QUESTIONS AND ANSWERS

NB: The topics written notes have their questions and answers in the note book. The Questions with
stars are topic that get repeated a lot depending on how much they get repeated i.e 1 star repeated in
1 paper, 2 stars 2 paper.
The Question and Answers will be written in terms of sections since you don’t have any module
outlines.

SECTION A
*Distinguish the following terms:
I. Pasteurisation and Ultra-Pasteurisation
II. Thermophilic and Thermoduric
III. Thermisation and Tyndallisation
IV. Psychrophilic and Psychrotrophic
V. Ultra-pasteurisation and Ultra Heat Treatment(UHT) [4x5]
*Explain the Age gelation phenomenon. (4 marks)
After weeks of storage to month storage of these products, there is a sudden increase in the
viscosity accompanied by visible gelation and irreversible aggregation of the casein micelles
into long chains forming three-dimensional network.
Actual cause not clear, however, some theories exist:
Proteolytic breakdown of casein: bacterial or native plasmin enzymes that are resistance to
heat treatment may lead to formation of a gel
Chemical reactions: Polymerisation of casein and whey proteins due to Maillard type or other
chemical reactions. Formation of Kappa-casein-β-lacto globulin complexes.
*Establish the relationship between mastitis and somatic cell counts and their
importance in quality of milk and milk products. (4 marks)
Milk from cows infected with mastitis generally have higher total bacteria counts and somatic
cell counts (white blood cells) than milk from uninfected cows.
Therefore, bacterial counts and somatic cells counts are used by dairy farmers and processors
as indicators of milk quality.
In general, the higher the counts, the lower the milk quality.
Milk from mastitic cows may have off-flavors and may undergo deterioration of the milk fat
and protein more quickly than milk from healthy cows.
There are regulatory standards for microbial numbers (total bacteria count) as well as quality
control and human health parameters (somatic cell count, and antibiotic drug residues) in
milk, as specified by the Grade A Pasteurized Milk Ordinance (2005).
*What is the significance of coliforms to the dairy industry? [5]
*What are the FIVE basic problems caused by dispersed air in milk (5 x 2)
The basic problems caused by dispersed air are:
• Inaccuracy in volumetric measurement of milk.
• Incrustation of heating surfaces in pasteurisers (fouling).
• Reduced skimming efficiency in separators.
• Loss of precision in automatic in-line standardisation.
• Concentration of air in cream, causing
-Inaccurate in-line fat standardisation,
- Incrustation of cream heaters,
- “pre-churning” resulting in
Loss of yield in butter production,
Adhesion of free fat to the tops of packages.
• Reduction of the stability of cultured milk products (expulsion of whey).
**Discuss the FOUR variables which must be carefully controlled to ensure satisfactory
results with a given detergent during Cleaning- In- Place (CIP). (4 x 3 marks)
The concentration of the detergent solution
The amount of detergent in the solution must be adjusted to the correct concentration before
cleaning starts.
During cleaning, the solution is diluted with rinsing water and milk residues. Some
neutralisation also takes place.
It is therefore necessary to check the concentration during cleaning. Failure to do this can
seriously affect the result. Checking can be done either manually or automatically.
The dosage must always be according to the detergent supplier’s instructions, as increasing
the concentration does not necessarily improve the cleaning effect – it may indeed have the
reverse effect due to foaming, etc.
Using too much detergent simply makes cleaning needlessly expensive.
The temperature of the detergent solution
Detergent temperature Generally speaking, the effectiveness of a detergent solution increases
with increasing temperature.
A blended detergent always has an optimum temperature which should be used.
As a rule of thumb, cleaning with alkaline detergent should be done at the same temperature
as the product has been exposed to, but at least 70°C.
Temperatures of 68 – 70°C are recommended for cleaning with acid detergents.
The mechanical effect on the cleaned surfaces (velocity)
In manual cleaning, scrubbing brushes are used to produce the required mechanical scouring
effect.
 In mechanised cleaning of pipe systems, tanks and other process equipment, the mechanical
effect is supplied by the flow velocity.
The detergent feed pumps are dimensioned for higher capacities than the product pumps, with
flow velocities of 1.5 – 3.0 m/s in the pipes.
At these velocities the liquid flow is very turbulent. This results in a very good scouring
effect on the surfaces of the equipment.
The duration of cleaning (time)
The duration of the detergent cleaning phase must be carefully calculated to obtain the
optimum cleaning effect.
At the same time the costs of electricity, heating, water and labour must be taken into
consideration.
It is not sufficient to flush a pipe system with a detergent solution. The detergent must
circulate long enough to dissolve the dirt.
The time this takes depends on the thickness of the deposits (and the temperature of the
detergent solution).
Heat exchanger plates encrusted with coagulated protein must be exposed to circulating nitric
acid solution for about 20 minutes, whereas 10 minutes’ treatment with alkaline solution is
enough to dissolve the film on the walls of a milk tank.

Briefly explain challenges of a high somatic cell count in raw milk [5]
Milk with a high somatic cell concentration can be harmful to human health and contains less
casein. This results in lower cheese yields.
In addition milk with a high cell count generally contains an increased amount of enzymes,
which have effect on the quality of the protein and the fat in milk.
The presence of these enzymes in milk increases the potential for off-flavours and odours.
Because the somatic cell content of raw milk is important for the shelf-life, flavour and the
yields (particularly of cheese), milk processors strive to obtain raw milk of the highest
hygienic quality from their producers.
Explain the role of the following ingredients used in ice-cream manufacture:
i) Sweetener, [3]
ii) Milk fat, [3]
iii) Stabilizer and [3]
iv) Milk solids not fat. [3]
Briefly explain five effects of mastitis on the dairy industry. [10]
Reduced milk yield per cow;
Deleterious effects on the composition of milk and consequently its rejection by processor
and consumer;
Presence of bacteria and other infectious agents which may be harmful to human health, but
also to the calf;
Mastitis therapy results in the presence of antibiotic residues in milk, rendering it unsuitable
for human consumption or further dairy processing;
Animals suffer from the disease and may even die. Often one or more quarters or even the
whole udder may become permanently dysfunctional.
Why are antibiotics not permitted in the dairy industry? [3]
They affect milk starter culture.
Allergic reactions.
Antibiotic resistance.
Identify 4 methods that can be used to control biofilms in the dairy industry [4]

Briefly explain importance of Bacillus cereus to the dairy industry. (5 marks)

Explain the symbiotic relationship between S. thermophilus and L. bulgaricus in yoghurt
manufacture [4]
Explain the symbiotic relationship between Streptococcus thermophilus and Lactobacilli
bulgaricus in yoghurt manufacturing [4]
Discuss the role of k-casein in stability of the casein micelle [4]
List the four principle factions of casein protein [4]
List the four principle factions of WHEY protein [4]
Highlight six major differences in the manufacturing processes of Roquerfoti and
mozzarella cheese [2x3]
Discuss standardisation and its application in the dairy industry [10]
Standardization of milk refers to the adjustment i.e. either raising or lowering of fat and / or
solids-not-fat percentages of milk to a desired value, so as to confirm to the legal or other
requirements prescribed.
If fine adjustment of the fat content of cream is required, or if the fat content of whole milk
must be reduced to a given level, skim milk must be added. This process is known as
standardisation.
The usual method of making standardisation calculations is the Pearson's Square technique.
Areas of Use:
Milk and milk cream
Fermented dairy products
Milk powder production
Cheese milk
Yogurt production
What are the limiting factors for heat treatment process [5]
Intense heat treatment of milk is desirable from the microbiological point of view. But such
treatment also involves a risk of adverse effects on the appearance, taste and nutritional value
of the milk.
Proteins in milk are denatured at high temperatures. This means that the cheese making
properties of milk are drastically impaired by intense heat treatment.
Intense heating produces changes in taste; first cooked flavour and then burnt flavour.
The choice of time/temperature combination is therefore a matter of optimisation in which
both microbiological effects and quality aspects must be taken into account.
Explain regenerative heating and cooling as used in dairy processing [5]
Milk at 4 oC is fed into a regeneration unit by a pump.
Cold milk is heated in the regeneration section by outgoing pasteurised milk.
Hot pasteurized milk flowing in a counter current direction on the opposite side of the milk to
milk regenerator plates.
In the section, cold milk is heated to 55 oC from 4oC by pasteurised milk.

I. 63 OC FOR 30 MIN
II. 72-75 OC FOR 15 TO 20 SEC
III. 63 – 65°C FOR 15 SEC
IV. 135 – 140 OC FOR A FEW SECONDS
V. 115 – 120 FOR 20 TO 30 MINS
SECTION B
30 questions from 2016 to 2020
***Distinguish the four types of membrane techniques used in dairy processing in
relation to pressure and membrane characteristics (structure). (12 marks)
*Discuss centrifugal techniques and their application in the dairy industry. [12]
PAGE 237
***Describe four methods used to chill milk in dairy farms and processing plants. [20]
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**Describe the principle of operation of a vacuum de-arator [5]
Vacuum deaeration has been used successfully to expel dissolved air and finely dispersed air
bubbles from milk.
Preheated milk is fed to an expansion vessel, in which the vacuum is adjusted to a level
equivalent to a boiling point about 7 – 8oC below the preheating temperature.
If the milk enters the vessel at 68oC, the temperature will immediately drop to 68 – 8 = 60oC.
The drop in pressure expels the dissolved air, which boils off together with a certain amount
of the milk.
The vapour passes a built-in condenser in the vessel, condenses, and runs back into the milk,
while the boiled-off air, together with non-condensable gases (certain off-flavors) is removed
from the vessel by the vacuum pump.
For production of yoghurt the vacuum vessel is not provided with a condenser, as milk
intended for yoghurt is normally also slightly (15 – 20%) concentrated.
Condensation of vapour is arranged separately.
**Discuss the genetic and environmental factors which affect milk composition. [20]
Page 16 to 20
*Examine the 5 stages of the continuous pasteurisation process of milk. [20]
I. Regeneration section
Milk at 4 oC is fed into a regeneration unit by a pump.
Cold milk is heated in the regeneration section by outgoing pasteurised milk.
Hot pasteurized milk flowing in a counter current direction on the opposite side of the
milk to milk regenerator plates.
In the section, cold milk is heated to 55 oC from 4oC by pasteurised milk
II. Heating section
 From regeneration unit, the warm milk is pumped to the plate heater.
The raw milk, under pressure flows through the heater section where steam heated,
hot water on opposite sides of the stainless steel plates continues to heat the milk to a
temperature exceeding the minimum pasteurization temperature
The plate heater heats the milk to pasteurisation temperatures of 72 oC.
III. Holding section
The milk from the plate heater which is at pasteurisation temperatures is then pumped
into the holding tubes.
The holding tube provides for the continuous holding of every particle of product for
at least the minimum required holding time.
IV. Cooling section
Then, milk from holding tube is then fed into the regeneration section where it is
cooled to 18 oC by incoming cold milk at 5 oC.
The pasteurised milk is then pumped directly into the cooling section where it is
cooled to 4 oC for final packaging
V. Regenerative heating and cooling
In many cases a product must first be heated for a certain treatment and then cooled.
E.g Pasteurisation of milk
Chilled milk is heated from, perhaps, 4 °C to a pasteurisation temperature of 72°C,
held at that temperature for 15 seconds and then chilled to 4°C again.
The heat of the pasteurised milk is utilised to warm the cold milk.
The incoming cold milk is pre-heated by the outgoing hot milk, which is
simultaneously pre-cooled.
This saves heating and refrigeration energy.
The process takes place in a heat exchanger and is called regenerative heat exchange
or, more commonly, heat recovery.
As much as 94 – 95% of the heat content of the pasteurised milk can be recycled

*What are biofilms, their significance and control in the dairy industry? (10 marks)
A biofilm is a complex surface attached community of bacteria held together by a self-
produced polymer matrix mainly composed of polysaccharides, secreted proteins and
extracellular DNA.
Significance
Microorganisms occurring in the dairy industry could be a source be a source of secondary
contamination in food products.
The main source of contamination of dairy products is often associated with the formation of
biofilms on the surfaces of milk transport pipes, milking containers and accessories in the
dairy industries.
The bacteria may detach from the biofilms and contaminate the milk as it passes surfaces.
Moreover, the biofilm bacteria can also increase the corrosion of metal pipes, reduce heat
transfer and increase fluid frictional resistance.
Some of the bacteria species involved in biofilm production are pathogenic to humans and
animals.
Pathogenic bacteria are bacteria that are harmful because they cause disease.
Staphylococcus aureus capable of producing biofilms are linked to chronic mastitis in dairy
cows.
Chronic mastitis can affect the dairy farmers negatively. This is because cows with chronic
mastitis produce more white blood cells in order to fight against the disease.
Non-starter lactic acid bacteria (NSLAB) are important for the flavour development during
the ripening process of cheeses.
However, NSLAB are suspected to lower the product quality by producing off flavours and
other manufacturing defects.
Control

*Discuss the role of stakeholders in ensuring the quality of dairy products (5 stake
holders) [20]
i) To the Milk Producer.
The milk producer expects a fair price in accordance with the quality of milk she/he
produces.
ii) The Milk Processor.
The milk processor who pays the producer must assure himself/herself that the milk received
for processing is of normal composition and is suitable for processing into various dairy
products.
iii) The Consumer.
The consumer expects to pay a fair price for milk and milk products of acceptable to
excellent quality.
iv)The Public and Government Agencies.
These have to ensure that the health and nutritional status of the people is protected from
consumption of contaminated and sub-standard foodstuffs and that prices paid are fair to the
milk producers, the milk processor and the final consumer.
*Briefly explain the rationale of the four main stages in milk powder production [8]
*Outline the main steps in Mozzarella cheese manufacture [12]
Explain principles behind hurdle technology and how it is applied in UHT milk. (8
marks)
PRESENTATION
What are the principles of hurdle technology and show how hurdle technology is
applied in TWO dairy products. (10 marks)
PRESENTATION
Providing the rationale of each stage of hurdle technology describe the butter
manufacturing process. (10 marks)
PRESENTATION
Describe how the milk cream separator works. (5 marks)
The milk is introduced through vertically aligned distribution holes in the discs at a certain
distance from the edge of the disc stack. Under the influence of centrifugal force the sediment
and fat globules in the milk begin to settle radially outwards or inwards in the separation
channels, according to their density relative to that of the continuous medium (skim milk).
As in the clarifier, the high-density solid impurities in the milk will quickly settle outwards
towards the periphery of the separator and collect in the sediment space. Sedimentation of
solids is assisted by the fact that the skim milk in the channels in this case moves outwards
towards the periphery of the disc stack.
The cream, i.e. the fat globules, has a lower density than the skim milk and therefore moves
inwards in the channels, towards the axis of rotation. The cream continues to an axial outlet.
The skim milk moves outwards to the space outside the disc stack and from there through a
channel between the top of the disc stack and the conical hood of the separator bowl to a
concentric skim milk outlet.
Discuss FIVE factors which affect efficiency of a cream separator (5 x 3 marks)
1. Position of the cream screw (or skim milk screw): Cream screw in or skim milk screw
out, higher fat percentage in cream and vice versa. The cream screw / outlet consists of a
small, threaded hollow screw pierced by a circular orifice through which cream emerges.
2. Fat % in milk: The higher the fat percentage in milk, the higher the fat percent in cream
and vice-versa. Since particularly all the fat in the milk is contained in the cream, the cream
from the separation of high fat milk has a higher fat content than that from low fat milk.
3. Speed of the bowl: The higher the speed of the bowl, the higher the fat percentage in
cream and the lower the speed, the higher fat loss in skim milk. The higher the speed, the
greater will be the centrifugal force, and the more rapidly will the skim milk leave the bowl.
4. Rate of Milk flow: The higher the rate of milk flow, the lower the fat percentage in cream
and the higher fat loss in skim milk vice-versa. When the rate of inflow increases, the
discharge from the cream outlet increases, as the skim milk discharge remains constant,
more cream containing the same amount of fat results in a lower fat% in cream. When the
flow of milk is high through the bowl too rapidly to allow for complete separation, thereby
results a higher fat loss in skim milk.
5. Temperature of the Milk: The lower the temperature of the milk the higher the fat % in
the cream and vice versa. Lowering of temperature increases viscosity of both cream and
skim milk, but that of cream increases (proportionately) more than skim milk. Hence the
quantity of cream discharge is decreased (due to clogging of the bowl) there by resulting a
higher fat % in cream. The lower the temperature, the higher the fat loss in skim milk and
vice versa.
THERE ARE MORE FACTORS!!
Briefly discuss the defects which can occur in powdered milk. [8]

Discuss the effects of homogenisation on the physical structure on milk [8]

The effect of homogenisation on the physical structure of milk has many advantages:
1. Smaller fat globules leading to no cream-line formation,
2. Whiter and more appetizing colour,
3. Reduced sensitivity to fat oxidation,
4. More full-bodied flavour, better mouth feel,
5. Better stability of cultured milk products.
However, homogenisation also has certain disadvantages:
1. Homogenised milk cannot be efficiently separated.
2. Somewhat increased sensitivity to light – sunlight and fluorescent tubes can result in
“Sunlight flavour” .
3. Reduced heat stability, especially in case of single-stage homogenisation, high fat
content and other factors contributing to fat clumping.
4. The milk will not be suitable for production of semi-hard or hard cheeses because the
coagulum will be too soft and difficult to dewater.
Write detailed notes on the application of membrane techniques in the dairy industry.
(20 marks)
MICROFILTRATION
1) BACTERIA REDUCTION -ESL (Extended Shelf Life) Milk
Microfiltration is today widely used in the production of high-quality market milk and ESL
milk.
As opposed to traditional heat treatment where the microorganisms of the milk are
inactivated and the chemical composition of the milk is changed, microfiltration physically
removes bacteria, spores, dead cells and impurities from the milk, leaving practically no
autolyzable cells and causing no undesired changes to the chemical composition of the milk.
2) CHEESE MILK
Improvement of cheese milk can be achieved using microfiltration.
The natural content of anaerobic spores in milk - such as clostridia - which can survive
normal pasteurisation and cause undesired gas formation in the cheese, can be reduced by
means of microfiltration.
Furthermore, microfiltration can avoid or significantly reduce the addition of normal
inhibitors (e.g. nitrate) thereby achieving preservative-free cheese and whey.
3) MILK & WHEY POWDERS
Microfiltration can improve the quality of milk and whey powder considerably through a
reduction of bacteria and spores.
As a consequence, heat treatment can be kept at an absolute minimum which - among other
things - contributes to a preservation of the functional properties of the whey proteins in the
powder.
4) CHEESE BRINE SANITATION
The chemical and microbiological quality of the cheese brine used for salting cheese products
is critical for the final quality of the cheese.
As brine may contain undesired microorganisms, cheese brine has traditionally been
subjected to different types of treatment such as heat treatment, kieselguhr filtration, UV
treatment or even addition of preservatives.
Microfiltration can easily replace any of these processes avoiding the many disadvantages
involved.
5) CASEIN STANDARDISATION OF CHEESE MILK
When it comes to obtaining process control and quality, a uniform and stable production
process is of the highest importance to any cheese manufacturer.
By using microfiltration, it is possible to fractionate casein and whey proteins and thereby to
standardise the concentration of casein in the cheese milk to obtain the correct ratio between
casein and fat.
6) CASEIN PRODUCTION
Microfiltration can fractionate milk proteins into casein and whey proteins.
The fractionated casein can be used in the production of high-quality casein and caseinate or
in the production of special casein-rich milk products.
The by-product of this fractionation (permeate) contains whey proteins in their natural form
which are unaffected by heat treatment, enzymes (rennet) or bacteria (starter cultures).
7) MILK FAT REMOVAL
• Protein Isolate
In the production of protein isolate - e.g. Milk Protein Isolate (MPI) or Whey Protein Isolate
(WPI) - where a protein level of more than 90% in the total solids is required, the fat content
constitutes a limiting factor.
The milk fat is concentrated to a very high level, and in order to achieve the final protein
concentration, removal of the milk fat is required. Microfiltration is the obvious solution for
performing this fat removal.
Discuss advantages of probiotic and prebiotic ingredients products in milk and milk
products. (10 marks)
Describe the main stages and processing conditions in cheese manufacture [10x2]
PRESENTATION
Analyse the two main technologies used in UHT milk manufacture [20]
Direct Heating
The product is heated by direct contact with steam of potable or culinary quality.
The main advantage of direct heating is that the product is held at the elevated temperature
for a shorter period of time.
For a heat-sensitive product such as milk, this means less damage.

Methods of direct heating- Injection

High pressure steam is injected into pre-heated liquid by a steam injector leading to a rapid
rise in temperature.
After holding, the product is flash-cooled in a vacuum to remove water equivalent to amount
of condensed steam used.
This method allows fast heating and cooling, and volatile removal, but is only suitable for
some products. It is energy intensive and because the product comes in contact with hot
equipment, there is potential for flavour damage.
Methods of direct heating- Infusion
The liquid product stream is pumped through a distributing nozzle into a chamber of high
pressure steam.
This system is characterized by a large steam volume and a small product volume, distributed
in a large surface area of product.
Product temperature is accurately controlled via pressure.
Additional holding time may be accomplished through the use of plate or tubular heat
exchangers, followed by flash cooling in vacuum chamber.
This method has several advantages:
Instantaneous heating and rapid cooling
No localized overheating or burn-on
Suitable for low and higher viscosity products
Indirect Heating
The heating medium and product are not in direct contact, but separated by equipment
contact surfaces. Several types of heat exchangers are applicable:
Plate
Tubular
Scraped surface

Discuss the applications of reverse osmosis in the dairy industry [15]

Supplement to Evaporation
Reverse osmosis can be applied as a supplement to evaporation. If a new evaporation line is
required or an existing line is to be extended, huge savings can be obtained by joining the two
technologies.
Reverse osmosis is a very efficient way of removing water from the milk or whey prior to the
evaporation stage.
By installing a reverse osmosis plant upstream of an existing evaporator, the capacity of the
evaporator can be increased considerably depending on the application and type of evaporator
in question.
Total Solids Increase
Reverse osmosis can be used to concentrate skim milk or whole milk in order to increase the
total solids content. This is – among other things - relevant for fermented products.
As reverse osmosis practically removes only water, the technology can be applied as an
energy-efficient alternative to evaporation or the addition of milk powder, which are the most
common ways of increasing the total solids content of milk.
Volume Reduction
Reverse osmosis can be applied to reduce the volume of milk or whey - e.g. for saving
transportation costs. Volume reduction based on reverse osmosis is an alternative to
Nanofiltration
Product Recovery
In order for a modern dairy production facility to be able to meet the many demands – both
economical and environmental – put forward by the surrounding society, waste recovery has
become increasingly important.
From the first cleaning flush, sweet “white water” is collected in a dedicated collection tank.
The sweet “white water” is concentrated to the required total solids content by applying
reverse osmosis, and the recovered solids can subsequently be returned to the production
process
- e.g. to increase the total solids in yoghurt milk. The by-product of this concentration process
- water - can also be utilised as described in the following section. evaporators are practically
water.
With an additional reverse osmosis treatment normally referred to as “Polishing”, this water
can be purified and re-used for cleaning purposes.
With further heat treatment or UV light treatment, it is even possible to use the water as
process water
Effluent Control
Some production facilities – such as large whey processing sites – have an excess amount of
water which must be discharged.
As water disposal is normally connected with emission taxes depending on the COD level
among other factors, subjecting the water to a reverse osmosis process can lower the COD
level and reduce emission taxes significantly.
* Reverse osmosis is a cost-effective way of removing water.

Examine the factors which are considered in justifying the estimation capacity of a new
dairy plant [20]
Describe the rationale of the main stages in ice-cream manufacture [20]
Describe how bactofuge works [4]
Bactofugation is a process in which a specially designed centrifuge called a Bactofuge is used
to separate micro-organisms from milk.
• The two-phase Bactofuge has two outlets at the top: one for continuous discharge of
bacteria concentrate (bactofugate) via a special top disc, and one for the bacteria-reduced
phase.
• The one-phase Bactofuge has only one outlet at the top of the bowl for the bacteria-reduced
milk. The bactofugate is collected in the sludge space of the bowl and discharged at preset
intervals.

Discuss eight (8) factors which affect efficiency of a milk separator [16]
1. Position of the cream screw (or skim milk screw): Cream screw in or skim milk screw
out, higher fat percentage in cream and vice versa. The cream screw / outlet consists of a
small, threaded hollow screw pierced by a circular orifice through which cream emerges.
2. Fat % in milk: The higher the fat percentage in milk, the higher the fat percent in cream
and vice-versa. Since particularly all the fat in the milk is contained in the cream, the cream
from the separation of high fat milk has a higher fat content than that from low fat milk.

3. Speed of the bowl: The higher the speed of the bowl, the higher the fat percentage in
cream and the lower the speed, the higher fat loss in skim milk. The higher the speed, the
greater will be the centrifugal force, and the more rapidly will the skim milk leave the bowl.
4. Rate of Milk flow: The higher the rate of milk flow, the lower the fat percentage in cream
and the higher fat loss in skim milk vice-versa. When the rate of inflow increases, the
discharge from the cream outlet increases, as the skim milk discharge remains constant,
more cream containing the same amount of fat results in a lower fat% in cream. When the
flow of milk is high through the bowl too rapidly to allow for complete separation, thereby
results a higher fat loss in skim milk.
5. Temperature of the Milk: The lower the temperature of the milk the higher the fat % in
the cream and vice versa. Lowering of temperature increases viscosity of both cream and
skim milk, but that of cream increases (proportionately) more than skim milk. Hence the
quantity of cream discharge is decreased (due to clogging of the bowl) there by resulting a
higher fat % in cream. The lower the temperature, the higher the fat loss in skim milk and
vice versa.
6. Mechanical Condition of separator : Unsatisfactory mechanical condition of the
separator causes greater fat loss in skim milk. These include
a) Vibration of the Separator : This reduces the efficiency of separation by disturbing the
counter currents of cream and skim milk (vibration is caused by installation on an
insufficiently firm foundation, the bowl being out of balance, bearings being worn out, the
axis of rotation not exactly vertical.
b) Condition of Discs : Discs in an unsatisfactory condition suffer a loss of skimming
efficiency due to the uneven flow of the counter current streams of cream and skim milk
between them. (An unsatisfactory disc is one which is out of shape, dirty scratched or rough).
c) Amount of Separator slime in bowl: If too much slime accumulates, the fat loss in skim
milk increases, this not only by a disturbance in the even flow of the currents of cream and
skim milk, but by reduction in the centrifugal force (because of decrease in effective diameter
of the bowl).
7. Amount of Water or skim milk added to flush the bowl: The greater the quantity of water
or skim milk added to flush the bowl, the lower the fat % in cream and vice-versa. The
addition of more water or skim milk will cause an increase in the amount of the cream, with
the same amount of fat and will show a lower fat content.
8. Other Miscellaneous Factors:
a) Size of fat globules: The greater the number of fat globules of less than2 microns size, the
greater the fat loss in skim milk and vice versa. Fat globules less than 2 microns are not
subjected to sufficient centrifugal force and so enters in to skim milk.
b) Presence of Air in the milk: The greater the amount of air, the higher the fat loss in skim
milk. This is due to disturbance of counter-current streams of cream and skim milk and
lowers the efficiency.
c) Acidity of the milk: The higher the acidity, the lower the efficiency of separation. The
higher the acidity, the lower the stability of casein particles, which inturn get precipitated and
clog the bowl, there by lowering the efficiency.
d) Degree and Temperature at which milk is agitated before separation
The higher the degree and temperature of agitation, the greater the fat loss in skim milk and
vice-versa. Agitation of hot milk causes the disintegration of fat globules into smaller ones
which escape the effect of centrifugal force, there by leading to more fat loss in skim milk.
Briefly describe the sunlight and oxidation flavour defects in milk [8]
OXIDATION
Milk fat oxidation is catalysed by copper and certain other metals with oxygen and air. This
leads to an autooxidation reaction consisting of initiation, propagation, termination.
RH --- R + H initiation - free radical
R + O2 ---- RO2 propagation
RO2 + RH --- ROOH + R
R + R --- R2 termination
 R + RO2 --- RO2R
It is usually initiated in the phospholipid of the fat globule membrane. Propagation then
occurs in triglycerides, primarily double bonds of unsaturated fatty acids.
During propagation, peroxide derivatives of fatty acids accumulate.
These undergo further reactions to form carbonyls, of which some, like aldehydes and
ketones, have strong flavours.
Dry feed, late lactation, added copper or other metals, lack of vit E (tocopherol) or selenium
(natural antioxidates) in the diet all lead to spontaneous oxidation.
It can be a real problem especially in winter. Exposure to metals during processing can also
contribute.
Characterized: metallic, wet cardboard, oily, tallowy, chalky; mouth usually perceives a
puckery or astringent feel
SUNLIGHT
Often confused with oxidized, this defect is caused by UV-rays from sunlight or flourescent
lighting catalyzing oxidation in unprotected milk.
Photo-oxidation activates riboflavin which is responsible for catalyzing the conversion of
methionine to methanal.
It is, therefore, a protein reaction rather than a lipid reaction. However, the end product
flavour notes are similar but tends to diminish after storage of several days.
Characterized: burnt-protein or burnt-feathers-like, "medicinal"-like flavour
Design a cleaning cycle for a sterilized milk processing plant [5]
Justify 5 platform tests that you would select in setting up a mil reception laboratory for
Midlands First Dairy processing plant [5x3]
I. ORGANOLEPTIC TESTS
The appearance of the surface of the milk and the lid is observed and inspected
instantly after removing the lid of incoming milk can or container.
Any abnormal colour of the milk, visible dirt and particles, changes in viscosity etc.
are observed.
Any abnormal smell is noticed by inhalation of air standing above the milk in the
upper part of the milk can.
II. LACTOMETER TEST
If the milk appears during organoleptic inspections to be too thin and watery and its
colour is "blue thin" it is suspected that milk contains added water.
Lactometer test serves as a quick method for determination of adulteration of milk by
adding water.
 The lactometer test is based on the fact that the specific gravity of whole milk, skim
milk and water differ from each other.
III. ALCOHOL TEST
In case there is any reason to suspect that milk is sour, alcohol test is used as platform
test for rapid determination of elevated acidity of milk.
Alcohol test is based on fact that the proteins in milk, which has become sour, e.g. as
result of lactic acid formation by bacteria become susceptible to alcohol precipitation.
If the mixing of equal quantities (e.g. 2 ml) of milk and 68% alcohol in a test tube
results in coagulation of proteins it indicates that milk is sour.
This milk is not fit for any processes applying pasteurisation, because the proteins in
milks having increased acidity have also loosed the stability to the temperatures used
for pasteurisation.
IV. CLOT-ON- BOILING TEST
Also, this test is used for rapid testing of increased acidity of milk. As stated above
heating will precipitate proteins of milk if it is sour.
This method is slower than alcohol test but very useful where and when an alcohol
test is not available.
This test is performed simply by heating small amount of milk in a test tube over a
flame or immersed in boiling water for five minutes. The result can be seen
immediately. If no coagulation occurs, it indicates that milk can stand heating
operations at the time of testing.
V. THE RESAZURIN TEST
A reductase dye test used to check bacteriological quality of raw milk
The resazurin test is conducted based either on the color produced after a stated period
of incubation.
There is an inverse proportion of amount of bacteria and the time taken for
decolorisation of the dye in milk
The most commonly used are the “10 min test" and the “one hour test“. Numerous
modifications proposed
VI. PHOSPHATASE TEST
The phosphatase test is used to detect inadequate pasteurisation and possible raw milk
contamination of milk and milk products.
The phosphatase enzyme, which is present and active in fresh milk, is completely
destroyed by the heat treatments applied both for low temperature pasteurisation and
for the high temperature short-time pasteurisation (HTST) processes.
The phosphatase activity is a measure of the quality of active alkaline phosphatase
present in milk and milk products.
Any active alkaline phosphatase present in the sample will liberate p.nitrophenol from
the added p.nitrophenyl phosphate.
The p.nitrophenol liberated is measured by direct comparison with standard colour
glasses in a comparator.