Principles of Grain Storage

1/29
Grain Storage Never Improves
Grain Quality

2/29
 Outline

• Insect and mold development

• Overview of storage basics
– Equilibrium moisture content (EMC)
– Dry matter loss (DML)
• Weather influence

3/29
 Development of Insects and Molds

• Two primary factors

– Temperature
– Relative humidity (or moisture content)
• Other factors
– Level of damage
– Initial infestation level of insects and molds
– Time
– Residual pesticides

4/29
Temperature Influence on Insects
(Source: Fields 1992)

Temperature (C) Effect

> 50 Death in minutes
> 35 Development stops
25 - 32 Optimum
19 - 25 Sub-optimum
5 - 16 Death in days
0 Death in minutes

5/29
Relative Growth Rate as a Function of
Relative Humidity
Maximum
Growth Rate

Fungi
Bacteria
Minimum

60 70 80 90 100
Relative Humidity, %

6/29
Relative Growth Rate as a Function of
Temperature

7/29
 Biological Losses
• Insect development (species dependent)
– Minimal under 16oC or above 40oC
• Mold development
– Maximum RH under 70%
• Mycotoxins
– Primarily aflatoxin that could develop in storage
(optimal growth 27-30oC and 85-95% RH)
– Other mycotoxins are typically present prior to
harvest

8/29
 Causes of Grain Spoilage
• Unsafe moisture content for climatic conditions
• Condensation
– Solar vs. shaded effect
– Day vs. night time effect
• Moisture migration
• Self-heating of stored product
– grain respiration, insect respiration
• Moisture infiltration
– bolt holes, cracks, spouts, bin sheets, vents

9/29
 Characteristics of US Corn

• Harvested wet
– Requires artificial drying
• Produced in cool climates
– Average summer
temperature 24oC
– Relatively dry climate
– Winter temperatures
frequently below 0oC

10/29
 Moisture Content Review

• Market moisture contents are usually wet

basis
• Mass of water divided by the total mass
(water plus dry matter)

11/29
 Maximum Recommended Moisture
Content for Aerated Grain in US
South Central North

Sold as #2 by 14 15 15
spring
Stored 6 to 12 13 14 14
months
Stored more 12 13 13
than 1 year
Adapted from MWPS, AED-20
12/29
 Adjustments to Recommended
Moisture Contents for Corn
• Reduce moisture content by 2 points for
non-aerated storage (i.e. 10 to 11% for
Southern US)
• Reduce moisture content by 1 point for poor
quality grain (mold, broken kernels, etc)
• Reduce the values for the Southern US for
hot, humid locations by 1 to 3 points

13/29
 Outline

• Insect and mold development

• Overview of storage basics
– Equilibrium moisture content (EMC)
– Dry matter loss (DML)
• Weather influence

14/29
 Storage Basics`

• Primary factors affecting storability

– Temperature
– Moisture content
• Predicted using equations for EMC and
DML

15/29
 Equilibrium Moisture Content
(EMC)
• Moisture content after an
infinite exposure to an
environment
– Increase RH, increase EMC
– Increase temperature,
decrease EMC
• Major factor in mold
development and storability

16/29
 Temperature Effect on EMC of
Maize
17.0

16.0 5
15
25
15.0
35
45
Moisture Content (% w.b.)

14.0

13.0

12.0
Minimal Mold Optimal Mold
Development Development
11.0

10.0

9.0

8.0

7.0
40 45 50 55 60 65 70 75 80 85 90
Relative Humidity (%)
17/29
 Temperature Effect on EMC of
Soybeans
17.0

16.0 5
15
25
15.0
35
45
Moisture Content (% w.b.)

14.0

13.0

12.0
Minimal Mold Optimal Mold
11.0
Development Development
10.0

9.0

8.0

7.0
40 45 50 55 60 65 70 75 80 85 90
Relative Humidity (%)
18/29
 Dry Matter Loss

• Also allowable storage time (AST)

– Time before 0.5% loss in dry matter
– Indication of when significant mold damage
occurs and economic loss
– Influenced by variety and damage level

19/29
 Allowable Storage Time1
300

250
13
Allowable Storage Time (days)

14
15
200
16

150

100
2 month storage
50

40 days to ship
0
10 15 20 25 30 35 40 45
Temperature (C)

1
Assuming minimal losses in US 20/29
 Cost of Lower Moisture Content
Corn
• $150/t, moisture content 15%
• 13% corn
– Shrink cost of 2.3%, = $153.45/t
– Drying cost of $7.86/t
• $161.30/t for corn at moisture content of 13%
• Does extra cost justify decreased risk?
– Recommend Southern US 13% for 1 yr storage

21/29
 Outline

• Insect and mold development

• Overview of storage basics
– Equilibrium moisture content (EMC)
– Dry matter loss (DML)
• Weather influence

22/29
 Weather Conditions in SE Asia

• Grain temperatures during export between

25 and 35oC
• Average summer ambient temperature
greater than 29oC in Malaysia
– Similar in the Philippines, Vietnam, and
Indonesia
• Not possible to cool below minimum insect
development temperature

23/29
 Average of Daily Weather
Conditions in Bien Hoa

Low Average High

Temperature 22.8 27.3 30.9

RH 61 80 98

EMC 12.3 16.0 24.9

24/29
 Aeration Strategies for the Tropics

• Work by Zeledon
– Investigated aeration of US corn in Central
America
– Determined aeration during mean weather
conditions most effective
• Sinicio and Muir
– Aeration of wheat in Brazil
– Investigated numerous strategies, temperature
differential lowest cost

25/29
Typical RH and Temperature in Central America

Dr. Manuel Zeledon

26/29
 Use of Historical Weather Data

• Hourly weather data best

• Produce hourly data from daily data using
equations
• Useful for sizing fans, aeration strategies
• Rapidly check average runtime each year

27/29
 Summary

• Temperature and moisture content two

controlling variables
• Probably limited to 13% moisture,
maximum of 14%
• Weather in SE Asia limits potential for
reducing temperature to safe limits
– Have to reduce moisture content

28/29
 Acknowledgements
• Michael J. Buschermohle, University of
Tennessee
• Dirk E. Maier, Kansas State University
• Samuel McNeill, University of Kentucky
• Manuel E. Zeledón, Universidad de Costa Rica
• Ron Noyes, Oklahoma State University

29/29