = five times the disease.
● 2 h of condensation = six times the disease.
● 3 h of condensation = eight times the disease.
So far, the link between temperature and moisture has been considered. Many
experimenters report an increase in silver scurf as a result solely of increased tem-
perature. In many of these cases, condensation and subtle changes in surface mois-
ture were not measured.
● The moisture may help maintain a wide population of naturally occurring antag-
onistic organisms that may prevent infection by dry rot (this is speculative).
● Any free surface moisture present during the grading process will act as a
‘lubricant’ as tubers move along the grader, resulting in less damaged or torn
skin and less opportunity for disease to enter.
In any case, dry rot has been included here to demonstrate that once in a while
something will come along that can ‘buck the trend’, so the store manager needs
to be aware of the potential for particular control strategies for one disease to com-
promise those of another disease. On balance, however, the recommendation to
prevent or remove condensation by appropriate ventilation control strategies should
be followed.
Warm crop temperatures favour the development of most storage fungi. Fungi
can be classified into two groups: (i) those that develop more during curing (wound
healing) and high temperature storage, such as dry rot, silver scurf, black dot and
blight; and (ii) those that develop at lower temperatures, such as skin spot and
gangrene (Fig. 4.12). While the interaction with moisture is probably ever-present,
moisture may persist at undetectable levels so is discounted in this section.
The objective of good store management is to use the risk criteria identified in
Ch4.4 and modify storage accordingly if certain diseases are to be controlled.
So, for example:
● A pre-pack crop that has already been designated high risk for skin spot may
have its holding temperature raised by 0.5–1.0°C to escape low temperatures
where the disease develops most rapidly. While full disease control is not
achieved, many businesses use this approach to keep a stock within specifica-
tion for sale.
● A processing crop that meets high-risk criteria for dry rot might be stored at a
slightly lower temperature to avoid disease development. This may require the
period it is stored to be reduced to meet target fry colours.
● A pre-pack crop meeting high-risk criteria for silver scurf or black dot may
have its curing period reduced or omitted to reduce the period of high tem-
perature during which each disease can develop.
Pre-pack Processing Curing
Dry rot
Skin spot
Our theme in this section has been to ‘remove the pathogen’ from our disease trian-
gle through good hygiene or to limit its growth through chemical applied to the
tuber. If we know where or how the pathogen enters the tuber, we can decide what,
if any, adjustments to store climate will minimize further infection or multiplication.
4.6.1 Bacteria
Pathogens can enter tubers via a number of routes. Some bacteria (e.g. ring rot) can
persist within the vascular tissue of the growing plant. The disease can therefore
move from seed to daughter tubers without ever leaving the plant. The same bacteria
can also persist for some time away from a host plant and can enter a crop through
damaged tissue, so good hygiene practices combined with careful, low damage potato
handling are necessary. It is this ability of a pathogen to enter a crop via a number
of routes, combined with the loss in crop value due to the infection, that make closed
systems for seed multiplication so important. Closed systems, such as the UK British
Potato Council’s (BPC) safe-haven scheme, produce crops within a ‘cordon sanitaire’,
so limiting the chance of disease entering a seed multiplication operation.
Other bacteria also travel within the vascular tissue (e.g. blackleg-causing Erwinia),
which explains the classic blackleg tuber symptoms of rotted tissue around the point
of stolon attachment. Bacteria can also enter the tuber through lenticels and wounds,
sometimes mixed with fungal pathogens such as blight (Phytophthora infestans).
Infection by Erwinia via lenticels follows the breakdown, or complete rotting, of
mother tubers, which exude bacteria that are then washed through the soil to coat
daughter tubers. Where soils are wet then lenticels can sit in an ‘open’ position.
Bacteria can then continue multiplication, if conditions are right, and cause rotting
either in the field or early on in the duration of storage (Fig. 4.13). This occurs both
for Erwinia soft rotting and for brown rot (Ralstonia solani); in both cases the avoid-
ance of irrigation and the bacteria that irrigation water may contain can greatly
increase the long-term storability of the crop. Bacteria may on entry, or soon after-
ward, enter a latent phase whereby they are ‘sheltered’ by the tuber’s formation of
a protective corky layer over the open lenticel. While bacteria in this phase do not
cause visible symptoms they can survive until appropriate conditions for develop-
ment are triggered, so they remain a potential risk to storability. In some cases an
Periderm
Locations where
bacteria can survive Fig. 4.13. Open lenticel allowing
and multiply access by bacteria.
Disease Control in Store 123
intermediate stage may be seen where bacteria have started to multiply and affect
tissue around the lenticel, but at some point environmental conditions, such as lack
of tuber surface moisture or low temperatures, become less conducive to disease
development. In these cases, the rot becomes dry and localized and is often referred
to as a bacterial hard rot. Entry of bacteria through wounds can similarly be halted,
with the resting organisms remaining a potential source of infection should environ-
mental conditions become favourable to disease development.
Bacteria may be present on equipment, on grading lines, on tonne boxes, or
in the exudates from decaying mother tubers. When tubers are handled and dam-
age occurs then the bacteria can develop within the damaged areas easily and
quickly (Fig. 4.2). Wound healing and deposition of suberin may help the tuber
avoid excessive weight loss but will not prevent bacteria that have already entered
from multiplying and developing.
Blight is usually regarded globally as the most devastating disease to affect
potatoes and massive reductions in crop productivity can and do result from foliar
infection during the growing season. In many cases though, it is the role of the
invading P. infestans blight spores as a carrier of bacteria that makes blight even
more devastating in storage. Bacteria are commonly found within laboratory cul-
tures of P. infestans and are difficult to remove. Once tubers become affected by
blight then two disease triangles, one for the fungus and one for the bacteria,
coincide. Both P. infestans and the soft rotting bacteria grow most rapidly at higher
storage temperatures and very significant losses during storage can result, particu-
larly in processing crops.
Having an understanding and appreciation of these entry points allows the
store manager to develop the risk assessments described previously (Ch4.4) and
follow ‘risk avoidance strategies’ to minimize disease development. One for soft
rotting is shown below.
4.6.2 Fungi
Fungal diseases can be divided into those associated with tuber damage and those
that are not.
For gangrene and dry rot, prevention of damage is paramount for disease
control, as these pathogens usually require a wound to gain entry. Damage mini-
mization is required during both harvest and loading and also during the grading
process. Wound healing after loading or grading is an important part of the control
strategy for these diseases. This is especially so for dry rot in graded seed. The rate
of suberin deposition and development of wound periderm is at its slowest
124 Chapter 4
5–6 months after harvest, which coincides with a peak in seed grading activities.
As well as careful handling, grader hygiene is essential to reduce disease develop-
ment. Diseases such as skin spot can develop after superficial damage to the tuber
skins by sand particles on machinery and neighbouring tubers. Again, adequate
wound healing is an essential part of the control strategy, and applications of
CIPC, if used, need to be timed carefully as they can slow the development of
periderm cells forming over wound tissue, predisposing the tubers to more severe
infection. Diseases that do not require damage for entry are either carried into the
store as latent infections (e.g. black dot) or, like silver scurf, can germinate and
penetrate the tuber periderm unaided (Ch4.5).
4.7 Ecosystems
An understanding of how one pathogen interacts with another can be useful for
decision making prior to, and during, storage. Read and Hide (1984) showed how
control of silver scurf could result in more severe symptoms of black dot. The causal
organism of black dot (Colletotrichum coccodes) competes for the same ecological niche,
the potato epidermis, as does that of silver scurf (H. solani). In various experiments
Read and Hide (1984) demonstrated that where silver scurf was targeted for control
then black dot developed readily in stored crops. This may not be particularly excit-
ing to most store managers and could in fact sound depressing. The store manager,
however, might begin to consider the risks of each disease developing in the pres-
ence or the absence of the other. This should become easier as diagnostics improve.
For example, where silver scurf-free seed is purchased and the washed fresh sector
market is targeted, then soil with black dot ought to be avoided to reduce the risk
of disease development. Similarly, for a crop already heavily infected with black dot,
fungicide treatment to reduce silver scurf would be unnecessary since this disease is
unlikely to out-compete the black dot. The crop would also be unlikely to reach a
premium market given the high disease levels in the first place!
Using one disease as protection against another sounds like a lose–lose situa-
tion. What is more promising for the future is the potential of non-infectious organ-
isms being used as biological control agents either in soils, as potatoes develop, or
in store, as tuber surface treatments (e.g. various proprietary blends on Trichoderma
fungi). Various contenders exist within each category, although this may change in
time as the rigorous checks on environmental and consumer impact currently
applied to agricultural chemicals are implemented for biological control agents.
This may or may not preclude their long-term use.
4.8 Hygiene
Store hygiene, or removing the ‘pathogen’ from the disease triangle in store, is
covered later (Ch9.2). What interests us in this chapter is the need for the store
manager to extend the principles of good hygiene to all other activities that take
place in or around stores. We have seen from the previous section on mode of dis-
ease entry that access to tubers becomes much easier once tubers are wounded.
Disease Control in Store 125
It therefore follows that removing or reducing the levels of pathogens from equip-
ment or processes that can cause damage is equally important. Such operations
include the transfer of crop to and from hoppers and the use of graders. The need
for hygiene measures and their cost–benefit require considerable judgement. The
store manager should be able to make a common-sense risk assessment, similar to
that described previously for disease brought in from the field, and might consider
the following before grading:
● Are conveyors and riddles well-maintained, low-damage machines?
● If they are covered in dried, hard, sandpaper-like soil deposits, the following
stock will damage easily.
● Has the grader recently handled a diseased stock? If so, the grader can be
considered dirty.
● Will the disease(s) in question affect saleability of the stock about to be graded?
● Will less well-controlled conditions during post-grading storage and dispatch
affect disease development?
● How long does the stock have to ‘hold out’ before market? If it is to be con-
sumed within a few days then cleaning the grader before handling might prove
more costly than beneficial.
The store manager should also consider the ‘host’ and how that influences the
likely need for cleaning a grader:
● How big and what shape are the tubers? Where they are large and long then
they will damage more easily.
● How tightly will they need to be graded? If the end product needs a very nar-
row size distribution and is to be graded from a stock with a wide size distribu-
tion, then expect a lot of damage.
With all store management and grading operations it is important to consider the
cost and the benefit. There are many occasions where the benefits of cleaning a
grader will be small; for example, when grading a disease-resistant variety after a
very clean stock. Likewise there will be cases where over-zealous cleaning will come
at a cost beyond the benefit; for example, where disinfectants/detergents remove
protective paint from equipment and shorten its working life or where the labour
requirement for spray washing each and every box outweighs the benefit in removal
of possible pathogens. There may therefore be occasions where a programme of
frequent grader cleaning will not pay for itself. Exceptions might apply to high-
grade seed production or import/export operations where hygiene is paramount.
If the store manager has worked through a risk assessment and has found that
the grader has already handled a stock with a particular disease, the next stock
126 Chapter 4
6000
5000
4000
Bacteria/tuber
3000
2000
1000
0
Not cleaned Wiped with Spray Allowed to Disinfected
damp cloth washed dry
Cleaning method
Fig. 4.14. Bacterial numbers (Erwinia per tuber) recovered from tubers after their
passage over a contaminated grading line cleaned in different ways. (From Clayton
et al., 2000.)
Disease Control in Store 127
30
25
15
10
0
Control Contaminated Sacrifice Grader Grader spray
tuber cleaned scraped washed
Sequential treatment
Fig. 4.15. Percentage of tubers infected by gangrene after their passage over a
contaminated grading line cleaned in different ways. (From Clayton et al., 2000.)
60
50
Percentage tubers infected
40
30
20
10
0
Control Contaminated Sacrifice tuber Scraped Grader spray
cleaned washed
Sequential treatment
Fig. 4.16. Percentage of tubers infected by dry rot after passing over an inoculated
grading line cleaned in different ways. (From Clayton et al., 2000.)
For silver scurf, the grader was contaminated by dust containing spores (Fig.
4.17) and by a wet slurry containing spores (Fig. 4.18). In both cases, clean tubers
passed over the uncleaned grader (control) became heavily contaminated. Where
inoculum was dry and dusty (Fig. 4.17), sweeping or vacuuming had only a moder-
ate effect on reducing contamination of clean stocks. Where it was caked on to the
grader as wet slurry (Fig. 4.18), attempts to remove the dry cake (by scraping with
a section of semi-circular guttering chosen to match the size of grader rollers)
resulted in high levels of infection, probably because the hardened cake became
broken and crumb-like and could stick to tubers more easily.
128 Chapter 4
70
60
40
30
20
10
0
Control (a) Sweep Control (b) Sweep and vacuum
Cleaning treatment
Fig. 4.17. Percentage of infection in progeny tubers after seed tubers were passed
over a grading line that was contaminated with dust containing silver scurf and then
cleaned by different methods. (From Clayton et al., 2000.)
60
50
Percentage tubers infected
40
30
20
10
0
Control Slurry Scraped Disinfected
contaminated
Cleaning treatment
Fig. 4.18. Percentage of infection in progeny tubers after seed tubers were passed
over a grading line that was contaminated with wet slurry containing silver scurf and
then cleaned by different methods. (From Clayton et al., 2000.)
Whenever there are concerns about the true costs and benefits of hygiene,
then a few common-sense questions should allow for a safe, cost-efficient and effec-
tive cleaning programme to be devised. These would include:
● Will development of a certain disease affect marketability?
● Is the building designed for easy cleaning?
● Have crops been stored without disease developing in the past?
Disease Control in Store 129
This chapter has dealt with the predisposing factors that allow diseases to develop,
their mode of entry, disease development and some mitigating actions a store man-
ager might take. In this final section the predisposing factors and possible control
solutions are brought together in Table 4.1 so that a store manager can find the
possible causes and appropriate solutions to a given disease problem.
There are some unavoidable agronomic omissions in the table; field factors
such as volunteer control and adequate length of rotation have been excluded to
save repetition. These are requisites for good control of most of the diseases listed.
4.10 Summary
Potatoes inevitably enter store along with a range of disease microorganisms either
already infecting the crop or posing a potential threat. This chapter has focused on
the following issues.
● As stores comprise a single airspace, at-risk stocks cannot be treated differently
from those in the rest of the store, apart from subjecting them to additional
ventilation.
● The crop itself largely dictates the RH in the voids between the tubers, so
there is little a manager can do other than ensure the crop is dry.
● The concept of the ‘disease triangle’ helps visualize the pathogen, host tuber
and micro-environment interaction.
● This interaction is well illustrated by the disease skin spot, which poses most
risk below the disease’s optimum growing temperature, as this is when the host
tuber’s resistance is low and allows the disease to gain entry.
Disease Control in Store 131
5.1.1 Clamps
132 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Store Design and Structure 133
Straw 150–200 mm
thick
Potatoes Plastic sheet
(dotted line)
Width 1.0–2.5 m
rodents. They have a low capital cost but high labour requirement. Although
there are a number of different types, they all require the potatoes to be heaped
into a long narrow pile typically 1.0–2.5 m in width. Height is dictated by the
angle of repose of the tubers, approximately 35°, giving a corresponding height of
0.35–0.88 m (Fig. 5.1). A covering of straw, 150–200 mm thick when compressed,
is laid over the top of the tubers. This stops greening, insulates the crop from hot
or freezing weather and minimizes subsurface condensation on the stored crop.
Ditches are dug into the soil on either side of the pile to reduce the risk to the
crop from flooding and wire mesh is inserted in the ground to deter rodents. After
2 weeks, once the initial high level of respiration following harvest has subsided,
the straw is covered with two plastic sheets which overlap at the apex of the clamp
to prevent rain penetration but have a gap between them to allow for ventilation.
The plastic sheet is then covered with friable soil 100 mm thick to prevent the
plastic being blown away by strong winds and to prevent freezing winds from
entering the clamp. In very cold climates a second layer of straw followed by more
earth is applied.
A more sophisticated version of the clamp is the ‘Dickie Pie’ in which one
or two ‘A’ ducts are placed parallel to the long sides (Fig. 5.2) and bales of straw,
placed around the perimeter, allow a greater bulk of potatoes to be stored.
These ‘A’ ducts are open at each end and can provide some ventilation of the
tubers, allowing the size of the clamp to be increased to between 4.0 and 5.0 m
in width. Blocking the ducts with a bale of straw or equivalent can close the ‘A’
ducts. The cross-sectional area of the duct should be a minimum of 0.013 m2 per
every 10 t stored.
Low-cost tropical potato storage is quite different from storage in more temperate
climates. Two harvests a year are common in some areas, so storage may only be
needed for 1–3 months. This allows potatoes to be stored in simple stores at tem-
peratures that would result in sprout growth if kept longer (Ch3.8).
Most low-cost storage aims to protect the potatoes from the sun, high air tem-
peratures and low RH while keeping predators and insects out. Air movement is
134 Chapter 5
Straw 150–200 mm
thick
Plastic sheet
(dotted line)
Soil 100 mm
Straw bales thick
Pile of potatoes
Duct Duct
Width 4–5 m
encouraged to keep the potatoes as cool as ambient conditions will allow. The bulk
of potatoes is kept small to prevent self-heating and a maximum depth of 1.2 m is
traditionally suggested. Storage lots should be reduced in size if the temperature
exceeds 25°C (Hunt, 1982).
The most basic store is a woven basket, covered over with straw, kept in the
residential house. The structure of the house, which provides a stabilized environ-
ment for the residents, and sometimes animals, against extremes in ambient tem-
peratures, also benefits the storage of potatoes. The weave of the basket allows for
some natural ventilation and thus reduces the likelihood of hot spots or localized
deterioration.
Where a free-standing low-cost store is used, it should preferably utilize night-
air cooling when the temperature is at its coolest and the RH is at its highest.
Ventilation relies on natural convection, using the tuber respiration heat to draw
cool ambient air through the crop. The materials used will be what is locally avail-
able, but should ensure that all surfaces in contact with the potatoes are in shade
during the day. The vertical walls should be solid so that ventilation is in a vertical
direction; sometimes a bought-in plastic sheet is used to ensure the walls are sealed.
The simplest system has a ventilated floor 0.6 m off the ground so that air can pass
through the tubers all the time and animals or rodents cannot reach the tubers at
the base of the store. To ensure ventilation is primarily during the night, the store
has flaps (Fig. 5.3) that can be opened in the evening and shut in the morning. This
can be particularly useful where there is a large diurnal variation such as at alti-
tude. These systems can work well and often potatoes can be stored for up to 3 or
even 4 months. If two harvests are grown per year, this can be enough to provide
a year-round supply of potatoes.
Bishop and Stenning (1997) describe a further development of the naturally
ventilated store using night ventilation where a solar collector is included in the
design. This consists of stones laid out on a sloping tray at store roof height, cov-
ered with a clear polythene sheet. The solar-heated tray and the store below form
a continuous sealed enclosure, with flaps at top and base to prevent ventilation
Store Design and Structure 135
Corrugated iron
or thatch
Lids for
chimneys
Baffle to force
air up through
Foundation blocks potatoes
Fig. 5.3. Naturally ventilated rustic tropical store with night-air cooling system.
during the day. In the cool evening, the flaps are opened, and the stored heat from
the rocks together with the metabolic heat from the stored crop results in natural
convection and cooling of the crop. The initial trials carried out in Kenya at sea
level near Mombasa gave some temperatures slightly lower than with night ventila-
tion without the solar collector but additional development work is required.
There are examples, such as in the Mexican highlands, of tropical ambient-air
ventilated stores of 3000 t. In most cases, such large stores are refrigerated and used
for seed.
When air below 100% RH passes through a wetted pad or fine mist, it will absorb
water, increasing its RH. The latent heat absorbed in this process also cools the
air. Evaporative cooling would appear to have considerable potential to cool pota-
toes while minimizing weight loss in warm dry climates.
One design of an evaporatively cooled store is a mud or blockwork walled
hut on top of a floor that is kept constantly wet (Fuglie et al., 2000). The floor is
made up of a series of low walls, with wet sand in between and covered with a
bamboo mat. Wire mesh-covered openings at the bottom of the hut walls, on the
two opposite sides, allow air to enter the base of the store to absorb moisture from
the wet sand before rising up through the crop. A wind-operated exhaust turbine,
located in the roof, encourages this air movement.
While the system did show considerable potential to reduce crop weight loss
and improve quality, the increase in value of the crop was negated by the cost
136 Chapter 5
of the stores. In addition, the potential was not as high as it might have been due
to the owners of the stores not keeping the floors wet at all times.
Another design used moistened permeable pads, fitted into the sides of the
stores, so that the air entering the stores was humidified (Hunt, 1990; Bishop and
Stenning, 1997). However, the additional resistance to airflow caused by the pads
negated their cooling potential in stores relying on natural ventilation. Although
the technology is simple these systems are susceptible to problems such as keeping
the walls uniformly wet, preventing algae from growing on the wet pads, holes
developing in the pads, or saline build-up in recirculated water. Good management
is therefore essential. For the pads to work, two or three times as much water has
to trickle down the pad as is evaporated and pad height has to be kept to below
1.5 m to ensure the whole pad is kept wet. For effective cooling of side-ventilated
stores forced ventilation is required. Evaporatively cooled stores in practice have so
far promised more than they have delivered.
Portal frame buildings in the UK are now the industry standard for potato storage
and processing. Their clear span allows operation of potato elevators and forklifts
free from obstructions (Figs 5.4 and 5.5). Their extensive use for industrial build-
ings means that they are competitively priced.
Fig. 5.4. Portal frame seed store with adjoining grading area. (Ordens, Portsoy,
Aberdeenshire, UK.)
Store Design and Structure 137
Fig. 5.5. Internal view of a clear-span, portal frame, pre-pack store. (Courtesy of
W. Leslie, Farm Electronics, Lincoln, UK.)
To reduce the high cost of vertical retaining walls, bulk stores, particularly in North
America, are often designed with sloping walls (Figs 5.6 and 5.7). The pressure
exerted by the crop on a 60° sloping wall is between 25 and 33% that on a vertical
wall (Waelti, 1989). The building is designed as a single structural entity, using
timber or steel frames, with the outward force of the crop being countered by a
reinforced concrete wall set into the ground. The floor may be made of either
tamped loam soil or concrete.
The stores have a central ventilation duct, with crop stored in bulk piles
between 5.5 and 7.3 m high on either side. The ventilation fans, media humidifiers
138 Chapter 5
Fig. 5.6. Bulk stores with sloping retaining walls. (Courtesy of M.J. Frazier, Idaho,
USA.)
Angled retaining
Potato walls
Intake Loading Lateral
louvres door pile
ducts
and air-blending chamber are all located within the extension at the front of the
building. Intake louvres are fitted in the front of the extension. The doors on either
side of the extension allow access to either side of the store for loading the crop.
Potatoes arrive at the store in bulker truck or trailer, fitted with a horizontal
conveyor discharge. This discharges on to a cleaner/sizer, feeding an elevator or
piler, which loads the store. If above-ground lateral ducts are used, these are con-
nected into the main duct and put in place, starting at the back of the store, as the
piler retreats.
For unloading, all the machinery is turned around. An elevator (‘hog’ or
‘scooper’) removes potatoes from the pile and elevates them to the truck directly or
via a cleaner.
Store Design and Structure 139
In continental climate areas of the world, like the mid-west USA, Turkey and
Eastern Europe, low temperatures in winter and high temperatures in summer are
the norm. By burying the building underground, the thermal mass of the soil above
not only insulates the stored crop from extremes of hot and cold weather but also
greatly reduces the effect of solar heat gain.
While the thermal concept of these buildings is sound, it does mean that the
building structure has to carry the soil that covers the building. This adds to the
initial capital cost. This, together with the increased availability of refrigeration and
the relatively low cost of electricity, has meant that these types of buildings are no
longer built. In some areas of the world, caves or old mine workings are used to
achieve the same effect but without the high initial cost.
Modelled after storage units used by Idaho farmers in the 1930s, cellars to store
25 t of potatoes in places like Afghanistan are being promoted by the Citizens’
Network for Foreign Affairs (Rowe, 2005). The stores consist of a hole dug in
preferably sloping ground, 3.5 m deep, 5 m wide and 8 m long, with a long slop-
ing passage to allow access (Fig. 5.8). Foundation walls are built around the sides
of the excavated hole, 2.5 m high on the high side and 2.0 m high on the low
side. Beams 5 m long (Fig. 5.9) sit on the foundation walls, to support the roof.
One metre long, 200-mm-wide boards, with gaps left between them for air move-
ment, are nailed on to the beams. Retaining walls, built 1 m high round the
perimeter of the hole, form the edge of the roof. Straw, 1 m deep, is placed on
top of the roof boards and is filled level with the top of the retaining walls. The
retaining wall on the lower side has drains in it to allow any rainwater leaking
into the straw to escape. Lastly, the straw and retaining walls are covered with
100 mm of loose soil.
An ‘A’-shaped perforated duct stretches the length of the store, and is fitted to
an air intake box at one end. This allows air within the store to circulate naturally,
down the intake box, along the duct and up through the pile. Two ventilation
140 Chapter 5
Width 4 m
Foundation
walls
Retaining Intake
wall box
Straw 1 m Ventilator
deep Floor Floor
Boards Loose soil A A
of pile of pile
Beams
Retaining Duct
wall
Length
Pile Pile 7m
Beams
Drains Duct
Foundation Undisturbed
wall Boards Roof cut
ground
Section on AA Door away
Sloping Plan
walkway
to store
Fig. 5.9. Underground store under construction. (Courtesy of Citizens’ Network for
Foreign Affairs, Washington, DC, USA.)
Store Design and Structure 141
Fig. 5.10. Indian multi-storey sack cold store. (Courtesy of W. Leslie, Farm
Electronics, Lincoln, UK.)
pipes, located in the roof, allow ambient air to circulate through the store when
cool night air is available. Ventilation is controlled by the store owner, using a
thermometer lowered into the store to check when cooling is required.
In parts of the world such as India where labour is relatively low in cost, potatoes
are often stored in manually loaded multi-storey refrigerated cold stores (Figs 5.10
and 5.11). Loading is either manual using ladders or by a form of ‘dumb waiter’-
type lift arrangement. The store illustrated has a steel framework five floors high fit-
ted with porous floors. The sacks, usually 50 kg, are stacked flat, in a series of piles
on the floor (Figs 6.29 and 6.30). Refrigeration cooling coils, suspended from the
roof, emit cooled air, which passes over the piles of sacks on the upper floor. This
forms a circulatory air movement within the store, with air being drawn up through
the gaps between the piles of sacks. Many stores rely only on the refrigeration cool-
ing coil fans to create the circulation of air. In the sketch shown (Fig. 5.11), based
on a design by Tolsma Techniek, The Netherlands, additional air recirculation fans
are fitted to improve the air movement through the sacks of potatoes.
Sacks
Refrigeration
cooling coils
Access ladder A Doors to rooms
Inlet
vent Loading bay
Refrigeration
cooling coils Air circulation
Access
ladder
Section on AA
Circulation
Sacks fans
Steel frame
with mesh
Loading
floor to
bay
carry sacks
Fig. 5.11. Refrigerated multi-storey sack store. (Redrawn from Tolsma Techniek,
Netherlands plan.)
with a second personnel door fitted to ease staff access. Doors are potentially a
source of air leakage so the fewer the better. The enclosed grading area that adjoins
the stores acts as an air lock. If the store door is open, the grading area door should
be kept shut. This reduces the risk of freezing ambient air damaging the stored crop
or warm moist ambient air condensing on cold potatoes held in the stores.
One layout popular in The Netherlands, used to separate potatoes into 200–500-t
batches, uses a series of bins opening out into a central corridor (Fig. 5.12). The
design shown contains 530 t of potatoes in each of six stores, or 3200 t in total.
A mobile elevator, fed from trailers or trucks, loads the bins. As duct lengths are
26 m long, above-ground tapered ducts are used to ensure uniform airflow through
the crop. When the bin is nearly full, removable timber battens, or half-height
doors, are closed in the front of the bin to act as a front retaining wall. Loading of
Store Design and Structure 143
Section on AA
Ventilation
airflow
B Potatoes
Grading area
Compartments
Fans Tapered ducts within room
A A
Room 1 Room 2
Room 3 Room 4
End retaining
walls
B
Cut-away plan of 6-room
store with grading area
Building section
on BB
Fig. 5.12. Building complex of bulk stores holding a total of 3200 t, with grading area
attached. (Redrawn from Tolsma Techniek, Netherlands plan.)
potatoes continues over the wall or doors. This prevents the potatoes rolling for-
ward and provides a vertical front retaining wall. The sealed door in front of this
retaining wall is then shut.
To unload the bins, the bottom timber battens are removed, or the bottom
half-door opened, allowing potatoes to flow out the bottom of the bin on to an ele-
vator conveyor feeding a central conveyor running the length of the central corri-
dor. The conveying system takes the potatoes from the bins to a grading system at
the end of the storage building complex.
The layout for a group of box stores for ware potatoes is similar to that of the bulk
bins, with each store holding 500–1000 t of potatoes. Each store should be sized so
that it can be filled in 4–6 days, to minimize the exposure to changes in weather.
Once the doors are closed, the internal climate can be controlled.
Air movement and cooling can be provided by a blending box/refrigeration
system fitted with vertical ducts which throw air over the top of a stack of boxes.
Boxes are stacked to maximize airflow through the pallet apertures.
144 Chapter 5
If a letterbox system for seed potatoes is required, the end retaining wall shown
in Fig. 5.12 is replaced by a letterbox duct (Ch2.8), with the duct being pressurized
by fans jetting air downwards into the duct. Boxes are then stacked against the duct
for ventilation.
Loading and unloading is by forklift truck.
Where possible, the grading and packing area should be under the same roof as
the stores it is drawing potatoes from. This allows the transfer of potatoes from
store to grading area to be unaffected by frost, snow or wet weather. It also allows
stores to be opened without wind adding to the likelihood of outside air entering
the store. If possible the grading area should be equidistant from all stores.
If loading of lorries to dispatch crop has to be done outside the grading area or
individual stores, the loading area should be designed to give shelter from the pre-
vailing winds. In exposed areas, buildings can either be surrounded by trees or sur-
rounded by an earth bund.
Building layout and colour should be selected to minimize solar heat gain. For
the northern hemisphere, the relative solar radiation on roof and walls can be
found from Table 5.1. The figures give the temperature difference that should be
added to roof and wall heat gain calculations to take account of solar heat gain
(Box 5.1).
Table 5.1. Allowance for solar radiation in °C. (From Searle, 1975.)
Type of surface East wall South wall West wall Flat roof
To calculate heat gain through a roof or wall, the ambient dry-bulb temperature for
that part of the world and a store set-point temperature are selected. These may
be 18°C and 3°C, respectively. The temperature differential across the structure
will therefore be 18 – 3 = 15°C. To take account of solar heat gain, the figures in
Table 5.1 provide an additional temperature differential that should be added to
this figure. For a dark roof, a further 11°C should be added. This increases the
temperature differential to 15 + 11 = 26°C.
Store Design and Structure 145
The table shows that dark roofs in sunny climates should be avoided and west-
and east-facing walls receive more solar gain than south-facing walls. There is zero
solar gain in north-facing walls. External temperature sensors should be on the
north-facing wall to avoid being affected by the sun. Where potatoes are stored in
boxes, empty boxes can be stacked on the sunniest side of the store to provide
shading for the wall. In tropical countries, a twin-skinned roof is often used so that
natural air movement carries away the radiant solar heat.
Stores, grading areas and grading equipment should be designed to minimize the
risk of crop being infected with spores or contaminated with exudates from rotting
tubers. Spores are present in the soil, which when dry may turn to dust, become
airborne and settle on floors, ledges and potatoes. Contamination by exudates
occurs on conveying, washing, grading, brushing and packaging machinery.
The quantity of dust brought into store can be minimized by effective cleaning of
crops by the harvester and into-store cleaning equipment (Ch2.2). The greatest
amount of dust is generated once crops have dried out and are moved for the first
time after becoming dry. By enclosing box-tipping equipment or drops in convey-
ing equipment within a dust hood, a proportion of the dust can be removed by dust
extractors. Boxes conveyed by forklift spill considerable quantities of dust on to the
floor. This is dispersed into the air as the forklift passes to collect another box. To
minimize dust dispersal, forklifts should be driven slowly, transport distance to the
tipping point should be kept as short as possible and floors should be regularly
vacuumed.
The structure of the store can help minimize the amount of dust in store. Normal
store construction has exposed horizontal sheeting rails and roof purlins (Fig. 5.13),
where dust can settle. Any air movement can cause the dust to become airborne
and potentially infect stored crop. Either sheeting rails and purlins should be rou-
tinely vacuumed, or, where hygiene is paramount, the inside of the store can be
lined, for example with an extruded polystyrene board laminated to a white, rigid
PVC sheeting, 1.5 mm thick, such as that made by Owens Corning Building
Products (UK) Ltd (St Helens, UK). This is fitted on the inside of the sheeting rails
and purlins to give a smooth, white, washable, dust-free surface. Where the clad-
ding and insulation are to be provided by composite panels, rather than using one
thick composite panel, two composite panels half the thickness can be fixed on both
outside and inside of the sheeting rails. This will significantly add to the price of
the building.
146 Chapter 5
Fig. 5.13. Sheeting rails and purlins where dust can settle. (Courtesy of W. Leslie,
Farm Electronics, Lincoln, UK.)
Store design should take into account ease of cleaning. This will mainly involve
suction cleaning (vacuuming), which may require long extensions to cleaning equip-
ment to reach the top sheeting rails or the use of a travelling gantry. A slight slope
on the floor allows water to drain when washing floors.
The RH in a potato store ranges between 85 and 100% (Pringle et al., 1997). In cold
weather internal store surfaces may become wet with condensation. Only insulation
with a high resistance to water vapour transfer should be used. If vapour penetrates the
insulation, it will reduce its insulation value. Values for thermal conductivity and resist-
ance to vapour transfer for various insulation materials are shown in Table 5.2. These
are average figures taken from trade literature and mostly relate to published standards
for thermal conductivity (BS EN 12667:2002; BSI, 2001) and water vapour resistance
(BS EN 12086:1997; BSI, 1997a). The water vapour resistance is the reciprocal of the
rate of water vapour transmission, which is a measure of the weight of vapour (kilo-
grams) flowing through a material per second with a pressure difference measured in
giganewtons. The resistance is therefore giganewton seconds per kilogram of moisture
(GN s/kg). A high value indicates a good resistance to water vapour penetration. Box
5.2 describes the terminology used to define the thermal properties of buildings.
Store Design and Structure
Table 5.2. Insulation material characteristics.
147
148 Chapter 5
Box 5.2. Terminology and values used to define building thermal properties.
(Based on CIBSE, 2006)
The thermal conductivity, λ, is the rate of heat transfer (W/m °C) through insulation
material 1 m thick with a 1°C temperature difference between one side and the
other. It is used to compare the conductivity of different insulation materials.
Sometimes values of its reciprocal, thermal resistivity, –1λ (m °C/W), are quoted by
insulation manufacturers instead. Thermal conductance (W/m2 °C) is the rate of
heat transfer per 1°C temperature difference through a material of thickness L. The
reciprocal is the thermal resistance (m2 °C/W).
The surface conductance is the rate of heat transfer between a surface and air
per 1°C temperature difference between the two. Its reciprocal is surface resis-
tance (m2 °C/W).
The thermal and surface resistances of all the components of a wall, floor or
roof are added together to find a total resistance for the structure. The reciprocal of
the total resistance is the thermal transmittance for the structure, called the
U-value (W/m2 °C).
In the UK porous insulation materials such as glass fibre or rock wool are
usually avoided in new potato stores, as they are liable to become soaked with
condensation and lose their insulation value if the vapour barrier used (e.g. poly-
thene sheeting) becomes punctured or the installation workmanship is
substandard.
Board insulation materials used for stores should be rigid extruded polystyrene
board (e.g. Styrofoam, Styrodur, Polyfoam, etc.) with a closed cellular structure,
which resists vapour movement. Alternatively they can be polyurethane foams
made into boards by injecting foam between aluminium foil, PVC sheets or plastic-
coated steel sheet vapour barriers. With the polyurethane foams, vapour can enter
the insulation via the exposed ends of the sandwich, so a sealant should be applied
on exposed or newly cut edges.
Polyurethane foam can also be sprayed directly on to the store sheeting. This
has the advantage of sealing the store from air leakage and is an excellent way of
renovating buildings that are nearing the end of their life.
Extruded polystyrene sheeting, similar to the material used in disposable coffee
cups, is sometimes used as insulation for its cheapness, but unless it is sandwiched
between vapour barriers it will lose a proportion of its insulation value over time
due to surface moisture absorption.
Recommended thermal transmittance values (U-values) for various storage
systems and store locations are shown in Table 5.3. Since ambient-air cooled ven-
tilated stores usually only store potatoes for 4–6 months into early spring, the U-
value specified is less than for refrigerated stores, where storage may last for 8
months into early summer. More insulation is therefore required to minimize heat
ingress due to the warmer summer temperatures. However, higher levels of insula-
tion for ambient stores are an advantage as 2–3 weeks can pass when no cool
ambient air for cooling is available. The better the insulation and store sealing, the
slower the crop will warm during periods of warm weather.
Store Design and Structure 149
Table 5.3. U-values for potato stores and thickness of foam and rigid board. (From Cargill,
1976; Cargill et al., 1989.)
Walls Roof
UK
Ambient 0.42 50 60 0.36 60 80b
Refrigerated 0.36 60 70 0.26 80 100
Continental
Cold 0.27 80 100 0.20 110 140
Very colda 0.20 120 140 0.14 160 200
PIR, polyisocyanurate.
a
Insulation higher than necessary for temperature control to prevent condensation on the internal
structure.
b
70 mm would be sufficient but sheet does not come in this size.
Basic U-values for different wall, roof and floor configurations can be calcu-
lated (Box 5.3). For more complex configurations the CIBSE guide (CIBSE, 2006)
should be consulted.
The traditional structure for potato stores in the UK, until the mid-1990s, was to
externally clad a portal frame building with insulation board, and then protect that
from the weather with corrugated plastic-coated steel sheeting (Fig. 5.14). This
construction avoids any cold bridges, as stanchions are within the outer insulated
shell. Air leakage between the boards (Box 5.3) is minimized by:
● Using tongue-and-groove insulated boards.
● Taping the joins between the boards.
● Using two layers of boards staggered to ensure joins in the boards do not
line up.
The tongue and groove is of limited benefit, as often the fit is not tight, the boards
can shrink over time and only the sides, not the ends, are grooved.
Sealing of every join with mastic or silicon sealant, while effective, is very
labour-intensive and needs constant supervision of labour.
Taping of joins is successful only if applied externally, prior to the boards
being fixed. If done on the inner face of the boards, the tape loses its adhesion over
time and hangs in arcs from the roof. Staggering the boards is successful but more
expensive to install compared with fitting a single thicker board.
150 Chapter 5
5 0.11
≥20 0.18
Continued
Store Design and Structure 151
Thus:
U-value = 1 / R = 1 / 3.129 = 0.32 W/m2 °C.
For more precise calculations of U-values, see the CIBSE guide (CIBSE, 2006).
Values for thermal conductivities of a number of common insulation materials are
given in Appendix 3.
Outside Inside
surface surface
resistance resistance
50-mm airspace
Outside Inside
building building
Corrugated
Board insulation plastic-coated
forms a complete steel sheeting
outer shell
No cold bridges
The great benefit from spray-on polyurethane foam is that it seals every crack or
gap in the store, so that leakage through the building fabric is prevented altogether.
So long as doors and louvres are well sealed, the building will be airtight. The foam
should be applied at ambient temperatures above freezing, using CFC-free foam
applied by a reliable contractor, who applies the correct thickness (usually 60–
80 mm in the UK) of material throughout. The final result will be a well-insulated,
well-sealed store.
Care has to be taken regarding the fire rating and insurance cover. If the foam
itself does not comply with the fire rating ISO 9705:1993 (ISO, 1993) or BS 476:
Part 7:1997 (BSI, 1997b), it will have to be spray-coated with an intumescent paint
which expands when a flame is applied to it. Only foams which are acceptable to
the client’s insurance company should be used.
The foam forms an insulated shell on the inside of the building, rather than
on the outside when board insulation is used. All stanchions, purlins and sheeting
rails must therefore be sprayed with foam to avoid cold bridges and prevent con-
densation forming on the portal frames.
The foam surface is rough, with the result that dust can collect on the vertical
surfaces. The walls may therefore require more frequent cleaning compared with
situations where the insulation has a smooth surface. Its colour can darken in time
to a dull yellow and the surface can be damaged though collisions with machinery
or boxes on forklifts. If power washers are used to wash the foam, they should be
limited to 3.4 MPa (34.5 bar) pressure if the foam is not to be damaged (W. Elder,
CPS, Fife, UK, 2005, personal communication).
forms of insulated cladding, have made this the preferred construction method for
new stores in the UK.
In the past a major weakness of this form of construction was the seal between
neighbouring panels and where the panels meet the floor, the roof sheeting, the
gables and the ridge. This has resulted in a number of buildings incapable of pro-
ducing a quality product or with excessive refrigeration operating costs (Box 5.4).
Recent composite sheeting now uses three beads (i.e. extruded lines) of mastic
to seal the joins between the panels. If there is any doubt as to the sealing method,
the contractor can be left to select how he wants to seal the building, with the build-
ing contract specifying that the building be subject to an air pressure test when
complete. Air leakage of 10 m3/m2/h at 50 Pa pressure for composite sheeting is
quoted as the worst case that these panels will achieve in practice (Kingspan
Insulation Ltd, Leominster, UK; http://www.insulation.kingspan.com/).
As with spray foam buildings, the client’s insurance company should be
approached to verify that the foam and type of construction of composite panel has
a fire rating acceptable to them.
Load-bearing walls taking the lateral force of potatoes in a pile should be within
the insulated shell of the store. This will minimize the possibility of potatoes being
affected by frost, low-temperature sweetening, or condensation forming on the
panels and the potatoes nearest the walls.
154 Chapter 5
Stores with leaks in their building fabric or which have no way of keeping doors
closed during cleaning, grading or packing are liable to:
● Condensation on the crop when warm humid air enters the store and meets
cool potatoes.
● Freezing injury to the crop in freezing weather conditions.
● Lack of crop temperature control.
● Require excessive cooling ventilation to compensate for warm air entering the
building.
The building shown in Fig. B.5.2 was manufactured and built by an enterprising
farmer, but the crops he stored were severely infected with silver scurf. Air was
found to be leaking into store between the joins in the composite sheeting. The
leaks have subsequently been sealed with mastic, a major undertaking. It is too
early to know whether the leaks have been adequately sealed and the disease
problems solved.
Fig. B.5.2. Composite panel building that experienced air leakage between
vertical sheeting.
Store Design and Structure 155
5.8 Floors
In the UK, floors for stores are rarely insulated. The primary considerations are to
ensure that the floor is strong enough to take the weight of forklift trucks with
industrial-type, solid rubber tyres, carrying or raising potato boxes. Often two boxes,
holding 1 t of potatoes each, are put on the top of the stack of boxes, as the forklift
has insufficient reach to put the top box up on its own. To cope with these loads,
floors should be reinforced with steel mesh or steel fibre to ensure that they do not
crack over time. A detail of a typical floor for forklift trucks is shown in Fig. 5.16a.
In bulk stores, the pneumatic tyres on front loaders are likely to exert less force
on the floor than forklifts so floor strength can be reduced.
The floor should have sufficient fall to ensure that wash water will move to drains,
but should be sufficiently level to ensure stacks of boxes remain near vertical.
In continental climates, where ambient temperatures can be very low, floors should
be insulated. This is usually done by installing an extruded polystyrene insulated
board, capable of taking a distributed load, on top of a 150-mm-thick concrete slab
(Fig. 5.16b). A further 200-mm reinforced concrete slab is laid on top of the insula-
tion. The important detail is to ensure that heat is not lost through the edges of the
store. Thermal insulation blockwork or insulation board placed vertically will pre-
vent this happening.
Steel column
Composite
Sheeting rail
panel Thermal insulation
Square mesh blockwork
Blockwork reinforcement
wall Load-bearing
Dowel bar
insulation board
expansion joint
Ground
Fig. 5.16. Detail of (a) an uninsulated and (b) an insulated floor designed to carry a forklift for
stacking boxes. (Courtesy of C.A. Johnston, CEng MICE, Aberdeenshire, UK.)
156 Chapter 5
In bulk stores the ducts are often under the floor to ease the removal of potatoes
from the store. Most underfloor systems are specified and installed by the suppliers
of the ventilation equipment (Ch6.4). Their involvement should be sought in the
early stages of building design.
5.9 Doors
Doors are commonly the major source of air leakage in potato stores. The best
arrangement is for there to be only one crop loading door into the store, and it
should open into a second building such as a grading area so that it acts as an airlock.
The door should be well-sealed and easy to open and close. In the UK it is preferable,
but not essential, for the door to be insulated. In continental climates insulation of
the door panels, or a secondary insulated curtain, is required. A personnel door
should be fitted to avoid having to open the main door for staff to enter.
Sliding doors designed for food chillers also make good doors for potato stores (Fig.
5.18). The track is so designed that when the door is nearly shut, it moves laterally,
so that it seals against the frame of the doorway on all four sides.
The speed of electrically operated doors can be a significant issue if stores open
to the outside. Slow-opening doors will tend to be left open by forklift drivers, so
that they can maintain a rapid unloading or loading rate. In some store com-
plexes, the doors of a number of stores are opened in the morning and stay open
until late afternoon. Environmental control is therefore lost during daytime.
Warm humid air can enter the stores and condense on the crop, and the cooling
has to work overtime during the night to cool the crop back down to
temperature.
If a forklift travels at 5 m/s and the door takes 10 s to open, the door has to
start opening 50 m before the forklift arrives if its entry is not to be impeded. If the
door takes only 5 s to open, only 25 m is required. Door opening by radio control
operated from the forklift cab will greatly improve the chance of doors being
opened only when access is required.
Fig. 5.18. Fridge-type sliding door. (Wood Farm, Norwich, Norfolk, UK.)
158 Chapter 5
Most old stores in the UK are fitted with sliding doors. Even when fitted with brush
strip seals these tend to allow considerable ingress of ambient air. They are particu-
larly difficult to seal at ground level. They should not be fitted to new stores.
Plastic strip curtains can be fitted inside sliding doors to both improve the seal-
ing of the door and act to reduce air ingress when the door is open, but their effec-
tiveness is limited (Box 10.1).
Stores and grading areas can be dangerous workplaces, so a full written assessment
should be made of the potential risks to staff (BPC, 2005a). These should be
updated regularly and staff training undertaken to ensure that the recommenda-
tions are part of routine store operation.
Harvesting, crop cleaning, crop sizing and the elevating of the crop into store
all involve the use of rotating machinery. These should all be guarded to ensure
fingers, hands or pieces of clothing cannot be drawn into moving machinery.
Guards that have to be removed for machine adjustment must be replaced
prior to restarting machines. Staff should receive training on the safe operation
of the plant.
In box stores, forklifts often pose the greatest danger to staff in the store. They
can appear suddenly through a plastic curtain doorway at frightening speed.
Forklift movement areas should be clearly marked on the store floor, only essen-
tial staff should have access to vehicle movement areas and high-visibility cloth-
ing should be worn at all times. Forklift drivers should be trained to keep speeds
down, to use lights and horns in areas of poor visibility and to use mirrors when
reversing.
Boxes may be stacked to heights of 7–8 m. A fall from this height on to concrete is
often fatal. Staff should only access the top boxes when this is necessary to install
sensors or to sample crop. Sampling of top boxes should follow guidelines described
in the work at height regulations (HSE, 2005b). Access should be via a fixed plat-
form, fixed ladders, staircases, access towers, scissor lifts, or a purpose-designed
Store Design and Structure 159
cage which allows staff to be elevated safely by forklift. A simple solution is multiple-
step short ladders (Fig. 5.19).
Boxes should be stacked so that any gaps do not exceed 200 mm, to pre-
vent staff falling between boxes. Staff should be protected from falling over the
edge of a stack of boxes by erecting high-visibility tape fixed to canes inserted
into the corner of the box one-in from the edge of the stack. An alternative
measure is to use a work restraint system, consisting of a lanyard fixed at one
end to the belt of the staff member and at the other to a steel rope. Stops on
the rope are adjusted to prevent the staff member reaching the edge of the
boxes. Stops should be altered each time boxes are removed from store. A fall
arrestor system could be used instead, but this adds to the cost and complexity
of the safety system.
Store lighting should be 50 lux or more to see dangers and obstructions when
carrying out sampling. The use of a head torch can be extremely useful for examin-
ing samples more closely (Fig. 5.19).
Samples of potatoes should be collected into bags or containers not exceeding 8 kg,
so that they can be safely passed down to a colleague below.
160 Chapter 5
Boxes are usually constructed of wood, which degrades and can be damaged over
time. In the UK, 1- t boxes should be constructed to BS 7611:1992 (BSI, 1992).
Failure of pallet bases or corner posts can lead to a column of boxes toppling.
Staircase- or short ladder-fixing systems should hook over the centre or corner
posts as damaged top boards can be pulled off if a side load is applied. A better
approach is to provide permanent access at the design stage of the store, to a top
gantry supported from the roof or to the top of a ventilation duct (Fig. 5.20).
Access to ventilation tunnels and fans is required for opening and shutting laterals
and for maintenance. Prior to personnel access, fans and equipment should be shut
down and an isolator switched to the off position. Duct doors should open out-
wards, so that fan pressure aids rather than prevents escape in the event of a fan
being switched on inadvertently when a member of staff is in the duct. Electrical
services and lighting in the duct should be completely waterproof, to avoid the for-
mation of condensation within luminaries and the associated risk of the bulbs fus-
ing and staff being put in danger.
Dust, especially when working on grading lines (Ch10.4), can be a major hazard. Staff
may become allergic to the dust and be unable to continue to work in environments
where dust is present. Employers may subsequently become liable for costly compen-
sation. It is the responsibility of the employer to ensure that dust levels are kept below
acceptable levels. In the storage complex, dust is usually worst when potatoes have
been stored and dried and are subsequently moved. Dust falls through boxes on the
forklift or is generated on cleaners or conveying machinery. Good design of the store
can minimize forklift travel distances and separate off dusty areas from where staff are
working. Where dust cannot be controlled, staff should be supplied with masks
Store Design and Structure 161
Fig. 5.20. Access to top of boxes built in at the store design stage. (Wood Farm,
Norwich, Norfolk, UK.)
162 Chapter 5
5.11 Summary
This chapter has focused on the different types of stores used worldwide and what
criteria should be considered when selecting and designing a store. The main
aspects to consider are the following.
● Low-cost in-field storage is feasible but is labour-intensive and vulnerable to
high crop loss through rotting and disease.
● Stores for the higher latitudes and cold continental areas need to be well-
insulated, well-sealed, should be able to store crop for 6–9 months and be
environmentally controlled.
● Stores for tropical areas can be lightweight structures, storing 1–2 t of potatoes for
2–3 months and use manually operated flaps to make use of night-air cooling.
● Evaporatively cooled small-scale tropical stores have yet to live up to their
potential.
● Different evolutions of store technology in different parts of the world have
produced different types of structure.
● While the USA and Scandinavia tend to use continuous humidified-air ventila-
tion systems with bulk storage developed in the 1970s and 1980s, The
Netherlands and the UK have kept to the non-humidified ventilation systems
used previously.
● In India and southern Asia, high temperatures the year round combined with
low labour costs have favoured the general-purpose refrigerated cold store for
keeping potatoes and other food produce.
● Store complexes with a grading building attached allow potato unloading,
grading and dispatch to be carried out without restrictions from freezing, snow
or wet weather.
Store Design and Structure 163
● Building location, aspect, colour and shelter influence store air leakage and
solar heat gain. Sealed stores, light in colour and protected by trees all ease
store environmental control.
● Building designs which minimize ledges for dust to settle, ease washing of
stores and minimize forklift travel distance help minimize dust dispersal, worker
discomfort and potential spore contamination of crops.
● To cope with the high store humidity and prevent condensation within porous
insulation materials, new stores tend to be insulated with foam-based insula-
tion materials.
● Composite-sheeted buildings are becoming the norm in the UK for new stores
but must be sealed between sheets, at eaves, apex and floor.
● Spray-on foam is popular for upgrading existing buildings and for sealing leaky
buildings.
● Floors must be designed to take the high point loads exerted by solid-wheeled
forklift trucks when putting two boxes up together.
● Well-sealed, good-quality doors are essential if stores are to be environmentally
controlled. They should be able to be open and shut rapidly where repeated
access is required.
● Risk to staff from rotating machinery, forklift movement and falling from the
top of stacks of boxes should be assessed and safe operating procedures put in
place.
● Staff exposure to dust can severely impair health, so dust extraction equipment
should be installed wherever dry potatoes are moved.
● Carbon dioxide concentration in stores should be monitored routinely to
ensure staff safety and to avoid blackheart in the crop.
● Fire escapes should be provided with lights to indicate exits.
6 Store Ventilation
6.1 Introduction
A ventilation system is installed into a potato storage facility to dry the crop,
remove crop respiration heat, prevent condensation that often occurs just under
the surface of newly stored potatoes (subsurface condensation) and allow cooling of
the crop when the weather permits. This chapter describes the procedures that
should be followed in designing a ventilation system. This involves calculations but
these are kept as basic as possible. More detailed supporting mathematics can be
found in Appendix 4.
164 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Store Ventilation 165
Air warmed
by potatoes
Cool air Expelled
enters warm air
Fan Main duct
Airflow up
Potato pile surface pile
Airflow in duct
Lateral ducts
In the mild climate of the UK, inlet louvres tend to be fitted vertically into the
external cladding of the store (Fig. 6.2). If there is risk of rain being carried into
store by the incoming air and landing on potatoes, then a rain hood is put over the
inlet. If the air first goes into a duct, the rain will drop harmlessly on to the floor
of the duct, so no hood is required.
In countries within a large continental mass, louvres are often fitted horizon-
tally under an overhanging entry duct, so that snow and ice cannot form on them
and prevent them opening or closing (Fig. 6.3). In North America, ram-operated
vertical insulated panels are used instead of blade-type louvres, as panels tend to
be less likely to freeze and jam. In periods of very cold weather, additional insulated
Fig. 6.3. Horizontal louvres under the building eaves protect blades from icing up.
(Courtesy of A. Johansson, Norrköping, Sweden.)
panels are sometimes fitted manually on the outside of the louvres to prevent them
freezing up, with the possibility of cold air leaking into store.
The louvres should seal tightly when closed, and no light should be seen when the
store doors and louvres are shut. Air speed through louvres is normally restricted to
between 3 and 5 m/s to minimize backpressure on the fans (Box 6.1). The higher the
air speed through the louvre, the greater the backpressure on the fan (Box 6.2); the lower
the air speed, the less the backpressure, but the more expensive the louvre. A compro-
mise between the amount of backpressure and the cost of the louvre has to be found.
The usual arrangement is for the inlet to feed the incoming air into a blending
chamber. The inlet louvre is usually placed opposite to a recirculation louvre
When a fan is used to move air through a ventilation system it has to overcome the
resistance caused by the friction between the flowing air and the items it passes
through such as louvres, safety guards, main duct and lateral ducts. If the airflow
has to suddenly contract or expand, or change direction, these too cause a resis-
tance. The sum of these resistances, or pressure losses, measured in N/m2 or
Pascal (Pa), is referred to as the system resistance. This system resistance causes
a backpressure on the fan. For any fan, the higher the system resistance, the
greater the backpressure and the less air will flow from the fan. A fan has to be
selected with sufficient backpressure capability to overcome the system resistance
of the ventilation system and provide the desired quantity of air.
Store Ventilation 167
Inlet and outlet louvres are usually sized so that the air speed through them is kept
to below 3–5 m/s. This keeps the backpressure through the louvre between 8 and
24 Pa (Table B.6.1), and so enables low backpressure propeller or axial fans to be
selected. The backpressures were calculated as shown in Ch6.3.
3 8
4 15
5 24
(Fig. 6.4). Alternatively a single flap or damper is sometimes used, with the inlet
opening as the recirculation inlet closes. Two alternative arrangements for feeding
the air into the distribution system of a bulk store can be used. In Fig. 6.4a, a fan,
with its impeller blades horizontal, draws air in from the blending chamber and
discharges it into the lateral ducts that branch off the main duct. In Fig. 6.4b, each
lateral duct has its own fan which sources its air from the blending chamber.
In airspace ventilation systems for boxes, fans on the outlet of the blending
chamber discharge the air through overhead ducts into the store. In positively ven-
tilated stores, the fan(s) discharge air into the letterbox duct, which in turn distrib-
utes the air into the stack of boxes placed against it (Ch6.8).
Inlet Inlet
Blending Blending
chamber chamber
Fan
Pile Pile
height height
Fan
Duct
Duct
(a) (b)
Fig. 6.4. Alternative forms of blending chamber: (a) large single fan supplies a number of
(underfloor) ducts; (b) one fan per (above-floor) duct.
168 Chapter 6
The exit louvres (Fig. 6.1) are usually located as high as possible in the building,
as the exhaust air, having been warmed during its passage through the crop
being cooled, rises to the apex of the store. In continuous humid ventilation sys-
tems, the outlets act like weirs in a river, allowing the warm exhaust air to spill
out of the store.
So long as the air always exits the louvres at more than 3 m/s it is unlikely that
rain will enter through outlet louvres. Exit louvres should preferably not be in both
gables, as when the wind blows, the louvre in the windward gable can become an
inlet while the louvre in the leeward gable becomes an outlet. Ventilating air
should only be allowed to enter the store via the inlet duct and blending chamber
so that its temperature can be controlled (Ch3.5 and Ch8.3).
If there is a risk of rain wetting potatoes near an outlet louvre, rain hoods
should be fitted on these.
Example
If a store is to hold 1000 t of potatoes, and the UK system of ventilation and a sin-
gle fan is to be used, then:
Fan airflow required = 1000 × 0.02 = 20.0 m3/s.
Airflow rates for positive ventilation of boxes are often higher to compensate for
leakage between the boxes.
To fully specify the fan, the backpressure it has to work against should be calcu-
lated. The total backpressure, or system resistance, is the sum of all the pressure
drops that occur as the air flows through the ventilation system. In the diagram
shown (Fig. 6.5), this includes the sudden contraction (1) as the air enters the inlet
louvre, the resistance of the louvre itself (2), the sudden expansion (3) as it leaves
the louvre, the loss due to a change in direction (4), the sudden contraction (5)
as the air enters the fan, the resistance created by the fan guard (6), the sudden
expansion (7) as it leave the fan, the loss due to a change in direction from verti-
cal downwards flow to horizontal flow (8) along the main duct, the loss due to
the air resistance (9) due to air flowing along the duct, the sudden contraction of
air (10) as it enters the lateral, the resistance to air (11) as it travels through the
lateral, the sudden expansion (12) as the air leaves the lateral, the resistance to
airflow caused by the pile of potatoes (13), the sudden contraction as the air
enters the exhaust louvres (14), the resistance of air as it flows through the louvres
(15) and finally the sudden expansion (16) as the air leaves the exhaust louvre.
These losses exclude items that could be present such as mesh grids over the
louvres to prevent birds entering the store, and rain louvres on either inlet or
outlet louvres or both.
Fig. 6.5. Components that make up the system resistance of a bulk ventilation system.
170 Chapter 6
1
Ps = k × ρV 2 , (6.1)
2
where
Ps = static pressure caused by the constriction
k = constant for the constriction
ρ = air density (1.23 kg/m3)
V = air velocity through the constriction.
Values of k for each of these constrictions are available (FläktWoods Ltd,
Colchester, UK; http://www.flaktwoods.com/). The selection of the precise value
for k depends on the tightness of bend, the size of the mesh guards, the ratio
between the main duct and the lateral, and the roughness of the duct. In the
example shown (Fig. 6.5 and Table 6.2), all the losses have been included to show
what goes to make up the pressure loss of the whole system and how the removal
of a guard, the fitting of a bell mouth to the fan or the elimination of a bend can
help reduce the backpressure on the fan. To ease routine calculation of total back-
pressure, some of the losses can be combined. If the fan being selected has been
tested with a sudden contraction at entry, the first sudden entry pressure drop
should be omitted.
What is evident from Eqn (6.1) is that backpressure is proportional to the
square of the air velocity. To minimize backpressure at any point, the air velocity
should be kept as low as possible. Air speeds of greater than 6 m/s cause a dispro-
portionate increase in backpressure; so for potato storage, air speeds in ducts
should be kept at or below this value (Fig. 6.6).
High backpressures can be avoided by:
● Specifying large-diameter fans.
● Restricting air speeds in ducts to 6 m/s.
● Avoiding restrictions such as bird mesh and guards in high-velocity areas.
● Minimizing changes in air direction, sudden expansions and sudden contractions.
If a fan causes air to flow through a series of ducts of different cross-sections, the
flow Q will be constant, but the velocity V of air will change depending upon
whether the duct has a small (A1) or a wide cross-section (A2) (Fig. 6.7). This gives
rise to the continuity equation below:
Q = A1 V1 = A2 V2, (6.2)
Store Ventilation 171
Air being forced down a duct by a fan has kinetic energy due to its movement and potential
energy if the exit to the duct is constricted. The total energy of the air in the duct is the sum
of the kinetic energy and the potential energy. This energy is normally measured as pres-
sure (N/m2 or Pa), potential energy being referred to as ‘static pressure’ and kinetic energy
as ‘velocity pressure’. The sum of the two at any point in the duct system is the total pres-
sure at that point. The equation for velocity pressure and static pressure is:
Total pressure (Pa) = Velocity pressure + Static pressure = 1_ ρV + Ps,
2
2
where
ρ = density of air (kg/m3)
V = velocity of air (m/s)
Ps = static pressure in the duct (Pa).
The backpressure against which the fan is working is the static pressure.
The static pressure is measured using a manometer, one end of which is connected to
the duct, the other to atmosphere (Fig. B.6.1a). The weight of fluid supported, divided by the
cross-sectional area of the manometer tube, gives the static pressure in Pa (N/m2).
Alternatively the static pressure can be quoted in mm of water, which assumes that the fluid
in the manometer is water, or that the manometer scale is calibrated to read in mm of water
even though the liquid in the manometer is dyed paraffin.
The manometer can also measure total pressure (Fig. B.6.1b) by pointing the intake
tube of the manometer into the airstream. This measures the sum of the velocity pressure
and the static pressure.
To measure the velocity pressure using the manometer directly, the tube facing into the
airflow is connected to one side of the manometer and the tube measuring the static pres-
sure is connected to the other side (Fig. B.6.1c). The manometer reads the difference
between the two measurements, the total pressure minus the static pressure, which gives
the velocity pressure.
In practice a Pitot tube (Fig. B.6.1d) is connected to the manometer as this is designed
to minimize turbulence, which could affect the readings.
Velocity
Location Free area, Flow, Q Velocity, pressure, Duct Pressure % of system
on Fig 6.5 System element A (m2) (m3/s) V (m/s) k Pv (Pa) length (m) drop, Ps (Pa) resistance
Chapter 6
Store Ventilation 173
70
60
50
Pressure (Pa)
40
30
20
Recommended maximum
10
air speed
0
0 2 4 6 8 10
Air velocity (m/s)
Fan
A1 V1 V2
Q A2
Duct 1
Duct 2
where
Q = flow of air (m3/s)
A1, A2 = cross-sectional areas of duct sections 1 and 2 (m2)
V1, V2 = respective air speeds in the two sections of duct (m/s).
Eqn (6.2) is used to determine the speed of air at any point in the ventilation sys-
tem, so that the pressure drop for each item that causes a pressure drop can be
calculated using Eqn (6.1).
Q 20.0
V1 = = = 4.0 m/s.
A1 5.0
The combined k value for a sudden contraction (k = 0.5), the resistance through a
bladed louvre (k = 0.025) and a sudden expansion (k = 1.0) is 1.525.
Using Eqn (6.1) with r = 1.23 kg/m3 gives:
1
Ps = k × ρV 2
2
1
= 1.525 × × 1.23 × ( 4.0 )2
2
= 15.0 Pa.
This procedure is followed for all locations along the airflow path (Fig. 6.5), points
1, 4–13 and 15. The calculations are shown in Table 6.2.
The total system resistance, or backpressure, that the two fans have to work
against is 201 Pa (Table 6.2). It is usually wise to add some extra backpressure
capability to take into account any unforeseen resistances such as bird screens or
guards. For this example we shall increase the design backpressure to 225 Pa.
Axial flow fans (Fig. 6.8) predominate in potato storage. They provide the large vol-
umes of air required, with a minimum of heat output from the fan. Old installations
may use centrifugal fans, but these were often used both for grain drying and potato
storage, so were fitted with motors about three times the size of those used in axial
flow fans for potatoes to cope with the 1000 Pa of backpressure required for grain
drying. While they will not use their whole power capability at the 150–250-Pa
backpressures found in bulk potato stores, their power use will be higher than that
of axial flow fans selected purely for potato storage. The use of a high backpressure
centrifugal fan not only increases the running costs of ventilation but the additional
heat produced by the electric motor will dry the air more than if an axial flow fan
is used instead. If a centrifugal fan must be used, the heat from the motor should
be kept out of the airstream by separating the motor from the ventilating air.
To select a fan, find one with a performance curve with the required volume
flow near the centre of the x-axis and a static pressure near the centre of the y-axis
(Fig. 6.9). Draw a line vertically upwards from the desired volume flow until it meets
the required static pressure, shown by the upward curving dotted line and marked
PsF. Mark this meeting point on the chart and determine the blade pitch angle from
the series of curving lines marked from left to right in degrees. The fan selection is
now almost complete. Draw the vertical line down until it meets the appropriate
pitch angle on the absorbed power plot. Read off the power on the y-axis.
Additional information may show the efficiency of the fan at this operating
point and its noise output in decibels. Repeat this for a number of different diame-
ters of fans and see which provides the flow and static pressure you want for rea-
sonable cost. Manufacturers produce selection programs on the World Wide Web
or on a CD to make this selection easier.
If variable speed control is being considered, this should be stated in the speci-
fication as the coils of the fan motor need to be wound to suit.
Example
On Fig. 6.9, for the fan duty we selected in the section above, select the flow as
10.0 m3/s because there are two fans providing the combined flow of 20 m3/s.
Draw the line up to point A, where the static pressure is 225 Pa (Table 6.2). Note
the pitch of the blades to the right of this point is 28°. Extend the line from A down
into the absorbed power plot, meeting the 28° blade pitch curve at point B. The
absorbed power taken of the y-axis to the left of point B is 4.6 kW.
Fan noise is of increasing importance as farming areas become ever more popu-
lated with rural dwellers. The whine from axial fans, similar to that from an air-
craft engine, can be very annoying to neighbours. When selecting fans, their noise
rating should be checked and the planning authority consulted to see if the noise
levels are acceptable. The slower the fan runs the lower its noise output (Table 6.3).
By choosing large-diameter fans to give low air speeds in ducts, which favours uni-
form airflow distribution, the additional benefit from low noise will be achieved.
The noise level to neighbours can be further reduced by fitting a noise baffle
on the fan inlet (Fig. 6.10) or by surrounding the building with a soil bund. Trees
do not make a very good sound barrier.
176 Chapter 6
0
30
0
0
20
p sF =
25
15
V p sF =
=
p sF
=
sF
sF
sF
p
Type D
p
400
0
Air Density 1.2 kg/m3
40
10
=
sF
1.5
p
96
350
50
=
sF
p
94 97
pF – Fan (Total) pressure (Pa)
300
dF
30
p
250 100 1.0
95 95
32°
200 20
95
91 96 A
150
24°
92
0.5
92
100 10
16°
91
50
8°
0 0 0.0
0 2 4 6 8 10 12 14 16 18
Pr – Absorbed power (kW)
7
6
5
32°
4
3
2 24°
1 16°
0
0 2 4 6 8 10 12 14 16 18
B
qv – volume flow (m3/s)
Fig. 6.9. Manufacturer’s performance curves for an axial fan. (Courtesy of FläktWoods Ltd,
Colchester, UK.)
715 59–63
960 68–72
1475 78–84
Store Ventilation 177
Soundproofing material
(e.g. rock wool)
Steel inlet
Fan cowl
Airflow
Mesh to keep
material in
place Fig. 6.10. Soundproofing a fan inlet.
Many fan guards are bolted directly on to the fan casing (Fig. 6.8) and are
designed to prevent fingers touching fan blades. These guards can impose large
and unnecessary backpressures on the fan. If the guard can be offset from the fan
by the length of a finger then the mesh can be sized to prevent a hand passing
through the mesh. If it can be offset by an arm’s length, then the mesh can be
sized to prevent a body passing through it. This will reduce the backpressure on
the fan significantly and allow the lowest backpressure fan possible to be used,
though a further mesh in a low air speed area of the ducting may be necessary to
keep birds out.
The traditional system for ventilated bulk storage is to have a fan blowing into a
main duct with porous lateral ducts coming off one or both sides of the main duct
(Figs 6.11 and 6.12). The laterals may be above or below ground.
178 Chapter 6
Laterals
Fans
Main duct
(a)
Laterals
(b)
Fig. 6.11. Plan of alternative main and lateral duct layouts for bulk stores: (a) central
main duct; (b) side main duct.
Fig. 6.12. Main duct with bulk potatoes on either side. (Courtesy of W. Leslie, Farm
Electronics, Lincoln, UK.)
Store Ventilation 179
Main duct
The main duct should be sized to allow the store manager to walk through without
bending. Cramped ducts deter managers from opening and closing laterals, vital to
ensure that the parts of the crop that need extra ventilation get it (Holloway, 1990).
A light switch and fan stop button should be installed inside the duct, to allow staff
inadvertently trapped within the duct to stop the fan should it start on automatic.
Alternatively, doors to ducts should open outwards so they can be opened with the
fan running. The duct lights should be waterproof so that they are not affected by
condensation.
Above-ground laterals
The use of above-ground ducts saves the expense of installing a ventilated floor.
They do not become clogged with soil and they can be tapered for uniform air dis-
tribution. Care has to be taken during store loading to prevent above-ground ducts
being pushed out of position due to the pressure of potatoes.
Above-ground ducts are less suitable for stores that are unloaded by front-end
loader, as the loader wheels can damage the ducts. If they are to be used with a
front-end loader unloading system, they should be spaced so that the loader can
work between the laterals.
Above-ground ducts are normally of an ‘A’ or semicircular cross-section (Fig.
6.13a and b), although plastic drainage piping can be used as a temporary measure
(Bishop, 1994).
Below-ground laterals
Below-ground ducts allow the store to be filled and emptied without the risk of
damaging the ducts. These range from:
● Simple channels covered by 50–75-mm-thick wooden slats (Fig. 6.13c).
● Concrete floors with vents cast in them.
● Ventilated hardwood floors that can be placed on top of existing concrete
floors (Fig. 6.14).
Mesh covered
Air gaps between
with hessian
boards
WPB plywood
50–75-mm-thick
timber boards
Fig. 6.13. Alternative designs of lateral ducts: (a) timber A-frame ducts: (b) weld mesh hoop
ducts; (c) underfloor ducts with base rising to form a taper.
180 Chapter 6
Fig. 6.14. Hardwood ventilated floors laid on a plain concrete base. (Courtesy of
Flach and Le-Roy Ltd, Huntingdon, Cambridgeshire, UK.)
Dual-purpose grain/potato floors need a fine mesh to prevent grain from falling
through the porous area. This mesh can get plugged up with soil, so needs cleaning
annually. The mesh is recessed in troughs about 10 mm below the surface of the
floor to protect it from damage by wheel lugs and the edge of unloading buckets.
If buckets are used to empty the store, potatoes sitting in the recessed troughs can
be sliced by the edge of the bucket, resulting in a small but significant proportion
of the crop being unmarketable.
Lateral spacings
The maximum lateral spacing for above-ground ducts should not exceed the
minimum pile height, if an acceptable uniformity of ventilation through the
potatoes is to be achieved (Fig. 6.15a). This should take into consideration that
in some years the crop being stored will not fully fill the store, so that the depth
of storage may be lower than the design specification. Narrower spacings
between laterals will improve the uniformity of air through the crop. With
below-ground ducts the recommended spacing is 0.8 times the pile height
(Rastovski and van Es, 1981), as they do not spread the air sideways as effectively
as above-ground ducts.
Laterals are spaced as shown in Fig. 6.15b. The ends of the laterals should ter-
minate at half the spacing between laterals from the sidewalls. The lateral nearest
the end wall should be no closer than half the spacing between laterals. These
Store Ventilation 181
Store wall
Store
W/2
wall
W/2 W
H = height of potatoes (and main duct)
W = H for above-ground ducts
W = 0.8 H for below-ground ducts
Fig. 6.15. Distance of laterals from load-bearing retaining walls: (a) elevation of main duct
and laterals; (b) plan of main duct and laterals.
Laterals
Fan
Velocity = Max Velocity = 0
Main duct
Duct
framing
(a)
Fig. 6.16. (a) Cross-sectional plan of main bulk ventilation duct showing how air
velocity falls due to air leaving through laterals; (b) plot showing how total pressure,
static pressure and velocity pressure change along the main duct.
0.5 1.13 89
1.0 1.35 22
2.0 2.00 6
Store Ventilation 183
Table 6.4 shows that where the flow of air from the main duct into laterals is more
uniform, the backpressure on the fan is increased.
Even the best designed main duct and lateral systems tend to have higher
airflows in the ducts furthest from the fan. Slides are usually put in the entrance
to each lateral, partly to allow selective ventilation of new crop being loaded
into store and partly to allow the exit airflow to each lateral to be adjusted.
Slides can be adjusted to make the airflow from each lateral more uniform.
This is a labour-intensive task as an adjustment in one lateral affects all the
others. If it is done once, and the slide settings marked, then adjustment there-
after is simplified.
Fan Fan
As long as possible
Fan
Fan As high
as possible Low
velocity
High
velocity
Main duct
Laterals Laterals
minimized if air jets through stationary air masses are avoided. Air jets are
minimized by:
● Selecting the largest-diameter fan that can be afforded.
● Expanding the air from a fan in a tapered transformation piece with an angle
of 11° to the direction of the airflow.
● Placing the fan as far from the first lateral as possible.
5 4:1
10 7:1
15 10:1
20 13:1
Table 6.6. Pressure drop due to resistance of potatoes (Pc) for different air speeds and for
different heights of pile (specific volume of potatoes = 1.5 m3/t).
USA/
Scandinavia 0.007/0.0035b 3 0.014 7.0 5 0.023 13.7
UK 0.02 0.04 28.7 0.067 57.2
Netherlands 0.04/0.02b 0.08 72.6 0.133 143.5
a
Approach velocity = (volume of potatoes/m2 floor area/specific volume of potatoes) × airflow rate per
tonne (see Box 3.10).
b
Highest of the two rates was used for calculations.
186 Chapter 6
STEP 1: SIZING OF MAIN DUCT The main duct is sized mainly by the height the
potatoes are to be stored, wide enough to allow a person to walk along it, and if
possible sized to suit the 1.22- or 2.44-m size of plywood sheet if this or similar
sheet material is to be used. Eqn (6.2) should be used to check that the air speed
is no higher than 6 m/s.
Airflow (Q ) is 20 m3/s. Required duct area is Q / 6 = 3.333 m2. Actual duct
area is 1.22 × 3.6 = 4.39 m2; this is above the required size so is satisfactory. Initial
air speed is 20/4.39 = 4.55 m/s.
STEP 3: SIZE OF LATERALS The inlet of the laterals should be sized to have an air
entry speed Vmax of 6 m/s or below. Since there are 18 laterals in total, the amount
of air in each lateral (Q o) is:
Q o = 20.0 / 18 = 1.11 m3/s.
The area of laterals at the start is:
Ao = Q o / Vmax = 1.11 / 6.0 = 0.185 m2.
A decision as to the width of the laterals has to be taken. If they are assumed to be
0.4 m wide internally, their initial depth can be calculated:
Depth (initial) = Ao / width = 0.185 / 0.4 = 0.46 m.
The area at the closed end of the lateral is taken from Table 6.5. The ratio of the
duct inlet/duct end cross-sectional area for a 8.4 m duct is about 6:1. Therefore the
area at the end is:
Initial area/ratio = 0.185 / 6 = 0.0308 m2.
If the width is kept constant at 0.4 m wide, the depth at the far end of the duct will
be 0.0308/0.4 = 0.077 m, or about 80 mm below the bottom of the duct covers.
STEP 4: SIZE OF LATERAL DUCT COVERS For tapered ducts the static pressure in the
duct should be even. An outlet velocity of 4 m/s is recommended (Rastovski and
van Es, 1981) to ensure the flow is even. As potatoes block off between 65 and 75%
of the air outlets, the outlet area has to be 3 to 4 times that needed to give 4 m/s
outlet air speed with no potatoes in place.
a) Free area per lateral
For the lateral ducts sized in step 3, the free area is calculated as follows:
Airflow per lateral (Q o) = 1.11 m3/s.
Free area required for the whole duct, if no potatoes in place, is:
Q o / Vmin = 1.11 / 4 = 0.2775 m2.
Free area required to take account of the potatoes is 0.2775 × 3 = 0.8325 m2.
Store Ventilation 187
As laterals have an internal width of 0.4 m, the total amount of gaps between
wooden slats should be:
Total gap = 0.8325 / 0.4 = 2.08 m.
b) Number, width and spacing of boards per metre
Gap per metre is calculated as:
Total gap / length of lateral = 2.08 / 8.8 = 0.236 m per metre of lateral.
If boards are 60 mm wide, and the space between them is X (m), then
(0.060 + X ) × n = 1.00 m (i)
where n is the number of boards per metre. As n × X = 0.236 m, then
n = 0.236 / X (ii)
Thus, substituting n from (ii) into (i):
(0.060 + X ) × 0.236 / X = 1.00 m,
or
X = [(0.060 + X ) × 0.236]
= (0.060 × 0.236) + 0.236X
= 0.01416 / 0.764 = 0.0185-m or 18.5-mm gaps.
If the boards are 60 mm wide, the gaps should be 18 mm wide.
Gaps should not be wider than 25 mm to prevent small tubers being pushed
into the gaps. They should preferably be greater than 15 mm to prevent them
being plugged up with soil.
STEP 5: SIZE OF OPENINGS IN MAIN DUCT TO GIVE REASONABLY UNIFORM AIRFLOW As the
main duct is a constant cross-section, the only way to obtain a relatively uniform
airflow into the laterals is to raise the static pressure in the duct by constricting the
outlets to the laterals. To minimize the variation to 1:1.35 (Table 6.4), the total
area of the lateral outlets should be equal to the minimum cross-sectional area Amin
of the main duct. This was calculated to be 3.333 m2 to keep the air velocity no
higher than 6 m/s. (In practice the main duct was made larger than this to allow
access, but for this calculation Amin is the figure to use.)
Number of laterals = 9 per side, 18 in total.
Size of outlet to lateral = Amin / number of laterals
= 3.333 / 18
= 0.185 m2.
If the laterals are 0.4 m wide and 0.46 m deep at the start (i.e. 0.184 m2), the whole
area is required to get sufficient outlet area.
Fig. 6.19. Pedestal fans for stores where no ventilation is installed. (Courtesy of
Martin Lishman, Lincolnshire, UK.)
Where bulk stores have no ventilation system installed, ‘pedestal’ ducts fitted with
fans, as shown in Fig 6.19, can be installed. The system consists of a vertical galva-
nized steel duct with a suction fan mounted on top. The duct is placed on the floor
of the store and potatoes are built up around it so that they support the duct in a
vertical position. There is no need for ancillary ducting. When the fan is running
it sucks air into the bottom of the duct and discharges it into the airspace above.
This draws air into the pile from the surface, ventilating the potatoes as it does so.
These tend to be used in small stores, where the installation of a floor ventilation
system would be uneconomic or where a building is rented. Their use is mainly to
dry the crop and prevent subsurface condensation. By manually opening and shut-
ting doors, cooling can be achieved but without the benefit of automation.
The supplier of pedestal ducts with fans in the UK (Martin Lishman Ltd,
Bourne, UK) recommends their use at 5 m spacing for a 2.5-m-high pile. At this
spacing the 1.1-kW pedestal unit will give 0.017 m3/s/t and the 2.2-kW pedestal
unit gives 0.025 m3/s/t, just below and just above the standard UK ventilation rate
of 0.02 m3/s/t. Each unit will ventilate 42 t.
The use of a porous evaporative humidifier located in the air supply duct may well
be impractical for the humidification of intermittent medium- and high-rate airflow
ventilation systems. To keep the approach air speed below 3 m/s requires either a
very large duct or for the porous evaporative humidifier to be formed into a large
box surrounding the fan. The alternative is to use a spinning disc humidifier or a
pressure jet, sonic humidifier (Fig. 6.21). These need very good filtration in the
water supply line to keep orifices clear of particles in the water, and pose the risk
of droplet carry over, which can wet the crop. Droplet arrestors are sometimes
incorporated to try to reduce this problem.
Air passing through a porous evaporative humidifier will pick up only the
amount of moisture it requires to reach approximately 97% RH (Munters, 2007),
so this system is self-controlling. In contrast, a spinning disc or sonic humidifier
Fig. 6.20. Water-soaked porous cellulose matrix on both sides of fan intake.
(Courtesy of W. Leslie, Farm Electronics, Lincoln, UK.)
190 Chapter 6
crops rapidly when cool air is available. If weight loss in the crop is found to be
excessive, it may be better to look at ways of reducing fan run time and redesigning
the ventilation system so that lower backpressure fans can be used (Ch6.4).
In UK box stores, overhead high-speed jet distribution systems are the most com-
mon form of airspace ventilation system. This is because:
● The space between the top of the boxes and the roof is used to distribute air.
No storage space is lost to provide ventilation ducts.
● The air-to-air friction between the high-speed jets and the air mass in store
causes a ‘roll’ of air to develop (Fig. 6.22).
● The incoming air is always cooler than the crop, so as its speed reduces it falls,
further encouraging the ‘roll’ of air necessary to induce air to flow back
through the ducts formed by the pallet apertures to the blending chamber.
In principle, overhead ventilation with cold air is wrong, as the top surface
potatoes are the first to be cooled by the cooling airstream. Once cooling ventila-
tion stops, natural convection takes over and warm air from potatoes below will
rise and may condense on these cooler surface potatoes. The risk of this happening
High-speed jet
Friction between
jet and stationary air
creates roll
Roll of air Duct
Fan
Roll of air
Fig. 6.22. High-speed jet causes a roll of air within the store.
192 Chapter 6
is indeed ever present. However, in practice, if the cooling air temperature can be
controlled so that the surface potatoes are never significantly cooler than those
below, then such condensation can be prevented.
When a jet of air flows through the stationary air mass (Fig. 6.22), turbulence and
friction occur at the interface. The force caused by this friction results in the sta-
tionary air mass starting to move. A jet of air will therefore induce an airflow
movement many times greater than the airflow volume of the jet. Air jets therefore
serve to induce a beneficial air movement within stores, which becomes a roll. The
location of exit louvres has little effect on the formation of the roll, so can be in
any part of the building cladding.
The units providing the high-speed jets are of three types:
● Ambient-air cooling with air blending.
● Floor-mounted refrigeration units.
● Combined ambient-air cooling with air blending and refrigeration (Fig. 6.23).
All are fitted with vertical ducts to discharge cool air at high level.
Fig. 6.23. Ambient-air/fridge cooling unit. (Welvent Unit, Warden Farming Co. Ltd,
Gainsborough, Lincolnshire, UK.)
Store Ventilation 193
Air jets are often used in airspace-ventilated box potato stores to induce a rolling
movement of air within the store. Should the jet hit an obstruction early in its trajec-
tory, the jet can be severely deflected (Fig. B.6.2). Likely obstructions are portal
frames or purlins. Such potential collisions should be avoided during the design
stage. If the problem is found after the store is built, a deflector placed under the
obstruction can be fitted retrospectively to minimize the effect.
Obstruction
Duct
Jet
deflected
Cooled surface
tubers
Airflow
Warm Top of
stack of
boxes
Uniform air distribution throughout the stack of boxes (Box 6.4) depends on:
● Jets flowing from the overhead ducts having sufficient momentum to reach the
end of the stack of boxes.
● Jets from the overhead ducts being angled so that they distribute the cool air
over the whole width of the store.
● Boxes being of the same height so that their pallet apertures form a continuous
duct from the front of the stack to the back. This ensures that a box of a dif-
ferent height does not obstruct the airflow returning from the far end of the
store through the ducts formed by the box pallet apertures.
With high-speed jet air distribution systems, airflow (Fig. 6.24) can be low in the
rear corners of the stack (Burfoot, 1997; Xu et al., 2002). This confirms observa-
tions by store managers that these areas often experience slow drying, condensa-
tion, disease development and poor temperature control. Airflow in these sectors
can be improved by jetting a small portion of the air to the corners of the store
(Fig. 6.23). By suspending additional large-diameter, low-backpressure axial fans
above the crop (Box 6.5), both the throw of the jets and the creation of the benefi-
cial ‘roll’ of air can be increased.
194 Chapter 6
Return
air Air drift
Fans
Boxes Fridge
inlet
(a)
Low airflow
Air drift
Fans
Boxes
Fridge
Air drift inlet
Low airflow
(b)
Fig. 6.24. Drift in the return air within the stack of boxes in a fridge-ventilated store:
(a) side elevation of store; (b) plan of store.
If a horizontal fabric or insulated steel duct is used to extend the length of the jet,
the beneficial air-to-air friction is lost. While the air delivered to the far end of the
store will flow back to the other end of the store, the amount of store air set in
motion will be very much less. Additionally, the air within the duct will be colder
than the warmer air above the potatoes, so condensation may form on the duct
and drip on to the crop below. Insulation on the duct may reduce, but not com-
pletely eliminate, the potential for condensation forming on the duct. The better
solution is therefore to install more axial fans to increase the ‘roll’.
The best solution to the problem of low airflow in the rear corners of the stack of
boxes mentioned above is to fit a ‘goalpost’ plenum for the boxes to be stacked
against (Figs 6.25 and 6.26). It is termed a ‘goalpost’ plenum as it looks like a foot-
ball goal post when the store is empty of boxes. This plenum reduces the tendency
of the air returning through the stack of boxes to converge (Fig. 6.24) as it
Store Ventilation 195
Airflow
Fig. 6.25. Goalpost duct built over fridge to minimize the problem in Fig. 6.24 of air
drift through stack of boxes.
Fig. 6.26. View of fridge and front face of duct from inside goalpost duct.
(Wroxham Home Farm, Norfolk, UK.)
approaches the intake of the fridge or blending box, and encourages the air to flow
along all the ducts formed by the box pallet apertures. Recent observations ( Joe
Macgrath, Norfolk, UK, 2005, personal communication) have shown more uniform
temperatures and consistent fry colours with this system. The duct does, however,
take up space that could have been used to store more boxes.
196 Chapter 6
The use of ‘goalpost’ or open-fronted plenums helps fans to direct air along the
ducts formed by the box pallet apertures. With typical UK 1-t boxes, 915 mm high
overall, the air will be passing within 0.4 m of the potatoes at the centre of the box.
The combination of the turbulence created by this airflow together with natural
convection will create as good ventilation of potatoes as any airspace ventilation
system can do. With the recent safety recommendations (BPC, 2005a) suggesting
that permanent access systems should be provided to allow sampling of the top
potatoes, such plenums can provide the dual function of providing a crop access
platform combined with improved air distribution.
Plenums can be under pressure or suction. If the store already has an ambient-
air cooling/refrigeration system, it can be housed within the plenum, which will be
under suction due to the fans in the fridge drawing air in through its cooling coils
(Fig. 6.25). Additional fans can be added to the top of the goalpost duct if the
airflow of the fridge/ambient-air system is found to be insufficient for rapid drying
and good temperature control. If no existing cooling system is in place, it can be
pressurized, with the incoming air coming from an air blending system.
The practice of storing potatoes in sacks is common in countries where the cost of
labour for handling the sacks is low. Many sack stores like this are to be found in
India. The stores may be up to five floors high, and have fridge cooling coils located
at the top of the building (Fig. 5.11, Fig. 6.29). The floors are made from hollow sec-
tion steel, to take the weight of the sacks and to be porous to the cooling air. Access
Store Ventilation 197
In a box potato store without any fans operating, natural convection from the stored
potatoes causes the air in the store to stratify, with warm air in the top and cooler
air below (Fig. B.6.3a). The boxes higher up the stack will therefore be warmer than
those below. Where a recirculation system is not part of the ventilation system,
small fans fitted with plastic hanging socks are often fitted to continually mix the air
within the store and so reduce temperature differences (Fig. B.6.3b).
4.5°C
Fan Polythene tube
4.0°C
Uniform 3.75°C
3.5°C
Boxes Boxes
3.0°C
(a) (b)
Fig. B.6.3. (a) Air stratification in store; (b) remedy, continuously running sock fan
reduces stratification.
Fig. 6.27. Extractor fan-ventilated store with sock fans to mix store air. (Deveron
Potato Growers Ltd, Banff, Aberdeenshire, UK.)
198 Chapter 6
When air enters a building, its trajectory is determined by two factors. One is the
velocity, or momentum, with which it enters the store; the other is the density of the
incoming air relative to the air within the store (Fig. B.6.4). The resulting direction
of airflow is the resultant of these two vectors. If the incoming air is very much
colder than the air within the store, it will tend to drop vertically as it enters. If it is
the same temperature as the air within the store it will travel horizontally across the
store. Between these extremes it will fall at an angle.
In the example shown, when ventilation starts initially, the jet will fall vertically as
the air is very much colder than the air in the store. However, as the store air is cooled
to the incoming air temperature, the inlet air direction will revert to a horizontal jet.
Since inlets, not outlets, determine airflow movement within stores (Randall, 1975),
the induced roll in Fig. B.6.4 will start anticlockwise, but will then revert to a clockwise
circulation when the air in the store has cooled to that of the incoming air tempera-
ture. It is not a good design feature for the pattern of ventilation within the store to
change as the difference between inlet air temperature and store air temperature
alters. It is better to ensure that air flows in the same direction at all times. This is
achieved either by ensuring that the inlet air speed in the direction that the air has to
go exceeds 5 m/s (Randall and Battams, 1979) or that the air is introduced at floor
level so that it rises due to natural convection due to the heat from the potatoes.
Vector diagram
Jet momentum vector
Inlet air at 7°C
to the individual floors is by ladder. Porters carry the sacks up the ladders, or use a
‘dumb waiter’-type lift and stack them in groups on the porous floors (Fig. 6.30).
While the authors have limited personal experience of these stores, warm
potatoes loaded into the cold store are likely to experience subsurface condensation
in the top bags. This may be minimized by rapidly batch cooling the crop to the
temperature of the store using positive ventilation prior to putting them into their
final storage location. Good air movement through the bags themselves, rather
than just between the stacks of bags, will also be beneficial.
Store Ventilation 199
Ambient air
Blended
air
Fig. 6.30. Sacks on porous floor in multi-storey refrigerated sack store. (Courtesy
of W. Leslie, Farm Electronics, Lincoln, UK.)
Exhaust
Roof louvre
Recirculation
Inlet
louvre
louvre
Inlet
louvre Blending Fan
chamber
Fan Foam
Letterbox duct bungs
Walkway
Air leaving
potatoes
Walk-in
duct Boxes being Boxes stacked
loaded against duct
Letterbox-type slots
with flaps to shut off flow
Fig. 6.31. Traditional letterbox duct with blending chamber above duct.
Box side
Gaps for
exhaust
air Box end
Airflow from
letterbox duct
If the duct is to ventilate a six-high stack of boxes, there will be six outlets (Fig. 6.33),
one near ground level and the rest located one above another, one box height apart.
Boxes for upward flow letterbox systems will cost more than those designed for
upward and downward flow, due to the extra timber needed for the solid pallet base.
When positive ventilation stops however, the warm zone will be above the cool zone,
which avoids any risk of condensation once natural ventilation starts. This system is
very suitable where high airflows are desired, as the air flowing in each duct only
ventilates one layer of boxes, so the air speed along the pallet aperture ducts should
not exceed 6 m/s. These systems are common for onion and bulb drying.
in the UK, leakage through gaps between the boxes is such that airflow declines
with distance from the letterbox duct. A row five boxes out from the duct will dry
in 4 to 7 days, while one of seven boxes may need to be turned if the crop is very
wet to ensure that the boxes at the end of the rows dry rapidly. Rows of ten boxes
should only be considered if the fan’s airflow can be directed at individual rows by
closing off other rows that are already dry, or where boxes are perfectly square and
precision made, so that they fit closely together.
Sidewall letterbox
Another form of letterbox duct is the sidewall unit, which simply recirculates air
within the store through the boxes (Fig. 2.17). This can be used to warm or dry
potatoes as a batch process.
Fig. 6.34. Open wall letterbox with socks on fans to aid uniformity of airflow through
boxes. (Ordens Farms Ltd, Aberdeenshire, UK.)
204 Chapter 6
Open wall letterboxes can have the fan either blowing or sucking; in the boxes
used in the UK, the leakage of air is so great (Box 6.8) that the difference is too
small to measure. In suction mode the diffuser sock shown in Fig. 6.34 is not fitted
(Fig. 6.35).
Fig. 6.35. Suction letterbox for open slatted boxes with plastic sheeting to seal
sides. (Courtesy of D. Dickson, Doune Park, Banff, Aberdeenshire, UK.)
Store Ventilation 205
Fig. 6.36. Pressurized plenum with fabric socks fitted to fans to aid uniformity of air
through boxes. (Wood Farm, Norwich, Norfolk, UK.)
Fig. 6.37. Side section of letterbox ducts with arrow size showing effect of socks on
uniformity of airflow from outlets: (a) plain letterbox; (b) letterbox with parallel-sided
sock; (c) letterbox with tapered sock.
206 Chapter 6
Suction walls (Fig. 2.19) have been a traditional form of positive ventilation for
cooling vegetables in porous crates or boxes for at least a century (Thompson et al.,
1992). Their use in the UK for ventilating boxes, under the trade name ‘Aspire’ or
‘Boxer’, is a more recent development. The basic system has two rows of boxes
with a plenum between them, with a canvas sheet stiffened by battens pulled over
the top and down the front face of the plenum. Either the rear of the plenum con-
nects with an aperture in a duct, which is under suction from an extractor fan
drawing air from the duct, or a portable extractor fan is placed at the front of the
stack. Air movement is horizontal (Fig. 2.19, Fig. 6.38).
In selecting this system for positive ventilation a number of factors have to be
considered:
● Boxes to be used in a suction wall system should be designed to suit the system,
with gaps in the ends of the boxes positioned to ensure that the air travels
through the potatoes in the box, not over or under them (Fig. 6.39).
● Extending the system to positively ventilate two rows of boxes each side of the
plenum instead of one row each side is unlikely to be successful. Boxes are
often not exactly the same height, so lining up gaps in open slatted boxes that
are 25 mm wide, with those of a neighbouring box, will rarely be successful.
The ducts, formed by 100-mm-high box pallet apertures, provide greater lati-
tude to any misalignment and potential obstruction to airflow.
Store Ventilation 207
● Since boxes are wider than they are deep, the ventilating air path is more than
twice the length with suction wall compared to letterbox ventilation.
● As with a letterbox system, a cooling front will develop, but this will be vertical
rather than horizontal. As with dual-direction letterboxes care is needed to
keep the temperature step in the cooling front to a minimum.
● Recent safety advice (BPC, 2005a) warns of the dangers of walking on boxes
within one box distance of the edge of a stack. This system either precludes
walking on the boxes or requires a harness safety system to be used when
inspecting or sampling potatoes.
● The canvas spanning the plenum between the two rows of boxes can appear
safe to walk on in the low light of a potato store, but hides a 5.6-m vertical
drop on to the concrete below; warnings signs on the canvas are essential but
may not be sufficient to prevent accidents happening. Its use should therefore
be avoided unless acceptable safety provisions are installed.
Most growers using this system rely on there being gaps between boards in the end
of boxes due to the timber shrinking following box construction. This is true of
many but not all boxes. The largest gaps in traditional UK ‘Angus’ boxes (Fig.
6.40) are at the top and base of the ends, so the majority of airflow will be over the
surface of potatoes, not through them.
The width of the plenum is usually sized to fit the diameter of the fan or to
allow personnel access. The width can however be reduced, so long as the air
speed entering the suction duct does not exceed 6 m/s. Since the air entering the
rows of boxes either side of the plenum can enter via the front, top or back of the
row, the gap between adjoining rows can be quite narrow, while still keeping the
maximum air speed at or below 6 m/s.
Fig. 6.39. Box suitable for suction wall ventilation. (G. Mackie, Darnabo,
Aberdeenshire, UK.)
208 Chapter 6
Posts 25 mm
1830 mm
above sides
Board flush
with posts 915 mm
Gangnail
plates Leakage
Long
Air from
slot
letterbox
duct
1220 mm
Leakage through
floor slats
Large board
(a) (b) to seal leaks
Fig. 6.40. (a) Traditional ‘Angus’ box; (b) closed-ended type box best for letterbox ventilation.
Portable versions of positive ventilation systems are discussed more fully in Ch2.8.
They can, however, form part of a permanent ventilation system. Wedderspoon
tents (Fig. 6.41) are used for initial drying of boxes in some cross-flow ventilated
stores, while suction wall and Posi-vent® systems are usually linked in with the ven-
tilation and refrigeration systems (Figs 2.18 and 2.19).
Single-stage positive ventilation is almost always used in bulk storage systems and
commonly used in letterbox ventilation systems for box stores (Fig. 6.42a). One fan
draws air in from outside, and forces air through ductwork and lateral ducts and
subsequently through the potatoes themselves. However, a two-stage ventilation
system can be used in preference in some circumstances.
A two-stage positive ventilation system uses two fans in series, one to bring the air
into the store and one to blow the air through the potatoes (Fig. 6.42b). This allows
two high-volume fans with a low backpressure capability to be used rather than a
single fan with a higher backpressure capability. Not only can these fans be cheaper
to purchase, but their running costs and the amount they heat up, and therefore
dry, the air can be less. In a comparison carried out (Pringle, 1989) between a tra-
ditional letterbox system using a fan with the relatively high backpressure capability
of 365 Pa compared with a two-stage system using fans each with a backpressure
capability of 120 Pa, the capital cost of the two fans was 80% that of the single fan
while their running costs were 70%.
Store Ventilation 209
Fig. 6.41. Tent drying system with operator protected by a travelling safety harness.
(Deveron Potato Growers Ltd, Banff, Aberdeenshire, UK.)
Bungs Bungs
(a) (b)
Fig. 6.42. (a) Single- and (b) two-stage positive ventilation systems.
For letterbox and suction wall systems, boxes should be designed to suit the system
installed. A guide to the structural requirements of UK 1-t potato boxes is to be
found in BS 7611:1992 (BSI, 1992). Boxes to suit letterbox ventilation systems
should be close-boarded on their ends, from the top of the corner posts to the bot-
tom of the bearers (Fig. 6.40).
For suction wall systems, the ends of the boxes should be open-slatted.
However, the lower 200 mm of the ends should be close-boarded, to prevent air
flowing sideways through the pallet apertures (Fig. 6.39).
Only with the Wedderspoon tent does box design not matter.
If the decision is to install a letterbox or suction wall system, yet only conven-
tional ‘Angus’ type boxes are available, these can be modified (Fig. 6.43) or phased
out over time and boxes bought thereafter can be specified to conform to the new
design.
caused by the pressure of potatoes plus the difficulty of stacking boxes precisely at
a height of 5.6 m and at speed means that there will always be gaps between boxes.
If growers only have Angus boxes, the installed airflow should be 0.08 m3/s/t so
that once the ventilation-system-specific box is phased in, a good drying airflow
approaching 0.04 m3/s/t will be available.
will get wet from condensation. The wet potatoes will then be vulnerable to dis-
ease. If the crop is in bags the condensation and disease may not be seen until the
bags are opened. Warming prior to removal from store is good practice (Ch6.10).
Regular inspection of the crop, usually on those tubers 300 mm below the crop
surface, will reveal when eyes are starting to open. Treatment should then take
place. Between two and five treatments may be required per season, the actual
number depending on length of storage, the temperature that the potatoes are
stored at and the dormancy characteristics of the variety stored.
In Europe the CIPC is mixed with a carrier or solvent. With some CIPC/solvent
formulations, fans within the store cannot be run for fear of a spark causing an
explosion, so dispersion of fog relies solely on natural convection. With other for-
mulations, fans can assist the distribution of the fog. The formulation label will
indicate what is allowed. The fog will not only be deposited on potatoes, it will also
be deposited on to the fabric of the store, the boxes, fans and refrigeration equip-
ment. Prior to fogging, measures should be taken to minimize the contamination
of fans and fridge components by protecting equipment with easily removable ply-
wood or steel panels.
CIPC residues on the tubers, especially in box stores, can be very uneven. Temperature
gradients in a stack of boxes alter the deposition pattern (Briddon et al., 1999; Xu and
Burfoot, 2000) with colder potatoes on top and warmer ones at the base resulting in
the highest proportion of CIPC deposited on the top box. Crop temperatures should
therefore be as uniform as possible. If fans are allowed to be used, these help to dis-
tribute the chemical, but they should be run at low speed if this is possible. Best uni-
formity of application is achieved if a dry fog is produced, which requires the ambient
RH during application to be low (SAM, 2005). Blowing high-humidity warm ambient
air, along with the CIPC, into a store that is colder is likely to result in the moisture-
Store Ventilation 213
laden air condensing on the potatoes during the treatment process. The moisture
from the products of combustion from burning of petrol in the fogger makes such
condensation even more likely.
Prior to applying the chemical, the air within the store should be recirculated for
an hour or two to ensure that all parts of the crop are as near to the same tempera-
ture as possible. The dew-point temperature of the ambient air should be checked
to ensure that it is below the temperature of the stored crop. If fans cannot be run
to assist CIPC distribution, they should be enclosed with plywood or sheeting to
prevent their contamination with CIPC crystals. The cooling coils of a fridge
should also be blocked off. Treatment can now start.
With the doors of the store closed, the application equipment, often mounted on
a vehicle for mobility, is positioned close to the store and a stainless steel pipe
connected from the fogger through a port installed in the outer wall of the store
(Fig. 6.44). The fan and burner unit of the fogger is started and CIPC injection
Fig. 6.44. Thermal chloropropham fogger with stainless steel flexible pipe inserted
into store.
214 Chapter 6
begins. With bulk stores using a no-fan formulation, the hot chemical is fed into
either the main duct or the mixing chamber. As the chemical exits the generator,
it cools to the temperature of the store and forms microscopic crystals that make
up the fog. The fog rises through the crop by natural convection, being deposited
on the crop as it goes. If fans can be run, use them to aid uniform distribution. If
fans are variable speed, they should be run at low speed. If there is more than one
fan, the lower air speed can be obtained by running only one fan.
With box stores, the most basic system allows the chemical to be fed into the
side of the store, so that the hot vapour rises into the headspace then falls down
through the stack of boxes. This tends to result in the highest levels of CIPC on
the top boxes and the least on the potatoes in the boxes at the base of the stack.
This effect is greatest if there is no air circulation.
Alternatively, the fog can be introduced into a stack (Fig. 6.45) so that the chemi-
cal rises up thorough the stack. Application in box stores results in 30% of the applied
chemical reaching the target (Duncan, 2006) and 70% being lost to the atmosphere
and deposited on boxes, store surfaces and any exposed fans and equipment.
Most chemicals require the store to stay closed for 24 h, although some require only
12 h. Potatoes having been treated cannot be processed for between 21 and 28 days,
depending on the chemical label. As the ethylene produced from burning petrol dur-
ing the fogging process tends to trigger increased respiration and affect fry colour,
growers would like to open their stores within 8 h of treatment.
Investigations to date to find out whether CIPC contaminates wash water sug-
gest that the chemical is more likely to stick to the soil on the potatoes than to pass
into the wash water (Duncan, 2006).
Canvas sheet
Fig. 6.45. Box stacking system using canvas sheet to minimize variation in
application of chloropropham (CIPC).
Store Ventilation 215
The best place to warm potatoes prior to grading is within the store itself. In store,
as the RH is between 90 and 98%, the dew-point temperature will always be slightly
below the temperature of the crop. Warming can therefore be carried out without
the warmed air causing condensation to form on the crop (Ch3.5). If the air used
to warm the potatoes has been warmed using an electric heater or an indirect gas
or oil heater, the temperature of the air will increase but its dew point will not.
With an airflow of 0.08 m3/s/t and a leakage of warmed air from boxes of approxi-
mately 50%, warming of potatoes from 5 to 10°C takes about 11 h (Pringle, 1993b).
A proportion of the heat will enter the potatoes while some will dissipate within the
store. This will have to be removed by ventilation or refrigeration, if the store is to
be kept at a fixed temperature. If the batch of potatoes is small compared with the
mass of potatoes stored, the increase in store temperature will be small. A fixed or
portable letterbox, or a tent with recirculation, can be used to warm batches of
potatoes in this fashion.
Potatoes in small 120-t bulk bins are easily warmed using a space heater blowing
warm air into the recirculating airstream. Warming the day’s out-take in large bulk
stores is less precise. Closing all but the last few laterals in the bulk store and recir-
culating warmed air through an amount of potatoes equal to a day’s grading can
achieve a satisfactory result.
Where it is possible to have a warming chamber between a box store and the grad-
ing area, this provides the best option. One door of the chamber opens to the store
while the other opens to the grading area. With the grading area access door
closed, boxes are stacked in the chamber for warming. The store access door is
closed, and the potatoes are warmed without introducing any outside air. The
grading side door is then opened (Fig. 9.15) and the boxes removed for grading.
This set-up ensures the dew-point temperature of the air for warming is below the
temperature of the potatoes, while keeping the warm air in the warming chamber,
so it does not leak heat into the main store.
216 Chapter 6
Potatoes removed without warming from cold store are very likely to get wet from
condensation (Fig. B.9.3). If the potatoes are then put into a room or grading area
and left to warm up naturally, the moisture on the potatoes will slow the warming
process. The room should be kept at the desired final temperature of the crop dur-
ing this period; usually 10°C. Warming will normally take 3–4 days.
6.11 Summary
Ventilation is the principal operation carried out on the stored crop. The subject
of this chapter is how to ensure the right amount of air flows uniformly through
the stored potatoes. The principal conclusions are as follows.
● Ventilation systems usually consist of an inlet, blending chamber, fan, distribu-
tion system and exit louvres.
● For buildings operating in areas where snow can be severe, inlets are often
placed horizontally under the building eaves.
● Air outlets are usually placed as high as possible, to allow warm exhaust air to
exit.
● In exposed sites inlets and outlets should be on the same side of the building
(i.e. within the same pressure zone) to prevent uncontrolled wind-induced
ventilation.
● Bulk stores may be ventilated using a main central duct discharging air into porous
laterals on either side or by single tapered ducts each with their own fan.
● System-designed porous concrete or timber floors provide both uniform air-
flows to bulk crops and allow unimpeded store emptying.
● Potatoes, unlike grain, offer minimal resistance to airflow so the majority of the
resistance the fan ‘sees’ arises from high-speed air passing through inlets,
blending chamber, ducts and outlets.
● To allow the use of low-backpressure fans, air speeds in ducts should be
restricted to 6 m/s.
● Uniformity of airflow through the crop is improved by keeping air speeds in
ducts low, avoiding the Venturi effect, using tapered ducts, restricting lateral
outlet area and restricting duct or floor outlet area.
● While restricting duct or floor outlets improves uniformity of air through the
crop, the increased backpressure reduces the total airflow supplied by the fan.
● Fan air capacity is proportional to tonnage stored. Its backpressure capability
is calculated from the sum of the resistances to airflow as the air passes through
the building apertures, ductwork and crop.
● Axial flow fans whine like aircraft engines; if residences are near their speed
should be kept low, intakes can be muffled and earth bunds erected.
● Media-type air humidification systems do not have to be controlled and
minimize the likelihood of droplet carry over.
● Spinning disc or sonic jet humidifiers need sophisticated water filtration and
control as well as droplet arresters to prevent droplet carry over.
Store Ventilation 217
In a refrigerated store, air being circulated through the potatoes is routed through
a heat exchanger cooled by refrigeration. When refrigerated cooling is taking place,
louvres and doors in the store are sealed. This contrasts with ambient-air cooling
systems, where heat from the potatoes leaves the store with the ventilating air.
The refrigeration system used for this is floor-mounted, has a vertical duct to trans-
fer air to the top of the store and a horizontal outlet to discharge air horizontally
across the boxes. A photograph of such a unit is shown in Fig. 6.23. Box 7.1 intro-
duces some terms used in refrigeration.
A direct expansion (DX) refrigeration system consists of two heat exchangers,
an evaporator (cooling coils) inside the building and a condenser on the outside
(Fig. 7.1). These are connected together by pipework to make a circuit. The
218 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Store Refrigeration 219
Refrigeration systems use a heat exchanger in the form of a cooling coil or evapo-
rator to remove heat from the store. A second heat exchanger, called a condenser,
is placed outside the store to dissipate the heat removed to atmosphere. The fluid
within the pipework of the refrigerator is called a refrigerant, which has the prop-
erty of being able to change phase easily from a liquid to a gas and back again.
Store Outside
Circulating
fan
Evaporator Condensor
Pipework
Thermostat
Outside Warmed
Store Compressor air outside
return air air
Expansion
valve
refrigerant is forced clockwise round the circuit by a compressor. The liquid refrig-
erant (Box 7.2) is forced at high pressure through an orifice (expansion valve) into
the evaporator, which is at low pressure. At this low pressure the liquid refrigerant
starts to change phase into a gas, provided it can obtain the latent heat it needs to
become a gas. This heat is obtained from the store return air passing through the
evaporator. The liquid refrigerant evaporating within the evaporator cooling coils
therefore cools the store return air as it passes between the coils of the
evaporator.
Once the refrigerant reaches the outlet from the evaporator it has totally
changed from a liquid into a gas. The compressor draws in the gas from this low-
pressure side of the circuit and compresses it prior to forcing it into the condenser
located outside the store. At this high pressure the gas starts to change phase into
a liquid, but has to get rid of latent heat to do so. To remove this heat, fans force
air from outside the store through the condenser. The refrigerant gas therefore
changes phase from a gas to a liquid (i.e. condenses), within the condenser, giving
out heat as it does so. This heat warms the outside air passing through the con-
denser. The refrigerant in liquid phase is then forced through the expansion valve
once again to repeat the cycle.
220 Chapter 7
When a fluid changes phase from a liquid to a gas, heat is required. The heat
absorbed in the phase change is hidden, termed latent, as it does not change the
temperature of the fluid, only its state.
The refrigerants used in refrigeration systems are selected for the efficiency
with which they change from liquid to gas at the design temperature (3.5°C for
potatoes). In the heat exchanger (cooling coils) of a DX refrigeration system, the
refrigerant evaporates from a liquid to a gas, taking in ‘latent heat of vaporization’
from the air passing through the heat exchanger as it does so. This is why the heat
exchanger is called an evaporator. In the heat exchanger outside the building, the
refrigerant condenses from a gas to a liquid, giving out its ‘latent heat of vaporiza-
tion’ as it does so. This is why it is called a condenser.
This process effectively pumps heat from inside the store via the refrigerant to
outside, so removing heat from the potato crop and store.
As the cooling coils of the evaporator are cooler than the store return air, heat is
transferred from the warm air to the cooling coils (Fig. 7.2). The temperature dif-
ferential (TD) between the return air and the evaporator coils is usually 6°C. The
return air does not cool by this amount, instead cooling by 2.5°C when the crop is
newly harvested and warm and by 1.5°C once the crop is cooled to 3–4°C. The
temperature difference between the air entering and exiting the evaporator is
called the air-on/air-off temperature difference. This should not be confused with
the TD discussed above.
The design TD determines the RH of the air circulating through the potatoes
(Table 7.1). The greater the TD, the more moisture will condense out of the air on
to the cooling coils. This will reduce the amount of moisture in the air, so its RH
will be lower. To minimize crop weight loss, the TD should therefore be low. This
Store Refrigeration 221
Cross-section of evaporator
Coil Cooling coils
temperature
(e.g. 4°C)
Air-on Air-off
(e.g. 10°C) (e.g. 7.5°C)
4–6 95–91
6–7 90–86
7–8 85–81 Increasing Increasing Decreasing
8–9 80–76
9–10 75–70
The evaporator consists of a series of pipes, or coils, which loop back and forth
across the width of the unit. As pipework on its own has a relatively small heat
exchange surface area, the evaporator coils are fitted with vertical aluminium
plates, or fins, 4–6 mm apart, to increase the heat exchange area. Being vertical,
any moisture in the air that condenses out due to the air being cooled by the evap-
orator will run down the fins to the drain in the floor. If however the fins are too
close together, surface tension can slow the rate that water runs off and the water
can turn to ice. This can block up the fins and prevent the fridge from working
(Fig. 3.23). For high-humidity storage atmospheres, fin spacing at minimum should
be 4 mm with 6 mm being preferable. These fins are delicate and can be damaged
by impact or overzealous pressure washing. A uniform airflow across the evapora-
tor is also essential if icing is to be avoided.
222 Chapter 7
The closer the store set-point temperature is to freezing, the more likely the evapo-
rator will be to ice up. For conventional DX systems, a minimum set-point
temperature should be 3.5°C. Most refrigeration systems use off-cycle defrost to
melt any ice that has formed. This simply shuts off the compressor for a quarter of
its running time, to let the evaporator warm up to allow any ice to melt. In sizing
a refrigeration unit, this requirement should be taken into account.
Alternatives to off-cycle defrost include electric heating elements beside the
evaporator or valves in the refrigeration system to allow the compressor to be oper-
ated in reverse, as a heat pump. This latter approach is called hot gas defrost. Both
electric heaters and hot gas defrost systems melt the ice more rapidly than off-cycle,
but cost more and add heat to the store. They should be installed if stores are to
be operated below 2.5–3.0°C.
7.1.7 Refrigerants
Evaporator Condenser
Motorized Condenser
air recirculation fan
louvre fully open
Main cooling
Evaporator fan Condenser
Motorized Condenser
air recirculation fan
louvre 80% open
Incoming
Compressor ambient air
(b)
Fig. 7.3. Floor-mounted fridge: (a) with louvres set for refrigeration mode;
(b) with louvres set for ambient-air cooling. (Courtesy of W. Leslie, Farm Electronics,
Lincoln, UK.)
the desired duct air temperature. Figure 7.4 is a photograph showing this system,
mounted on a skid unit, being brought into the store during installation. The fans
in view are those used to cool the condenser.
DX refrigeration units are often selected for being relatively low in cost and modular
in construction. Floor-mounted units mounted on a skid unit are favoured in preference
Store Refrigeration 225
Fig. 7.4. Floor-mounted fridge unit being installed. (Courtesy of W. Leslie, Farm
Electronics, Lincoln, UK.)
Table 7.2. Comparison of direct expansion (DX), two-stage water/glycol and chilled-
water systems.
Air-on/air-off
Temperature temperature Store relative
System differential (°C) difference (°C) humidity (%) Suits
to roof-mounted units in box stores as they can be placed in position by forklift and
draw air through the boxes of potatoes. Being at ground level they can be serviced
even when the crop is in store. They can incorporate ambient-air cooling as described
above. To install, they need only a hole left in the store wall for the condenser to
project outside. Once the fridge is in place the hole is sealed. A removable wall fills the
gap. Alternatively, the unit comes in two sections, with the evaporator and refrigera-
tion unit inside the building connected by two pipes to the condenser mounted outside.
DX units used to store potatoes over a period of 6–8 months can produce a potato
firm enough to satisfy the demanding requirements of supermarkets (Table 7.3).
Two-stage water/glycol systems have the benefit that the TD across the cooling
coils can be reduced proportionally from 6°C to 0°C. The lower the TD the higher
226 Chapter 7
the store RH will be, since less moisture in the air will condense on the cooling
coils. The rate of cooling will however reduce as TD reduces, so in intermittently
ventilated systems the cooling will be stopped at a preset value. In continuously
ventilated systems, there will almost always be some cooling taking place to remove
respiration heat.
The system suits multi-store units as the one refrigeration system can pro-
vide cool water for a series of stores. This can provide a lower capital cost sys-
tem than having DX systems in each store, if all the units are working
simultaneously. In stores that are emptied sequentially, individual DX units may
provide the lower operating costs. In the UK most growers have a separate DX
refrigeration system per store rather than a water/glycol multi-heat-exchanger
system.
Chilled-water coolers are the preferred system for the long-term storage of veg-
etables but can also be used for potato storage. In one unit installed at the
Potato Marketing Board’s packhouse at Sutton Bridge, Lincolnshire, UK, a
chilled-water unit using a Baudelot evaporator was installed in a 2700-t box
store and compared with a conventional DX unit located in an identical, adjoin-
ing store. The chilled-water cooler had an airflow of 0.01 m3/s/t and the fans
ran continuously, the air being distributed through the stack of boxes via their
pallet apertures from a pressurized ‘goalpost’ style duct, the height of the boxes.
The DX unit was intermittent in operation, with an airflow of 0.025 m3/s/t,
with air from the distribution fans being discharged over the top of the stack of
boxes.
Over 2 years of monitoring (Mawson et al., 1992, 1993; Statham and
Cunnington, 1993), where the store was operated for 31 weeks and 35 weeks
respectively, the results were as shown in Table 7.3.
The wet cooled store did reduce weight loss in both years, but the financial
value of the saving in product for sale and the marginally improved quality of the
wet cooled stock of potatoes did not compensate for the 15–20% additional capital
cost of the wet cooling system compared with the DX cooler or the potential risk
in the slower drying of the newly harvested crop.
Refrigeration systems tend to run with few problems. The most likely is for one of the
switches that monitor refrigerant pressure, either the high-level or the low-level switch,
to cut out and stop the compressor working. If this occurs, the control system should
also shut off the recirculation fans. In some systems this is not the case and the fridge
appears to be working because the circulation fans are operating, but the compressor
is stopped. Leakage of refrigerant is one cause of these switches tripping. There is usu-
ally a light on the control box to indicate that the switch has tripped.
Icing up of the evaporator with well-designed units is rare. It is best avoided
by keeping store temperatures above 3.5°C, and preventing store air RH rising to
near 100% due to the ingress of warm humid ambient air through leaks or open
doors. Another cause of high store RH, sprout growth, will increase the risk of
icing if it is not halted.
Refrigeration systems have to be able to remove all the heat that enters from out-
side or is generated within the building, as well as being able to cool the crop after
harvest or cool the stored crop back down to the set-point temperature should the
crop temperature rise for some reason. These heat loads include (Fig. 7.5):
Insulation
Heat
from lights
Radiant heat
and leakage Heat of respiration
COLD
Refrigeration
Heat unit Heat from
Stored potatoes
warm air
Ground heat
The best method of illustrating how a plant is sized is to do an example. The one
chosen is to size the refrigeration plant for a 930-t pre-pack store similar in design
to, but smaller than, that used as the basis for the costing exercise in Ch13.
The 24-m-long by 18-m-wide building, with 6.5-m eaves height and 15° pitch
roofs, holds 930 t of potatoes in boxes. The building is medium in colour, and has
roof, wall and floor U-values of 0.25, 0.32 and 0.36 W/m2 °C, respectively. Air
leakage is assumed to be 0.5 air changes per hour. Two air distribution fans of
2.0 kW each run continuously to circulate the store air. Potato respiration heat out-
put is 10 W/t at the storage temperature of 3.5°C (Ch1.5).
The maximum heat load is probably in early summer for a fully loaded store,
when the average external temperature over the 24 h is taken as 18°C, while the
store set-point temperature is 3.5°C.
A high level of insulation and good sealing of the store will help keep the plant size
to a minimum. The higher the outside temperature, the larger the plant cooling
capacity will need to be. A plant to keep potatoes well into the late spring/early
summer will therefore be larger than one to store crop until early spring.
The heat transfers through walls, roof and floor are as in Table 7.4, using the
equation:
HF = U × A × ΔT,
where
HF = heat flow (W)
U = U-value (W/m2 °C)
A = area of building fabric (m2)
ΔT = temperature difference between outside and inside of the building (°C).
A supplementary temperature increment is added for solar gain (Ch5.4,
Table 5.1).
The maximum total heat transfer through the building fabric and floor is
therefore 7.97 kW.
Store Refrigeration 229
The heat gain due to leaking air is found using the equation:
HL = r × Cp × q × ΔT,
where
HL = heat from air leakage (W)
r = density of air = 1.23 kg/m3
Cp = specific heat of air = 1.005 kJ/kg
q = airflow = 0.5 air changes/h
ΔT = temperature difference outside to inside = 14.5°C.
If the volume of the building is 3336 m3, then
Heat from air leakage = 1.23 × 1.005 × (3336 × 0.5) / 3600 × 14.5
= 8.30 kW.
The metabolic heat from the stored crop is constant and is determined by the
store’s set-point temperature (Fig. 1.18):
In addition to the above heat loads, the plant must have capacity to cool the crop
after harvest and have a reserve capacity at the warmest time of the year that will
enable it to cool the crop should it warm up for some reason. This could be
because the plant stopped working for a day or the power supply failed.
A reasonable estimate of crop cooling capacity for box potato stores is to size
the plant to cool the crop by 0.5°C per day to avoid any risk of subsurface conden-
sation. If the crop enters at 18.5°C and is cooled to 3.5°C, cooling will take 30 days.
Only at the later stages of cooling, when the store and crop temperature is signifi-
cantly cooler than outside, will the plant have to cope with both cooling of the crop
and removal of heat entering the store through the fabric and through gaps.
In early summer, the plant can be sized to cool the crop by 0.5°C over a
period of 24 h, while still coping with the heat load from warm outside tempera-
tures and solar heat gain.
m × 1000 × c × dT
Rate of heat removal to cool the crop = ,
t
where
m = mass of potatoes in store (t)
c = specific heat of potatoes (3.80 kJ/kg)
dT = temperature reduction over cooling period (°C)
t = time of cooling period (s).
As the store holds 930 t of potatoes,
930 × 1000 × 3.80 × 0.5
Rate of heat removal =
24 × 3600
= 20.45 kW
What has not been included in this calculation is the cooling required to cool the
potato boxes which, at 100 kg per box, is a significant load.
The total heat gain that the plant will have to deal with is the sum of the above
heat loads.
If the refrigeration plant can only work for 83% of the time, to allow off-cycle
defrosting of six periods of 20 min each, the plant will have to be increased in size
appropriately:
Cooling load
Plan size to allow for off-cycle defrost =
0.83
50.02
=
0.83
= 60.27 kW
Therefore
Final cooling capacity = 60 kW or 65 W/t.
The specification can be looked at for different times in the year, at cooling, during
winter storage and in early summer, and the plant size adjusted to the highest
demand. What refrigeration engineers fear most is thermal runaway: where the
crop starts to heat up for some reason and its respiration rises to an extent that the
refrigeration system cannot regain control of the store temperature. The only solu-
tion then is to remove the crop from the store.
7.8 Summary
● Chilled-water coolers also chill a tank of water, but then produce a spray or
trickle of chilled water which comes in direct contact with the store air. The
air is therefore both cooled and humidified by the water.
● The coefficient of performance of a refrigeration system is the cooling capacity
in kW of the system divided by the energy consumption of the fridge compres-
sor, water pumps if present and air circulation fans.
● DX cooling systems are favoured in the UK over water/glycol and chilled-
water systems due to their low cost and ability to produce a firm potato.
● The required cooling capacity of a refrigeration plant is calculated by adding
the heat loads on the building to the cooling required to reduce the crop
temperature.
● An approximate cooling capacity for box potato stores in the UK is 65 W/t.
8 Store Environmental
Monitoring and Control
©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton) 233
234 Chapter 8
joined at each end, and made from different metals. UK storage manufacturers
prefer thermistors, as they are low in cost, have a high resistance so that connecting
leads can be different lengths yet have minimal effect on sensor accuracy; they are
also reliable. Thermocouples are favoured by those carrying out experimental work
as they can be cut to the length required for the experiment, provide good accuracy
and are low in cost so can be discarded if necessary after use. Some European ven-
tilation suppliers use thermocouples as standard. Resistance thermometers are only
used where high accuracies are required.
Temperature sensors accurate to ±0.2°C are normally specified for potato
storage applications. Thermistors are inserted into 4-mm stainless steel tubes and
sealed to exclude moisture. Thermocouples can be similarly ‘potted’ or can be left
as an exposed, welded tip (Fig. 8.1).
Connecting wires have to be long enough to reach from the point being moni-
tored back to the control box/data logger. Cable lengths of 60 m are not uncom-
mon. Cables must be kept clear of elevators and forklifts, so are commonly hung
from the store roof, becoming accessible to staff for insertion into the potatoes only
when the crop or boxes are loaded into store.
Air RH is increasingly being measured using capacitance film sensors (Fig. 8.2).
They give a read-out directly in percentage RH. However, when used outside they
can fail completely, possibly due to condensation forming on the sensor or, where
the store is near the coast, due to salt spray from the sea. When used inside the
store, they are particularly prone to dust, so should be protected with a fine filter.
Ventilation manufacturers using these sensors should ensure self-checking routines
are installed in the monitoring software to inform the store manager when a fault
has occurred.
The recent availability of plug-in replacement heads allows routine replace-
ment of the vulnerable sensing portion. Non-plug-in units need to be disconnected
each year and sent to the supplier for recalibration.
Thermocouple
Copper Plastic-covered
Wire ends copper/constantan
welded at wires
tip Constantan
Bead
thermistor Thermistor
Wet- and dry-bulb sensors, fitted with a fan to draw air over the sensors (Fig. 8.3),
can be used to determine RH from a table or formula (CIBSE, 2006). Like the
capacitance RH sensors, these too have limitations. The reservoirs of sensors
located outside will freeze in subzero conditions, sometimes causing the capillary
action of the wick to fail once the ice melts. Busy store managers can forget to top
up the reservoir. When used within the store, dust in the air can collect on the wet
wick, causing it to become coated in a wet paste. Filtration of the aspirated air will
reduce but not totally prevent this problem. The monitoring software should again
be designed to identify such failures.
Store monitoring must serve a purpose. What monitoring tells the store manager
is described in Table 8.1.
Variations in temperature of the crop in different parts of the store, near
cold walls or by leaky doors can cause poor fry colours or localized condensa-
tion. Temperatures across the store should optimally be within 1°C of each
other, less if possible. A large temperature difference between the top and base
236 Chapter 8
Suspension
chains
Wires to
control box
Wet-bulb
Filter sensor
Airflow Airflow
Airflow
>3 m/s
Computer
cooling fan
of a cooling front is likely to reduce with further cooling, but is undesirable over
long periods when no ventilation is occurring. Recirculation will reduce this dif-
ferential. Since there is little a store manager can do about store RH other than
to keep doors shut, the value of internal store RH measurement is primarily to
indicate store leakage.
Store Environmental Monitoring and Control 237
Bulk potato storage ambient-air cooling systems are almost always provided with
air blending. Blending allows the temperature of the cooling air to be controlled so
that it is never more than 4°C (Box 3.7) cooler than the crop temperature. It also
allows ventilation when the ambient air is below freezing. Blending control is illus-
trated by the plot in Fig. 8.4. As the ambient air temperature (Ta) cools, ventilation
starts at point A when Ta crosses the ‘ventilation initiation line’, 1.0°C below the
crop temperature line. Ventilation continues as Ta cools, until it meets the low
temperature limit 4°C below the crop temperature line. At this point (B) the intake
louvres will start to close and the recirculation louvres open to blend the incoming
cold air with the warmer store air (Fig. 8.5). The duct temperature sensor then
controls the position of the two louvres to ensure that the duct temperature Td is
always 4°C below crop temperature Tc. As the ambient air temperature rises,
blending will stop at C and the inlet louvres will return to being full open. As the
ambient air continues to warm, ventilation will stop at D when Ta meets the venti-
lation initiation line.
If no low limit is put on the temperature of the incoming air, the base of the
pile can become very much cooler than the top. This presents a condensation risk.
As the crop sensors are in the top of the pile or boxes, air cooler than the top of
the pile but warmer than the base could be blown into store, resulting in condensa-
tion forming on the potatoes near the floor.
Crop temperature(Tc)
1°C Ventilation initiation
line
Temperature
4°C
A B C D
Time
Fig. 8.4. Graphical representation of the control of ambient-air cooling and blending.
238 Chapter 8
Tc
Fan
Pile
Duct temperature
sensor
Duct temperature Td
Fig. 8.5. Control of intake and recirculation louvres to achieve blending of inlet air.
Example
Assume the ambient air is at 10°C, 75% RH, while the crop temperature is at 9°C.
The wet-bulb temperature of this air is 7.8°C. If the 10°C, 75% RH air is used to
ventilate the crop, as it leaves the humidifier it will be near 7.8°C and so will effect
cooling of the crop.
As with intermittent ventilation systems described above, the system is fitted with
blending, to ensure that the ventilation air is never too much cooler than the crop.
To avoid the risk of blowing air warmer than the potatoes into the bottom layer of
the pile, a temperature sensor is put into a litre bottle of sand located in the main
duct. This varies in temperature in a similar manner to the potatoes at the base of
the pile. Ventilation is stopped if the wet-bulb temperature of the air entering the
main duct is warmer than the sensor in the bottle.
An alternative to using the bottle filled with sand is to have a sensor located in
the base of the pile, but this has to be put into position as the pile is being filled.
It cannot easily be put in once the store has been filled.
Store Environmental Monitoring and Control 239
Connection box
Rod thermostat
Wet towel
acts as wick
Water
Plastic reservoir
Fig. 8.6. Inlet air wet-bulb sensor for continuously ventilated, humidified-air
ventilation systems.
The control system installed is dependent on the ventilation configuration (Fig. 8.7)
used in any particular store. These include:
● Direct entry of ambient air into store with air either blown into store (Fig.
8.7a) or sucked out of store (Fig. 8.7b).
● Entry of ambient air via a blending system, with air either blown into store
(Fig. 8.7c) or sucked out of store (Fig. 8.7d).
The air can be brought into store and forced through the crop by means of:
● An airspace ventilation system where air circulates around the boxes.
● A single-stage positive ventilation system.
● A two-stage positive ventilation system.
In the most basic of ventilated box stores, ambient air enters the store directly from
outside (Fig. 8.7a and b). The air is either blown into or sucked out from the store. In
the latter, fan heat is discharged to outside. In the former it blows into store, raising
the inlet air temperature by about 0.1–0.2°C, slightly reducing its cooling potential.
Direct air entry systems have a number of limitations:
● They may result in air very much colder than the crop flowing over warm
potatoes, which may result in subsurface condensation in the crop, particularly
in the top boxes.
240 Chapter 8
Blending
Boxes chamber Boxes
(a) (c)
Boxes Boxes
(b) (d)
Fig. 8.7. Box store ventilation and blending configurations: (a) ventilation without
blending, air blown in; (b) ventilation without blending, air sucked out; (c) ventilation
with blending, air blown in; (d) ventilation with blending, air sucked out.
● If controls are added to prevent ventilation with very cold air, valuable air
available for cooling may be lost.
● The incoming air path is unpredictable as its direction is dependent on its
speed and its temperature compared with that of the store (Box 6.7).
To modify existing direct entry ventilation systems, the frost thermostat (Fig. 8.8)
fitted to the control box can be altered on a weekly basis to prevent the incoming
air from being too much colder than the crop.
If fans blow into the building, it may be possible to add a blending system to
the existing ventilation system. If fans draw air out of the building, an inlet blending
system can be specially constructed and fitted to match the size of the inlet louvres
(Fig. 6.28).
The availability of blending prevents the problem that can occur where venti-
lation is prevented initially due to cool air being unavailable, only to be prevented
subsequently due to ambient air being too cold to introduce into store.
As with the blending system for a bulk store, the system (Fig. 8.7c and d) allows a
maximum crop/duct differential to be set so that the cooling air is never cooler than
the crop by more than a fixed value, usually 4.0°C. This will minimize the chance of
subsurface condensation forming on the potatoes, especially those in the top boxes.
Single-stage positive ventilation (Fig. 6.42a) systems for boxes fitted with blending
are controlled in the same way as for bulk storage ventilation systems.
Store Environmental Monitoring and Control 241
In two-stage positive ventilation systems (Fig. 6.42b), two sets of fans in series are
used, one set to bring the air into the store and one to blow the air through the
potatoes. Where ambient air enters the store directly, not via a blending chamber,
the incoming air is blended to some extent with the store air. The second fan then
takes the partially blended air and forces it through the crop. So long as the incom-
ing air is forced to mix with the store air before it enters the second fan, the system
will achieve a degree of blending of incoming air.
Where refrigeration is fitted, most control boxes allow the store manager to select
from:
● Ambient-air cooling only.
● Refrigeration only.
● Ambient or refrigeration cooling.
242 Chapter 8
The third setting instructs the controller to try to cool the store using ambient air
if cool air is available and using refrigeration if it is not. Since this system is ‘blind’
to time of day or to a cool spell of weather coming soon, in the UK it tends to use
refrigeration more often than is needed.
A restriction may be put on the refrigeration to allow it to operate only during
the times when low-tariff electricity is available.
Where control boxes are linked by modems to the World Wide Web, long-
range weather forecasts can be utilized to decide when refrigeration can be delayed
when a cool spell of weather is approaching. Such weather information could
include ambient dew-point temperature information to allow growers to plan when
to empty their stores without the risk of them becoming wet with condensation.
Potato Clamp
surface
Cables
70 mm
Twin matched
thermisters
300 mm
example 0.5°C. A table of suggested values is given elsewhere (Table B.3.1). These
are arbitrary figures, which may need adjusting with experience.
If warm ambient air enters the headspace of stores through leaks or open
doors, recirculation ventilation of the pile can lead to warmer headspace air con-
densing on the cool potatoes at the base of the stack (Ch3.5). Prevention of this
problem is simply achieved by monitoring the dew-point temperature of the head-
space air, and comparing this with the temperature of the potatoes at the base of
the pile. So long as the dew-point temperature of the headspace air is, at minimum,
1°C below the lowest temperature of the base of the pile, the 1°C providing a
margin of safety, recirculation can be carried out without the risk of condensation.
The same control can be used to warm potatoes without causing condensation.
An alternative control is only to allow recirculation to be carried out if the
headspace air is at, or below, the temperature of the crop at the base of the bulk
store. Air that is cooler than the crop will not condense on warmer potatoes. This
requirement will restrict the times when recirculation is possible more than when
the dew-point method is used.
Condensation can form on the roof or on the coldest potatoes due to sudden falls
in ambient air temperature (Ch3.5). Figure 8.10 shows the output from a sample
tuber fitted with a skin resistance sensor (Fig. 8.11) located in the top boxes of a
well-sealed, continuous, low-rate, humidified-air ventilated refrigerated store in
December 1991. The high resistance at the start shows that the tuber is dry. The
reduction in resistance shows that condensation is almost certainly occurring.
A white mould was subsequently seen to develop on the surface tubers, which suggests
that condensation did occur.
14.0 40
12.0 35
Skin resistance
10.0 dropping
Store temperature (°C)
30
Skin resistance (MΩ)
8.0
25
6.0 Ambient temperature
Cold spell 20
4.0
Store temperature 15
2.0
0.0 10
−2.0 5
−4.0 0
8 9 10 11 12 13 14
Date
The ambient air temperature* had cooled to 6°C below the crop storage tem-
perature, causing heat to be lost from the headspace air and condensation to form
on the coolest potatoes. The problem is easily overcome by replacing the lost heat
using roof-space heaters.
While many stores have roof-space heating fitted, store managers rarely know
if they ever come on. Control is often by thermostats set on the heaters themselves
and these are often accessible for checking only once the crop is in and the ther-
mostats come within reach of staff walking on the crop.
A better arrangement is for the heaters to be controlled by the main control
box. Heating should be triggered when the headspace air becomes colder than the
top of the crop by about 2.0°C. This differential should be fine-tuned from experi-
ence. An interlock is needed in overhead cooling air distribution systems to ensure
the heaters do not come on when cooling is in progress.
There should be indication as to when the roof-space heaters come on, as their
operation is likely to be during the middle of the night when most store managers
are in their beds.
When ambient temperatures fall with the onset of winter, the store insulation and
crop respiration heat may be insufficient to maintain the required set-point
*The ambient air temperature plotted in Fig. 8.10 came from a meteorological station 50 miles
(80 km) away, which may account for the coldest weather not coinciding with the condensation
starting. A second explanation is that the door was opened, which allowed warm air into store,
but this was ruled out by the store manager.
Store Environmental Monitoring and Control 245
When crops are ventilated with ambient air immediately after harvest, the aims are to:
● Prevent respiration heat from causing subsurface condensation.
● Dry surface moisture on potatoes and dry adhering soil.
● Stop ventilation if ambient air is likely to rewet the crop (i.e. crop temperature
below dew-point temperature of the ventilating air).
● Maintain a warm crop temperature to speed wound healing and to prevent
subsequent condensation during the next warm weather front (Ch3.5).
Ventilation is normally continuous immediately after harvest, reducing to periodic
recirculation once the crop is dry and as crop respiration rate reduces. The venti-
lating air will vary in temperature, sometimes being warmer and sometimes being
cooler than the crop. Blowing warm air on to the crop can result in moisture from
the air condensing on the potatoes, undoing the benefit from drying. Limits should
therefore be incorporated into control boxes during this period to prevent conden-
sation occurring.
the air will not become saturated when it meets the potatoes, so condensation on
the crop will not occur.
Example
If the potatoes are at 10°C, the saturated air moisture content at their temperature
will be 0.0076 kg/kg dry air. Ambient air at 20°C, 45% RH has a moisture content
of 0.0065 kg/kg, so moisture in the air will not condense on the crop directly; while
air at 15°C, 75% RH with a moisture content of 0.0079 kg/kg will.
Both these control strategies act in a similar manner to prevent ventilation, intended
to dry the crop and remove respiration, from causing condensation on the crop.
To avoid the complexity of dew point or moisture content control, some grow-
ers prefer the simpler system of avoiding the use of ambient air for drying, relying
on a refrigeration system to do both the drying and the cooling. This scenario is
discussed in Box 8.1.
If a period of cold weather occurs during the drying and wound healing period,
the crop will be cooled to below the average temperature for the time of year. The
dew-point temperature, or air moisture content, control will increasingly restrict
ventilation when warmer temperatures re-establish. To prevent this happening, a
low temperature set point, 4°C below the temperature of the crop, should be incor-
porated into the control system to switch louvres from ventilation to recirculation.
This will maintain the crop temperature at near the average ambient temperature
for the time of year, while allowing ventilation with ambient air to be maximized.
In addition it will speed the rate of wound healing.
A more basic but inferior method of control to the one above has the fans and the
louvres switched to manual and controlled by two thermostats. The frost override
thermostat (Fig. 8.8) switches ventilation to recirculation when the temperature of
ambient air falls too low and a second, specially fitted high temperature thermostat
switches ventilation to recirculation when the ambient air temperature rises too
high. In between, the fans ventilate the crop continuously. A time clock can be fit-
ted to operate the fans in recirculation mode on a periodic basis if ventilation is not
taking place.
To operate such a system with existing control boxes, the frost thermostat is
adjusted to 4°C below the temperature of the crop entering the store, while the
high temperature thermostat is set at 4°C above the crop temperature.
Compared with ‘dew-point temperature’ or ‘air moisture content’ control, this
approach is not guaranteed to keep the crop free from condensation. The store
manager can contribute to improving its reliability by monitoring weather forecasts
which include dew-point temperature information.
Store Environmental Monitoring and Control 247
Box 8.1. Relative cost of drying potatoes using ambient air and refrigerated air
Some growers would rather dry their potatoes using their refrigeration system than
use ambient air. This saves having to fit an ambient-air cooling system.
Fridge system
With a fridge system, again having an airflow rate of 0.02 m3/s/t, if the air leaves
the wet crop at 17.5°C and RH of 95%, and the fridge cools this air to 15.0°C,
0.00153 kg of moisture will be removed per cubic metre of air passing over the
fridge cooling coils. To remove the 20 kg of moisture, this will require 13,072 m3
of air or 182 h, 7.6 days, of continuous refrigeration and air recirculation to dry
the crop.
* This assumption is based on there being 9860 potatoes, average diameter 57 mm, in a tonne,
with a 0.2-mm-thick layer of water on their skins and in the attached soil.
248 Chapter 8
Dew-point control or air moisture control can also be used when warming potatoes.
This is more fully discussed in Ch6.9.
In airspace-ventilated stores, where incoming air flows over the top boxes, the fans
will run for very different times depending on where the controlling crop sensors
are placed. Sensors may be:
● In the roof headspace.
● Approximately 70 mm below the top surface of the potatoes in the top boxes.
● Deep down, near the centre of the top boxes.
● In the top and base layers of the stack of boxes.
When cooling starts, the roof headspace sensors will cool rapidly. Second to cool will
be the sensors 70 mm below the surface of the potatoes in the top boxes. Last to cool
will be the sensors deep down in the top boxes and in the lower layers of the stack.
With UK rates of ventilation (0.02 m3/s/t), it will take 3–4 min of fan running
to flush the whole store. Ambient-air fan operation, and therefore energy use, will be
minimized if the air in the store is flushed with cool ambient air, then the fans
stopped. Heat exchange between crop and air will take place by natural convection.
Since there is no forced air movement, the rate of cooling is likely to be slower than
when fans are operating continuously. If fans are run continuously when cool ambi-
ent air is available, the cooling rate will be a maximum, but a proportion of the fan
energy used will be wasted moving air from one part of the store to the other, with-
out necessarily passing through the potatoes. A compromise has to be found.
The aim must be to ensure that when the fans are running, they are removing heat
from the crop. In a bulk store, all the ventilating or recirculating airflow passes through
the crop, so that heat transfer is high. In airspace-ventilated stores a proportion of the
airflow passes through potatoes in the boxes, but unlike bulk stores, a proportion will
circulate over the potatoes and through gaps between boxes. If heat transfer to the
ventilating air is low, the ventilating air will exhaust from the building little warmer
than the temperature at which it enters. The optimum is for the exhaust air to leave at
the same temperature as the crop. This brings in the concept of cooling efficiency.
The efficiency of the cooling process can be assessed, in part, by monitoring
the temperature of the air exiting through the exhaust louvres and the temperature
of the air entering the store from the blending chamber or air inlet. The cooling
efficiency can be obtained (Box 8.2) by dividing the temperature difference between
Store Environmental Monitoring and Control 249
For example, if the crop is at 10°C, the duct inlet air is 6°C and exit air is 7.5°C, then
7 .5 − 6 .0
Cooling efficiency = × 100 = 38%.
10.0 − 6.0
the air leaving and entering the store by the temperature difference between the
crop and the ventilating air at entry. If all the air flows through the potatoes, cool-
ing efficiency will approach 100%. If half the air bypasses the potatoes, the cooling
efficiency will be 50%. What the efficiency measure does not take into account is
the cooling by evaporation that takes place during ventilation. It will however give
some indication as to how much air is flowing through potatoes and whether fans
should be run continuously or in short bursts.
8.7.3 Energy use in cooling using ambient air compared with refrigeration
If cool ambient air is available, ambient-air cooling uses considerably less energy
than that needed to run a refrigeration system (Box 8.3). In continental climates
ambient air for cooling is usually more regularly available than in maritime cli-
mates like the UK (Ch3.6), where there may be weeks of mild weather when air
suitable for cooling is unavailable. The availability of cool air below 4°C, particu-
larly in the autumn, appears to be reducing in recent years.
In making an assessment as to whether to wait for ambient air or to resort to
refrigeration, it is important to take the following aspects into account:
● Cooling costs in the UK using ambient-air and refrigerated cooling are approx-
imately £1.50/t and £6.00/t, respectively, for 7 months’ storage; 1.2% and
4.6%, respectively, of the value of a crop selling at £130/t.
● If refrigeration is delayed to the period of the night-rate (Economy 7) tariff,
usually 00.30 to 07.30 hours, the cost of electricity drops to approximately
67% of the standard day rate.
● If the crop could sprout and lose value for want of temperature control, it is
better to use refrigeration if it is available rather than try to save money by
relying on ambient-air cooling instead.
● Energy costs are likely to rise in future.
Temperature, RH and wet- and dry-bulb sensors provide only partial information
of the store microclimate. A number of other sensors (Table 8.2), when fitted to a
250 Chapter 8
Ambient-air cooling uses fans to draw cool ambient air, when it is available, through
the crop to cool it. The cool air is free, only the electricity required to run the fan has
to be paid for. In contrast, a refrigerated store is sealed, and the refrigeration sys-
tem has to remove the heat from the recirculating air using a compressor, which is
the main power user in a refrigeration system. This uses significantly more energy
than just running a fan.
With any refrigeration system, the temperature difference between the air
entering the evaporator (air-on temperature) and the air leaving the fridge (air-
off temperature) is constant, usually about 2.5°C. If the crop is at 10°C, the air
entering the fridge cooling coils (air-on) will be about 10°C and the air-off tem-
perature will be 2.5°C cooler, i.e. 7.5°C. (This difference will reduce by about a
degree as the crop nears 3.5°C, but it will not vary in the same way ambient-air
ventilation does.)
Ambient-air cooling usually starts automatically when ambient air tempera-
ture drops to 1–1.5°C cooler than the crop. Cooling will normally continue until the
ambient air falls to 4°C below the crop. After this point, blending should start to
ensure that the differential between the cooling air temperature and crop is not
excessive as this could result in condensation on the crop.
The rate of cooling with ambient air is therefore likely to be similar to using the
fridge. The problem with ambient-air cooling is that there may well be no cool air
available when it is required.
Presuming that there is air available, the power rating of the fans and fridge
will be as follows.
Ambient-air cooling
As in Box 8.2, the power used to supply a ventilating fan operating against a back-
pressure of 120 Pa is about 5 W/t of crop stored, regardless of the ambient air
temperature.
Refrigeration
Using a refrigeration system to cool a crop from an initial temperature and relative
humidity of 10°C, 95% to air at 7.5°C, 95% requires 6.43 kJ of enthalpy to be
removed per cubic metre of air. At an airflow of 0.02 m3/s/t, this comes to 6430 ×
0.02 = 129 J/t. As in Box 8.2, if the coefficient of performance (COP) is 2.5, the
electrical energy needed to power the fridge cooling is 129 / 2.5 = 51.6 W/t.
The fridge cooling costs are therefore 51.6 / 5 = 10 times that using ambient-air
cooling.
data logger, can provide a complete picture of what is occurring in store. These
additional sensors include:
● Carbon dioxide sensors.
● Tuber skin resistance sensors.
● Simulated potato to monitor tuber condensation.
● Simple solar heat gain indicator.
● Anemometer.
● Wind vane.
Store Environmental Monitoring and Control 251
A carbon dioxide monitor should be installed in all processing stores and in any
store that is very well sealed. The information that it provides will indicate when
and if flushing of the store is necessary. This avoids routine flushing, which may
often be unnecessary, and the associated risk of the ventilating air causing conden-
sation to form on the crop.
The tuber skin resistance sensors shown in Fig. 8.11 have been used extensively
in experimental work (Pringle and Robinson, 1996) to determine when condensa-
tion is occurring in a store and why it is occurring, and to help determine whether
condensation has contributed to the subsequent development of disease in the
stored crop.
The sensor used (Fig. 8.12), measures the electrical resistance of the tuber skin
between the two wires 30 mm apart girdling a sample tuber, placed within a mass
of potatoes. It was developed from the idea of a large grid, invented by Rasmussen
(1989), to measure the drying of crop in store, and was miniaturized to allow
detection of condensation in different layers of a box or pile. Details of its electrical
circuitry are given in Box 8.4.
252 Chapter 8
Tuber Resistance of
Wire condensation
Plastic
spacers Tinned copper
wire
Velcro Velcro
Rp = potato
30 mm
tuber
Resistance of
Plastic skin and flesh
spacer
(a) (b)
Fig. 8.12. Skin resistance sensor – technical details: (a) sensor laid flat to receive tuber;
(b) sensor wrapped round tuber; (c) electrical circuit for skin sensor.
Example
If Vx = 5 V, Rf = 1.0 kΩ and a voltage Vi = 2.1 V is monitored at the data logger, then
X = 2.1 / 5 = 0.42
and
Rp = 0.42 × 1.0 / (1 − 0.42) = 0.72 kΩ.
Some data loggers can provide the required AC excitation voltage. If it is not avail-
able, a separate AC voltage source should be used. A fixed resistor can be clipped
across the sensor wires to check the sensors are working before they are wrapped
round the tubers.
Store Environmental Monitoring and Control 253
The results need careful interpretation. While the sensor does show drying or
wetting of the tuber surface, it also shows skin cells losing moisture during ventilation
and recovering once ventilation stops. This complication has restricted its use to
those carrying out experimental work and has stimulated the concept of a simula-
tion potato, which has no skin to confuse the output.
A sensor that monitors surface moisture and condensation, but not skin resistance,
would solve the problems of using real tubers discussed above. It would not indi-
cate how rapidly the skin of potatoes dries after harvest, but it would monitor con-
densation on the crop thereafter.
For such an instrument to be successful, it must have the following properties:
● Its thermal mass should be similar to that of the potatoes it is monitoring, as
its thermal lag, related to its mass, plays a significant part in why condensation
forms on stored tubers.
● The sensing grid should not come into contact with surrounding tubers.
● Air within the crop must be free to circulate across the sensing element to
evaporate moisture when microclimate conditions change.
A sensor to this specification was developed (Martino and Gow, 1994). It used wet
flour dough packed tightly within a plastic cylinder to mimic the thermal charac-
teristics of a real potato and used an etched copper grid condensation sensor to
monitor condensation. In limited trials its performance was comparable with the
skin resistance sensor but without the confusion of variable skin resistance. A number
of companies have developed their own versions. A modified version of the con-
densation sensor concept, utilizing newly available film sensors used for measuring
condensation on building surfaces, is shown in Fig. 8.13.
Moulded thin-
walled plastic
cylinder A Condensation
Condensation Protrusions sensor
sensor End
cap Protrusion
CS
Hollow Cable
Filled with
wet dough
Hollow
Hollow Condensation
sensor (CS) A
End section on AA Side view
In monitoring the performance of the whole store, it is important to know why the
headspace temperature of a store is rising. This could occur when the store door is
left open on a warm day, allowing warm air to drift into store. Alternatively, it
could occur on a sunny day with the door tight closed. A simple sensor indicating
the effects of solar radiation is therefore required. A temperature sensor mounted
within an inverted jar, or similar translucent container, with no base fitted, is all
that would be required. If fitted on the external south-facing side of the store, any
reading of the sensor in the jar which exceeds an external, shaded, store sensor by
a margin of 3–4°C could be assumed to indicate that solar heating was taking
place. This sensor would then contribute to the store performance analysis.
The placement above the store door of a large digital display repeater showing
external dew-point temperature compared with internal crop temperature or a
traffic-light-style warning light (Fig. 8.14) will warn forklift drivers and store staff
that condensation on the crop may occur if store doors are opened. The forklift
(a) (b)
driver will soon learn at what time of day he can open the door without risk of
condensation forming on the crop inside. When circumstances like a waiting lorry
require the door to be opened regardless of the warming, it will serve to ensure the
door is left open for the shortest possible period.
In any store data-logging system, indication of fan, fridge or louvre operation is usu-
ally taken from the controller. If a maintenance engineer or store manager has
switched the isolator of any piece of equipment to off, then the log of data will suggest
that the equipment is running when in practice it is not. The same problem arises if
fans burn out, louvres jam or refrigeration high- or low-pressure sensors trip.
One approach to this problem is to fit temperature sensors or motion sensors
to each piece of rotating equipment. This can lead to considerable quantities of
additional wiring and more items to be monitored. The other approach is to moni-
tor all aspects of the store climate and identify any failures in equipment through
the failure to achieve the desired result. Should a crop fail to cool when the fridge
is running, for example, this may indicate that the fridge compressor motor has
tripped. Whole-store monitoring is simpler than fitting multiple lengths of wire, but
it does need good analysis to ensure faults are quickly rectified.
In many stores, particularly processing stores, fans are regularly run to flush the
store to prevent carbon dioxide levels rising to levels that could spoil the fry colour
of the potatoes. This may well be unnecessary as there may be sufficient air leak-
age to prevent this happening. The flushing may also risk causing condensation
on the crop, and may be better avoided if unnecessary. By installing a carbon
dioxide sensor in store, flushing can be carried out only when actually required.
If the level is logged, this can indicate for future use when high levels of carbon
dioxide are likely.
14
Ambient
12
10
Temperature (°C) 8
6 Crop temperature
Dew-point temperature
4 exceeds crop temperature
2
External dew-point
0
15 16 17 18 19 20 21
(a) Date
12
Condensation starts
10
Skin resistance (MW)
8
Base boxes
6
4
Top boxes
2
0
15 16 17 18 19 20 21
(b) Date
Fig. 8.15. Condensation on the stored crop due to air leaking into store: (a) crop
temperature versus ambient air dry-bulb and dew-point temperature; (b) drop in skin
resistance indicates condensation occurring on stored crop.
the latter being omitted from the graph for clarity. Figure 8.15b* shows that conden-
sation occurred on the potatoes in the top boxes first, then in the base boxes later
during this period. Ambient air must therefore have drifted into store during this
period, causing the condensation to occur. Subsequent examination of the store
found a louvre blade jammed open when the louvres were closed.
By monitoring a store, the store manager has access to all the data collected, but
not how it compares with other stores. His store may be using more energy than
*Skin resistance measurement was carried out using a short pulse of DC voltage so resis-
tances monitored were higher than true resistances measured when using AC voltages.
258 Chapter 8
other stores without his knowledge. By fitting a modem and telephone line to his
data logger, or the personal computer used as the controller/logger, he can make
the logged data available to a grower group manager. Comparative energy usage
or ambient-air cooling/refrigeration mix can be compared between the stores
within the group. Reasons for excess energy usage can then be found.
One UK company, Proctors Ltd, encourages growers to fit modems as stand-
ard and to contract into a scheme whereby store performance is checked at 08.00
hours each morning. Any fault or problem is usually found before any damage can
be done. Often the problem can be sorted remotely. If a service engineer is required
to visit the store, he goes there knowing what the problem is, so he can take the
tools or equipment he needs. This can save long drives to remote locations only to
find that a special part is required.
An additional benefit from such a system is that the store manager can access
store records through his palmtop computer or mobile phone when he is away
from home. The increasing power of these devices means that the usefulness of this
information can be improved over time.
The information collected can be integrated into the traceability and quality
assurance systems discussed in Ch12, providing information that is often the miss-
ing part of the jigsaw when seeking explanations for problems. The knowledge
gained should result in better store management in the future.
8.12 Summary
● A more basic condensation control system during crop drying and wound
healing restricts incoming air temperature to ±4°C of crop temperature.
● The precise location of sensors in box stores that are used for control can have
a significant impact on ambient-air cooling fan running times and energy costs.
● Box stores differ from bulk stores in that a proportion of the ventilating or
recirculating air goes over or between boxes, so does not pass through the
potatoes themselves.
● The concept of cooling efficiency for box stores is useful for determining how
much air is going through potatoes and whether intermittent ventilation or
continuous ventilation during cool weather is the more efficient in energy use
and effectiveness.
● The use of additional sensors such as skin resistance sensors, simulated tuber
condensation sensors, carbon dioxide monitoring, solar radiation indicators,
anemometers and wind vanes can greatly assist troubleshooting in stores.
● Fitting of modems to store data loggers or personal computer-based controllers
allows ventilation and refrigeration equipment suppliers to remotely monitor
store equipment and save on any repair costs.
● The value of data logging of store climate is greatly enhanced if data can
be partially analysed automatically and then compared, using modems and
the World Wide Web, with data from other stores. It can also be used for
traceability.
9 Store Management
9.1 Introduction
At the end of each winter storage period, significant quantities of soil and dust car-
ried in with the crop during the previous harvest remain within the store, in lateral
260 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Store Management 261
ducts, on floors, on ledges and walls, on boxes and machinery. As this dust is likely
to contain pathogens that can contaminate the crop about to be harvested, the
store should be thoroughly cleaned.
Removal of soil and dust from store will minimize the risk of disease being
carried over from one year to the next and is particularly important if parts of the
crop were diseased in the previous season. Cleaning of the store must be completed
in time for the new harvest to begin.
Vacuuming (Fig. 9.1) is much preferred to brushing as it removes both heavy and
light dust particles. Brushing removes the heavy large particles while dispersing the
remaining lighter fraction containing spores and the resting bodies of bacteria, only
for them to settle back on to the floor and sheeting rails (BPC, 2001b).
While store preparation is mentioned separately from store cleaning, it is often
best to do the cleaning and inspection at the same time. The process of vacuuming
dust from louvres, fans and equipment will reveal bent blades, frayed sensor wires,
damaged fan impellers, loose bolts, rat-chewed cables and gaps in the store fabric.
The action of cleaning forces staff to inspect the equipment at the same time.
While vacuuming the store is normally sufficient to remove the risk of infection
from the previous year’s crop, washing may be justified for high-health pre-basic
Fig. 9.1. Vacuuming the store is preferable to brushing. (Slackadale, Aberdeenshire, UK.)
262 Chapter 9
seed storage (BPC, 2001b). While composite panel stores and Styrofoam sheet-
insulated stores can be washed with a pressure jet washer, spray foam insulation
can be damaged if jet pressure is too high. Sprayer pressures should therefore not
exceed a pressure of 3.4 MPa (34.5 bar) if spray foam-insulated stores are to be
cleaned (Ch5.7).
Fogging equipment like that used for CIPC treatment can also be used to disperse
a disinfectant in the store after it has been vacuumed and washed. This is usually
only carried out for pre-basic seed storage.
If box tippers do not rotate the boxes through a great enough angle, some tubers
can be left in each box. Internal box bracing can make this problem worse by trap-
ping tubers under the bracing. If some tubers remain, they may contaminate the
new crop with last year’s decayed tubers. Increasing the rotation of the tipper is a
simpler solution than removing a handful of tubers from hundreds of boxes by
hand. Unless very high-health seed is being stored, removal of soil and crop residues
is usually sufficient to minimize any disease carry over from one year to the next.
Cleaning boxes with a water spray hand lance is a major undertaking. If it is
done properly it can take a day to wash 30 boxes. While it is difficult to justify finan-
cially washing all boxes every season, it is prudent to mark with chalk any particular
boxes that have stored rotting stocks, which may contain an abrasive film of sharp
soil particles intimately mixed with, for example, dry rot spores or other pathogens.
These should be cleaned thoroughly before re-use. Where cleaning is to be carried
out every year, a conveyor-based automatic washing system can be installed.
Boxes can be left outside to allow ultraviolet light to kill pathogens living on
the timber. However, in the UK climate, box life was reduced by 40% when stored
outside rather than inside (Pringle, 1993a).
Some weeks before the crop is loaded into store, the store fabric and doors should
be examined to find any damage, holes or problems. Ventilation equipment and
the refrigeration system should be inspected and operated, sensors calibrated, and
control and logging systems checked. This can be combined with the cleaning of
the store as both cleaning and inspection can be done simultaneously.
To check for gaps in the store fabric, the store doors should be shut during
daytime and any daylight showing through doors, cracks or louvres identified.
Large holes can be sealed using sheet insulation, while smaller gaps can be filled
with polyurethane foam, squirted into the gaps using spray-cans, or an opaque
mastic or silicon sealant sold in tubes, injected using a sealant gun.
Store Management 263
the tubes to ensure that they are all at a uniform temperature. Liquid paraffin can
be used instead of water if the water could damage the sensors.
The temperature reading on the store control box or monitor is then read off
and compared with the reading on the reference thermometer. A correction table
should be made up to show the offset of each sensor. Any offset is usually constant
over the range of monitoring. However, it is best to calibrate sensors using liquid
at a temperature within the normal store operating range (e.g. 5–14°C in the UK).
If the temperature of the water or liquid paraffin is being increased gradually by
the addition of warmer liquid, the liquid should be stirred well to ensure that the
probes are all at the same temperature.
If it is impossible to bring all the sensors together, the next best alternative is
to take the handheld electronic thermometer round each sensor, measure the tem-
perature and compare it with the reading on the control box (Fig. 9.4). Two people
will be required to do this, as the sensor may be a considerable distance from the
control box.
In both box and bulk stores, sensors should be located in the top and bottom of
the stack, with more sensors located in the top of the crop than at the base (Fig.
9.5). With the overhead throw systems commonly used in box stores, excessive
temperature differentials between cool air jet and crop surface can result in
Fig. 9.4. Switchgear boxes for two separate stores with control boxes to left and right.
Store Management 265
Temperature
profiles
Plastic pipe laid on
sloping face during
loading
Fig. 9.5. Suggested location for temperature sensors in box and bulk stores.
condensation in this top layer. If this differential exceeds 0.5°C in the top 300 mm
of the crop, this indicates that the incoming cool air is likely to cause condensation
in this top layer. The use of two sensors (Fig. 8.9), selected as being within ±0.1°C
of each other, always used as a pair, will give valuable feedback about the likelihood
of condensation if located under the path of the cooling jets, particularly where the
jet starts to lose momentum and flow over the top surface of the potatoes.
In bulk stores, sometimes the sensor to measure the base of the pile is not put
in the pile itself but in a 1-l plastic bottle filled with sand, placed in the ventilation
duct. This saves having to install the sensor in the crop as it is being loaded. The
thermal mass of the sand means that its temperature will approximately follow the
temperature of the potatoes in the base of the pile. The alternative is to lay a tube
down the face of the pile during loading (Fig. 9.5) so that the base sensor can be
inserted once the store is full.
The base temperature of the pile is monitored by the controller to ensure that
the ventilating air dew-point temperature never exceeds this value.
Checking the external and blending duct temperature sensors and thermostats is
usually complicated by their out-of-reach location. Calibration is carried out on one
sensor at a time and usually needs two people, plus possibly two mobile phones if
distances between monitor and sensors are large. The external temperature sensor
and the duct temperature sensor are checked by comparing them with the handheld
electronic thermometer in the same way as for checking internal sensors individu-
ally. However, as they are usually fixed to the gable of the building and in a blend-
ing chamber, respectively, they may require a ladder or forklift cage to enable them
to be reached. A risk assessment should be carried out prior to undertaking this task.
While this calibration is time-consuming and difficult, it is vital as a 1.0°C error in
these sensors can result in considerable loss of potential cooling time.
266 Chapter 9
When carrying out these checks, ensure that the sun or any exterior floodlight
cannot affect the temperature of the external sensor.
To check the frost override thermostat (Fig. 8.8), rotate the switch-on/off tem-
perature setting knob to increase the set temperature until you hear a click and
then turn it down until you hear another click. Turn the knob back and forth to
find the setting between clicks. Check the temperature on the thermostat dial and
compare it with the handheld electronic temperature sensor. These should be the
same if the thermostat scale is accurate.
If the frost thermostat is being set at 0°C and it has an exposed sensing element
(Fig. 8.8) which is waterproof, then the frost thermostat can be set by immersing its
sensing element in an ice/water mix. As mains voltage is fed to the thermostat, the
mains power should be switched off during this operation if there is any chance of
moisture coming into contact with a live terminal. The knob is rotated until it is in
a position between the two clicks, and this position marked for future reference.
External RH is measured by the control box using either a capacitance film
sensor (Fig. 8.2) or wet- and dry-bulb sensors (Fig. 8.3). Capacitance sensors should
either be fitted with new sensing heads annually or the whole device sent to the
supplier for calibration, although kits of salt solutions can be supplied to users to
carry out the checks themselves.
The thermistors of a wet-bulb sensor are checked with the wick removed using
the electronic thermometer. The wick is then washed in warm soapy water and
rinsed in clean distilled water to ensure it takes up water properly. It is then fitted
over the wet-bulb thermistor and suspended in the reservoir of distilled water.
RH sensors can be checked for approximate accuracy using a whirling hygrom-
eter (Fig. 9.6) and compared with the reading on the control box display. As whirl-
ing hygrometers are at present available only with glass thermometers, they should
not be used inside the store in case they break and the glass or mercury contami-
nates the crop.
Capacitance film RH sensors used inside the store should be protected from
dust using a filter. Wet- and dry-bulb RH sensors similarly need good filtering and
regular cleaning to prevent the wick clogging up with dust.
Rotate for
1 min
Dry-bulb thermometer
Handle
Wet-bulb thermometer
Water cooled by evaporation
reservoir
Checking that the frost thermostat stops fans and shuts louvres
when fans and louvres are switched to manual
To check that the store is adequately protected against being ventilated with freez-
ing air, the fans are started and the louvres opened using the manual switches on
the switch panel beside the control box. The adjustment knob of the frost thermo-
stat is rotated until the arrow of its pointer is greater than the ambient air tempera-
ture; a click should be heard. The fans should stop and the louvres shut. If the frost
thermostat works only when the switches are set to auto, there is no frost protection
when fans are set to work on manual. This is a serious flaw in the control system
and means that if the fans are switched to manual and unintentionally left on over
a frosty night, the potatoes could suffer frost damage.
Checking blending
The ventilation supplier will probably check operation of the blending system using
fixed resistors that plug into the ambient, duct and crop temperature sensing ports.
Unless these are left for you to use, it may not be easy to check whether the blend-
ing system is working properly. Blending should start when the crop/duct differen-
tial exceeds the set value, usually 4°C. Experience has shown that in many cases
blending will not operate when the ambient air is below 0°C, as the frost override
thermostat located in the duct is often cooled to below 0°C by unmixed freezing
air and stops ventilation altogether (Pringle, 1989). This can only be found by
monitoring the blending process when ambient air is below freezing. It can be
avoided by placing the frost thermostat after the fan, so that the air is well mixed
before it reaches the sensor.
268 Chapter 9
Potatoes can be harvested and loaded into store at rates of 10–25 t/h. Long
transport distances may mean that crops may not start coming into store until a
couple of hours after lifting starts, whereupon 50 t may have already been har-
vested. Should a problem be found in the crop as it enters the store, it will be
too late to divert it elsewhere, especially as another lorry with 24 t of potatoes
may soon be due.
It is vital therefore to sample crops in the field a day or two ahead of harvest.
These need to be washed in a bucket of water if the evaluation is done in the field.
A plastic-coated egg basket is useful for this purpose (Fig. 9.7). Markets can then be
found for crops showing rots or blight, so that they can be sold straight off the field.
The agronomist or fields person works ahead of the harvester, assessing skin
set and disease. Skin set indicates whether the crop is ready to harvest. The disease
assessment will help to assess the best market for the crop. If there is more than
1–2% rots or physical disorders, such as skin surface cracking, or disease is so bad
that the crop is not worth storing, the crop can be bypassed or harvested for imme-
diate sale or stock feed.
Bucket with
wash water
Fig. 9.7. Wire basket for washing samples
in the field.
Store Management 269
The equipment used for loading bulk stores is described in Ch2. The key manage-
ment aims are as follows.
1. If blight or rots are likely to cause progressive crop breakdown in store, sell crop
off the field or keep separate from the main store.
2. Remove any rots, mother tubers or haulm that are present.
3. Adjust cleaning equipment to remove as much soil as possible while minimizing
damage to the crop.
4. Stop harvesting if crop starts to smear with soil or crop temperature falls below
9°C.
5. Move loading head constantly to avoid the formation of soil cones.
6. Load pile in layers (Fig. 9.8) to minimize amount of roll-back of tubers.
7. Have fans running immediately when the crop starts to enter store (Fig. 9.9).
8. Open up lateral ducts as they are covered with crop (Fig. 9.10).
9. Keep surface of pile as level as possible to ensure uniform ventilation.
10. Soil cones reduce ventilation to tubers within the cone, which slows drying
and cooling of tubers in this area. This can result in localized rotting. Pockets of
potatoes with blight or rots can start to rot down (Box 9.1), causing hot spots which
can lead to large-scale rotting.
The same principles (1) to (4) above should be applied for storage in boxes. However
with boxes, instead of harvesting having to be stopped if smearing is occurring,
270 Chapter 9
Low drop
Pile
Fig. 9.8. Loading bulk pile in layers to minimize drop damage and tuber roll-back.
Fig. 9.9. Axial flow fans for a bulk store. (Courtesy of Farm Electronics, Lincoln, UK.)
empty boxes can be put on trailers for loading direct from the harvester. Smearing
will be less and the boxes can be blown with air to dry the soil so that when the
potatoes are put over the cleaner later the soil falls off. It may take a week of drying
to get the soil to this stage.
Store Management 271
Hot spots in potatoes can occur both in boxes and in bulk storage, but their consequences
are particularly serious in bulk storage. The problem starts with a tuber starting to rot due to
a blight or soft rot infection that invaded the growing tuber (Fig. B.9.1a). Rotting may occur
whatever the store management or may be exacerbated by tubers remaining warm and wet
after harvest. The rotting tuber collapses under the weight of potatoes above (Fig. B.9.1b),
resulting in an anaerobic mass of exudates though which ventilating air cannot penetrate.
The metabolic heat from rotting exudates results in warm moist air rising from the hot spot
and condensing on the cooler sound potatoes above (Fig. B.9.1c). They become warm and
wet, causing further rotting to occur and further breakdown of tubers. The hot spot increases
in size with more and more tubers collapsing, resulting in the surface of the pile starting to
slump (Fig. B.9.1d). This, and the exudates flowing into the laterals and main duct, may be
the first indication of rotting that the store manager sees.
Prevention, by hot box testing at-risk crops and diverting them elsewhere and rapidly
drying newly loaded potatoes, is the only cure for hot spots. Temperature monitoring of the
crop should give early warning of hot spots occurring, so that the store can be emptied
before rotting has progressed too far. Processing stores, kept warm to keep sugar levels low,
are the most prone to hot spots developing.
Continued
272 Chapter 9
Load from
potatoes above
Diseased tuber
Slime creates anaerobic
collapses
conditions
(a) (b)
Pile
Condensation Slump surface
Warm air
rises
Heat Rotting mass
Fig. B.9.1. Sequential development of a hot spot in a bulk pile: (a) tuber rots to
a bacterial slime; (b) slime contaminates other tubers; (c) heat from rots causes warm
air to rise and condense on tubers above; (d) collapsing tubers cause slump.
Store managers are often advised by agronomists to treat batches of crop in store
in different ways. If the store has a single airspace, it is not possible to treat any
part differently from the rest, except to provide more air to some potatoes than to
others.
Were it possible to cool one part of the crop more quickly than the rest to slow
the multiplication of a certain disease, then convection currents from the warmer
potatoes in store can result in the cooler part of the crop being soaked with
condensation (Ch3.5). This unintended result can make disease more, rather than
less, likely.
If any rots are present, it may take up to 6 weeks to dry the rotting tubers to
prevent them from causing sequential rotting. It is important to evaluate whether
it is worth saving such a crop or whether it is better to sell or dispose of the crop
straight from the field (Box 9.2).
Boxes loaded into store should be stacked in such a way that ensures ambient
air or recirculated store air can be forced through the newly loaded boxes within
hours of being harvested. While this may be easily achieved with positive ventilation
Store Management 273
systems, the airflow in airspace-ventilated systems will bypass the first boxes loaded
unless steps are taken to prevent this.
In stores holding a single variety, boxes should be stacked in a wall across the
direction of airflow (Fig. 9.11a). The recirculating air in this arrangement has to
flow through the boxes to return to the intake of the fan. In seed stores, where
boxes are stacked in rows to ease access to specific varieties, the only approach is
to try to prevent air bypassing the rows. This can be done by building temporary
stacks of boxes across the store to prevent air bypassing the rows (Fig. 9.11b). Any
boxes containing particularly wet potatoes can be used for this purpose, as they will
get more ventilation than those in the longer rows.
As the store becomes full, it is common, though not particularly good practice,
to fill the end of the store with boxes placed at right angles to the main rows of
boxes. If this is done, spaces should be left between every second or third box to
allow air to pass between these boxes prior to entering the pallet apertures of the
boxes in the rows (Fig. 9.12).
As stated in Ch2.4 and Ch2.5, chemical treatments must be based on a fully justi-
fied crop risk assessment, with the chemical used targeted at the disease of concern.
It is preferable to concentrate on non-chemical disease control measures, such as
rapid drying of crops, gentle handling, removal of respiration heat and minimiza-
tion of temperature differentials of crops in store.
If liquid chemicals are being applied, every effort should be made to dry off as
much liquid as possible before the potatoes go into store. Ventilation immediately
Rows
(a) (b)
Fridge
Main stack
of boxes
Boxes at right
angles to main
stack to fill
access passage
Door
Return airflow
Cold air from fridge falls here flows through
gaps between boxes
Fig. 9.12. Gaps left between last boxes in, to allow return airflow to enter main stack
pallet apertures.
after loading will help to remove the remaining moisture. While the chemical is
likely to prevent development of the target disease, any remaining liquid may well
make the development of a non-target disease more likely.
Stores should be sized so that harvesting equipment can load them within 5–8
days. This minimizes the risk period when doors are open and ambient air or ven-
tilation can cause condensation to form on the crop when it is still warm and dis-
ease multiplication can be rapid (Box 9.3).
Harvesting and store loading in a Scottish seed store was interrupted for 2 weeks
due to rain. The grower decided to start cooling the half-filled store during this
period as it was dry and the wounds had cured. Soon after harvest resumed, the
farmer discovered that the first half of the crop into store was now wet. A few weeks
later it had developed severe silver scurf. The later harvested crop was dried using
ambient air and was unaffected by disease.
The warm humid air from the newly lifted potatoes had condensed on the
cool, dry crop, thoroughly rewetting the latter. This provided perfect conditions for
silver scurf spores present to infect the crop and any infection already in the crop
to multiply. The result was a highly contaminated, low-value stock of seed.
to heal their skins and should be stacked in such a manner that plenty of air passes
through the sacks to remove respiration heat and any associated condensation.
From the start of store loading, ventilation should be switched on and the store
temperature maintained so that it tracks the temperature of the crop in the ground
(Fig. 9.13). The reasons for this are:
● Ventilation removes respiration heat and so prevents temperature differences
developing which can cause subsurface condensation.
● Keeping potatoes in store at the same temperature as newly harvested crop
minimizes the possibility of convection currents forming, which can result in
condensation forming on the cooler potatoes.
18
Store temperature during
loading should follow crop
15 temperature in ground
12 at risk
from
disease
9
Cooling rate for
leaky stores
Warm potatoes
6 at unloading to
minimize grader
3 damage
Cold to prevent sprout
growth or disease
0
Sep Oct Nov Dec Jan Feb Mar Apr May
Fig. 9.13. Target over-winter store temperatures for seed and pre-pack potatoes in
the UK.
276 Chapter 9
● Ventilation dries wet crops and removes moisture from adhering soil.
● Keeping the potatoes at harvest temperature, normally 10°C and above in the
UK, ensures wound healing is rapid and allows ambient-air ventilation with
little risk of crop condensation.
● Rapid healing of wounds helps minimize weight loss in the crop by preventing
evaporation of moisture from wounds.
At loading, differences between sections of a pile or potatoes in stacks of boxes
should not exceed 3–4°C, while differences in temperature of surface and subsur-
face potatoes should be restricted to 0.5°C (Box 3.7).
Drying is fastest at warm temperatures as warm air has greater water-carrying
capacity than cool air. Positive ventilation dries crops more quickly than airspace
ventilation.
Once the skins of tubers are dry, continuous ventilation can be replaced by
periodic recycling of air through the crop to remove respiration heat and ensure
that the temperature of the crop is uniform throughout. The tubers should be dry
throughout the pile or box, rather than just the surface layer. In positively venti-
lated systems, tubers nearest the incoming drying airstream will dry first, while
those next to where the air leaves the box may take up to 1–2 weeks to dry. In
airspace-ventilated stores the top layer of the top boxes will dry first but potatoes
lower down in the boxes and the stack of boxes may still be wet. Extensive recircu-
lation may be necessary to dry these deeper potatoes. Unnecessary ventilation
however increases evaporative weight loss and loss of skin bloom, so should be
undertaken only when necessary.
Wound healing is fastest at 20°C, and reduces to almost zero at 7°C (Ch1.5).
To evaluate the speed of wound healing, the manager can cut a few tubers in half,
place them on the corner post of boxes or on a piece of board and visually monitor
the progress of wound healing of the exposed flesh.
If crop temperatures at harvest are 18°C or above, cooling can be started before
wound healing has finished. Particular care has to be taken to prevent temperature
differentials forming in boxes if the crop is still respiring at a high rate (Box 9.4).
Drying of potatoes is almost always carried out using the ambient-air ventilation
system rather than the refrigeration system. Ambient-air drying uses considerably
less energy. However, some stores do not have ambient-air ventilation so have to
make do with the refrigeration system. On occasions where the air outside is foggy,
the refrigeration system can be used to continue drying when drying using ambient
air would have to stop.
The four plots in Fig. B.9.2 show what can happen when a store is ventilated soon after har-
vest with fans set to manual. There was therefore no control on inlet air temperature. Ambient
air temperature varied considerably over the period (Fig. B.9.2a), with unblended, ventilating
air being blown over the top of the boxes. As the weather was very wet, ventilation was often
stopped. This caused the relative humidity (RH) in store to rise into the nineties (Fig. B.9.2b).
The air was blown over the top of the stack of boxes, causing large temperature differences in
the top layer of boxes. On one occasion, on the night of 12 September, the temperature
25
20
Ambient temperature
Temperature (°C)
15
10
0
09 Sep 10 Sep 11 Sep 12 Sep 13 Sep
(a) Date
110
Ventilation Store internal RH
Air
100
recirculation
90
Relative humidity (%)
80
70
60
50
40
09 Sep 10 Sep 11 Sep 12 Sep 13 Sep
(b) Date
20
19
Potato
18 Box
17
Temperature (°C)
16
15
14
13 3.0°C
Cooled surface difference
12
layer
11
10
09 Sep 10 Sep 11 Sep 12 Sep 13 Sep
(c) Date
800
Skin drying
700 Skin due to
drying ventilation
600 due to
Skin resistance (kW)
ventilation Return to
500 base-level
resistance
400 Condensation
event
300
0
09 Sep 10 Sep 11 Sep 12 Sep 13 Sep
(d) Date
difference between the top tubers and those 350 mm below was 3°C (Fig. B.9.2c). At this point
the top boxes became wet due to condensation (Fig. B.9.2d). This was evident when the skin
resistance of a sample tuber fell below the base level of 270 kΩ. The previous peaks in skin
resistance were due to air drying the top potatoes during ventilation.
The crop, which was harvested with no sign of disease or rots, eventually started to rot
after a series of ‘ventilation-induced’ condensation events.
While this was an extreme case, the store manager was completely unaware that there
was a problem and did not know how it was caused.
Store Management 279
within the store while the air circulation system distributes the air through the crop.
This is a manually controlled system, with the store manager opening and closing
doors as weather conditions dictate. There are no automatic controls to prevent
ventilating the crop with freezing air or air with high dew-point temperature,
which may condense on the stored crop. There is therefore considerable risk in this
approach.
9.7 Cooling
Once the crop is dry and wound healing complete, or at least in progress, cooling
can begin. As mentioned previously, cooling should only start once the doors have
been shut for the winter. The rate of cooling is determined in part by the store
equipment and external climate and in part by the store manager. Two alternative
rates of cooling are recommended, one for well-sealed stores and one for stores that
are less well-sealed or which are regularly opened to source potatoes, usually seed
(Fig. 9.13). The fast cool helps dormancy to be maintained and slows the multipli-
cation of any established disease. The slow cool aims to prevent any ambient air
that does leak into store from condensing on the crop and causing disease to
develop owing to the presence of moisture.
Store managers can influence the rate of cooling to an extent. They cannot speed
cooling for individual, at-risk batches of crop (Box 9.2), nor can they cool crops at
the same rate as vegetables are cooled (Box 9.5).
They can restrict cooling:
● To a maximum of 0.5°C/day to minimize the possibility of temperature dif-
ferences occurring in the crop, particularly in box stores.
● For stores regularly opened to ambient air, where cooling should not overtake
the natural cooling of the atmosphere as winter approaches.
Too rapid cooling can cause temperature differences within boxes to occur in box
stores. The store manager should monitor crop temperature differences during
cooling and slow the rate of cooling should temperature differences within boxes
exceed 0.5°C. By setting the crop/duct differential to a maximum of 4°C (Box 3.7),
the likelihood of temperature differences within boxes exceeding 0.5°C is minimized.
To slow cooling when using refrigeration systems, the simplest way is to switch the
unit off for a period each day.
There will always be some crop weight loss as there is continuous breakdown of
carbohydrate through respiration to provide cell nutrients and energy to maintain
life. The minimum theoretical weight loss of potatoes at 3.33°C has been calculated
as 0.072% per week (Hunter, 1985). These figures assume saturated air. At this rate,
weight loss for a 30-week period will be 2.16%, or 21.6 kg/t of crop stored.
Even with cooling air humidified to 95–98%, cooling will always result in
weight loss in the crop. The amount of weight loss can be minimized by:
● Ensuring leakage of warm air into store is minimized (Box 9.6).
● Opening doors only when absolutely necessary.
● Using personal doors for entry rather than the main store doors.
Store managers or agronomists sometimes try to treat potatoes like they would
treat newly harvested raspberries or broccoli. They want to cool them over a few
hours to 3–4°C, both to reduce their respiration rate and to protect them from
developing disease. However, raspberries may have a value of £2500/t and a res-
piration rate of 580 W/t at 20°C, while potatoes have a value of £130/t and a respi-
ration rate of 70 W/t. Potatoes may be harvested at 200 t /day, while a typical
harvesting rate for raspberries is 1.5 t/day. It is uneconomical, impractical and
unnecessary to cool at such speed. Potatoes have relatively low respiration rates,
and their thick skins are their primary barriers to disease. The store manager
should concentrate on keeping potato skins free from damage, aiding the healing
of any wounds that do occur, and ensuring the skins stay dry during the 30 days
that it takes to cool the crop down to the long-term storage temperature.
Store Management 281
Recirculating air through the crop in stores without humidifiers also leads to
weight loss. The need to recirculate air through the crop can be minimized by:
● Setting crop/duct differential to a maximum of 4°C so that differentials >0.5°C
in the crop do not occur.
● Using sensors measuring temperature differences in the crop, rather than time
clocks to control when recirculation occurs.
Comparing energy consumption with other stores or for the same store in previous
years will give an indication as to whether the store is working at maximum efficiency.
This has been tried on an experimental basis (Bishop, 1992). A validated computer
study (Box 9.6) suggests the optimum settings for optimum energy efficiency.
With stores cooled using refrigeration only, the quantity of condensate pro-
duced from the drain under the cooling coils should match to the amount of mois-
ture lost from the crop. Condensate in excess of the actual moisture loss from the
crop may be caused by ambient air leaking into store. Excessive moisture loss in
DX refrigeration systems may be caused by the TD between the cooling coils and
the store air exceeding the recommended 6.0°C (Ch7.1).
Even with the advent of computer-based data logging it is useful to keep a manual store record sheet beside the store controller. This
ensures that:
● The store is visited daily.
● The requirement to write down temperatures, fan and fridge times forces the store manager to check that the plant is working
correctly.
● Problems such as sticking louvres or burnt-out fans are quickly identified.
● On the store person’s day off, others can see how the plant has been performing.
● Indications of rotting, such as the presence of insects, can be noted on the sheet.
Fans Fridge
Crop temperature (°C) Air temperature (°C)
Hours/ Cum. Hours/
Date Time (hours) Set point Ave. Min.a Max.a External Store ave. Cum. hours day hours day Comments
2/10 08.30 10 12.5 11.7 s3 13.2 s9 9.2 12.8 221 – 23 – Amb. air
cooling
3/10 08.15 10 12.1 11.2 s2 12.7 s9 8.3 12.4 234 13 23 – As above
4/10 09.30 10 11.6 10.8 s2 12.3 s7 13.2 11.8 241 7 31 8 Fridge
cooling
7/10 08.10 9 10.3 9.5 s2 11.2 s7 12.2 10.5 241 0 85 54 Rots in box
Chapter 9
Row1Col3
a
s3 denotes that sensor 3 is the coldest; and s9 that sensor 9 is the warmest.
Store Management 283
To aid inspection of the top of boxes or pile, a fixed safe access should be pro-
vided and safe areas marked by hazard tape (Ch5.10).
Regular sampling of tubers for disease development over the storage period will
not only help to determine suitable markets for the crop but may also help to identify
problem areas within the store that need to be modified for the following season.
The use of a modem (Ch8.11) fitted to the control box and data logger allows
cooling performance and energy use to be compared with other stores and will give
an early indication of poor performance and equipment failure.
If the store is on low-tariff electricity, the control box will usually be configured to
run refrigeration only during the night and in periods during the day when cheap
electricity is available. It is normal to set limits which will override these restrictions
should the crop warm over a set limit, by 0.5–1.0°C for example.
Potatoes in store represent a large thermal mass. This can be used to delay cooling of
the crop until ambient air cooler than the crop becomes available. The use of refrigera-
tion can then be minimized. A judgement has to be made as to how much the crop
temperature can be allowed to rise between periods of cooling, with the amount being
less with short dormancy varieties than long. Any diseases present should also to be
taken into account, as the warmer the crop, the faster these will multiply. As the end
of the storage period approaches and ambient air temperatures start to rise, store refrig-
eration equipment may not have sufficient cooling capacity to enable it to bring the
crop back down to temperature if the crop temperature rises too much. Energy saving
through the use of the crop’s thermal mass has therefore to be done with care.
CIPC is the most common chemical applied to crops in store to control sprouting
(Ch3.8). It and other sprout suppressants must never be used on seed potatoes, as
otherwise the seed will not grow. Even storing seed in boxes or stores that have
been contaminated with CIPC in the past may prevent uniform sprouting after
planting. It is strongly recommended that growers subcontract the application of
CIPC to experienced contractors.
Where fry colours are found to be poor, as indicated by a low Agtron or Hunter L
value, crops can be reconditioned by warming them for a period of 2 to 6 weeks in
store to improve their fry colour. In a series of experiments (PMB, 1991–1995) over
284 Chapter 9
50
Storage at 10°C Recondition:
20°C
40 15°C
20
Minimum
10 acceptable
fry colour
0
0 6 12 18 24 30 36
Storage term (weeks)
4 years on cv. Pentland Dell potatoes, the Agtron value of crops stored at 5°C was
raised by up to 16 points when the store temperature was increased to 20°C (Fig.
9.14). Reconditioning will affect the entire crop stored in the one airspace. The safest
way to warm the crop is to use roof-space heaters with recirculation ventilation
switched on. If the controller has a control mode for safely warming the crop with
ambient air without causing condensation on the crop, this can be used instead
(Ch8.6).
Reconditioning will not work on longer stored crops, where senescent sweeten-
ing has set in.
Warming potatoes in the store allows the crop to be warmed without risk of wet-
ting the crop. In box stores this is best done if the store has a separate warming
chamber (Ch6.10), accessible via canvas doors (Fig. 9.15) from both the store and
the grading area. The store-side canvas door is opened to allow boxes to be
stacked prior to warming. The door is then closed and warming started. Once
warming is complete, the boxes are removed using the second door opening into
the grading area.
Potatoes in bulk bins can be easily warmed using a space heater. Warming
the day’s out-take in large bulk stores is less easy. The only way this can be done
Store Management 285
is by opening up the last few laterals in the bulk store, and ventilating with
warmed air. This, however, prevents ventilation of the rest of the pile while warming
is taking place.
If potatoes at 3–4°C are taken out of store and the dew-point temperature of the
ambient air is above that of the temperature of the crop, the potatoes will get wet
from condensation. The warming airflow will first have to evaporate the water that
has condensed on the crop (Box 9.8) and, once it has dried, only then will the crop
start to warm. The warming process will be considerably longer than if the crop
was warmed within the environment of the cold store and the possibility of disease
development will have been increased.
Radiant heat applied to tubers on a roller table for 1 min immediately after store
has been shown to reduce damage (Bishop et al., 2000) and has the benefit of heat-
ing only the very outside of the tuber, so being economical on heat energy. The
temperature profile under radiant heat was also measured, which showed an
increase in the outside flesh temperature of 8–12°C depending on tuber colour.
286 Chapter 9
A letterbox warming system fitted with an electric heater and an airflow of 0.08 m3/s/t
was installed in the grading area of a multi-store complex. The heater raised the
temperature of the recirculated grading-area air by about 2.5°C. The batch of pota-
toes became wet when removed from a cold store held at a temperature below the
dew-point temperature of the ambient air. The warming system had then to dry the
crop before warming started. Figure B.9.3 shows how the potatoes nearest the
incoming warm air dried almost immediately and started to warm thereafter. In con-
trast, the potatoes in the layer where the air exited the potatoes were first evapora-
tively cooled by the air. Only when they were dry did they start to warm. The wetting
of potatoes extended the warming process by about 9 h.
14
Potato temperature where
Warmed air
12 warm air enters boxes
entering potatoes
Temperature (°C)
10
8 2.5°C lift
6
Potato temperature where
Grading-area air
4 warm air leaves boxes
Fig. B.9.3. Warming of potatoes that had previously become wet from condensation
on removal from cold store.
Similar radiant heat treatments have been used to reduce black spot (Trenckmann,
1988) but this has not been adopted commercially. Since wound healing is neces-
sary following grading, tuber temperature should be held above 7°C optimally for
1 week, but for 3 days at a minimum.
9.13 Summary
This chapter has concentrated on what actions the store manager can take to
ensure that the crop is stored in as good condition as possible. The key actions are
as follows.
● A month before harvest, the store is cleaned and inspected for damage, leaks
or faulty equipment.
● If boxes are used for storage, these are emptied of last year’s debris.
● Temperature and RH sensors are cleaned and calibrated ready for inserting
in the crop.
Store Management 287
● Ambient-air cooling control and fridge operation are tested prior to the store
being filled.
● Crops are sampled before harvest to help decide whether to store or sell
straight from the field, with sales staff alerted as to what action is intended.
● Crops identified by hot boxing samples as being likely to rot in store should be
sold immediately or dried separately from the main crop.
● Stores are loaded within a period of 5–8 days to minimize disease risk to crop
in store at this particularly vulnerable stage.
● Minimization of damage has priority over speed of harvest.
● If liquid chemical is being applied to the crop entering store, skins should be
dried immediately after application using ventilation.
● The temperature maintained in the store during loading should follow that of
the temperature of potatoes being lifted from the ground.
● Harvesting should stop if soil on tubers is wet and starts to smear, or crop
temperature falls below 9°C.
● Elevator heads in bulk stores should be kept moving to prevent soil cones
forming within the pile.
● Ventilation should be started immediately when potatoes or boxes enter store.
● In bulk stores, crops that could rot should be kept out of the main pile; in box
stores vulnerable loads can be kept separate from the main storage area and
blown with high volumes of air to prevent hot spots occurring.
● Potatoes grown in hot countries which are to be stored in bags in refrigerated
stores should be cooled down to store temperature in batches prior to stacking
within the main store.
● During store loading and wound healing, the control box should be switched to
wound healing mode to maximize ventilation while preventing condensation.
● Stores should be closed and sealed as soon as possible after loading.
● Once wounds are healed, the control box can be set to cooling. The rate of
cooling of 0.5°C/day should allow cooling to take place without the risk of
condensation forming on the crop.
● A record sheet should be kept of internal and external temperatures, crop
temperature differences and ventilation and refrigeration run times to ensure
that the store is operating optimally.
● A modem fitted to the control box and data logger allows remote monitoring
by ventilation equipment suppliers and energy use to be compared with other
stores.
● CIPC should be applied to crops in processing stores when eyes show signs of
opening. Stores should be opened for ventilation, preferably within 12 or even
8 h of treatment.
● Crops with sugar levels too high for processing can be reconditioned by main-
taining the stored crop at 15°C for a period of 15 days.
● Potatoes to be graded dry should be warmed, preferably within the storage
area, to 8–10°C prior to handling to minimize damage.
10 Seed Grading and Preparation
for Planting
10.1 Introduction
Seed has special requirements for storage, grading, dispatch and pre-planting prep-
aration that differ from the production of processing or pre-pack potatoes:
● Seed needs to be put into store so that individual boxes and varieties can be
removed throughout the storage period.
● In the UK seed is usually grown in the cooler areas of the country where
aphids, the transmitters of viruses, are few. In wet, cool autumns, potatoes may
be harvested with considerable amounts of soil, which can impede subsequent
ventilation.
● Due to the small tuber size, small voidage and presence of soil, positive ventila-
tion to force air between tubers is often advantageous.
● Seed may be required soon after harvesting, either for export or for chitting on the
customer’s farm, or may have to be stored over winter for springtime delivery.
● Seed may require up to five grader screens to split it into the size ranges
required to achieve uniform planting.
288 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Seed Grading and Preparation for Planting 289
● By selling split graded seed, the remaining fractions may have to be returned
to store until requested by another customer.
● Following grading, seed may spend weeks or even months either in transit or on
the recipient’s farm waiting for soils to dry or warm sufficiently for planting.
● Most seed supplied locally is required over a short period of approximately 4–6
weeks, putting pressure on the grading process.
Boxes Boxes
Fork Fork
lift lift
(a) (b)
Fig. 10.1. Stacking arrangements to allow removal of individual rows: (a) spaces
between each row allow individual rows to be removed; (b) two to three rows tightly
stacked, with boxes removed by first using a forklift side-shift, if fitted.
290 Chapter 10
As ventilation of air though boxes should start immediately the crop comes
into store, to prevent subsurface condensation, the ventilation system should be
designed to allow this. Positive ventilation systems (Ch6.8) should therefore either
be designed to allow rows of boxes to have 150-mm gaps between them, or to be
only two or three boxes wide, so that the box can be moved sideways prior to the
forklift reversing (Fig. 10.1b).
If airspace ventilation systems (Ch6.6) are used for storage of seed, the first
rows in should be in the centre of the store where the airflow from the ventilation
system is greatest (Fig. 10.2). Air through the first boxes in can be further improved
by blocking off the air return path with either a wall of empty boxes or a tempor-
ary wall of boxes containing seed that requires to be dried. Poor air distribution
can be compensated for to some extent by increasing the volume of recirculation
airflow to above the UK figure of 0.02 m3/s/t.
In other ways the storage of potatoes for seed is similar to that for the pre-pack
or processing market but with some distinct differences. No sprout suppressant will
be used, as the ability of the seed to grow subsequently would be affected by its
application. There are fewer restrictions on pesticide use in seed as compared with
ware, so fungicide treatments prior to storage, or after grading, are common.
The crop may be pre-graded into different size grades during the storage period
to remove the grading bottleneck in the spring. Minimal re-grading and inspection
is then required prior to putting the seed into bags or boxes for dispatch.
In a recent survey in the UK on seed storage (BPC, 2005b), the most common
size of seed store was 1000–1250 t (~25% of those sampled) with a typical loading
capacity of 100–200 t/day (~60%), giving a time to fill the store of 8 days or less
Fans Fridge/ambient-
air cooling unit
Intake
Wall of boxes,
(dotted), placed
to force air Overhead airflow
returning to from fridge/
fridge/ambient ambient unit
unit to flow
through boxes
in centre rows
Boxes being
loaded into
store
Centre
Plan of seed store rows
(~60%). In the UK, box storage for seed is universal, with traceability, in addition
to separation, being a major issue. After storage nearly 80% is warmed before grad-
ing. A failing in many large stores is that they take more than 8 days to fill, and so
cannot be rapidly closed, wound healed and then cooled.
Seed should always be dispatched at a temperature above average ambient
(usually 8–10°C in the UK) to prevent condensation during transit.
Most of the equipment for grading seed is the same as that used for grading ware.
The primary difference is the grader itself. In ware production, the grader simply
removes oversize and undersize material (e.g. <45 mm and >85 mm), requiring two
screens. In seed the size grades may be:
● <25 mm.
● 25–35 mm.
● 35–45 mm.
● 45–55 mm.
● >55 mm.
To achieve this number of grades, using a grader with one chain mesh screen
after the other on a horizontal plane (Fig. 10.3a) would lead to a very long
machine. To conserve space, therefore, graders are used that have mesh riddles
(screens) stacked vertically (Figs 10.3b and 10.4) with potatoes cascading down-
wards through the screens, with the largest tubers being removed first and the
smallest being removed last. A compromise between the chain mesh screen grader
and the vertically stacked riddle grader is to use a stepped riddle grader (Figs
10.3c and 10.5), which, although longer than the vertically stacked riddle grader,
allows the falling tubers to land on a rubber belt rather than cascading through the
series of oscillating riddles below. While manufacturers of the step grader claim it
does less damage than the vertically stacked screen grader, due to the rubber belt
being a softer landing for tubers, both types of grader are popular with seed pro-
ducers. As seed tubers tend to be considerably smaller than ware, the potential for
damage is less than for ware.
292 Chapter 10
Large tubers
Large tubers
Small tubers
Small tubers
(a) (b)
Small tubers
Large tubers
Direction of crop flow
Mesh screen direction of movement
Riddle oscillates back and forth
(c)
Fig. 10.3. Types of grader: (a) chain mesh screen; (b) vertically stacked riddle;
(c) stepped riddle grader.
Fig. 10.4. Vertically stacked riddle grader. (Courtesy of D.T. Dijkstra, B.V.,
The Netherlands.)
Seed Grading and Preparation for Planting 293
Fig. 10.5. Step grader with riddles removed. (Slackadale, Aberdeenshire, UK.)
12 m
Dividing conveyor
Steel
dividers
Soil
conveyor
Grader Reject
tubers
Roller inspection tables conveyor
25–35 mm 35–45 mm 35–45 mm 45–55 mm
Raised platform Over 55 mm
for grader for ware
Elevator
Soil extractor
Potatoes from
store or field
tipped into
hopper
Fig. 10.6. Layout of a seed grader system. (Redrawn from illustration by D.T. Dijkstra, B.V.,
The Netherlands.)
Fig. 10.7. Twin box tippers feeding a flat belt. (Courtesy of Haith Tickhill Group,
Doncaster, South Yorkshire, UK.)
Seed Grading and Preparation for Planting 295
the separate fractions from the belt and channel the tubers on to a series of five
roller tables, which rotate the tubers so that inspection staff can see the whole sur-
face of the tubers and remove any that are blemished or damaged. The roller
tables feed the seed into boxes, big bags or sacks (not shown), each with a close
graded range of seed size. As the 35–55 mm size range contains the majority of the
seed, two roller tables are allocated to either 35–45 mm or 45–55 mm, depending
on which size predominates.
The inspection tables are usually housed in a well-lit cabin, illuminated to
500–750 lux at the inspection table, so that inspection staff can be kept warm and
free from the dust that is generated by forklift movement, tipping of boxes, filling
of sacks or bags, or from any point where potatoes drop.
A soil and clod conveyor takes soil from under the grader and roller tables,
while the reject tubers conveyor removes damaged or diseased tubers picked off the
tables by the inspection staff.
Seed crops in storage for longer than their dormancy period are usually kept at
3–4°C to prevent sprouting. There are, however, risks of inducing damage to the
crop if it is handled or graded at this temperature. When potatoes were subject
to a standard bruising test, those at 3°C had a bruise incidence of 83%, while
those at 21°C suffered only 8% bruising (Wiant et al., 1951). Potatoes dropped
1.1 m at 5°C had 77% splitting, while at 8°C splitting was reduced to 38%
(McRae et al., 1975). Warming of potatoes to 8–10°C prior to handling or grad-
ing is therefore commonly recommended for potatoes that are below this
temperature.
296 Chapter 10
If the whole store is being emptied, the whole crop can be allowed to rise in
temperature prior to grading. However, since seed may be graded mid-storage to
reduce the bottleneck in the spring or to satisfy orders throughout the storage
period, the store is normally kept at 3–4°C and batches of crop warmed prior to
grading. If the warming is done rapidly overnight, this tends to dry out the skin of
tubers, making fingernail-type scuff damage more likely. Giving the skins time to
regain their original moisture and elasticity can reduce this problem.
The crop not only needs to be warm to prevent damage, it also needs to be
warm so that any wounds inflicted during grading are healed rapidly. Periderm
formation in tubers develops more rapidly at warmer temperature (Ch1.5).
Seed, due to its small tuber size and therefore small mass, is less liable to suffer
damage during grading than ware. This allows the use of vertically stacked screen
graders or step graders, which require potatoes to be elevated to high level only for
the tubers to cascade downwards through riddles to near ground level. Ware graders
are designed to be flat to reduce such impact damage, which is possible due to the
limited number of ware size grades.
Seed is graded dry, so there is considerable friction between tuber skins and
surfaces where soil has built up and dried. In contrast, ware is often graded after
it has been washed, so that tubers are slippery and any soil on the grader is soft
and non-abrasive.
Damage is likely to occur in:
● Bulk hoppers if the exit is constricted.
● Cleaning coils (Fig. 10.8) if the steel is grooved by hard stones or small stones
jam between the coils.
● Drops into the boot of an inclined elevator.
● Drops from the inclined elevator on to a belt or grader riddle (Fig. 10.9a).
● Any change of direction from one belt to another, particularly if belt guides
are located beneath where tubers land on the belt (Fig. 10.9b).
● Increased drops on to a grader riddle when the top riddle has been removed
(Fig. 10.9c).
● The tail of a grader screen due to soil build-up (Figs 10.9d and 10.10).
● Drops into the boot of a bag filler (Fig. 10.9e).
● Drops into the sack, box or big bag (Fig. 10.9e).
In the experience of the authors, where grading lines have been designed, supplied
and installed by the one company, damage points are few. Problems arise when
growers install unmatched pieces of equipment in the line or put in pieces of board
to bridge gaps or divert tubers (Fig. 10.11). Even the softest piece of rubber foam
padding will build up with soil, which may then dry to the consistency of sand-
paper. The skin of tubers landing on such surfaces will then tear, giving the famil-
iar ‘thumbnail’ marks or scuffing that become obvious 2 or 3 days later (Fig. 10.12).
This not only spoils the appearance of the seed but provides access points for dis-
ease to invade.
Seed Grading and Preparation for Planting 297
Tubers
Tubers D
Bag filler
Sack
D2
Spring-
D1 loaded
platform
(e)
Fig. 10.9. Some potential damage points on a seed grader: (a) keep elevator incline
<25° to keep drop height (D) low; (b) try to avoid 90° belt-to-belt transfers; (c) if
remove top riddle to reduce number of tuber sizes, this increases drop D; (d) soil
build-up on tail of riddles causes scuffing; (e) minimize drop to intake of bagger (D1)
and place empty sack on spring-loaded platform to minimize drop (D2).
Fig. 10.12. Tubers with ‘fingernail’ or scuff damage. (Courtesy of Potato Council,
Oxford, UK.)
Where uniformity in size of tubers in the daughter crop is critical, cup-type potato
planters (Fig. 10.13) are commonly used to ensure seed is planted at regular spac-
ings. If the cups of the planter are much larger than the smallest tubers in the batch
Seed Grading and Preparation for Planting 299
of seed, two tubers can be carried by the one cup and planted together in the drill.
Alternatively, if the cup is smaller than the largest seed in the batch, the tuber can
fall off the cup giving a miss. The results are doubles and misses, leading to a
variable-sized daughter crop.
Split grading of seed, where seed is subdivided, for example into two fractions,
one 35–45 mm the other 45–55 mm, allows planter cups to be sized closer to the size
of the batch of seed used. Doubles and misses are minimized and a more uniform
crop is obtained. For the seed supplier, it may mean having to grade the seed, take
out the fraction the customer wants and put back into store the remaining fraction.
This means warming the crop to be graded, putting the fraction to be sold into bags
or boxes, allowing them and the remaining fraction to wound heal, then returning
the unsold fraction of warm tubers back to store where they will be cooled back
down to the temperature of the store.
In the UK, under the Seed Potatoes Regulations, the government sets the percent-
age tolerances for disease, skin blemishes and dirt (DEFRA, 2006). These vary
depending on whether the seed is Pre-basic, Basic, Super Elite, Elite, A or A/S.
These are shown in Table 10.1. This provides a quality assurance standard for
customers, so that they know what quality of seed to expect. The tolerance for
Group I diseases or pests is nil.
The individual, group and collective group tolerances work as follows. In
Group III for Pre-basic stocks, for example, individual tolerances of up to 1.0% of
300
Table 10.1. Individual tolerances (%) and group tolerances (%) for Pre-basic, Basic, Super Elite (SE), Elite (E), A and A/S stocks of
potatoes. (DEFRA, 2006.)
Specified diseases or pests; damage Individual Group Allowable % Individual Group Allowable %
or defects tolerances tolerances surface cover tolerances tolerances surface cover
Chapter 10
*Tolerances to be applied in the case of class A seed potatoes.
Ecc, Erwinia carotovora subsp. carotovora; Eca, Erwinia carotovora subsp. atroseptica.
The table excludes the Group I diseases of wart disease (Synchytrium endobioticum), eelworm (Ditylenchus destructor), potato cyst nematode (Globodera
spp. infesting potatoes), and diseases and pests not established in the UK.
Seed Grading and Preparation for Planting 301
black scurf alone or 5.0% of common scab alone can be tolerated, but they cannot
jointly exceed a maximum of 5.0%. In the same way, a tolerance in Basic stocks
in Group III of 4.0% is acceptable as is a tolerance of 2.0% in Group IV, but the
collective tolerance of Groups II, III and IV must not exceed 4.0%.
In a survey carried out in the UK (Heywood et al., 2006), 39% of seed was treated with
chemicals into store or prior to dispatch. Methods of chemical application into store
are dealt with in Ch2.5. Systems of treatment of seed out of store are similar in design,
but it is even more important that treated tubers should be thoroughly dried before
dispatch. For seed, unlike ware, chemicals in the form of dusts can be applied.
As seed is almost always graded dry, dust in the grading area and on the floor
of the store passageways is a major problem. Dust is categorized by particle size
302 Chapter 10
as inspirable and respirable. The inspirable dust is filtered out in the nose and
throat, while the respirable dust can enter the lungs. The latter is therefore the
more likely to lead to long-term respiratory illness. The Occupational Exposure
Standard (OES) for organic dust (HSE, 2005c) is 10 mg/m3 time-weighted
average for an 8-h shift. A study of personal exposure to total inspirable dust
in six on-farm grading areas recorded dust levels averaging 28 mg/m3 (range 3
to 148 mg/m3), well above the OES limit (Robertson, 1993). The OES for the
respirable fraction of the dust is 5 mg/m3. Only 9% of readings recorded
exceeded this limit. Highest personal exposure to dust occurred at inspection
roller tables and at bagging-off points. Dust from the potato grading process
and dispersed into the air by forklift movement produces a most unpleasant
environment in which to work, and one which commonly exceeds acceptable
OES levels.
Dust control equipment, in the form of air extraction hoods, can be fitted
wherever tubers are subject to drops during their passage along the grading line.
Locations include the box tipper, transfer points from one conveyor to the next
and where bags are filled. The air extracted is fed to a dust filter to remove the
dust and allow the air to be recycled. Dust extraction systems help to reduce dust
levels but rarely are fully effective. A cabin fitted with windows installed over the
roller tables, supplied with a downward flow of filtered warm air, can improve
the atmosphere for inspection staff. For full dust control, a steam or mist genera-
tor to moisten tubers at the start of the grading process should be fitted. This
approach may increase the likelihood of disease development on the tubers, so
the graded seed should be thoroughly dried using positive ventilation to prevent
any disease developing.
For short periods of working, face respirators designed for dust can be used.
These must fit the face well, be kite-marked to a recognized standard and have an
exhaust valve to ease breathing out and to prevent spectacles steaming up.
Box 10.1. Strip curtains only partly reduce air exchange between store and outside
Plastic strip curtains are often used to keep warm ambient air out of refrigerated
stores while allowing forklift access to fetch potatoes for grading. This is unlikely to
be very effective. With the store door closed (Fig. B.10.1a), the store is full of cold
air that is denser than the warm ambient air outside. If the door is opened (Fig.
B.10.1b), even with the strip curtains in place, the dense cold air will flow out of
store and the less dense, warm ambient air will enter. If the crop temperature is less
than the dew-point temperature of the ambient air now within the building, the pota-
toes in store, particularly in the top boxes, will become wet with condensation.
Ambient at
Store air
12°C
at 3°C
Potatoes at 3°C
Fridge
(a)
(b)
Fig. B.10.1. Cold store with: (a) main door tight closed; (b) main door open but
plastic strip door present.
This also puts an additional burden on the cooling system, as the air within the store
has to be cooled back down to the set-point temperature. The best routine is to open
the store when ambient temperatures are as near to the store temperature as possible
and to remove the day’s grading over as short a period as possible. In the UK, this
is early in the morning or late in the day.
This problem of doors being open is worst with large seed suppliers, where
high-volume grading is taking place and a number of stores are being accessed
simultaneously. There is a tendency for doors to be kept open all day so as not to
slow forklift operation. If such a system is practised, it may be better to keep seed
at a warmer temperature so that it remains dry rather than trying to keep it cool,
where repeated wetting through condensation is likely. However, if the weather is
warm, this can lead to early dormancy break and sprout development.
304 Chapter 10
unloading, they are not airtight. When travelling, air will leak through the edges of
the curtain and ventilate the load.
In frosty conditions, therefore, the pallets or boxes should be encased in insula-
tion material, such as corrugated cardboard or fleece, and then sheeted with canvas
or plastic tarpaulin to cocoon the load to prevent air ingress. As it is difficult to seal
the pallet apertures of boxes, or 1-t or 20–25-kg bags on pallets, the canvas should
be put over the floor of the flatbed prior to loading so that it can be pulled over
the load to meet at the top with the canvas from the other side (Fig. 10.15). Some
specialist transport companies have an insulated cocoon, which both seals and
insulates the loads when conditions are frosty.
The other common form of transport is the flatbed lorry. When double
sheeted, they provide better sealing than the Tautliner (Pringle and Thompson,
1987). An insulating layer such as corrugated cardboard, plastic fleece, bubble
wrap or straw can be used as the insulating medium between the two sheets
of canvas for the top and sides of the load. In severe weather conditions, of
temperatures below −5°C, seed should not be transported in uninsulated lorries.
Above this temperature, the chance of frosting potatoes on seed transported on
journeys of less than 36 h is low.
Seed can also be transported loose in bulker lorries for just-in-time direct filling
of planters in the field. Since planting is unlikely to be carried out in periods of
frost, insulation is not required.
Containers are sometimes used for the transport of seed. The cheapest con-
tainers are simply steel boxes holding around 20 t (Fig. 10.16). How airtight
Canvas
envelops
entire load
Canvas
up for
transit
Canvas
down for
loading
Cross-section
Fig. 10.15. Wrapping seed to prevent it being frosted during transit. (Courtesy of
Potato Council, Oxford, UK.)
306 Chapter 10
Fig. 10.16. Loading big bags into a steel container. (Courtesy of S.G. Baker Ltd,
Angus, UK.)
they are depends on how much damage they have sustained in use. As steel is a
good conductor of heat, seed tubers are at risk from frost damage. So long as the
journey is short, the seed has been well ventilated and is warm and dry, and the
box is lined with corrugated cardboard, they are suitable for short journeys.
Physical damage can be incurred on the tubers if drop heights during loading are
excessive. For longer journeys refrigerated containers will provide a more reliable
form of transport.
Traditionally, the chitting of seed in trays was carried out in greenhouses (Fig. 10.18),
with high light levels partially compensating for the lack of temperature control.
A 3-kW blower heating system for every 10 t of seed provides sufficient frost protec-
tion for the south-east of England. In trials carried out by one of the authors using
Fig. 10.17. Sprouts should be green and short, not white and long.
308 Chapter 10
such a system, the temperature of the seed was maintained between 2.5 and 7.5°C
for approximately 50% of the time with the remaining 50% being above 7.5°C
(Bishop and Maunder, 1980). Since there is little control of temperature, sprouting
can become excessive if planting is delayed by wet weather. Greenhouses are now
rarely used.
An insulated store with lights can provide better temperature control than green-
houses. The artificial fluorescent strip lights (65 W/t) are hung on the side of, or
between, stacks of potato trays and moved from one stack of trays to the next. As
the capital cost of the lighting is so high, it is sensible to keep the lights on 24 h/day
once sprouts start to appear and to move them twice a day to ensure that all the
trays get their share of light. If lights are moved twice per day, one hanging flores-
cent tube is needed per eight stacks of trays (FEC, 1985). To simplify moving the
lights, they are either hooked on to the top tray or suspended on wires or tracking.
The store is fitted with a recirculation ventilation system rated at 0.04–0.05 m3/s/t
of seed, to ensure that the temperature within the chitting shed is uniform. In the
trials mentioned above, the temperature in the insulated shed was between 2.5 and
7.5°C for 84% of the time, with the remaining 16% being above 7.5°C (Bishop
Seed Grading and Preparation for Planting 309
and Maunder, 1980). The heat produced by the lights provides frost protection,
but also makes cooling the shed more difficult. A further level of sophistication is
to have a refrigerated store with lights, which from the same trial kept the seed
within 2.5–7.5°C for the whole period.
The time required to fill trays (three per 50 kg) is estimated at 120 person-
minutes per tonne and the rate for filling the planter from trays can be up to 20
person-minutes per tonne in comparison with under 5 person-minutes per tonne
from bulk bags (Bishop and Maunder, 1980). The filling time can be less than
stated above if well mechanized.
Although producing green sprouted seed gives the greatest yield and early harvest
advantage, in many cases having the eyes open just prior to planting can suffice.
Refrigerated storage and a good recirculation ventilation system to ensure potatoes
are at a uniform temperature are required to achieve this. This avoids the high
labour input of putting the seed into trays, the cost of the lights, and the problem
of sprouts being broken off in the planter.
Alternative, less labour-intensive systems than tray systems have been invented over
the years. The Blackburn crate had a zigzag arrangement of mesh so that potatoes
could be fed into the crate mechanically using a tractor front-end loader fitted with
a box rotator. The spaces between the hanging mesh allowed light to get to most of
the tubers. While these were reasonably successful, if sprouts grew too much the
tubers became trapped between the mesh and would not come out. A similar system
using plastic hanging nets suffered from the same problem. Every few years a new
‘solution’ to mechanized chitting systems is produced, but none but tray systems
seems to have endured.
A spray foam-insulated chitting shed is shown in Fig. 10.19. The shed contains 300 t
of seed held in trays 750 mm × 450 mm × 165 mm, with three trays holding 50 kg.
There are 50 trays per pallet, so the loaded pallet has dimensions of 1.5 m × 1.2 m
× 1.8 m. Pallets are stacked two high to give a stack of trays 3.6 m high. Gaps, 500 mm
wide, are left on all four sides of each pallet to allow inspection of sprouts and to take
the lights. Two 1.8-m-long fluorescent lights, each rated at 70 W, are suspended one
above the other in the gaps to provide the light (Fig. 10.20). This provides well above
the 65–70 W of light per tonne stated above. Lights are on continuously. The flores-
cent tubes should be protected by perspex casings to prevent damage to tubes and the
risk of injury to staff.
310 Chapter 10
Fig. 10.19. Chitting shed for 300 t of seed. (Hamilton and Sons, East Lothian, UK.)
The tubers were put into trays on 17 January (H. Hamilton, West Lothian,
Scotland, 2005, personal communication) and were planted on 7 April. They
were therefore kept in the chitting shed for a period of 11.4 weeks. A refrigeration
system (Fig. 10.21) is located at one end of the building, which serves to keep the
temperature between 3 and 4°C.
Some of the better seed suppliers produce reports on washed samples of seed, car-
ried out prior to grading (Pseedco Ltd, Perth, Scotland, 2007, company information
sheets). The benefit of inspecting pre-graded material is that this includes material
which will be rejected on the inspection line, but which will give a true reflection of
potential problems that could develop following planting, such as rotting or blackleg.
An example of a washed and pre-graded report is shown in Box 10.2. As well as
312 Chapter 10
Box 10.2. Example of a Washed and Pre-Graded Report (SAC Ltd, Edinburgh, UK)
Where 0 = nil, 1 = trace (1%), 2 = trace to 1/16th (1–6%), 3 = 1/16th to 1/8th (6–12%),
4 = >1/8th (>12%).
Disease % plugs
Black scurf 14
Black dot 0
Silver scurf 4
Skin spot 0
Blight 0 Bruising 2
Dry rot 0 Growth cracks 0
Gangrene 0 Pest damage 0
Soft rot 0 Superficial rots 0
Other 0 Green 6
Loose skin 30
Cuts 2
Scuffs 16
Wounds 10
Thumbnails 0
Seed Grading and Preparation for Planting 313
details of variety, size, field name, producer and generation, the report indicates the
percentage of:
● Disease (common scab, powdery scab, black scurf, silver scurf, black dot, skin
spot).
● Damage-induced soft rotting.
● Blight-induced soft rotting.
● Damage-induced dry rot or gangrene.
● Blackleg-induced rots.
● Mechanical damage.
● Misshapes.
If the customer knows that there is a trace of blackleg in the seed, he knows
not to grow it on land that could be subject to flooding or where irrigation applica-
tion rates are uneven. Similarly, if there is some silver scurf present, he knows that
he should harvest the subsequent crop early, ensure that it is dried and kept free
from condensation during store loading and wound healing, and make certain the
store is closed up within 3–4 days so that cooling can start as soon as possible.
Documentation delivered along with the seed or with the invoice should advise
customers to inspect the contents of boxes, bags or trucks on arrival and, if necessary,
to dry the seed if it is moist. This should prevent bags being unloaded and left
unopened in the back of a shed for weeks, with the associated risk of disease development
should planting be delayed. The supplier should keep a sample of the seed dispatched
so that he has evidence of the quality of the seed should a dispute arise (Ch12.15).
Experiments by Firman et al. (2004) have indicated that early planting of seed into
cold soils produces more stems than would otherwise be expected. The effect is
similar to that of storing seed at low temperature to encourage multiple sprouting
and the production of small tubers. Optimally, seed should be planted into soils
above 7°C so that growth is rapid and time to emergence is short.
10.8 Summary
Seed storage and preparation for dispatch have aspects that differ from the produc-
tion of ware pre-pack or processing potatoes.
● Seed either needs boxes to separate out the numerous varieties and genera-
tions or a coordinated strategy so that one grower grows all the seed of one
variety for a group.
● Seed storage requires ventilation with an evenly distributed stream of air follow-
ing harvest to ensure that the heat generated from immature crops is removed,
so that subsurface condensation is prevented and the crop is rapidly dried.
314 Chapter 10
● Seed designed to maximize the production of large tubers or small tubers can
now be produced to order using chronological age manipulation techniques.
● If access to seed stores for material for grading is required on a regular basis,
door design or access management should aim to prevent warm humid air
entering stores resulting in condensation on the stored crop.
● Warming of seed prior to grading is necessary if crops are stored at 3–4°C,
both to minimize grader damage and to allow subsequent wound healing.
● As seed may spend many weeks after grading awaiting suitable planting condi-
tions, wound healing along with ventilation to prevent subsurface condensation
following grading are essential.
● Unless dispatched in refrigerated transport, seed should leave the grading area
or store at a temperature above the likely dew-point temperature of the ambi-
ent air through which it will be travelling.
● Seed transported in refrigerated containers or lorries should be allowed to
warm to above the dew-point temperature of the ambient air before opening
doors, particularly in seed sent by sea to warm or tropical countries.
● Chitting of seed for ware production uses heat to initiate apical dominance,
and light and cool storage to control subsequent sprout development.
● The use of pre-grading reports on washed samples, together with instructions
stapled to seed deliveries, helps the customer with subsequent seed manage-
ment, planting and crop husbandry.
11 Packhouse and Processing
Facilities
Potatoes arriving at a grading line, processor or packhouse may arrive from one or
more sources. They may come from:
● The field in a tipping trailer, bulker lorry, boxes or bags.
● Bulk or box potato stores without being cleaned.
● Bulk stores, but via a cleaner/grader located on the farm.
● Box stores with on-farm cleaning/grading prior to dispatch using static, farm-
owned equipment or mobile packhouse-owned systems.
● Dutch-type storage bins adjoining the grading area.
● Box or sack stores on site.
Potatoes for processing may in addition be washed on the farm to minimize the
amount of soil for disposal as a waste from the factory.
©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton) 315
316 Chapter 11
When potatoes are harvested, adhering soil, haulm, damaged and diseased tubers
(known as brock or cull potatoes) will be present along with marketable tubers. As
tubers dry, increasing amounts of soil fall off the tubers whenever they are con-
veyed or handled. If tubers are stored for a period of months, a proportion will
become diseased and unmarketable and tubers will shrink in diameter by a few
millimetres due to evaporative moisture loss. Since packhouses want only market-
able material, cleaning and grading on the farm avoids transporting soil and brock
to the packhouse only for it to be returned to the farm for disposal. However, if
on-farm grading equipment is old and worn, it can cause damage or loss of bloom
to the crop. If this is the case the packhouse may prefer to accept the soil and
brock along with ungraded potatoes, to ensure that the skins of the potatoes keep
their bloom and suffer minimal damage. This waste material is likely to be con-
sidered as ‘trade waste’, which has then to be disposed of at a licensed waste
treatment facility.
Washing of potatoes on farm is only carried out if the crop is to be processed into
French fries, crisps or for ready meals within 5–7 days, as disease can rapidly multiply
on wet skins. Washing will remove any adhering soil and, if carried out in conjunc-
tion with grading, will allow simultaneous removal of brock and separation of
tubers into different size grades. The wash water has then to be passed through a
wastewater treatment plant to remove soil and reduce biological oxygen demand
(BOD) to an acceptable level for either discharge to watercourses or reuse.
Alternatively it can be applied to land using low-volume irrigation. The soil filters
out inorganic soil particles and provides microorganisms which break down the
sugars, starch and organic matter present. It is usually easier and cheaper to dis-
charge dirty water to farmland than to treat it sufficiently to discharge it to sewer
in built-up areas.
Brock potatoes can be fed to livestock on the farm, sold for stock feed or con-
verted to starch if facilities are available.
Potatoes removed directly from cold store will become wet with condensation if
their temperature is below the dew-point temperature of the ambient air. Where
potatoes are being washed prior to immediate use or sale, this is of no consequence.
Packhouse and Processing Facilities 317
In contrast, where potatoes are to be kept in bags or boxes for some weeks after
removal from store, any wetting can result in disease initiation. Ware that is to be
sold dry and which is put in Kraft bags some weeks prior to use is particularly
vulnerable.
Potatoes removed from cold store at 3°C are likely to suffer damage or bruis-
ing during handling or transport. To prevent damage, crops should be warmed to
at least 8°C before they are handled. To prevent condensation after they leave
store, they should be warmed to a temperature above that of the dew-point tem-
perature of the ambient air. A warming arrangement should preferably be designed
into the store, as removal of cold crops from store to a warming area will result in
the crop becoming wet before warming can begin (Ch9.12).
The first decision for grading and packing potatoes is whether this should be done
on the farm or in a central facility. The central facility may be cooperatively
owned, a private stand-alone company or part of a retail potato marketing group.
The advantages and disadvantages of on-farm or off-farm grading are considered
in Table 11.1.
Packhouses are divided into two areas: (i) a dirty area at intake where potatoes
contaminated with soil and haulm enter the packhouse for cleaning; and (ii) a clean
area where the cleaned material is sized, inspected and packed. Staff in the dirty
area usually wear coveralls, while staff in the clean end wear white coats and white
hats, and have to wash hands and change clothes before work. The sampling of
material for quality control is carried out at reception, at critical control points
during the process and prior to dispatch.
A packhouse may include some or all of the following:
● Reception for trailers, lorries, boxes and bags.
● Dry cleaning equipment to remove stones, soil and haulm.
● Washing plant followed by tuber drying equipment.
● Grading equipment to split material into different sizes.
● Inspection tables for examination of material prior to packing.
● Packaging machinery to put material into bags or boxes.
● Quality control laboratory and shelf-life assessment room.
● Pallet stacking equipment for stacking packs or bags on to pallets.
● Cool or chilled areas to hold produce prior to dispatch.
● Storage space for packaging material.
● Printers to produce labels with daily packing codes and display until dates.
● Offices, meeting rooms, staff canteens, changing accommodation and wash
rooms.
318 Chapter 11
Avoids transport of soil and brock Need either to transport soil and brock back to
to and from packhouse farms if permitted or to send them to landfill or
a composting facility
Relatively easy to dispose of dirty Will require wastewater treatment system, if
wash water and soil if washing washing carried out on site and plant is in a
carried out built-up area
Old or low-technology machinery may Can invest in high-technology,
spoil bloom and damage potatoes high-throughput, low-damage
grading and packaging machinery
Provides jobs for farm staff If grading is seasonal, staff may be under-
during winter employed during periods when grading is not
taking place. This can be avoided if casual labour
is readily available
Use of family labour can keep Labour will mostly be hired, at or above minimum
costs down wage, with rest rooms and transport for staff
required
Gain value from existing farm buildings Costly investment in new buildings in possibly high
if they meet supermarket standards rateable value area
Can take out over- and undersize Can establish specialist lines to use the over- and
material to sell locally for specialist undersize material (e.g. punnets, bakers)
high-value markets
Growers with good reputation may get Require intake monitoring system that ensures
premium prices quality material is rewarded and poor material
is penalized
If rural labour is scarce, seasonal staff Town location may ease the getting of staff or be
may be difficult to obtain unless attractive to casual labour
working conditions are pleasant
Quality assurance and traceability Can invest in high-technology computer-based
may cost more per tonne due to quality assurance and traceability systems
small scale of operation
Facility may be too small for Large facilities can supply 365 days per year and
supermarkets to deal with can buy in material to bridge gaps in crop
availability and supply Christmas rush
Farmer has to market produce as Large facility can have its own marketing manager
well as grow it and agronomists
Conveyors or boxes are used to move the produce and waste between these, with
skips or boxes placed by the machinery to accept soil, discarded potatoes, stones
and haulm for disposal.
The packhouse lines illustrated in Fig. 11.1a and b are designed for packing
potatoes into polythene bags, over-wrapped 600 mm × 400 mm × 300 mm free-flow
trays, or boxes for bakers. The free-flow trays hold produce loose, allowing custom-
ers to select the potatoes they want. In the supermarket, rolls of polythene bags are
placed by the free-flow trays, so that customers can fill the bags with the potatoes
they select. In Fig. 11.1a and b, all produce movement is from top to bottom.
Packhouse and Processing Facilities 319
Soil and
Automatic
waste box
box tipper
Bulk hopper
Soil extractor
Waste
conveyor
Pre-soak tank
Wash and barrel wash
line
diverter
Brush washer
Inspection
table
Grader
size 1 Class II
conveyor
Grader
size 2
Grader Dirty
size 3 area
Dividing wall
or curtain
Oversize
Clean
area
Fig. 11.1. (a) Packhouse cleaning and grading line; (b) packhouse packing line.
(Redrawn from plan supplied by R. Balls, consultant, Bedford, UK.)
The first stage in the packhouse system (Fig. 11.1a), the cleaning and grading line, has
potatoes delivered to the packhouse in 1-t boxes, automatically tipped into a bulk hop-
per, then passed over a dry cleaning system to take out soil. The potatoes can be either
delivered by conveyor to the inspection tables directly, or diverted via a pre-soak tank,
barrel washer, brush washer and sponge drier, if potatoes are to be washed.
320 Chapter 11
Rolling lid
box tippers
Empty trays
placed on
conveyor here
Surge Surge
belt belt
Pocketed belt
Check
weigher Polybag
filler Clean
Close free- Empty trays area
flow tray Check on pallets
wrapper weigher
or baker Rotary
box lid table
Rotary
table Place bags
in trays
Roller
Place bags conveyor
Roller in trays
Roller conveyor
conveyor Trays stacked
Trays stacked on pallets
on pallets
Pallets removed Pallets removed
by forklift when Trays by forklift when
full full
The potatoes go over an inspection table, where inspection staff remove rots, mis-
shapes, blemished or green potatoes and transfer lesser-quality Class II potatoes to the
centre belt. The Class I potatoes pass over three grader modules with square mesh
screens, which grade the tubers from small to large. The size grades are diverted into
three sizes (denoted by # on Fig. 11.1a) of grades, to fill separate 1-t interim holding
boxes. The Class II potatoes avoid the grading system and are conveyed to their own
box. A fourth oversize grade may go for catering or for processing.
The waste conveyor carrying soil, stones and reject potatoes delivers the waste
to a box or skip situated beside the intake. A dividing wall, or curtain, separates
the dirty operations of soil extraction, inspection and grading from the clean area
Packhouse and Processing Facilities 321
where clean, sized crop is delivered into separate boxes. Once past the curtain,
there is only clean, marketable produce.
The second stage of the packhouse system (Fig. 11.1b) is all in the clean area. The
clean, sized produce from the cleaning and grading line is delivered from its
interim holding box to box tippers, fitted with rolling lids to minimize the height
of drop when the 1-t boxes are being tipped. The surge belts ensure an even flow
of potatoes is delivered from the tippers.
In the left-hand line for filling supermarket free-flow trays or baker boxes, the
weigher feeder supplies the multi-head weigher, typically having 12 heads, which
delivers accurately weighed lots into the pockets of the pocketed belt cross con-
veyor. The cross conveyor is timed so that each preset pocket lot is delivered into
a lined supermarket free-flow tray passing below on its roller conveyor. As the free-
flow tray capacity is bigger than the pocket lots, the system is set to stop the tray
conveyor to receive a number of pockets per tray; e.g. a 10-kg tray would receive
5 × 2-kg pockets. The lining sheet on filled free-flow trays, or lid flaps on the baker
box, is closed over the top, check weighed, possibly checked for metal contamina-
tion, labelled, and stacked on to pallets. The tray conveyor has a free-rolling end
section, which allows the boxes or trays to accumulate and make palletizing inde-
pendent of conveyor speed.
In the right-hand section of the left-hand line (Fig. 11.b), shown for filling poly-
thene bags or paper sacks, the pocketed conveyor is reversed and discharges into
plastic or paper bags. There are many variants of bag filling systems, ranging from
bags manually placed on to the holders, to equipment which forms the bags from
flat rolls of pre-printed plastic. A labeller automatically prints or adds a stick-on
label to the full bags. These show contents, weight, pack and sell by date and bar-
code. The discharged full bags run along a flat belt conveyor and through a check
weigher and metal detector. Any underweight bags or ones containing metal are
removed. The bags drop on to a rotary table, where they are packed into trays and
palletized. In very large installations the filling of bags into trays and tray palletiz-
ing is fully automated.
The right-hand box tipper and packing line (Fig. 11.b) depicts a very basic
system used for manually filling polythene or paper bags and trays, or for filling
bakers into various packs. This line is very flexible and can deal with almost any
style or weight of pack but does not have the automatic filling that the left-hand
line provides. As with the left-hand packing line, it starts with a box tipper followed
by a surge belt feeding a cross conveyor. This delivers a steady stream of tubers
one layer deep on to the main belt conveyor from which operators pick tubers and
bag or box them, the bag or box being placed on a weigh scale. After labelling the
rotary table conveys the polythene bags of potatoes for packing into customers’
plastic trays, usually 600 mm × 400 mm × 300 mm if the packs are small, or
directly on to the pallet if the packs are large.
For all packhouses, whatever the level of sophistication, there are a number of
questions that need to be addressed. These are summarized in Table 11.2.
322 Chapter 11
Reception Is the crop always received in the same way (i.e. bulk,
boxes, similar sized containers, etc.)?
Is incoming material to be sampled routinely?
Are facilities needed for incoming drivers to rest, wash
and eat?
Cleaning Is soil removed in a dry state before tubers are washed,
essential if the wash water is to be kept clean?
How is the reject material taken away?
How is soil in the wash water kept in suspension?
How is the contaminated wash water treated?
Can the throughput be altered so that pickers on the
roller tables or belts are not overburdened?
Grading What quality control is required?
Where does the second grade product go?
Is individual operator accountability required?
Packing How are the packing materials supplied to the operators
and machines?
Will many different packing configurations be required?
Dispatch Does the end product need to be stored in more than
one way?
Does the area need to be chilled as part of a cool chain?
Are all dispatch vehicles the same height and size?
Is a final quality check required?
In the UK, packhouse operation comes under regulation (EC) No. 852/2004,
Hygiene of Foodstuffs (EC, 2004), which is monitored by the local authority’s
department of environmental health.
The legislation for packhouses covers aspects of hygiene, the Hazard Analysis
and Critical Control Point (HACCP) safe food production system and operator
safety from machines, noise, dust and excessive working hours. While HACCP
is not mandatory for packing potatoes, operators of such primary production
facilities are encouraged to follow HACCP procedures as far as is possible (EC,
2004). Other legislation covers wastewater discharge consents, disposal of solid
wastes and any environmental impact on the surrounding area. The clients of
the packhouse may also have their own additional requirements, which should
be addressed.
Some of the equipment used in packhouses or the intake of a processing plant has
been discussed in Ch2, Harvesting and store loading, and Ch10, Seed grading and
preparation for planting. Further details of equipment may be found in these chap-
ters. Quality assurance aspects are dealt with in Ch12.
Packhouse and Processing Facilities 323
11.4.1 Intake
The primary objective at intake is to ensure that the cleanest sample of potatoes pos-
sible enters the packhouse or processing plant. Soil, stones and waste organic matter
can be put back to the land if separated from the potatoes at the farm. Once this
material is brought to the packhouse or plant, in the UK they come under commer-
cial waste regulations and have either to be processed into a useful product or treated
(e.g. separated, composted, etc.) prior to disposal at a licensed waste disposal site. The
potatoes being delivered to the cleaning and grading line will arrive in bulker lorries
with conveyor discharge, tipping lorries or trailers, boxes or bags. In the UK pre-
pack potatoes will normally arrive in boxes or bags to minimize damage.
The bulker lorry, fitted with a bottom conveyor, discharges potatoes at a rate that the
conveyor supplying the cleaner can accept. In contrast, a tipping lorry or trailer can
rapidly discharge its load into a bulk reception hopper and get on the road again to
collect more crop. The reception hopper acts as buffer storage. While this minimizes
the waiting time for the tipping lorry or trailer, it can lead to potato damage if the
drop to the hopper floor is too high. The size and dimensions of the hopper must
match the trailer or lorry height when tipped and provide a reserve of tubers so that
the cleaning line can still work even with an intermittent supply. Typical capacities are
between 3 and 15 t of crop (Figs 2.5 and 2.7). The hopper is boat-shaped, having a
long and preferably wide moving belt in the base. This belt can either be at a slight
angle to the horizontal along its full length to feed on to the line or be in two parts,
with a section of horizontal belt followed by another inclined upwards.
The supply from the bulk hopper must be readily adjustable so as to provide
an optimum rate of supply to the sorting, sizing and packing operation. The rate
of discharge can be adjusted using a manually operated variable speed motor but
often proximity sensors or electronic eyes are used at buffer points (Fig. 11.2) to
start and stop conveyors instead.
Sensor prevents
potatoes exceeding
this height Elevator boot
Fig. 11.2. Use of a proximity sensor to control flow of potatoes into the intake boot
of an elevator.
324 Chapter 11
There are two types of box tipping systems: one which rapidly tips the potatoes
into a hopper with a slow-moving discharge, the other which slowly tips the con-
tents of the box on to a moving conveyor belt (Fig. 11.3). In the first, the reception
hopper acts as a buffer, so that there is time for the forklift truck to fetch another
box and place it in the tipper before the first box is empty.
In the second, either there will be a gap in supply as boxes are changed over,
or two tippers are used. While one is discharging potatoes, the empty box in the
other can be replaced with a full box. In the UK, these are gradually replacing the
buffer hopper type, as they have been found to do less damage to the crop. Their
tipping speed is controlled using a proximity switch to ensure a uniform discharge
rate to the cleaning and washing equipment. The forklift driver is made aware that
there is an empty box by a flashing light or claxon.
Where possible box tippers discharge at a high level, so that the crop does not
need to be elevated to discharge on to the grader or inspection tables. The result is
a flat grading system, with a minimum of drops that could otherwise do damage.
Some box unloading systems now have the facility to read a barcode or a
RFID (radio-frequency identity device), which is discussed further in Ch12.
Fig. 11.3. Box tipper discharging potatoes delivered from a farm store. (Taypack,
Inchture, Dundee, UK.)
Packhouse and Processing Facilities 325
Box tippers or rotating buckets fitted to a forklift truck or tractor avoid the need
for a stationary tipper. Some need the base of the pallet box to be closed to prevent
the pallet falling off the forks when rotated. Others have an arm above the top of
the box to prevent it falling. The discharge height will be less controlled and if a
bucket is used its edge will cut a small but significant percentage of tubers, which
will have to be removed during inspection. In addition, equipment mounted on
tractor front loaders is awkward to manoeuvre.
If the packhouse requires that potatoes should be delivered in boxes ‘as dug’ (i.e.
harvested directly into boxes and not handled thereafter), potatoes arriving straight
from the field may have large amounts of adhering soil present. This may vary from
moist and sticky to dry and crumbly, depending on the ground conditions at lifting.
Any soil on crops arriving from store should be dry, assuming the store is well-
ventilated. Potatoes removed from bulk stores by bucket can also contain soil. If, in
contrast, crop has been pre-graded on farm or removed from a bulk store using an
elevator fitted with a soil extractor, the amount of adhering soil will be minimal.
Where a considerable quantity of soil is present on delivered potatoes, a dry
cleaning system should be used (Fig. 11.4). The separated soil can then be put into
boxes or skips for return to the farm of origin (Fig. 11.5). This avoids the wash
water becoming too contaminated with soil.
Fig. 11.4. Star wheel dry cleaner followed by endless screen grader. (Courtesy of
RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
326 Chapter 11
Fig. 11.5. Discharge of undersize tubers (left) and soil (right) from dry cleaning
equipment.
There are a number of ways to remove soil and trash from potatoes. In all
cases a compromise has to be reached between the level of cleaning and the poss-
ibility of inflicting bruises or damage to the tubers.
Most modern harvesting systems will remove the majority of stones and clods lifted
with the crop but some may still remain. Stones and clods, especially those with
sharp edges, damage potatoes during transport and when the crop passes over
cleaners and conveying equipment. Stones can also become lodged in cleaning
equipment and graders, causing damage to equipment.
One separation system, which is also used on some harvesters, conveys the
tubers and stones on a rubber pintle (or hedgehog) conveyor belt (Fig. 11.6), which
is inclined so that one side is higher than the other. The denser, smaller stones set-
tle between the pintles and are carried to the end of the conveyor, while the lower-
density larger-diameter potatoes stay on top of the pintles and roll sideways over
the conveyor on to a flat belt conveyor running parallel to the pintle conveyor.
This system is simple and provides a reasonable degree of separation at intake vol-
umes as high as 60 t of dirty product per hour.
An alternative is a rubber-coated rotating drum used to bounce potatoes and
stones into different trajectories, so that they land on two separate belts. As potatoes
and stones have different rebound characteristics, good separation can occur below
Packhouse and Processing Facilities 327
Adjustable angle
to horizontal
Stone Movement
conveyor
Movement
Stones kept by pintles
from rolling off belt
Potato
conveyor
Movement
Conveyor feeding
Movement
potatoes and stones
on to pintle belt
Fig. 11.6. Sloping rubber pintle belt for separating stones from tubers.
the damage threshold (Feller et al., 1987). When used in commercial situations and
on some varieties, these machines have sometimes been found to cause damage so
are now less popular than they once were.
In the past, X-ray separators were used to separate stones and clods from
potatoes by sensing their relative permeability to X-rays, but these were complex,
had health and safety implications when servicing radioactive components, and the
fingers used to deflect potatoes and stones into the two separated streams tended
to damage the potatoes.
A recent development uses a passive capacitance sensor, which acts similarly
to the X-ray system in sensing the density difference between stones and potatoes
but without the radioactive components that posed the health risk. It causes stones
and clods to drop through the floor of the conveying system while leaving the
potatoes to pass unchecked.
If potatoes are to be washed, the use of an upward flow of water ensures that
all potatoes, regardless of their dry matter, will rise and the stones sink (Fig. 11.7).
The potatoes can then be removed from the water free of stones.
Brushes are sometimes used in wet cleaning systems after a soak tank to assist
in reducing soil adhesion to the tuber surface.
Washing potatoes increases the attractiveness and potential sale price of blemish-
free potatoes but shortens their shelf-life. Fingers and Fontes (1999) claimed that
the average shelf-life of potatoes is 30 to 40 days in perfect conditions but reduces
to 7 to 15 days once they have been washed. Washing is therefore carried out just
before sale or processing to minimize the development of disease blemishes or rot-
ting. Were it not for the consumer preference for washed product, potatoes would
be better sold unwashed.
Packhouse and Processing Facilities 329
A heat exchanger may be fitted in a soak tank to act as a hydro-cooler to cool the
potatoes to 3–9°C, to act as the start of the cool chain between packhouse and
supermarket shelf.
Barrel washers
There are a variety of machines available on the market suitable for washing and
drying potatoes (Clarke, 1996), but the most common is the barrel washer in which
the tubers are rolled around inside a cylindrical barrel or drum (Fig. 11.8). The
amount of cleaning depends on the speed of rotation of the barrel, the nature of
its inside surface, and the residence time of the tubers within the washer. The resi-
dence time is adjusted by either changing the angle to the horizontal of the barrel
axis or adjusting the discharge door opening to hold them back. The barrel is usu-
ally about 15% under water so that the potatoes keep falling back into the water.
The barrel provides the agitation, while the water does the cleaning. Tubers can
experience a high level of damage if they are ‘thrown’ against the sides of the bar-
rel but the insertion of a ‘brake sail’ made of a thick plastic material can minimize
the damage by slowing the fall of the cascading potatoes. This both reduces the
potential damage to the crop and reduces the variations in mechanical load imposed
on the barrel by the cascading potatoes (Geyer and Oberbarnscheidt, 1998). It is
possible to have a dry barrel cleaner with sides made of bars to allow the dry soil
to fall through.
Fig. 11.8. Barrel washer fed from a bulk hopper at rear and discharging to an
inspection table in the foreground. (Courtesy of Potato Council, Oxford, UK.)
330 Chapter 11
Fig. 11.9. Sprayed make-up water rinses wash water from clean tubers. (Courtesy
of RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
Packhouse and Processing Facilities 331
bacteria. All the equipment used to hold the wash water should therefore be either
made from stainless steel or coated with a material which will resist attack by any
bactericides added.
A decision has to be made on whether to let the soil settle below the barrel
washer or to try to keep it in suspension by recirculating the wash water using
high-volume water pumps. The tanks below barrel washers are fitted with remov-
able sealed doors so that, if settlement does occur, the water in the tank can be
drained and the wet soil removed using a shovel.
Plant to treat the water prior to reuse as top-up water or discharge to water-
courses will include (Geyer, 1996):
● Sieves to remove large items of organic matter such as haulm and small tubers.
● A series of gravity settling tanks or hydro-cyclone soil/water separation equip-
ment to remove the larger soil particles.
● Lagoons for removing very fine clay particles if these are present in the areas
from which potatoes are sourced.
● Monitoring equipment to ensure water to be discharged meets the water dis-
charge requirements (DETR, 1997).
If the water is to be discharged to a watercourse it will need to be treated in a bio-
logical treatment plant to reduce its BOD and COD to levels agreed by the environ-
ment agency concerned. In some cases, where there is the possibility of diseases
such as brown rot in the water, it is heated to 70°C for 10 min prior to being
placed in the settlement tank.
One way of reducing the heaviest and most easily settled soil and large pieces
of organic matter is to use a trailer fitted with a sieve with a settling tank below
(Fig. 11.10). This allows the soil and organic material to be returned to the field of
origin, where it is tipped, spread out and ploughed in.
If there is plenty of farmland surrounding the packhouse, the final wastewater
discharge can be through a low-volume irrigation rain gun, pulled back and forth
across a field. The soil surface acts as size filter to prevent fine silt entering streams
Fig. 11.10. Dirty wash water screen mounted on a trailer to allow return of settled
soil to the field. (Courtesy of Haith Tickhill Group, Doncaster, UK.)
332 Chapter 11
and as a biological filter to reduce the BOD and COD of the wash water. This
option is not available where packhouses are located in built-up areas or in indus-
trial estates. In any new development, the requirements of the local water authority
as to acceptable wastewater BOD/COD and suspended solids levels should be dis-
cussed before the final site for the packhouse or plant is chosen.
Potatoes should be dried after washing to remove surface moisture. This is usually
achieved using sponge rollers (Fig. 11.11), although more rarely air knives, which
blow the water off the potatoes, are used.
Modern grading and packaging systems are designed to be as flat as possible. This
avoids the large drops that occur when elevators discharge on to belts and reduces
the need for buffer hoppers where potatoes, clods and stones can rumble about and
abrade each other. A consequence of this approach is that potatoes must be
Fig. 11.11. Sponge rollers for removing surface water from washed potatoes.
(Courtesy of RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
Packhouse and Processing Facilities 333
discharged from boxes or trailers at high level (Fig. 11.12), so that there is room
below grader screens for potatoes to fall on to belts for feeding to bag fillers without
the need for secondary elevators.
Although all drops in potato handling should be less than 200 mm, the size of
drop usually increases as the buffer hopper empties. Work carried out by O’Brien
et al. (1980) on fruit and vegetable crop filling systems found that fitting a height
sensor to an elevator with adjustable height control was the most effective way to
limit damage. Where the use of a proximity sensor is not possible, a telescopic ‘zig-
zag’ fall breaker should be considered. By constantly changing the direction of the
potatoes, their downward velocity is slowed.
While rubber cushioning materials do reduce impacts on tubers as they land,
any damp soil from the tubers tends to build up on the rubber, dry to a sandpaper-
like texture and then abrade the skins of the tubers that follow. Static solid areas
of any material over which potatoes have to roll or slide should therefore be kept
to a minimum.
When cushioning is used, it must combine high surface wear resistance with
retention of resilience over a long period (McRae, 1990). Armstrong et al. (1995)
suggest that cushioning material should absorb at least 60% of the impact energy
to minimize product rebound, it must be durable and easy to clean, and the uni-
formity between different production lots should be high. Bollen and Dela Rue
(1995) evaluated a number of cushioning materials and stated that ‘closed cell PVC
foam was the best material with polyethylene foam and neoprene rubber exhibiting
adequate characteristics over the energy range of the tests’. An instrumented sphere
(Zapp et al., 1989) mounted on a pendulum was used to test the various padding
materials at six energy levels between 0.3 and 1.8 J.
Significant damage arises in high flow rate systems from tuber to tuber colli-
sions. While cushioning cannot reduce these, minimizing churning in hoppers or
in the boots of elevators, or preventing changes in direction of conveyors, can.
Frictional or scuff damage to potatoes during handling is an area which has
received only limited interest from researchers. The dynamic coefficient of friction
has been investigated by Schaper and Yaeger (1992) for 25-kg lots of tubers and by
Bishop (2007). Both teams showed significant differences in tuber damage between
a range of surfaces for clean and dirty, and dry and wet tubers. Wet potatoes will
slide more easily than dry and so receive less frictional damage. If tubers are both
wet and dirty, particularly with soil having high clay content, there will be even
more lubrication between the tuber and the handling surface and subsequently less
scuff damage. The potential for scuffing is increased when tubers are transferred
from one conveyor to another if tubers are rotating in the opposite direction to that
of the second conveyor. On landing, the frictional force on the skin is increased
compared with tubers landing without spin (Bishop, 1990).
As most potatoes for the ware or processing sector are washed, the problem of
dust occurs only between trailer or box discharge and the washer. For more infor-
mation on dust see Ch10.4.9, which discusses the problem in relation to seed pota-
toes that are graded dry.
The size of a potato can be measured by its dimensions or by its weight. Some siz-
ing can be done by ‘vision’ grading methods using camera technology and this is
considered under Ch11.10.
Size grading has been, and still is, the most common method of grading potatoes,
even though some dimension methods such as screen mesh sizers have an accuracy
of only ±40% as opposed to an optical weight grading system with an accuracy of
±9% (Glasbey et al., 1988).
The most common form of size grading for ware potatoes is the endless screen,
where the tubers pass over a series of conveyor belt-type square meshes. The size
of aperture in the screens increases with each screen, so that the smallest tubers fall
through first on to flat belt, cross conveyors. There is normally gentle agitation, but
the size of aperture through which the tuber falls very much depends on the orien-
tation of the potato if the variety is elliptical rather than round. If multiple size
grades are required, this type of grader can become very long.
A second form of size grading is the riddle system, where the sieves or riddles
are stacked one above the other with the largest mesh riddle uppermost (Fig. 10.5).
This system is more suited to multiple grades than the endless screen, as it takes up
less horizontal space. The tubers are thrown forward over the riddles by the oscil-
lating action of the drive mechanism, and fall from one riddle to the next until their
Packhouse and Processing Facilities 335
size prevents them falling further. The potential for damage has meant that riddle
graders are losing popularity for ware potatoes and are used predominantly for
seed where the small tubers are less prone to damage.
Diverging roller graders, while expensive relative to endless screen graders, mini-
mize damage to the potatoes. They consist of a series of rollers, rotating like those
on a roller inspection table, but with the gap between the rollers increasing as the
rollers move laterally from one end of the grader to the other. Potatoes being con-
veyed on the moving rollers fall through the gaps when these exceed the size of the
potatoes. Flat conveyor belts below the grader, running at right angles to the flow
of potatoes on the grader, remove the different size grades of potatoes for packing.
Expanding roller graders are similar in principle to diverging roller graders,
but they incorporate diablo section rollers, similar in shape to the diablo roller used
on a potato harvester. These better mimic the square hole of a square mesh riddle
and expand in steps rather than the gradual expansion of the diverging roller
grader. Simply winding a handle can alter the step increase.
An alternative method of size grading is by weight, where the potatoes are fed on to
a conveyor made up of plastic cups, with each tuber having its own cup (Fig. 11.13).
The tubers and cups are passed over a weighing mechanism and routed depending
on weight. This method has been popular in the apple industry for many years.
Although a more costly method than traditional size grading, this system has become
common for specific markets such as for bakers. Weight sizing has the potential to
be more precise as the weight increases as the cube of the radius of a tuber.
11.10 Inspection
Fig. 11.14 Roller-type inspection table to allow all sides of the tubers to be
inspected. (Courtesy of RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
Packhouse and Processing Facilities 337
Fig. 11.15. Tuber inspection using flat belts. (Courtesy of RJ Herbert Engineering
Ltd, Cambridgeshire, UK.)
defects to be removed. Initial work, carried out by Malcolm and De Garmo (1953)
with artificial wooden potatoes, suggested that each operator could inspect a
maximum of 4.2–5.0 tubers per second at an optimum speed past the operator of
0.1–0.15 m/s and a rotation speed of 6–12 revolutions per metre of travel. Hunter
and Yaeger (1970) carried out field trials with real tubers that had been artificially
blemished, concluding that the feed rate could be adjusted to give 0.8 t of defects
to be removed per hour per operator. In tests done by McRae (1985), there was
90% efficiency of defect removal with a total flow rate of 4.3 t/h when there was
a 20% defect level, which can be expressed as the removal of 0.8 t/h.
Work by Bishop and Mortimer (1999) also showed that the forward velocity of
the tubers can be 20% less at the edge as opposed to the centre of the roller table
because of the drag effect of the table’s side walls. Their work also found that the
speed of rotation is influenced by the loading level and whether size grading has
occurred prior to the roller table, as tubers of different sizes are more likely to
‘mesh’ and so not rotate. Overall the investigations indicate that the most efficient
manual removal of defects will be when a roller table is working below maximum
capacity, where less than 20% of the total tubers are expected to be removed and
some size grading has been previously carried out.
Both intensity of illumination and colour of the light are important if maximum
inspection efficiency is to be achieved. The level of illumination should be no less
than 500 lux (Zegers and van den Berg, 1988). While increased levels of lighting to
338 Chapter 11
2000 lux showed no benefit, it is probably worth installing higher levels than 500 lux
to compensate for subsequent deterioration in lighting level due to dirt and age of
tubes or bulbs. In practice the illumination level is often below 500 lux. In work car-
ried out by Hyde (1991), the levels varied in practice between 350 and 700 lux.
Zegers and van der Berg (1988) also suggested that for effective inspection the
colour-rendering index of the light should be 85 or over, where natural light has
a colour-rendering index of 100. A low colour-rendering index indicates a prepon-
derance of a particular colour, which makes it more difficult to see certain defects
on the tuber surface. The most common colour-rendering index found by Hyde
(1991) was 62, for a cool-white fluorescent tube. Details of light output and ren-
dering index are available from lighting data sheets.
The illumination source should be located so that inspection is never carried out in
a shadow or in a light source of varying intensity. Where the fluorescent tubes run paral-
lel to the direction of crop flow, the resultant light intensity tends to vary across the table.
Balls (1986) describes a system where a 4.8-m-long fluorescent tube bank fitted with
reflector above a 3.3-m-long table provided 1300 lux at the centre and 1000 lux near
the sides of the roller tables. Balls also recommended that maintenance staff should
replace the lights after every 6000 to 7000 h, rather than wait until they fail.
Considerable work has been carried out on automatic grading systems at the
research level, where diseases, bruises and damage have been identified using near
infrared reflectance (NIR) techniques (Tao et al., 1990; Gall et al., 1998). Current
commercial systems almost universally use camera technology to grade potatoes.
There are two common methods of presenting a flow of potatoes to camera sys-
tems. The first is a cable transport system, developed in the USA, which uses a sys-
tem of parallel plastic cables to support a flow of potatoes. These pass through a
camera unit which can grade and size individual tubers using image analysis tech-
niques. The system rejects or grades using solenoid-driven fingers which knock
tubers off the cable system on to an appropriate cross conveyor.
The other is a roller table transport system, which presents tubers to a camera
and lighting system mounted overhead. The tubers are separated in the valleys
between adjacent rollers using a singulator (Fig. 11.16) to provide separated tubers.
The rollers rotate under the camera’s active field of view (Fig. 11.17) to allow multiple
images of rotating tubers to be captured. This permits a full inspection of each tuber
surface.
Almost all camera-based vision grading systems use proprietary software to
detect the periphery of each tuber and hence its size and shape characteristics. If
colour cameras are employed, they also use the colour information to assess the
tuber surface for damage or disease. This is achieved by relating the red, green and
blue components of the sequential captured images to identify damage or disease
based on pre-programmed, algorithm-based thresholds.
Rates of throughput have increased over recent years with high-end machine
throughputs in excess of 30 t/h, depending on the size spectrum of the potatoes
flowing through. One machine quotes 16.5 t/h for potatoes sized 45+ mm, and the
same system will run in excess of 24 t/h with material sized 55+ mm.
Packhouse and Processing Facilities 339
Fig. 11.16. Singulator for separating flow into discrete tubers, as required for
visual grading. (Courtesy of RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
Fig. 11.17. Vision grader. (Courtesy of RJ Herbert Engineering Ltd, Cambridgeshire, UK.)
340 Chapter 11
Although there has been much interest in these systems and a number have
gone into commercial packhouses, the uptake for potato grading has been limited.
They tend to be more popular in the grading and packing of fruit, where its higher
value compared with potatoes can justify the high cost of vision grading.
11.12 Packaging
Potatoes for the ware market are sold in bags containing 1 kg, 2.5 kg, 5 kg, 10 kg or
25 kg. All bags are pre-printed with the farm, packhouse or supermarket name on
the label. The 1-, 2.5- and 5-kg bags are made of low-density polyethylene (LDPE)
film, with small holes to allow some ventilation. Alternatively nets made from poly-
propylene can be used. These are more expensive, but give better ventilation,
reduce the likelihood of rotting if the product is washed, but allow more evapora-
tive weight loss. The larger bags are made of natural Kraft, which is a strong
brown paper made from wood pulp. Semi-bleached and fully bleached Kraft, off-
white and white in colour respectively, are less commonly used for fresh produce.
Hessian, sisal or woven polypropylene bags can also be used.
In the UK, the potatoes packed in LDPE plastic bags are usually washed. Potatoes
put into Kraft bags are unwashed and are often marketed as ‘Value Pack’, when skin
quality is not good enough to display the contents as a washed sample in see-through
plastic bags. Double-walled paper bags may be used for strength, and can be filled
directly on the harvester or in the packhouse for supply to wholesalers or retail outlets.
To protect the potatoes in plastic bags from damage, and to facilitate handling,
the bags are put into either reusable polypropylene crates or trays for regular custom-
ers or 20- or 25-kg Kraft bags where there is no system for getting the crates back.
The equipment commonly used for filling the larger 20–25-kg sacks is the cleated-
belt, elevator bagger (Fig. 11.18). To satisfy the Weights and Measures (Packaging
Goods) Regulations 2006 (DTI, 2006), the local authority environmental service’s
trading standards department offers a service for calibrating weighing equipment
and can visit packhouses to check weigher accuracy, should they get a complaint
from commercial purchasers or the public. The most recent regulations specify that
the average weight of bags should be as stated, superseding the previous require-
ment that every bag should be at or above the stated weight.
There are two approaches to ensure that the customer receives the stated weight in
a bag. The first uses a low-cost cleated-belt elevator bagger, which slightly overfills the
bag. The operator then places the bag on a calibrated platform weigher for check weigh-
ing (Fig. 11.19) to ensure that the bag is at or above the weight stated on the bag.
The second type of bagger fills a hopper suspended on calibrated load cells,
which measure the weight of potatoes in the hopper prior to discharging the weighed
batch of potatoes into the bag. More sophisticated designs have a second elevator
for small tubers, which is used to top up the bag to the correct weight. This ensures
Packhouse and Processing Facilities 341
that the bag holds the required weight and is not overfilled. Once the target weight
is obtained the sack is sealed or stitched.
Systems for filling plastic bags or punnet boxes are even more sophisticated. If
a small bag of 1 kg is being filled and it weighs 990 g, another tuber is needed
which may weigh 80 g, giving a 7% overweight for which the packer is not paid.
To reduce this ‘giveaway’, a multi-head bagger is used. Between 20 and 25% of
the required pack weight is fed into one of a bank of weigher heads, typically nine,
and the weight of tubers in each head is measured. A computer decides the opti-
mum combination of heads to give a weight just greater than the final desired pack
weight, and these are emptied into the bag or punnet (Fig. 11.20). The automatic
bagger will record the number of bags/punnets filled and can be interfaced with a
personal computer to monitor production data.
All bags for the supermarket trade, and for many of the other retail outlets,
have to be labelled post packing with contemporary data, including ‘display until’,
‘use by’ or ‘sell by’ date and a barcode which accords with the store till reader.
These data cannot be pre-printed on the bag and have to be added post filling,
either by printing directly on the bag or as a stick-on label; the latter have to be
prepared and printed daily.
In a recent survey carried out by the British Potato Council the preferred way
of buying fresh potatoes was the 2.5-kg bag (BPC, 2004b). The plastic bags are first
filled with 2.5 kg of washed potatoes. These are then put into reusable plastic trays
(Fig. 11.21) or Kraft paper sacks, commonly known as ‘outers’, to ease handling
and provide cushioning from damage. A third machine stacks the Kraft sacks on
a pallet (Fig. 11.22) while a fourth wraps the stack of bags with a film wrapper to
stabilize the bags on the pallet.
Fig. 11.20. Multi-head weigher for filling pre-packs. (Courtesy of Newtec, 5230
Odense M, Denmark.)
Packhouse and Processing Facilities 343
Fig. 11.21. Pre-packs being packed into returnable plastic trays. (Taypack,
Inchture, Dundee, UK.)
Increasingly potatoes are being packaged for specific selling lines, such as boxes
with a known number of baking potatoes (Fig. 11.23) for a catering outlet or 750-g
punnets of salad potatoes for a supermarket customer. The covering films are usu-
ally permeable and sometimes contain a light filter, as punnets may be displayed
in a supermarket under illumination as high as 1000 lux. Potatoes have to com-
pete with simple-to-prepare rice or pasta, so any packaging system which identifies
an idea for a meal and simplifies meal preparation, thereby increasing sales, may
be attractive to supermarkets (Fig. 11.24). Another example, which requires food
processing equipment, is partially cooked roasting potatoes in an aluminium tray,
where one has only to remove the over-wrap before placing in the oven (Fig. 11.25).
Packhouses have to invest in the required packaging or cooking machinery, which
should only be done after discussion with potential buyers.
The use of modified atmosphere packaging (MAP), where the gas composition in
the packaging is altered to extend shelf-life or inhibit disease, is rarely used for fresh
potatoes. When it is used, it is primarily for punnets where the tubers have unset
Fig. 11.24. Microwave-ready tray of salad potatoes, herbs and pats of butter.
skins. Where potatoes are sold peeled for sale to catering outlets, MAP is the nor-
mal mode of packaging.
A limited amount of biodegradable packaging is now on the market that will break
down completely, or almost completely, into carbon dioxide and water over a
period of months. This has considerable potential, as waste packaging can be put
into the kitchen receptacle for food waste for subsequent collection by the local
authority for centralized composting or for composting at home. One of the alter-
native materials used for biodegradable packaging is potato starch, which provides
an outlet for potatoes rejected for the fresh market. At the time of writing, corn-
starch seems the preferred feedstock but there is some commercial work with a
combination of starch types. The present higher cost of this material compared
with oil-based LDPE results in it being used primarily for higher-value organic
product lines.
The packed potatoes are held in a dispatch area until enough produce is available
to fill a lorry. The product is most likely to be washed tubers, held in 2.5-kg bags,
in free-flow trays or in Kraft bags stacked on a pallet. As the potatoes are likely to
be damp from being washed, blemish disease or rots may develop, particularly if
the holding area is not kept cool. Free-flow trays (also called reusable plastic crates,
RPCs) have many advantages in being reusable, easily cleaned, strong and water-
resistant. However, because of their highly permeable sides, they can allow the
product to warm if exposed to warm ambient air.
Potatoes cooled by passing them through a hydro-cooler (i.e. cooled water
tank) prior to packing can be kept at low temperature by wrapping the crates or
bags on the pallets with plastic film. This both stabilizes the crates on the pallet and
restricts warm air from penetrating the crates or bags during their stay in the dis-
patch area. In trials carried out on 2.5-kg bags of potatoes at 4.0°C, packed six to
an RPC and exposed to an airflow 1–2 m/s in a room at 20°C, the flesh tempera-
ture of the tubers warmed up by 3.5°C in 1 h without a film wrap and by only
1.7°C when encased in film wrap. In the former case a 0.5–1.0°C temperature dif-
ference occurred across the width of the pallet.
Packhouse and Processing Facilities 347
11.16 Summary
This chapter has focused on packhouse systems, their layout and the equipment
used. From the discussion above, a number of conclusions can be drawn.
● The packhouse must be able to receive potatoes from either field or store.
● On-farm cleaning and sizing allows soil, stones and organic matter to be
returned to either the field where they were grown or elsewhere on the farm.
● On-farm packing allows the grower to market over- and undersize potatoes as
bakers or salad potatoes, which might otherwise be disposed of as out-grades
by the packhouse.
● Into-store cleaning and sizing at harvest help to ensure a clean sample can be
provided at packhouse intake.
● In UK law, any soil, stones and organic matter that enters an off-farm pack-
house is immediately classified as a waste. Subsequent waste disposal should be
accompanied by waste transfer notes for delivery to licensed waste treatment
facilities.
● The use of old on-farm cleaning and sizing equipment may cause damage and
loss of bloom if used for cleaning or grading material prior to delivery to the
packhouse.
● Centralized packhouses can afford more sophisticated cleaning and grading
equipment and have economies of scale which are difficult to achieve with on-
farm packing.
● In the UK most packhouses are supplied with potatoes in boxes or bags.
● Packhouses have a dirty reception area to receive product and a clean area
after washing to grade, inspect, pack, store and dispatch product. The two
areas are separated by a wall or curtain.
● The intake area will have a covered area to protect delivered boxes from
rain, to allow a buffer stock of boxes to stored; it may have a cleaning and
grading line, and box tippers to discharge material on to the cleaning and
packing lines.
● If potatoes are accompanied by soil, this should be removed using a dry clean-
ing system prior to washing.
● After dry cleaning, the potatoes are delivered into a soak tank, passed through
a barrel washer and then excess water removed by a sponge drier.
● A hydro-cooler may be fitted in the washing system to cool the potatoes to
between 3 and 9°C, to act as the start of the cool chain between packhouse
and supermarket shelf.
● Sizing is carried out using a continuous mesh screen grader followed by inspec-
tion and packing into 2.5-kg or 1-kg polythene bags.
● In larger packhouses, weight or optical graders may be used.
● Automation of packhouse systems simplifies traceability and quality control
and reduces staff costs.
Packhouse and Processing Facilities 349
● Bags are loaded into trays, which are stacked on pallets and stabilized using a
plastic film wrapper.
● Pallets may be stored in a chill prior to dispatch.
● A number of different selling lines, targeted at the convenience buyer, are
being tried to add value to potato sales.
● MAP is used for sales of peeled potatoes or new potatoes with unset skins.
● Biodegradable packaging enables it to be disposed of along with food waste
but at present is used mainly for higher-value organically grown product.
● Reject potatoes are best fed to livestock and should be free from plastic, soil
and stones.
12 Quality Assurance
12.1 Introduction
350 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Quality Assurance 351
The chapter concludes with a discussion on the relative merits of paper trail
versus computer-based quality control systems and emphasizes how important it is to
maximize useful information collected while minimizing form-filling paperwork.
12.2 Quality
The packhouse’s quality control system starts with the packhouse procurement
manager assessing potatoes while they are still in the field or store. Quality control
then follows the crop through the packhouse and finishes by monitoring samples of
potatoes dispatched to the supermarket distribution centre or merchant.
The most appropriate definition for the quality of potatoes is ‘suitability for a
particular use’ (Abbott, 1999) or more fully ‘quality is the extent to which the
product comes up to the users expectations’. It is important to define quality so that
customers know precisely what they are buying. A customer buying ‘value-pack’
potatoes in 10-kg Kraft bags may not be concerned about skin blemishes, which in
a pre-pack could result in the product being left on the shelf unsold.
There are emotional as well as analytical considerations. A bright shiny skin
or ‘bloom’ may attract the purchaser to buy, but is of little real benefit if the
potatoes are to be peeled. Such qualities may require sophisticated instrumenta-
tion to measure. The relative importance of different quality attributes can change
from harvesting and handling, to purchase from a grower to purchase by the final
consumer. The harvesting team may concentrate on minimizing damage and
picking off any rots, while a buyer may be looking for a clean, soil-free sample
with a good bloom. The final customer may search the supermarket shelves for a
bag of small sized, good-tasting potatoes, which look attractive on the plate.
The easiest quality standards to use are those like size, which is simple to
control and easy for quantifying out-grades. Temperature of product is normally
another, but in packing potatoes compared with cooking food, is of lesser import-
ance. Disease and damage are much harder to define, as removal of excessively
damaged or diseased material on a conveying system usually involves a rapid
visual assessment by pickers on the inspection line. What is often required is a
simple accept/reject approach, which the ‘would I buy it?’ approach solves
(Ch12.11). This is by definition subjective, and has to be backed up by disease
and damage assessments so that staff on one shift can apply the same standard
as that on the next. Such a standard may alter depending on the general quality
of the potatoes coming on to the market at the time. Shoppers may select a
blemish-free sample in preference to one covered in silver scurf, but if all items
on sale have silver scurf they may just select the samples that look best.
Research on consumers’ approach to taste quality is limited. However, BPC
and other trade associations do run taste panels on new varieties and are actively
funding work on what biological and genetic issues contribute to taste (Winfield
et al., 2005). There are increasing numbers of studies of consumers worldwide to
identify what influences customers to choose a particular variety, size grade and
pack (Krueger, 1994; Moskowitz, 1994). The combination of consumer panels
together with an increased understanding of the science behind taste, texture,
352 Chapter 12
colour and cooking performance is helping to identify the aspects of potato quality
that are required for the different markets (Connor, 1994).
● Crop production practices should not adversely affect soils, watercourses, bio-
diversity or the wider environment.
● Waste materials, pesticide runoff and reject potatoes with associated botanical
matter should be minimized and their disposal should not harm the environment.
● Operator health and safety practices should be documented and operators
trained and tested in their operation.
The quality assurance scheme therefore encourages good crop management and
the production of product to a defined quality standard while discouraging prac-
tices like the routine prophylactic use of chemicals, the disposal of chemicals to
watercourses, the burning or burying of plastics and the use of excessive and
unnecessary packaging.
Fig. 12.1. Tuber samples from intake material are washed, peeled and cut in half to
assess any disease, damage, bruising or hollow heart. (Taypack, Inchture, Dundee, UK.)
354 Chapter 12
Fig. 12.2. Pre-packs stored under lights for 8 days to replicate a supermarket
display cabinet. (Taypack, Inchture, Dundee, UK.)
On the walls will be charts for identifying disease, tables showing maximum levels
for blemishes and rots, and charts for identification of the fry colour of French fries
and crisps.
Adjoining the laboratory is the quality control office, where the quality assur-
ance manuals are kept. It is here that the most up-to-date reference manual is kept.
This is where the paper records, if paper is still used, of previous samples of prod-
uct are kept and the computers holding the data of material processed over the
preceding year are located. In the laboratory or office, there will be a series of
shelves (Fig. 12.2), holding bagged samples of produce kept at room temperature
until their ‘use by’ date expires. At the room temperature of 20°C, these samples
give early warning of potential rotting or development of blemish diseases in pro-
duce sold, so that the retailers can be warned early of potential problems with their
produce on display.
Samples will be brought to the quality control laboratory by fieldsmen, the
potato procurement manager, or by agronomists and quality control staff who are
constantly on the road visiting farms and stores.
HACCP is entirely concerned with food safety and is a legal requirement within
the EU (EC, 2004). If potatoes are sold to a packer rather than to a merchant,
agent or broker (Fig. 12.3), the produce will be packed using HACCP procedures.
Quality Assurance 355
Merchant or
1
te agent/broker
Rou
Route 2
Grower Retailer
Ro
Ap upp
ut
pr lie
e
s
HACCP
ov r
The packhouse will be subject to regular inspection by the supermarkets they sup-
ply to ensure that these procedures are being maintained.
The main objective of any HACCP system is to ensure that, in a continuous-
flow operation with a series of stages, produce cannot move on to the next stage
unless it has met the requirements for the previous stage or group of stages. The
requirements, or critical limits, form critical control points. Should the produce fail
a critical limit, there is an action programme designed into the system to divert the
product from the production line. HACCP is therefore a systematic approach to
the identification, evaluation and control of food hazards.
There are seven principles, or steps, in this systematic approach (CCFH, 1997).
1. Conduct a hazard analysis.
2. Determine the critical control points.
3. Establish critical limits.
4. Establish monitoring procedures.
5. Establish corrective action.
6. Establish verification procedures.
7. Establish record-keeping and documentation procedures.
A key requirement for a critical control point is that the measurement must be able
to be carried out quickly, to ensure that unacceptable product is immediately
diverted away from the normal production line. This is simple where produce has
to achieve a specified temperature but is more difficult if, for example, produce has
to achieve <2% bruising.
A detailed description of how to implement HACCP procedures in any spe-
cific situation is beyond the remit of this book, but it is important to stress that
any HACCP procedure must be fully supported by the senior management.
There will be occasions when material fails to reach the standard required, which
will incur loss of income. It is, however, better to lose some income than to lose
a reputation for quality produce. The stages in a HACCP study are summarized
in Table 12.1. An example of a flow diagram for pre-packing potatoes is shown
in Fig. 12.4.
One of the weaknesses of some HACCP plans is that once they are developed
they are not updated. So it is important to build into the plan a verification sched-
ule to validate and review the procedures.
Various organizations provide training in HACCP procedures and certifica-
tion is available from organizations such as the Royal Institute of Public Health
and Hygiene (London, UK).
356 Chapter 12
Table 12.1. Development of a Hazard Analysis and Critical Control Point (HACCP)
plan.
Action Detail
Pre-cleaner
Boxes Box
to remove Washer Grader
in tipper
dry soil
Inspection
Bags into Trays on
roller tables Bagger
trays to pallets
or belts
12.7 Traceability
Growers supplying packhouses or processors will have their own traceability sys-
tems. These document:
● Seed source, variety and generation used when planting a field.
● Field name or number used to grow the crop.
● Seed treatment, herbicides and other pesticides applied to the growing crop.
● Irrigation applications.
● Harvest date.
● Final destination or location in farm store.
Packhouse or processor staff purchasing crop from, or packing crop for, the grower
require access to these data should a problem occur, but will not necessarily store
the information itself. Packhouse or processor staff will start monitoring the crop as
it is delivered for packing or processing, or will visit stores filled with their source
material.
Quality Assurance 357
In the UK the BPC Ware Prescription is the statutory standard for quality and size
and its quality regulation states that the total of damage, blemish, misshapen or dis-
eased tubers must not exceed 5% of the sample by weight (Gall et al., 1998). While the
quality of potatoes should agree with the BPC Ware Standard, the seller may deliver
up to a maximum of 10% faults. According to the London International Financial
Futures and Option Exchange (LIFFE), where the proportion of faults exceeds 5%, the
seller ‘shall make an allowance to the buyer of 1% of the settlement price for each 1%
or part thereof of the faults over 5%. Dry matter shall be a minimum of 18%.’
Bulk crops have to be identified by the field or the store they came from. Unlike
potatoes in boxes they cannot be identified once they have been moved, so strict
recording of loads is necessary.
Boxes are always labelled (Fig. 12.5). Information required for stacking in store
should be printed large enough for the forklift driver to read from his seat. In seed
crops the label will usually include the:
● Variety.
● Generation (e.g. Super Elite generation 2, i.e. SE2).
● Name or number of field.
● Name of grower.
The label is either stapled to boxes in the field as they are loaded or as the
boxes enter the store or packhouse. The loss of a label can render the box contents
worthless.
Boxes can be identified by:
● Label with information printed on it.
● Barcode label plus some printed information.
● RFID fitted to the box.
The label system is cheap, simple, easy to read and can be in a colour to differenti-
ate varieties or generations, but the amount of information is limited. Individual
boxes are rarely identified. Most of the information has to be kept elsewhere.
Barcode labels can now be produced on a portable computer fitted with a printer
as the boxes leave the field or enter the packhouse or store. The label can have
some typed information on it so that farm staff can identify the boxes visually. The
barcodes allow individual boxes to be identified, detailed data of the contents of
each box to be kept on computer and rapid reading of box number prior to it
being tipped to empty.
An RFID fixed to each box allows the box to carry its own information. It acts as
a transponder, being able both to transmit and to respond to an external electronic
receiver/transmitter. It therefore needs no battery. Details of the box’s contents,
weight and history can therefore be stored on the RFID itself and the information
subsequently read by another receiver/transmitter fitted on the box tipper at intake
to the packhouse or by an operator. The RFID can also be used to identify owner-
ship and history of the empty boxes. Though more expensive than the label and
barcode systems, it has greater capacity for storing and conveying information.
However, staff without a receiver cannot identify the contents of the box. The use
of RFIDs is widespread in the retail trade where it is a proven technology. Their
size can be as small as the full stop on this page, but normally with potatoes they
are larger and have a bigger aerial so that the transmission range is greater.
Quality control requires critical limits to be set and crop to be diverted away from
the product stream should these limits be exceeded. These limits may be <2% area
Quality Assurance 359
of tuber skin to be covered with silver scurf lesions, or <10% common scab, or
<1% greening. Because the location of each growing tuber in the ridge is different
(Fig. 1.14), each tuber experiences different environmental conditions. To get an
average with a low standard deviation, a large number of tubers have to be sam-
pled. Sampling predicts the average quality of a batch and possibly the quality dis-
tribution (Abbott, 1999), but it does not identify that a few tubers in the sample
may be totally rotten or severely deformed while some may be of very high quality.
Averages may need verbal statements added.
The number of tubers required for detecting a disease or defect is shown in
Table 12.2. For example, if the number of rots should not exceed 2% of the prod-
uct, 150 tubers should be assessed. This would best be sampled in three separate
sub-samples of 50 each to act as replicates. The results will be in the 95% confi-
dence level.
20 15 45
10 30 90
5 60 180
2 150 450
1 300 900
360 Chapter 12
Fig. 12.6. Size gauge to check samples are within specification. (Taypack, Inchture,
Dundee, UK.)
Random samples of tubers are checked for size (Fig. 12.6) and the various grades
weighed to find out the percentage in each grade. The actual sizes used are
matched to the intended market.
The traditional damage index is based on the amount of tuber flesh or pulp that
will be lost when a tuber is peeled to remove the damaged area (Robertson,
1970).
A sample of 100 tubers taken at random is divided into four categories:
● Undamaged.
● Scuff damage.
● Peeler damage (<1.5 mm deep).
● Severe damage (>1.5 mm deep).
The damage index of the sample is calculated as in Ch2.3.1.
While the above damage index is simple to understand and determine in the
field, store or grading area, it fails to identify how the damage is being caused and
omits any mention of bruising. A more rigorous approach is given in Table 12.3
(Bouman, 1995, 1996).
Quality Assurance 361
Surface
Damage Severity measurement Depth (mm) Definition
% of tuber
surface
Scuffing None 0 0 The skin (epiderm) is damaged
Slight 0–10 0 and removed partly or totally
Moderate 10–20 0
Severe >20 0
Area of
tuber (cm2)
Cuts None 0 0 Part(s) of the tuber is (are)
Slight 0–1.7 0–1 sheared, or shearing may
Moderate 1.7–5.1 1–2.5 be accompanied by loss of
Severe >5.1 >2.5 tissue
Crushing None 0 0 Rupture of tuber tissue caused
Slight 0–1.7 0–1 by compressive forces
Moderate 1.7–5.1 1–2.5
Severe >5.1 >2.5
Splitting None 0 0 The tuber has splits in the
Slight 0–1.7 0–1 flesh, propagated from the
Moderate 1.7–5.1 1–2.5 surface
Severe >5.1 >2.5
Bruising None 0 0 Subsurface damage to
Slight 0–1.7 0–2 the tuber tissue, which
Moderate 1.7–5.1 2–5 subsequently causes blue/
Severe >5.1 >5 grey to black discoloration
of the flesh
When tubers are hit by stones or other tubers, dropped on to a hard surface or
exposed to pressure damage from the weight of crop above, bruising damage (blackspot)
362 Chapter 12
to cells below the skin can occur. Blackheart in contrast is caused by depleted oxygen
in the atmosphere surrounding the tubers, causing cells in the centre of the tuber to
die off. Both are invisible to inspection staff unless the tuber is sliced open. Bruised
tubers are therefore very difficult to remove on a packhouse inspection line; so whole
crops are often rejected even if only a few bruised tubers are present.
Bruising may take 3–4 days to develop into its black or greyish appearance
and can have a serious impact on crisp quality (Fig. 12.7). To identify potential
bruising from mechanical damage, samples can be put into a hot box (Fig. 12.8)
and kept at high RH by circulating air over a water bath. Bruises show up within
12–14 h. Samples of 25 tubers are placed in the hot box overnight. By morning the
bruises should be visible. The hot box (Fig. 12.8) is maintained at a temperature of
34–36°C and RH of 95–98%. It has shelves to hold the sample trays, a heater ele-
ment to keep the box warm, a wet wick to produce the high RH and a circulating
fan to reduce temperature differentials within the box.
To assess bruise damage, the tubers should be peeled to reveal bruising. The
categories are:
nil bruising = no bruise
slight bruising = removed in less than two peels (<3 mm)
severe bruising = needs more than two peels to remove (>3 mm).
The hot box method is simple, effective and can also be used to determine the risk
of crop breakdown in store.
Another way of increasing the speed of bruise blackening is to hold potatoes
in oxygen maintained at 150 kPa (1.5 bar) pressure at 37°C. Bruising will become
apparent within 7 h (Duncan, 1973). When the oxygen was humidified the time
was reduced to 5 h (Melrose and McRae, 1987). However, owing to safety and cost
Fig. 12.7. Crisps with blemishes due to bruise damage. (Courtesy of Potato
Council, Oxford, UK.)
Quality Assurance 363
Fig. 12.8. By placing samples of potatoes at intake in a hot box overnight, bruising
or rotting becomes visible within 12–14 h. (Taypack, Inchture, Dundee, UK.)
considerations, compressed air at 300 kPa (3 bar) was used instead. This showed up
bruising as rapidly as when oxygen was used (McRae and Melrose, 1990). However,
due to the complexity of these methods and the relative simplicity of the hot box,
growers and packhouse staff routinely use the hot box for rapid bruise detection.
Growth deformities and pest damage are normally estimated from visual inspec-
tions of samples, with the occurrence of each being calculated as percentages.
12.11.6 Disease
Since the growing and storage conditions of the individual tubers in a crop are
never completely uniform, disease and blemishes will occur in some tubers but not
364 Chapter 12
in others. To assess the severity of a single disease, the presence or absence of the
disease on a number of individual tubers is recorded and expressed as a percentage
incidence. Where the tubers have a blemish disease, such as silver scurf, the per-
centage of surface area affected by the disease on each tuber is recorded and used
to calculate an average percentage area affected.
12.11.7 Bloom
Bloom is the ability of the skin to reflect light and so give a bright, shiny appear-
ance rather than a dull, matt look (Ch1.3). While this is purely cosmetic, supermar-
kets want a bright bloom as consumer research suggests that customers prefer
potatoes with a good bloom. Bloom meters (Bowen et al., 1996) are available (Fig.
12.9) only on an experimental basis at present, but these may become standard
pieces of equipment in future.
Potatoes that rot on the supermarket shelf are one of scenarios that packhouse staff
fear most. Organisms that cause rot can be present within the flesh of tubers at
harvest, but may not show up as rots until days or weeks later. When the possibility
of rot is present, the same hot boxes used to accelerate bruising, but kept at 20°C
rather than 36°C, can be used to accelerate rotting. Samples are usually kept in
the hot box for 12–24 h.
With all these tests, it is possible to lose sight of the critical test: would I, as a
supermarket customer, buy these potatoes? One packhouse uses this as their stand-
ard. Potatoes are given three qualities:
0 = rots present
1 = poor grade
2 = buy grade.
This provides a simple accept/reject decision, which is easy for staff to operate. It
needs to be backed up by the previous tests, as growers will not like material they
supply to be rejected on such a system without more detailed explanation.
Sugar content is crucial in potatoes destined for the crisping or French fry markets.
Assessments are made prior to harvest as levels reduce with time and so dictate
harvest date. Levels increase in store with time or if held at too low a temperature,
resulting in darker fry colours the longer they are stored.
assessment provides valuable feedback to the grower, which can lead to changes in
husbandry in future seasons.
With certain high-risk crops such as new potatoes, lightly set salad potatoes or
loose skin bakers, growers are required to identify specific risks. These have a red,
amber and green classification so that packhouse staff can deal appropriately with
the crop.
Growers not in the core group should adopt this approach, as this practice
may increase the chance of their being asked to join the core group of suppliers. It
also informs other buyers of the quality of the material being supplied and reduces
the likelihood of unwarranted rejection of loads in time of excessive supply.
Monitoring crop in, soil and trash removed, over- and undersize material, dis-
carded potatoes and final product dispatched is a mass balance exercise (Fig.
12.12). While not strictly necessary for quality assurance, it is a vital management
exercise. It allows growers to be rewarded or penalized on the basis of percentage
of saleable product of the potatoes they have supplied. It also allows targets for
improvements in crop quality to be set and feedback as to whether these targets
have been met. ‘If you don’t measure you can’t manage.’
Quality Assurance 367
Fig. 12.11. Tuber dry matter determination: (a) core sampler; (b) balance for
weighing core sample. (Courtesy of Martin Lishman, Lincolnshire, UK.)
368 Chapter 12
Potatoes
Pre-
and soil in Washer Grader
cleaner
(X tonne)
Bagging
Roller table Dispatch (Y tonne)
unit
Grading by sieve has been the traditional method of sizing potatoes and suits
continuous-flow production. Weight grading is also suitable but is more expensive
and tends to be used for the more valuable grades of potatoes such as bakers. Optical
grading, using cameras to measure tuber size, offers the potential to include disease
monitoring on the continuous flow of produce. This meets the HACCP requirements
of being able to automatically divert sub-standard tubers from the main flow of
produce. Disease recognition is, however, still in the development stage.
Quality Assurance 369
Until optical grading systems become more common, sampling will continue to be
carried out visually by inspection staff using tables like Table 12.4 above. This
sampling has to be carried out at intake on sample batches, as it is not possible to
do as a HACCP critical control point.
Most packhouses will use a quality checklist similar to that shown in Table 12.5.
The end user may have their own format with the range of factors reduced to those
likely in that particular season.
The list is extensive but in most cases a zero can be put in most of the boxes.
The system is designed to ensure that the same result would be obtained by who-
ever carries out the assessment, so that it is objective rather than subjective.
Grower
Date Variety Destination Identity no. A.J. Monks
06/04/08 Maris Piper AJC Ltd AJM 2398 & Sons
Factor Sample 1 Sample 2 Sample 3 Sample 4
Sample number 2398/1 2398/2 Etc.
Time (hours) 10.30 10.40
Correct variety ✓ ✓
Correct label ✓ ✓
Correct label position ✓ No
Display until 12/04/08 12/04/08
Intact seal ✓ ✓
Stated weight (g) 1000 1000
Actual weight (g) 1045 1095
Number of tubers 19 17
Size range (mm) 35–45 35–45
Oversize 0 1
Undersize 0 0
Temperature 8°C 7°C
Skinned 8 7
Misshape 0 0
Damage 3 1
Pressure bruising 0 0
Bruising 0 0
Green 0 0
Slugs 0 0
Soft 0 0
Soil adhesion 0 0
Sprouting 0 0
Growth cracks 0 0
Silver scurf 0 5
Black scurf 0 0
Black dot 0 0
Skin spot 0 0
Scab 0 2
Checker’s initials RSJ RSJ
A great deal of information is therefore collected on a routine basis. There is, how-
ever, a tendency for data to be collected by individual staff members but not shared.
The procurement manager may have a clear assessment of the quality and type of
crop available to him but this information may not be available to the sales force.
Opportunities for sale of specific crops may therefore be lost. The quality control
staff may be recording increased amount of damage on crops at intake, but the
procurement manager or growers may be unaware of this until it is too late to
tackle the problem.
Much information that could be useful in identifying problems of disease devel-
opment or adverse sugar levels, such as store climate records and crop inspections,
Quality Assurance 371
may never be used. The advent of store data loggers often results in store climate
data being logged automatically but rarely examined. The results of inspections of
stored crop by store managers may remain in their pocket notebooks, but never
accessed unless a major crisis occurs.
A well-set-up quality assurance system will aim to provide maximum informa-
tion to all staff with the minimum of unnecessary form-filling. It should:
● Provide staff with a method of making collected data immediately available to
other staff within the organization.
● Identify the origin or previous store location of any particular batch of
potatoes.
● Allow quality control staff to find where and why problems occurred.
Where possible the system should not require information collected to be retyped
into a database, as this can result in delays in making the information available. It
is usually better to have paper forms, which can be instantly inserted into a file.
The problem with even the best paper-based system is that the information is
likely to be held in different locations, requiring the troubleshooter to visit numer-
ous offices. Information may be distributed among the procurement manager, the
quality control department, salesmen and the grower’s farm office. This is where
computers can help.
With computers, all permitted staff can access the collected data in the comfort of
their own office. If the system is set up well, troubleshooting can be easy.
Successful computer systems require staff to enter information into the sys-
tem as it is collected, preferably on to a hand-held computer, which may also
provide a prompt for the information required. The computers have to be
robust to cope with being dropped in mud or rained upon. The computer
shown in Fig. 12.13 can transmit data to the packhouse via a telephone link, can
be used as a mobile phone, can have a Global Positioning System installed, and
can be fitted with a barcode or RFID reader. With such a system the informa-
tion fed in can become immediately available to other staff. In contrast, paper-
based systems are unavailable to others until entered on to the computer
network. With busy people, instilling a regime that updating records is more
important than solving the latest crisis is difficult. A combination of sufficient,
but not excessive, data recording, backed up by office staff available to assist
record-keeping, should be sufficient to ensure material is entered into the com-
puter system on a regular basis.
With any record-keeping system, it must be simple, it should only record infor-
mation that will be used and it must have the backing of staff. The ability to
upgrade the system in response to suggestions by staff is vital.
372 Chapter 12
12.17 Summary
Quality assurance, due diligence and traceability have become key components of
any food packaging organization. With particular reference to a packhouse for pre-
pack potatoes, this chapter has discussed the following:
● Quality assurance, the scheme adopted by the packhouse to ensure product
quality is maintained.
● A traceability system between field and product dispatch to facilitate trouble-
shooting and reassure the public that due diligence is being practised.
Quality Assurance 373
● Quality control, the sampling and monitoring system used to ensure product
lies within the quality quoted.
● HAACP, the system whereby material on a continuous-flow packing line can
be diverted from the line should it fail a critical control point test.
● The use of labels on boxes, or documentation with bulk loads, to track product
once it leaves the field.
● Sampling procedures for monitoring potatoes dispatched by the grower and
taken in by the packhouse.
● The benefit of monitoring packhouse intake, output, discards, soil and trash,
to establish a mass balance, useful for improving management.
● Sampling of the final product together with shelf-life monitoring.
● Improving accessibility by staff troubleshooters to information collected and
recorded by other packhouse staff.
13 Marketing and Costs
The sale of potatoes directly from the field has advantages and disadvantages. The
advantages are:
374 ©CAB International 2009. Potatoes Postharvest (R. Pringle, C. Bishop and R. Clayton)
Marketing and Costs 375
The onset of cold winter weather may force growers to provide storage, or storage
may be a marketing decision. Storage may be built to:
● Protect the crop from frost over the winter period.
● Prevent the crop sprouting prior to sale.
● Take advantage of the rise in potato price as time from harvest increases.
● Allow sale of crop at the optimum price over the storage period.
● Satisfy processors’ and packhouses’ requirements for a continuous supply of
material 365 days per year.
If storage is in field clamps or low-cost structures, the decision to store will cost little
and so is easily made. If it involves the building of a sealed, insulated, environmen-
tally controlled store, this is a major investment decision. The annual cost of a store
built specifically to supply potatoes at the end of a storage season, when prices are
high, may become financially uncompetitive if imports from countries in other cli-
matic zones undercut the price of the stored material.
Prices of potatoes can rise or fall by large amounts in years of shortage or glut;
the crop has an ‘inelastic’ response to demand in that a small shortfall in supply
can result in a disproportionate rise in price. However, high prices cannot be relied
upon in years of shortage as shoppers may simply change to rice, pasta or flour-
based foods. In the UK in 2004 the price of ware potatoes ex-farm was only £6/t
more in May than at harvest the previous October. Growers would have obtained
only £6/t for storing potatoes (Fig. 13.1). In 2003 and in 2005, however, the differ-
ence was £58 and £42/t, respectively.
There are few alternative uses for redundant potato stores, so they should be
built or modernized only if they fit into the supply infrastructure. In the UK the main
376 Chapter 13
220
200 2003
180
Ex-farm price (£/t)
160
140
120 2005
100
80 2004
60
40
1-Jul
29-Jul
26-Aug
23-Sep
21-Oct
18-Nov
16-Dec
13-Jan
10-Feb
10-Mar
7-Apr
5-May
2-Jun
Week from 1 July
influence over the last 20 years on farm storage has been the increasing size and spe-
cialization of potato and packing enterprises. Economy of scale is the driving force.
While world potato production appears static, this is due to the 1.8% per annum
decrease in production within industrialized countries counteracting the 3.8% per
annum rise in production in developing countries. Table 13.1 shows the main potato-
producing countries, in decreasing order of importance, for the different regions of
the world. Data of imports and exports of primary commodity show where competi-
tion from other countries can threaten the profitability of producers who store pota-
toes. In the industrialized countries 12% of production is exported, while in developing
countries the quantity is less than 2%. In developing countries potatoes are largely
consumed where they are grown. What the table does not show is competition from
growers within their own country, where different climatic regions allow potatoes
direct from the field to compete with material coming out of store.
Near the equator, two or sometimes even three crops of potatoes can be grown
each year. This reduces the length of time that harvested potatoes need to be
stored. With two harvests per year, with some being early potatoes and some left
in the ground after they are ready, storage need be for only a couple of months.
This allows simple, low-cost, rustic stores to be used (Ch5.1). Sample weather data
for different climatic areas are included in Ch3.6 and in more detail in Appendix
2 to assist strategic decisions as to whether the climate allows storage in the field
or when storage should be provided.
Marketing and Costs 377
Table 13.1. Potato production, growth rate, relative importance and trade for the main potato-
producing countries of the world. (From FAO, 2006.)
Trade (×103 t)
Production (×103 t) Annual growth (%) Yield (t/ha)
Imports, Exports,
Country 2005 1996 1996–2005 2005 2005 2005
Crops which were once seasonal are increasingly available all the year round.
Strategies to provide a continuous supply of potatoes include:
● Storage for 6 to 9 months during which sprout growth is suppressed and disease
development minimized by ambient-air cooling, refrigeration or sprout suppres-
sants to control sprout growth.
● Storage for 1 to 2 months using non-environmentally controlled rustic stores
or cellars to protect crops from insects and animals.
● Transportation of produce from: a growing area at different latitude or a
growing area at different altitude.
● Combinations of the above.
If the growing costs in the home country and exporting country are similar, the
cost of transport compared with the annual operating cost of storage will determine
which is the more competitive. If the producer country has lower production costs,
due to low mechanization and labour costs (Fig. 13.2), this will favour imports
against storage at home.
Since crops lose moisture during storage, which tends to reduce bloom and
firmness, the transported crop is likely to have superior skin finish to the stored
crop. It may therefore attract a premium. This too has to be taken into account
when evaluating the costs and benefits from building new storage.
Fig. 13.2. Low production costs and seasonal difference allow Egyptian exports to
compete with UK-produced potatoes. (Courtesy of D.C. Inglis, Velcourt Ltd, Suffolk, UK.)
Marketing and Costs 379
To calculate what storage and grading will add in cost to every tonne of potatoes
sold, the capital fixed cost of the building and equipment, and the variable costs of
energy, equipment maintenance and labour should be added. This total sum is
then divided by the total tonnage that passes through the facility to find a cost per
tonne stored and graded.
Fixed costs are those which are borne by the producer regardless of whether or not
a crop is stored or graded. Fixed costs for a store and its associated grading equip-
ment include:
● Capital cost of the building and ventilation equipment.
● Capital cost of the grading equipment.
● Interest on the money borrowed to pay for the building and grading equipment.
● Insurance cover for the building and equipment.
● Any rates or fixed payments to government or local authority.
All the above costs are converted to an annual charge to the business, as it would
be unrealistic for the business to pay the whole cost of an investment in a single
financial year. The annual charges are calculated as shown below.
Depreciation
£100,000
Depreciation per annum = = £5,000
20
Interest charge
In the year of purchase a sum equal to the whole capital cost of the building or
machine has to be borrowed. If the machine is written off over a period of years,
the amount of the loan outstanding in the last year of write-off is zero. The aver-
age interest charge per annum is therefore the average of the capital sum bor-
rowed in the first year and last year multiplied by the interest rate paid to the
bank for the loan.
380 Chapter 13
Insurance
The insurance is proportional to the capital cost and type of building or machine.
An approximate cost of insurance is 1% of the capital cost of the building or
machine, though a more precise figure should be sought for detailed costings.
Employer’s liability insurance may have to be included if this is a completely
new facility.
3.0 + 0.5
Repair cost = £30,000 × = £1050 per annum
100
For buildings, repairs and maintenance costs are approximately 1% of capital cost.
Straight-line depreciation can be criticized for the annual charge not following the
curved drop in value of an investment that occurs in practice. Instead, the following
formula can be used:
⎛ r (1 + r )n ⎞
Annual charge = C ⎜ ⎟,
⎝ (1 + r ) − 1⎠
n
where
C = capital investment
r = rate of interest (e.g. 0.08)
n = years of repayment.
This annual charge combines both depreciation and the interest paid on borrowed
capital, so a single figure replaces the two used in the calculation in Ch13.5.2
above. Table B.13.1 (SAC, 2005/6) gives annual charges to service the capital and
interest per £1000 borrowed for rates of interest between 5 and 20% and deprecia-
tion life of 1–40 years.
Year 5 6 7 8 9 10 12 14 16 18 20
1 1050 1060 1070 1080 1090 1100 1120 1140 1160 1180 1200
2 538 545 553 562 569 576 592 607 623 639 655
3 367 374 381 388 395 403 417 431 445 460 475
4 282 289 296 302 309 316 330 343 357 372 386
5 231 237 244 251 257 264 278 291 305 320 334
6 197 203 210 216 223 230 243 257 271 286 301
7 173 179 186 192 199 206 219 233 248 262 277
8 155 161 168 174 181 188 202 216 230 245 261
9 141 147 154 160 167 174 188 202 217 232 248
10 130 136 142 149 156 163 177 192 207 223 239
11 120 127 134 140 147 154 169 183 199 215 231
12 113 119 126 133 140 147 162 177 192 209 225
13 106 113 120 127 134 141 156 171 187 204 221
14 101 108 114 121 129 136 151 167 183 200 217
15 96 103 110 117 124 132 147 163 179 196 214
20 80 87 94 102 110 117 134 151 169 187 205
25 71 78 86 94 102 110 128 146 164 183 202
30 65 73 81 89 97 106 124 143 162 181 201
40 58 66 75 84 93 102 121 141 160 180 200
For example, the annual charge to service the interest and capital repayments on £8000
repayable over 20 years at 7% would be £94 × 8 = £752.
382 Chapter 13
Table 13.2. Estimated annual repair cost (as percentage of purchase price). (From Nix,
2008.)
average power used as 40% of maximum. A typical forklift for 1-t boxes would have
a maximum power of 40–45 kW.
For example, for a 40 kW forklift truck and with diesel costing £0.45/l:
40
Fuel cost = 40 × × 0.25 × £0.45
100
= £1.80 per hour.
Consumables
Various consumables like potato bags, sprays, lubricating oil, etc. will be required
during storage and grading. These have to be included in the operating costs.
Labour cost
Labour cost can be either a variable or a fixed cost. For seasonal work like potato
grading it is usual to employ labour that would otherwise not be employed. Some
staff will be employed whether or not grading is practised. Since this labour could
be doing useful work elsewhere on the farm, it is best to cost labour as a variable
cost. After carrying out the calculation to find the total operating cost of the store
and grading area, the implications of employing fixed or seasonal labour should be
considered before making the final decision to invest in a grading system.
The costings in Table 13.4 relate to the 1950-t bulk store shown in Fig. 13.3a. This
is a bulk store fitted with ambient-air cooling, but no refrigeration, with potatoes
stored 4 m deep. The store fabric is plasticized steel sprayed with 60-mm foam
insulation in the walls and 80-mm in the roof. Airflow is 0.02 m3/s/t, with air
cooling being intermittent and not humidified. Maximum air speeds in the main
Bulk Bulk
Duct Fan/recirc. Cooling unit Cooling
unit unit
Passage for forklift
Fans and
ducts
Fig. 13.3. Plan and elevations of: (a) a 1950-t processing potato bulk store;
(b) a 1860-t box pre-pack store; and (c) a 1512-t box seed store.
384 Chapter 13
and lateral ducts are restricted to 6 m/s and the laterals are tapered, all to ensure
uniform air distribution from ducts.
The cost of the foundations, floor and lateral ducts make up 32% of the cost
of the store. The relatively low building cost per tonne of potatoes stored compared
with the other two stores is due in part to the maximum utilization of space that
bulk storage offers and that expensive storage boxes are not required.
Marketing and Costs 385
The costings in Table 13.5 relate to the 1860-t box store shown in Fig. 13.3b. This
type of store is typically used for ware potatoes for the pre-pack market. This is a
spray foam-insulated, portal frame building, with an ambient-air/fridge cooling
unit located at the back of the store. Incoming ventilation air can be mixed with
store air to ensure the incoming air is not too much cooler than the crop.
Alternatively, the refrigeration system can be switched on with the ventilation sys-
tem switched to recirculation. Air distribution is over the top of the boxes, with the
return air encouraged to return to the intake of the air/fridge cooling unit via the
‘ducts’ formed by the pallet apertures of boxes making up the stack.
The boxes add a considerable additional cost of storage over bulk storage, and
are justified in part by the improved skin finish and reduction in damage that boxes
provide.
The costings in Table 13.6 relate to the 1512-t positively ventilated box store
shown in Fig. 13.3c. This type of store suits seed potato production, where tubers
and voids are small; with positive ventilation a significant advantage to ensure air
flows uniformly through the whole crop. Since seed is usually grown in poorer
soils, in cool climates and often harvested from wet soils, good drying capability
is required. The increasing adoption of earlier harvesting of seed to minimize dis-
ease development on growing tubers requires well-ventilated storage to remove
the high respiration heat associated with early lifting. The wall ducts and the
access passage up the middle of the store do however occupy expensive building
space. Since seed is sold throughout the year, ease of access to rows of different
varieties is convenient. The relatively low tonnage of crop that can be housed, the
386 Chapter 13
additional cost of the positive ventilation and the cost of boxes all make this the
most costly form of storage.
In addition to the capital cost of the store is the annual insurance premium. As
with all fixed costs this is paid whether or not the store is used. Insurance companies
have concerns relating to fire danger from foam-filled composite panels or foam-
sprayed building cladding. Prior to a building being erected, it is a wise precaution
to contact the grower’s insurance company to get an estimate of the likely pre-
mium. Insurance companies may have preferred constructions or materials that
will be cheaper to insure. Where an actual figure for annual insurance premium
cannot be obtained, a figure of 0.5–1% of the capital cost can be used.
In the UK and similar mild weather maritime areas, heat is rarely required to keep
the crop from cooling below the set-point temperature. The heat generated from
the potatoes themselves, provided the store is sufficiently well insulated, is usually
enough to keep the crop at the desired level.
Heaters are commonly installed in the roof space of stores to maintain roof-
space temperatures during cold nights, to prevent surface cooling of the crop, as
this can result in subsurface condensation. Such heating in the UK is for short
periods only so its energy cost can be largely ignored.
The annual repair and maintenance of the building fabric is usually assumed to be
approximately 1% of the building cost (Nix, 2008).
13.7.5 Forklift operating cost for loading and unloading the store
The management of a 1500- to 2000-t potato store will vary on the level of inspec-
tion carried out, whether temperature monitoring is manual or automatic and
whether problems occur. The main inputs will be at loading and unloading and if
applying CIPC to the crop. For a pre-pack store the management input could be
about 1.5 h per weekday. If the manager’s annual average cost including National
Insurance, Employers’ Liability Insurance, a premium over the minimum wage and
some overtime is £20,460 (SAC, 2005/6), the hourly rate would be £11.22.
Having calculated the fixed and variable costs, these can now be put together to
determine the store operating costs. The store chosen for this example is the pre-
pack store in Ch13.6 (Table 13.5) for 1860 t, with an ambient-air/fridge cooling
system and potatoes stored in boxes. We shall use the single amortization figure
method rather than the straight-line depreciation plus interest on half-capital
method.
The period chosen to depreciate or amortize the store over is based partly on
the actual life of the building, which may be 30–50 years, and partly on the risk
that growing and storing potatoes may become unprofitable. Where long-term
profitability appears secure a life of 20 years may be chosen. This was the period
chosen for Table 13.8. Were there more doubt about the long-term viability of
potato growing on the farm, a shorter life of 10–15 years should be taken.
Marketing and Costs 389
Table 13.8. Calculation of store operating costs for 1860-t pre-pack store.
Fixed costs
Amortized cost of store over £94 / 1000 × £229,663.43a 21,588.36 11.60
20-year life, 7% interest
Amortized cost of boxes at £94 / £1000 × £87,420 8,217.48 4.42
£47.00 each
Building insurance, 1.5% 2,296.63 1.85
Total fixed costs 17.25
Variable costs
Electricity for ventilation and 9.32
cooling
Repair and maintenance at 1,584.70 0.85
1% of building cost (excl.
plant and equipment)
Repair and maintenance at 1,067.96 0.57
1.5% of cooling plant and
equipment capital cost
Forklift operating cost 6,092 3.28
Store management cost 2,861.10 1.54
Total variable costs 15.56
The total store operating cost per tonne stored, assuming a depreciation period
of 20 years, is therefore £32.81. This assumes that 100% of the potatoes stored are
sold. If only 95% are actually sold, the cost would increase proportionally to
£35.19/t. To justify the cost of putting up a store, the sale price in June/July must
therefore be £34.54/t more than the grower would get for selling straight from the
harvester. From the prices in Fig. 13.1, the average increase in price over the 3
years was £29.30, so storage until July did not pay. A better approach would be
to store until May only, when prices were somewhat higher and the electricity and
management costs would be less.
The store operating cost therefore produces a single figure which allows capital
investments to be compared with potato prices and transport costs, and so aids
investment planning.
The operating cost of grading is carried out in the same way as for storage. The
building, the plant and the equipment are depreciated as before, with the building
assumed to have a life of 20 years and plant a shorter life of about 7–10 years. The
electricity costs can be obtained by adding up the power ratings of the electric
motors and lights. This will give the approximate consumption in kilowatt-hours
390 Chapter 13
for the energy used every hour grading is taking place. The cost of labour is added.
Insurance, repairs and maintenance, and forklift costs are added and a final operat-
ing cost per hour obtained. If the potato throughput is known, the cost per tonne
can be calculated by dividing the grading plant operating cost per hour by the
throughput of potatoes.
13.10 Summary
Growers should erect storage buildings only if there is clear financial benefit from
doing so or the crop would be at risk from the weather. The risk of having to
accept low prices from selling crop straight off the field may be justification enough,
but other factors should be considered.
● The difference between the selling price of potatoes at the end of storage and
at harvest should cover the operating costs of storage.
● This calculation must take into account that a proportion of the stored crop
will be rejected due to blemishes, misshapes and disease.
● The decision to invest in new storage should consider the changes in potato
growing and export worldwide as it may be cheaper to ship potatoes from
another climate zone than to grow and store at home.
● In higher latitudes only one crop a year is possible, so storage is necessary to
supply potatoes when growth does not occur. Nearer the equator, crops may
be planted sequentially, and at different altitudes, to deliver potatoes to market
all the year round.
● To obtain a single figure per tonne for the cost of storage, the store operating
cost should be calculated.
● The store operating cost combines both building and equipment fixed and
variable costs.
● The fixed costs include annual depreciation, interest payments on borrowings
and insurance.
● The variable costs include annual energy use, repairs and maintenance, con-
sumables and labour.
● Depreciation and interest charges can be calculated separately or a single
amortization charge, which combines both depreciation and interest payments,
can be used.
● The store operating cost is lowest for bulk storage of processing potatoes, and
highest for box storage of seed. The cost of pre-pack potatoes lies between
the two.
Appendix 1: Metric–US Imperial
Conversion Tables
Imperial–Metric Conversions
Imperial A Metric B
391
392 Appendix 1
Imperial A Metric B
NB: Imperial thermal resistance (R) = 1– × 5.6732, where λ is in W/m °C and R is in ft3 °F h/Btu.
λ
Column 1 A Column 2 B
Physical Properties
Metric Imperial
Notation
Unit/prefix Name
°C degree Celsius
°F degree Fahrenheit
bar = 0.9869 atmospheres
Btu British thermal unit
bu (US or UK) bushel
cwt (US or UK)* hundredweight (20/ton)
ft foot
gal gallon
ha hectare
hp horsepower
in inch
J Joule
k kilo (×103)
km kilometre
l litre
lb pound
m metre
M mega (×106)
m milli (×10−3)
mm millimetre
μ micro (×10−6)
oz ounce
Pa Pascal (N/m2)
t tonne
therm unit of heat = 100,000 Btu
ton ton
W Watt
w.g. water gauge
yd yard
Asia
Bangladesh Dhaka 25°N 90°E 10 Jan 12 25 7 31 46 Jun 26 32 22 36 72
China Beijng 40°N 117°E 44 Jan −10 1 −23 14 50 Jul 21 31 15 41 72
Wuzhou 23°N 111°E 120 Jan 8 16 1 28 72 Jul 26 32 22 38 78
India Hyderabad 17°N 78°E 38 Dec 15 28 8 33 57 May 27 40 19 44 44
Indonesia Medan 4°N 99°E 25 Jan 22 29 18 34 80 May 23 32 18 36 78
Iran Isfahan 33°N 52°E 1590 Jan −4 8 −19 18 64 Jul 19 37 9 42 28
Israel Haifa 32°N 35°E 125 Jan 9 18 −2 26 61 Aug 24 32 18 37 70
Japan Hakodate 42°N 141°E 34 Jan −7 0 −22 13 76 Aug 18 26 9 33 85
Kazakhstan Kazalinsk 49°N 62°E 68 Jan −15 −9 −33 5 84 Jul 18 32 10 41 47
Republic of Seoul 38°N 127°E 70 Jan −9 0 −22 12 65 Aug 22 31 14 37 76
Korea
Nepal Kathmandu 28°N 85°E 2230 Jan 2 18 −2 25 80 Jul 20 29 18 33 84
Pakistan Islamabad 34°N 73°E 508 Jan 2 16 −4 24 44 Jun 25 40 14 48 23
Syrian Arab Damascus 34°N 36°E 609 Jan 2 12 −6 21 69 Aug 18 37 13 45 34
Republic
Turkey Ankara 40°N 33°E 850 Jan −4 4 −25 15 78 Aug 15 31 4 38 40
Africa
Algeria Algiers 37°N 3°E 25 Jan 9 15 1 24 71 Aug 22 29 18 42 65
Egypt Alexandria 31°N 30°E 7 Jan 11 18 3 28 66 Aug 23 31 18 41 70
Malawi Zumbo 16°S 13°E 1154 Jun 13 28 6 36 54 Nov 23 37 17 49 48
(Mozambique)
Morocco Marrakech 31°N 8°W 466 Jan 4 18 −2 28 77 Aug 20 38 14 47 53
South Africa Pretoria 26°S 28°E 1534 Jul 3 19 −4 24 52 Jan 16 27 9 35 59
Uganda Kampala 0°N 33°E 1190 Aug 16 25 12 29 78 Jan 18 28 12 33 66
Latin America
395
Argentina Bahia 39°S 62°W 75 Jul 4 14 −7 26 68 Jan 17 31 6 42 52
Blanca
continued
396
Coldest Warmest
temperatures (°C) temperatures (°C)
Appendix 2
Australasia
Australia Adelaide 35°S 139°E 40 Jul 7 15 0 23 70 Dec 15 28 6 46 36
New Zealand Napier 39°S 177°E 2 Jul 5 13 −3 22 77 Jan 14 24 5 34 62
Aluminium 160
Asbestos sheet 0.23–0.40
Bitumen 0.16
Brick 0.81
Clay roof tiles 0.85
Clay soil 1.50
Concrete, aerated 0.12–0.20
Concrete, no fines 0.60–0.90
Concrete slabs 1.44–2.0
Concrete tiles 1.10
Cork, granulated 0.04
Cork slab 0.05
Expanded polystyrene 0.037
Extruded polystyrene 0.029
Glass 1.05
Glass fibre 0.036–0.04
Granite 2.50
Gravel 0.30
Hardboard 0.08
Limestone 1.50
Loam soil 1.20
Mineral wool felt 0.039
Mineral wool slab 0.045
Plasterboard 0.16
Plywood 0.14
Polyethylene 0.50
Continued
397
398 Appendix 3
Surface Resistance
The surface resistance depends on the surface colour and air speed over the sur-
face. However, typical surface resistances (sum of inside and outside) follow.
Roof 0.15
Walls 0.176
Appendix 4: Theoretical
Derivation of the Dimensions
of Lateral Ducts used in Bulk
Storage
In a grain store the airflow through the grain creates sufficient backpressure to
nearly equalize the airflow coming from both ends of the main duct. Potatoes cre-
ate much less backpressure, so uniform airflow must be achieved through more
precise design of the ductwork. The aim is to obtain a reasonably uniform flow,
even though potatoes are not present or are only part loaded. The calculation
below (Rastovski and van Es, 1981) is designed to set the parameters for good duct
design. Its conclusions are used in Ch6.4.1.
The aim is to have the same airflow flowing to each lateral from the main duct and
for each lateral to supply a uniform airflow along its length. For air to flow through
the duct outlet there must be a pressure difference between the inside of the duct
and outside. This initial theory looks at how static pressure within the duct causes
the air to flow in the laterals and how the duct should be designed to provide uni-
form air distribution.
The relationship between the static pressure in the duct and the air speed
through a hole in the duct is given by:
1
ΔPs = k rv 2 (A1)
2
where
ΔPs = static pressure difference between inside and
outside of the duct (Pa)
k = constant friction coefficient for the hole
r = density of air (1.23 kg/m3)
v = velocity of air flowing though hole (m/s).
399
400 Appendix 4
2ΔPs
v= (A2)
kr
The flow of air through an outlet is given by:
Q = Av, (A3)
where
Q = airflow (m3/s)
A = area of outlet (m2).
Combining Eqns (A2) and (A3) gives:
2ΔPs
Q =A (A4)
kr
which allows the flow of air through the outlet to be found from the static pressure
difference and the size of the outlet.
Tapered ducts
For the duct shown in Fig. A.1, changing from A1 to A2 in cross-section, the total
pressure (Pt1) at outlet 1 is:
Pt1 = Ps1 + Pv1,
where
Ps1 = static pressure in the duct (Pa)
Pv1 = velocity pressure in the duct (Pa).
The total pressure in the duct declines from outlet 1 to outlet 2 due to friction
between the duct wall and the air. If this loss is given by the value ΔP, then:
Pt2 = Pt1 − ΔP. (A5)
Outlet 1 Outlet 2
Pt1 Pt2
Ps1 Ps2
v1 ΔP v2
Q1 Q1-Q0
A2 Q0
Q0
A1
Fig. A.1. Section of duct between two outlets showing static pressure at each outlet.
Derivation of the Dimensions 401
1
Pt 2 = Ps2 + ρ v 22.
2
Eqn (A5) can therefore be rewritten as:
1 1
P s2 + ρ v 22 = P s1 + ρ v12 − Δ P .
2 2
The difference in static pressure between outlet 1 and 2 is then:
1
( )
P s1 − P s2 = ΔP − ρ v12 − v 22 .
2
(A6)
Since pressure losses always arise when the air velocity changes, the entire velocity
pressure will not be converted into static pressure but only a part (h) of it.
Eqn (A6) then becomes:
1
(
Ps1 − Ps2 = ΔP − hr v12 − v 22 .
2
) (A7)
In air ducts η will usually vary between 80 and 90%, i.e. η = 0.8 to 0.9.
By expressing air velocity in terms of airflow/area of duct (Eqn (A3) ), Eqn (A7)
can be expressed as:
1 ⎡⎛ Q ⎞ 2 ⎛ Q − Q ⎞ 2 ⎤
Ps1 − Ps2 = ΔP − hr ⎢⎜ 1 ⎟ − ⎜ 1 0
⎟⎠ ⎥ . (A8)
2 ⎝
⎢⎣ 1A ⎠ ⎝ A 2 ⎥⎦
The static pressure in the duct will remain unchanged, i.e. Ps1 − Ps2 = 0, if the veloc-
ity of air in the duct declines in such a way that the resulting gain in static pressure
equals the loss in total pressure along the duct, i.e.
1 ⎡⎛ Q ⎞ 2 ⎛ Q − Q ⎞ 2 ⎤
ΔP = hr ⎢⎜ 1 ⎟ − ⎜ 1 0
⎥. (A9)
2 ⎢⎣⎝ A 1 ⎠ ⎝ A 2 ⎟⎠ ⎥⎦
A duct which conforms to Eqn (A9) is called an equal pressure duct and will dis-
tribute air uniformly along its length. Its cross-section declines steadily in the direc-
tion of the airflow, so it is tapered, being widest at the fan end and narrowest at
the far end.
Eqn (A9) can be rearranged to determine the cross-sectional area A2 of the
duct at the second outlet:
Q1 −Q 0
A2 = . (A10)
(Q 1 A 1 ) − 2Δ P hr
2
402 Appendix 4
5 4:1
10 7:1
15 10:1
20 13:1
To achieve equal airflow from each outlet, Eqn (A10) would have to be used to
calculate the taper between each outlet. Manufacturing a duct with the taper vary-
ing along its length is too complicated for potato storage, so a duct with a uniform
taper is all that is required. While not stated in their text, Rastovski and van Es
(1981) appear to have used a value of 10 Pa for the resistance of the duct (ΔP ). This
enabled them to recommend the following tapers (Table A.1).
The ratios in Table A.1 can be used for lateral duct design, or where a fan
blows into a single tapered duct (Ch6.4.1). If the lateral of the single fan duct is an
underground duct, its width will be constant and its base will taper vertically
upwards, being a maximum at the inlet and a minimum at the far end of the duct.
Its cross-sectional area should accord with the values in Table A.1.
In a constant cross-section duct, such as the main duct in a bulk store (Fig. A.2)
having lateral duct outlets along its length, the velocity of the air flowing along the
main duct will keep decreasing as air flows out from the laterals, until the velocity
will be zero at the end of the duct furthest from the fan.
The total pressure at the beginning of the duct is:
1 2
P ti = P si + rv i . (A11)
2
Ak Q0
Fig. A.2. Duct of constant cross-section.
Derivation of the Dimensions 403
where
Ak = constant cross-sectional area of duct (m2).
The static pressure difference between inside and outside of duct ΔPs determines
the velocity of air vo discharged through an opening. This pressure rises as the hole
size decreases, i.e.
1
ΔPs = k ρ v o2 ,
2
where
vo = velocity of air through outlet to laterals.
The static pressure at the inlet is therefore determined by:
1
ΔPi = k rv02 .
2
The static pressure at the end of the duct is then equal to the total pressure (i.e.
static pressure plus velocity pressure) at the inlet minus the loss due to the friction
between the airflow and the duct:
1 1
ΔPe = k rv 02 + hrvi2 − ΔP . (A14)
2 2
The ratio between the static pressure at the end and beginning of the duct becomes:
ΔPe ΔPe hv i2
= 1− + .
ΔP i ΔP i k v 02
Since the velocity is equal to Q/A, this ratio can be written as follows:
ΔPe 2ΔPA 02 ⎛ A0 ⎞ h
= 1− +⎜ ⎟ × , (A16)
ΔPi krQ 2 ⎝ Ak ⎠ k
where
Ao = cross-sectional area of all discharge openings (m2)
Ak = cross-sectional area of duct (m2)
404 Appendix 4
where
Q e = volume of air discharged at the end of the duct (m3/s)
Q i = volume of air discharged at the inlet (m3/s).
In a smooth-walled duct the pressure loss in the duct ΔP is usually relatively small,
so the value
2
2ΔP ⎛ A 0 ⎞
×
k r ⎜⎝ Q i ⎟⎠
can be disregarded.
Eqn (A19) then is simplified to:
2 2
⎛Q e ⎞ ⎛ A0 ⎞ h
⎜⎝ Q ⎟⎠ = 1 + ⎜⎝ A ⎟⎠ × k . (A20)
i k
Since resistance factor k ≈ 1 for the discharge through an opening and h = 0.8–0.9,
the ratio of airflow coming from the first and last outlet in the constant cross-section
0.5 1.13 89
1.0 1.35 22
2.0 2.00 6
Derivation of the Dimensions 405
duct is mainly determined by the ratio of the combined area of all the outlets in the
duct, Ao, to the cross-section of the duct, Ak. Table A.2 shows the ratios of airflows
in the last compared with the first duct for different ratios of Ao/Ak.
If the air velocity in the duct is limited to 6 m/s, the backpressure caused by
the size of outlet used to feed the laterals will be as in Table A.2. These values were
calculated using Eqn (A18) assuming that the inlet air speed to the main duct is
6 m/s, the ratio Ao/Ak is as shown in the table and k = 1.
From Table A.2 it is clear that there is a compromise between having a high
static pressure in the duct due to the outlet holes being small, to obtain a relatively
uniform airflow between first and last outlet, and the benefits of keeping the static
pressure low so that low backpressure fans can be used. The compromise is to have
the total lateral outlet duct area equal to the cross-sectional area of the main duct,
which gives a static pressure of 22 Pa and an airflow ratio of 1.35.
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Index
Adhering soil 30, 37, 61, 68, 131, 245, 276, Amortization 381, 388, 390
293, 316, 325 Anaerobic conditions 15–16, 80, 85, 110
Agitation of web 39 Apical dominance 3, 8, 314
Air Application of sprout suppressants 101, 260
approach velocity 92–93 As dug 38, 269, 325
blending 192, 196, 223, 237 Axial flow fans 50–51, 174–175, 216, 270
blending chamber 138, 200 Axial rollers 30, 39
diffusers 204
distribution Backpressure calculation 169–170
box stores 91, 196 Bacteria 9, 14–15, 105–106, 109–113, 120,
bulk stores 167, 177–188 122–123, 126, 131, 261, 331
jets 69, 193 Bacterial
systems 86, 91, 177, 181, 191, diseases 15, 27, 111–112
193, 244, 289 rotting 110, 114–115
entrainment by jets 192 soup 112
jets 184, 192–193, 264 Bags 17, 47, 62–63, 159, 196, 198, 212,
hitting an obstruction 193 217, 274, 287, 290, 293, 295–296,
leakage 59, 63, 69, 81, 96, 132, 137, 301–302, 304–306, 309, 313, 315,
142, 148–149, 153–154, 156, 317–318, 321, 323, 333, 340,
163, 204, 217, 223, 228–229, 342–343, 346, 348–351, 382
247, 255 Barcode 321, 324, 342
saturation 66, 71, 93, 245 labels 358
static pressure 171, 399–401 reader 342, 371
stratification 197, 242 Barrel washer 319, 329–331, 348
Airflow BASIS 43
calculation 168 Batch drying 49–53, 56
resistance 87, 136, 166, 169, 216 Biochemical processes 6, 18
Air-on/Air-off temperature 75, 220–221, 250 Biodegradable packaging 346–347, 349
Airspace ventilation 96, 102, 167, 191–199, Biological oxygen demand (BOD) 316,
211, 217, 239–240, 273, 276, 290 330–332
Ambient-air ventilation 62, 65, 68, 96, 135, Black dot 9, 21, 114–115, 121, 124, 131,
250, 276 313
417
418 Index
Blackheart 16, 58, 160, 163, 362 Calibrated load cell 340
Blemish diseases 65, 307, 346, 354, 364, 369 Calibration of chemical applicators 46
Blended ambient-air cooling 192, 237 Caraway sprout suppressant 100
Blending Carbohydrates 2, 18, 280
chamber 166–168, 190–191, 196, Carbon dioxide
200–201, 216, 241, 248, 265 monitoring 69, 259
control 237 sensor 250–251, 255
duct temperature sensor 265 Carvone sprout suppressant 100
systems 196, 199, 209, 239–240, 267 Catalytic converter on fogger 26
Blight 3, 17, 109, 114–115, 121–123, 131, Cave stores 132, 139
268–269, 271, 313 Cell
Bloom 10, 28, 38, 55, 101, 276, 316, 348, division 4, 19, 99
351, 359, 364, 378 sugar 4, 10, 16, 18, 27, 58, 62, 98,
BOD see Biological oxygen demand 101
Bold crops 7 Cellar stores 90, 132, 139, 378
Bow in boxes 210 Cellular starch 2, 12, 28, 62
Box Centrifugal fan 174–175
designs 117, 210 CFC-free foam 152
pallet apertures 193, 195–196, 203, 206 Chacocine 26
stacking 91, 214, 273 Chain tuberization 15–16
tipper 248, 262, 293–294, 302, 321, Chemical applicators 45–46
324–325, 358 Chemical oxygen demand (COD) 330–332
Bree 272 Chilled-water cooling systems 223
British Potato Council (BPC) ware standard Chills 221, 232, 346–347, 349
25, 42, 91, 99–100, 105, 107, 113, Chitting
122, 126, 158, 196, 207, 261–262, house 8, 307
290, 342, 351, 357, 387 systems 288, 309
Broccoli 280 Chlorine dioxide 330
Brock potatoes 316 Chlorofluorocarbons (CFCs) 152, 222, 388
Brown rot 15, 109, 111, 122, 331 Chlorophyll 26
Bruise susceptibility 14–15 Chloroplasts 26
Brush cleaners 39–40 Chlorpropham (CIPC) 26, 43, 99–101, 124,
BS 5750 352 212–214, 262, 283, 388
Bud end 4 Chronological age 6–7, 28, 288, 291, 307,
Buffer hopper 46, 324, 332–333 314
Building CIPC carrier-solvent 212
complexes 141–144 Clamps 90, 132–133, 375
fabric 59, 63, 137, 149–154, 228, Cleaners 38–40, 43, 46, 48, 54–55, 62,
386–387 113, 138, 160, 270, 293, 315, 323,
Bulk 325–327, 329
bins 142–143, 215, 284 Cleaning
hopper 34, 36, 38, 40, 293, 295–296, boxes 34, 38, 54–56, 112, 125, 145,
319, 323, 329, 332 262, 315, 319
storage 48, 77, 91, 93, 101–102, equipment 38–40, 55, 104, 145–146,
112, 162, 172, 174, 177, 208, 269, 317, 325–326
237–238, 240, 271, 384–385, routines 126
390, 399–405 stores 107, 111, 146
store ventilation system 165, 172, 174, Climate 1, 20, 57–103, 122, 133, 135, 139,
240 143, 145, 155–156, 165, 249–255,
trailers 31–34, 38 259, 262, 279, 369–371, 375–376,
Bulker lorry 315, 323 385, 390
Index 419
Climatic zones 57, 88–90, 390 Controller 76, 88, 97, 242, 245, 255,
Clods 30–32, 36–41, 55, 269, 293, 295, 258–259, 265, 268, 282, 284
326–327, 332 Cooling
Closed systems of seed production 122 coils 69, 75, 97, 141, 196, 213,
COD see Chemical oxygen demand 218–221, 225–226, 231, 247,
Coefficient of performance (COP) 218, 250, 279, 281
222, 232, 247, 250 costs 249–250, 387
Cold walls 235 efficiency 248–249, 259
Collapsed cells 9 potatoes 74, 280
Common scab 14, 17, 301, 313, 359 zones 73–74
Composite panels 145, 152–154, 262, 386 COP see Coefficient of performance
Compressor 219–220, 222, 227, 232, 250, Cordon sanitaire 122
255, 276, 279 Corky cells 21
Computational fluid dynamics 85 Corrugated plastic coated steel sheeting
Computer-based quality control systems 351 149, 151
Condensation 1, 19, 21, 24, 26, 28–29, Cost
76–88, 105, 108, 110, 111, ambient air cooling 383–386
118–121, 234, 235, 237, 240, and returns-storage 374–376
242–246, 249–251, 253–259, 265, Cracking of tubers 15
274–278, 281, 284–287 Crisps 14, 25, 62, 99, 101, 316, 353–354,
Condenser 218–220, 224–225, 231 362, 365, 368
Continental climate 88, 98, 139, 155–156, Crop
249 condensation sensor 253, 256, 259
Continental web cleaner 39, 293, 327 cooling 19, 21, 24, 59, 62–64, 69–76,
Continuity equation 170 80, 85, 88, 90, 91, 93, 94,
Continuous humidified-air ventilation 96–98, 103, 237–241, 248–251,
systems – box storage 93–94, 162 255, 256, 258, 259, 279–281
Continuous, low-rate, humidified-air monitoring 258, 366–368
ventilation cooling systems 70, respiration 16, 69, 94, 160, 164,
74–75 244–245
Continuous ventilation 85–86, 93, 95, 188, senescence 12, 284
233, 259, 276 set-point temperature 220
Continuously ventilated humid air systems temperature differential 69, 76–77,
95, 188–189, 233 258
Control Cull potatoes 316
air moisture content 245–246 Cup planters 298–299
ambient air cooling 76, 233, 237, 241, Curing 110, 116, 121, 129–130
259, 267, 287, 378 Curtain-sided lorry 304
box 220, 227, 234, 240–242, 244–246, Cushioning materials for handling
264, 266–267, 281, 283, 287 equipment 333
continuous humidified-air ventilation
cooling systems 189 Damage
crop cooling 237–241 index 42, 359–361
dew-point temperature 245–246 lenticels 16, 28, 109
intermittent ventilation systems 238 Damp crop 61
refrigeration 62, 76, 90, 220, 227, Data logger 234, 250, 252, 258–259, 283,
231, 241–242, 246, 258, 262, 287, 371
267, 283, 378 Day-degrees 8, 16–17
RH sensors 245, 266, 268 Decaying mother tubers 32, 123
software 76 DEFRA 12, 299–300, 352
ventilation 88 Dehydration tubers 1, 22–23, 29
420 Index
Dew-point temperature 60, 65–67, 76–79, Duct sensor 237–238, 265, 267
81–82, 84, 87–88, 96, 103, 118, Dummy potato monitoring tuber
211, 213, 215, 242–243, 245–246, condensation 250, 253
254, 256–257, 265, 268, 279, Dust
285–286, 303, 306, 314, 316–317 build up in store 145
Diablo rollers on harvester 335 control 301–302
Dickie pie storage 133–134 exclusion 145
Digital display 254 extraction hoods 302
Di-isopropylnaphthalene (DIPN) 100 DX refrigeration see Direct expansion
Dimethylnaphthalene (DMN) 100 refrigeration
Direct entry ambient air cooling 239
Direct expansion (DX) refrigeration 75, Early emergence of growing sprouts 7
218–220, 222–226, 231–232, 281 Electric heating 222, 279
Disease Electricity and fuel costs 380, 382, 389
contamination 111–112, 316 Electronic temperature sensor 266
development 12–13, 42, 55–56, 62, 69, Elevators 31–34, 36, 46–47, 53–54, 61, 102,
95, 101, 104–105, 107–109, 136, 138, 142–143, 234, 287, 293,
111, 113, 115–118, 121, 296–297, 323, 325, 332–334, 340
123–126, 129, 131, 193, 281, Endless screen grader 325, 335
283, 285, 302, 313, 370, 378, Energy 2, 18–19, 58, 64, 74, 96, 109, 171,
385 181, 222, 232, 247–250, 257–259,
ecosystems 104, 124 276, 279–281, 283, 285, 287,
germ tube 117–118 333–334, 379, 384, 390
initiation 65 Energy management 283
inoculum level 106 Enthalpy 247, 250
limitation strategies 109 Erwinia
multiplication 65, 92, 97, 108, 110–111, atroseptica 111, 300
122, 131, 272, 274, 279 carotovora 86, 109–111, 126, 300
ramification 117–118 Ethylene 26, 97, 100, 214
triangle 9, 104–109, 112–113, Ethylene generator 100
122–124, 130 Evaporation 10, 14, 19, 22–24, 28, 40, 58,
Dispatch areas 141, 346–347 64, 72–74, 77, 86, 92, 95, 190, 231,
Display until date 317, 342, 352 249, 266, 276, 279
Dolly tubers 15–16 Evaporative
Door-opening speed 157 cooling 63, 103, 135–136, 162, 286
Dormancy break 6–7, 20–21, 27–28, 98, humidifier 94, 189–190
100, 231, 303 Evaporatively cooled tropical stores
Dripping moisture 61 135–136
Droplet arrestor 91, 189 Evaporator
Dry Evaporator fins 221
-bulb temperature 67, 71–72, 78, 144, Evapotranspiration 14
233, 235–238, 258 Exhaust louvres 169, 248
matter 10–12, 14, 28, 307, 327, 357, Expansion valve 219
359, 365–367 External temperature sensor 145, 265
accumulation 4, 14–15 Extractor rollers 30
rot 86, 109, 113–115, 120–121, 123, Extruded polystyrene board 145, 148
126–127, 131, 262, 312–313 Eyes on tuber skin 4
Drying
crops using refrigeration 276, 279 Fabric horizontal ducts 194
potatoes using ambient air 247, 276 Fall breaker on conveying equipment 145,
tents 50, 196 326
Index 421
Humid-air ventilation systems 188–191 Irrigation 14, 122, 313, 316, 331, 356
Humidification 24, 91, 96, 188–191 ISO 9001:2000 352
Humidified air 24, 71, 74–75, 95, 103,
223 Jelly end rot 13
Humidifier 61, 91, 94, 96, 116, 137,
189–190, 216, 238, 281 Kraft bags 317, 340, 343, 346, 351
Humid ventilation systems 168 k value 174
Hunting of fans 267
Hygiene 112–113, 122, 124–128, 131, Labeller 321
145–146, 260, 322, 355 Labour costs 54, 162, 346, 378, 382
Latent heat 74, 135, 190, 219–220
Icing up of refrigeration coils 227 Lateral ducts 85, 138, 166–167, 177, 179,
Illumination for inspection 337–338 184, 186, 208, 269, 384
Imazalil 43 Laterals 160, 177–188, 215–216, 271, 285,
Individual, group and collective group 384
tolerances 299 Laterals, above-ground 138, 179–180
Infection 14, 19, 28–29, 45–46, 103, 105–106, Laterals, below-ground 177, 179–180
109, 114, 117–118, 120–124, Lateral spacings 180–181
126–128, 131, 261, 271, 275 Lenticellular outgrowth 13, 16
In-field sampling 268–269 Lenticels 4, 6, 14, 16, 18, 22, 28, 80, 109,
Inoculation 110, 117–118, 127 122–123, 131
Inoculum 9, 106, 111–112, 120, 127 Lesion 108, 117–118, 131, 359
Insects in store 90, 282, 378 Letterbox 50–51, 76, 144, 167, 199,
Inspection 37, 40, 55, 115, 212, 260–262, 207–208, 215
281–283, 290, 293, 301–302, 309, system 51, 76, 144, 200–208, 210, 217
311, 320, 325, 329, 335–340, 348, Life cycles 106–107, 109, 115, 117
351, 355, 362–363, 368–371, 388 Lifting (harvesting) 15, 24, 32, 53, 65, 90,
tables 295, 317, 319, 324 92, 94, 101, 106, 109–110, 114,
Insulated 118, 123, 268, 325, 352, 375, 385
doors 156 Load-bearing insulated panels 64, 153, 181
floors 155 Location of crop temperature sensors 259,
Insulated sheets/boards 149, 152, 155, 262 264–265
Insulation 16, 33, 60, 64–65, 69, 86, 137, Logging systems 255–257, 262
145, 149–152, 155–156, 194, 228, London International Financial Futures and
244, 262, 304–305, 383, 387 Option Exchange (LIFFE) 357
Insulation materials 146–149, 151, 163 Long-term storage 62–63, 223, 226, 280
Insurance 152–153, 379–380, 382, 386, Loose straw 81
388, 390 Louvre position sensors 237
Intake louvres 138, 165–166, 223, 237–238, Louvres 27, 59, 75–76, 84, 95–96, 137–138,
258 152, 165–169, 173–174, 191–192,
Interest charges 379, 390 196, 200, 209, 216, 218, 223–224,
Intermittently ventilated 24, 29, 60, 62, 231, 237–238, 240, 246–248, 255,
63, 71, 75, 85, 88, 90–93, 95, 257–258, 261–262, 267–268
189–191 226, 237–238, 242–243, Low-cost structures 132–136, 375
258, 259 Low-tariff electricity 242, 283
Internal air RH 234 Low-temperature
Internal blackening 16, 59, 69 storage 16, 27, 103
Internal bruising 32 sweetening 7, 101, 153
Into-store cleaning 34–40, 43, 49, 55, 145,
348 Main duct 138, 166–167, 169–170, 177–187,
Invertebrate pests 26 214, 238, 271, 399, 402, 404–405
Index 423
Two-stage positive ventilation systems 206–208, 210, 213, 217, 225, 228,
208–209, 239, 241 235, 261, 273, 290, 320, 337, 348,
Two-stage water/glycol refrigeration 354, 383, 385, 400
systems 225–226, 231–232 Warm front 66, 81–82
Warming
UK Assured Produce scheme 352 chamber 215, 284
Underground cellars 139–141 crops 284–286
Undersize material 291, 366 potatoes in store 272, 284–285
Uneven cooling 69 seed 8, 295
Uniform airflow 142, 175, 181, 216, 221, Washed seed inspections 115
399 Washing the store 261–262
laterals 182, 184, 404–405 Water
Unsealed stores 90 management 330–332
Unventilated bulk 59 soaked porous cellulose matrix 189
Up-and-over doors 156 stress 11–12, 14
Urethane foams 147, 152 vapour
U-value 148–151, 228, 387 resistance 146
transmission 146
Vacuuming 113, 127, 131, 146, 261 Water vapour pressure deficit (WVPD) 23
Value packs 340, 351 Weather fronts 49, 63, 80, 88, 96, 245
Variable costs 379–380, 382, 386, 388, 390 Web 30, 39, 68, 175, 242, 259, 293, 327
Variable frequency drives 36, 94 Wedderspoon tents 208–210
Variable speed fan 214 Weed hosts 9
Velocity pressure 170–171, 181–182, Weights and measures 340
401–403 Wet
Ventilated trailers 48–49 bulb temperature 67, 71–73, 94, 116,
Ventilation 190, 238
air 32, 70, 91–93, 101–102, 164, 190, and cold harvest 130
202, 231, 238, 385 harvesting years 203, 211
ducts 83, 94, 137, 160, 182, 191, 265 potatoes 61, 212, 273, 334
system 21, 29, 48–49, 51–52, 56, 59, wick 67, 72, 235, 362
62, 69, 76, 84, 86, 88, 90–97, Whirling hygrometer 266
102, 118, 138–139, 162, Wick – wet- and dry-bulb sensor 235, 249,
164–169, 172–174, 181, 266
188–189, 191, 196–197, Wind exposure 63
208–211, 216–217, 238–241, Work at height regulations 158
248, 258, 267, 273, 276, 290, Work environment 160, 162
308, 385 World potato production data 376–377
resistance 166, 169, 172, 174 Wound
Venturi effect 87, 183, 216 healing 19, 21–22, 24, 28, 36, 41, 50,
Vermin 65 62, 80, 88, 90, 102–103,
Vertebrate pests 26 107–111, 118, 121, 123–124,
Vision grading 334, 338, 340 223, 245–246, 256, 258–259,
Voids filled with soil 62 268, 276, 279, 286–287,
Void space 58, 102 313–314
period 23–24, 246, 275
Walls 10, 24, 51–52, 64–65, 69, 84, 113, Woven baskets 134
117, 134–139, 142–145, 148–153,
180–181, 186, 199, 203–204, X-ray separator 327