Michael T. Talbot oL i
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Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences
University of Florida / John T. Woeste, Dean
Grain Drying and Storage on Florida Farms
Michael T. Talbot
The drying and storage of agricultural products date back to the
beginning of civilization. Drying of food and food products, the subject
of this publication, is one of the oldest methods of preservation. Other
preservation options are available, such as acid preservation, ensiling,
and oxygen free storage for grain, if the product is to be used as livestock
feed. For more information about these subjects refer to the extension
publication entitled High Moisture Grain Storage.
In agricultural work, drying refers to the removal of moisture until
the moisture content of the product is such that decrease in quality from
molds, enzymic activity (respiration and heating), and insects will be
Drying farm products offers the farmer the following advantages:
It permits an early harvest, which reduces the field loss of products
due to weather, insects, rodents, birds, and natural shattering, in
addition to permitting the farmer to prepare the ground for the
Following crop earlier in the year (or for double-cropping).
It allows planning the harvest season to make better use of labor
because harvesting will not be dependent on fluctuations of the
moisture content of grain in the field.
It allows long-term storage with little deterioration.
It permits taking advantage of possible higher prices a few months
It provides maintenance of seed viability, since the possibility of
the natural heating of grain with subsequent reduction or destruc-
tion of germination is decreased.
It enables the production of a better-quality product.
Michael T. Talbot is an Assistant Professor, Agricultural Engineering Department,
University of Florida, Gainesville, Florida 32611.
FUNDAMENTALS OF GRAIN DRYING AND STORAGE
The major objective in drying grains is to reduce the moisture content
so that spoilage will not occur before the grain is used. The objective of
proper grain storage is to maintain the characteristics that the grain
possesses immediately following harvesting and drying. Thus, viabil-
ity should be maintained for seed grain, milling and baking qualities
should be preserved for industrially used grain, and nutritive proper-
ties should be sustained for grain fed to animals.
During storage, the quality of grain cannot be improved. Grain im-
properly produced, harvested, or dried will remain of low quality no
matter how well it is stored. Many complaints about poor quality grain
refer to improper storage, but too early harvesting (high moisture con-
tent), improper combine settings (high cylinder speeds), and too rapid
drying (high drying temperature) are also possible culprits.
The principal causes of loss in quality and quantity in grain during
storage are fungi, insects, rodents, mites, respiration, and moisture
migration. Although storage problems are common during bad harvest
years, many problems result from poor dry grain management prac-
tices as well. Proper aeration and insect control, along with adequate
observation, minimize these dry grain difficulties. For more informa-
tion on these subjects, refer to the extension publications entitled
Management of Stored Grain with Aeration and Pest Management
Strategies for Storing Grain in Florida.
The moisture content of a grain is an indicator of maturity or quality.
Two methods are used to represent moisture content: wet basis and dry
basis. The moisture content on a wet basis is obtained by dividing the
weight of water in grain by its total weight. The moisture content on a
dry basis is obtained by dividing the weight of the water in the grain by
its oven dry weight. Market pricing structure is based primarily on the
wet basis moisture content. Additional information on methods of ob-
taining samples and of determining moisture content can be found in
the extension publications entitled Grain Sampling and Grain
For proper drying and storage, the moisture content of the grain is
controlled. The three most important grain moisture content levels are
the harvest moisture content, the initial storage moisture content, and
the equilibrium moisture content.
The harvest moisture content will depend on many factors, but this is
the starting point for the drying process (or storage if the grain is field
dried to an acceptable initial storage moisture content). The storage
moisture content is the moisture content recommended for safe crop
storage. The maximum moisture contents of several crops for satisfac-
tory air-drying with heated air and for safe storage in tight structures
are shown in Table 1.
The third moisture content level mentioned above is the equilibrium
moisture content. Grain contains moisture, and depending on the
amount of moisture in the surrounding air, grain will gain or lose
moisture. The amount of moisture in the surrounding air is determined
by the air temperature and relative humidity. The grain is in
equilibrium with the surrounding air when there is no moisture flow
between it and the grain. The moisture content of the grain when it is in
equilibrium with the surrounding air is the equilibrium moisture con-
tent. The equilibrium moisture content is important specifically in the
drying and storage of grains. The equilibrium moisture content deter-
mines the minimum moisture content to which the grain can be dried
under a given set of drying conditions (temperature and relative
humidity). Since the humidity and temperature of surrounding air
change throughout storage, the storage moisture content will also
change. The equilibrium moisture content of several grains is shown in
Table 2. Note that Table 2 is for a constant temperature of 77 F. The
equilibrium moisture content also varies with temperature, but not by
a great amount. Table 3 gives the approximate moisture content for
shelled corn in equilibrium with air at various humidity and
temperature combinations. Other grains exhibit a similar pattern.
Drying Temperature and Relative Humidity
Air is a mixture of several gases, and it contains heat. When air is dry,
it occupies a given space. As it occurs naturally, air is rarely completely
dry. Air shares space with water vapor. The amount of space occupied
by the water vapor is a major concern when drying grains. The heat con-
tent of air is also important because heat is needed to change liquid
water (in the grain) to vapor. The vapor then mixes with the drying air
and is removed.
Air serves two basic functions in grain drying. First, the air supplies
the necessary heat for moisture evaporation; second, the air serves as a
carrier of the evaporated moisture. Both functions are essential,
regardless of the drying system used. The amount of moisture which
can be removed from grain depends on the relative humidity and
temperature of the drying air, the airflow rate, and the moisture con-
tent of the grain.
Air temperature determines heat content and to a large extent the
total water-carrying capacity of the drying air. As air is heated, the heat
content and the water-carrying capacity both increase. Relative
Table 1. Drying and Storage Recommendations
Oats Corn Soybeans Sorghum Wheat
Maximum moisture content of crop at harvesting
for satisfactory drying with heated air 25% 35% 25% 25% 25%
Maximum moisture content of crop for safe
storage in a tight structure 13* 13% 11% 12% 13%
Moisture content (%) at harvest and 25% 5.0 8.7 11.0 9.2
pounds of water per bushel 20% 2.7 4.7 6.6 5.3 5.2
which must be removed for 18% 1.9 3.3 5.1 3.9 3.7
safestorage 16% 1.1 1.9 3.5 2.5 2.1
Maximum safe temperature of heated air entering
crop for drying when crop is to be used for:
Seed 1100F 1100F 1100F 1100F 1100F
Commercial Purposes 1400F 1300F 1200F 1400F 1400F
Animal Feed 1800F 1800F 1800F 1800F
Preferred depth of crop for batch
drying with heated air 16-24" 16-24" 16-24" 16-24" 16-24"
12% for seed
Table 2. Grain Moisture Content (Percentage Wet Basis) in Equilibrium with Air at 77F and Various Relative Humidities
Relative Humidity, Percentage
10 20 30 40 50 60 70 80 90 100
Material Equilibrium Moisture Content, Percentage Wet Basis
Oats 4.1 6.6 8.1 9.1 10.3 11.8 13.0 14.9 18.5 24.1
Shelled Corn 5.1 7.2 8.5 9.8 11.2 12.9 13.9 15.5 18.9 24.6
Soybeans 5.5 6.5 7.1 8.0 9.3 11.5 14.8 18.8
Sorghum 4.4 7.3 8.6 9.8 11.0 12.0 13.8 15.8 18.8 21.9
Wheat 5.2 7.5 8.6 9.4 10.5 11.8 13.7 16.0 19.7 26.3
As an example of how this table may be used, select an average relative humidity of 40% for the 770F temperature. In the table it is noted that
9.8% is the equilibrium moisture content for sorghum.
Table 3. Equilibrium Moisture Content of Shelled Corn for Various Air Temperatures and Relative Humidities
Relative Humidity, Percentage
10 20 30 40 50 60 70 80 90
Temp. deg. F Equilibrium Moisture Content, Percentage Wet Basis
20 9.4 11.1 12.4 13.6 14.8 16.1 17.6 19.4 22.2
25 8.8 10.5 11.9 13.1 14.3 15.6 17.1 19.0 21.8
30 8.3 10.1 11.4 12.7 13.9 15.2 16.7 18.6 21.1
35 7.9 9.6 11.0 12.3 13.5 14.8 16.3 18.2 20.8
40 7.4 9.2 10.6 11.9 13.1 14.5 16.0 17.9 20.5
45 7.1 8.8 10.2 11.5 12.8 14.1 15.7 17.6 20.5
50 6.7 8.5 9.9 11.2 12.5 13.8 15.4 17.3 20.2
55 6.3 8.2 9.6 10.9 12.2 13.5 15.1 17.0 20.0
60 6.0 7.9 9.3 10.6 11.9 13.3 14.8 16.8 19.7
65 5.7 7.6 9.0 10.3 11.6 13.0 14.6 16.5 19.5
70 5.4 7.3 8.7 10.0 11.4 12.7 14.3 16.3 19.3
75 5.1 7.0 8.5 9.8 11.1 12.5 14.1 16.1 19.1
80 4.6 6.7 8.2 9.6 10.9 12.3 13.9 15.9 18.9
85 4.6 6.5 8.0 9.3 10.7 12.1 13.7 15.7 18.7
90 4.4 6.3 7.7 9.1 10.4 11.9 13.5 15.5 18.5
95 4.1 6.0 7.5 8.9 10.2 11.7 13.3 15.3 18.4
100 3.9 5.8 7.3 8.7 10.0 11.5 13.1 15.1 18.2
As an example of how this table may be used, select an average temperature and relative humidity for a given period, say 60% relative humidity
and 500F temperature. In the table it is noted that 13.8% is the equilibrium moisture content. This means that for these conditions the 13.8%
shelled corn will neither lose nor gain moisture from the air.
humidity is an indication of the amount of water vapor in the air. Low
relative humidity indicates the air has high water vapor holding
capacity (high drying potential), whereas high relative humidity
indicates low water vapor holding capacity (low drying potential).
The temperature of the drying air also affects dried grain quality.
Grain to be fed or milled can be dried at 150 F or higher, while grain for
seed should not be heated above 110 F to prevent reduced germination.
High heat often cracks seed coats, leading to grain breakage in han-
dling. Maximum safe drying air temperatures for several grains are
listed in Table 1.
Grain Response to Drying
When grain is placed in a drying or storage structure and air is forced
through the grain, a drying zone is established at the point where the
air enters the facility (Figure 1). The drying zone moves uniformly
through the grain in the direction of airflow. The rate at which it moves
depends on the volume, temperature, and relative humidity of the air
and on the moisture content of the grain. The same phenomenon occurs
in all types of dryers, although it is better defined in batch-in-bin and in-
The grain next to the air entry point normally dries almost to its
equilibrium moisture content with the entering air. In many cases, the
. : :Dry Grain : .
Figure 1. Grain is dried from the point of air entry with the drying front
moving in the direction of air flow. The wetter grain occurs
where the air leaves the grain layer.
grain at the bottom overdries because the relative humidity of the air
is extremely low. Grain where the air leaves the dryer is the last to dry,
assuming width of drying column and depth of grain are uniform.
One of the greatest problems in on-farm, in-bin drying systems is that
of overdrying the grain near the perforated floor. A simple solution is to
control the relative humidity of the drying air to obtain the desired
equilibrium moisture content level. In addition to an upper
temperature limit set on a thermostatic control, many installations
have a humidistat wired in series with the thermostat to cut off heat
when relative humidity drops to 55%. This arrangement is very effec-
tive in preventing overdrying and loss of quality, provided the calibra-
tion of the humidistat remains correct. Unfortunately, humidistats
generally must be calibrated at least once every season.
Proper Airflow and Drying Depth
Air must be forced through the grain mass at rates sufficient to
remove moisture fast enough to prevent mold and stay ahead of
spoilage. This rate varies, depending upon the type drying system used,
and is normally expressed in units of cubic feet per minute (c.f.m.) for
each square foot of drying floor. The fan or fans selected must be capable
of providing this airflow rate while operating against the static
pressure head created by the grain resistance to airflow.
The drying fan is the key to drying capacity for all dryers. Insufficient
fan capacity is a common problem in bin drying systems. Extra initial
investment for adequate fan capacity will improve dryer performance
and may avoid the necessity of purchasing a bigger fan later. Fan selec-
tion for grain drying and aeration will be covered in the extension
publication entitled Fan Selection for Grain Drying and Aeration.
The rate of airflow and the depth of the grain are also factors which af-
fect the width of the drying zone. A high rate of airflow or a shallow
grain depth can greatly increase the width of the drying zone. The zone
can be made so extensive that it includes the entire depth of grain being
dried. In reality this is what is done with many commercial dryers,
where the drying column depth is only about 18 inches and the airflow
rate is as high as 80 or more c.f.m. per bushel.
There are several other methods used to minimize the effects of the
drying zone. Reducing grain depths to 2 feet or less (which automati-
cally allows greater airflow) is one method already mentioned for bin
drying. The use of stirrers is another method which has been employed
recently. Bin recirculators are also used to keep the grain mixed during
drying. Constant movement of the grain or recirculation, thin columns,
high airflow rates, and directional airflow are used singly or in com-
bination in commercial dryers to obtain uniform drying and to avoid
problems with drying zones.
SYSTEMS FOR GRAIN DRYING
The best known methods of drying are batch-in-bin, in-storage or
layer fill, column batch, and continuous flow (Figures 2 and 3). In addi-
tion, there are modifications like in-bin stirrers, in-bin recirculators,
dryeration, low-temperature, and natural air-drying. Solar drying
combines supplemental heat and natural air-drying similar to in-
storage or low-temperature drying. ITailer and wagon driers are
classed as batch-in-bin dryers.
Dry in Layers
Figure 2. Schematics of several batch drying processes.
\X XI X/
meant, it is difficult to arrive at the one best system for a farm. Many
factors influence the selection, such as volume to be dried, harvesting
rate, volume to be stored, various grains and other crops to be handled,
and available labor. Select Graing a storage system depends on
I, Dry Grain ^->- Exhaust Air
Figurhe3. type illustration size of farmthe 3 types of continuous cash grain, beef feeding,
swine production, etc.
'Ibtal annual production
With so many different methods and ratescombinations of drying equip-
ment, it is difficult to arrive at the one best system for a farm. Many
factors ing storafluence faselction, such as volume to be dried, harvesting
rate, volume to be stored, various grains and other crops to be handled, experience and preference
and available labor. Selecting a storage system depends on
SThe producer is faced with the problem of drying- cash grain, beef feeding,ly as it
swine harvested. The system selected or available for his use will great-
ly influence drying rate. Sometimes a combination of drying systems
using existing equipment or a relatively small investment in new
equipment will enable the farmer to reach his harvesting goals.
Studio Personal experience and preferquipment which will do the requiredjob at
the lowest fixed cost per bushel dried annually will in the final analysis
be the producer is faced with the problem of dryr. Planning grain as rapidly as it
strcan be willharvested. The system selected or availablcatie for his use will great-nning
ly influence drying rate. Sometimes a combination of drying systems
using existing equipment or a relatively small investment in new
equipment will enable the farmer to reach his harvesting goals.
Studies have shown that equipment which will do the required job at
the lowest fixed cost per bushel dried annually will in the final analysis
be the most economical for the producer. Planning on-farm drying and
storage will be covered in an extension publication entitled Planning
On-Farm Drying and Storage.
When a bin is used to dry a batch of grain, usually not more than 2 to 3
feet in depth, it is called batch-in-bin drying (Figure 2). The batch of
grain is placed in the bin, leveled, dried in a relatively short period of
time, and removed. Temperatures of 120 oF to 160 F are generally used
with only thermostatic control. In the case of seed drying, the max-
imum temperature permitted is around 110 F. Airflow per bushel of
grain in the batch is fairly high since the depth of grain in the bin is
limited. The batch of grain is cooled before removal from the bin.
In-Storage or Layer-Fill Drying
In-storage drying is used almost exclusively in metal storage bins
(Figure 2). The bin is filled, one layer at a time. After a layer is almost
dry, another layer is placed in the bin and the process continues until
the bin is filled. Generally, bins are equipped with fans to move a
minimum of 5 c.f.m. per bushel at 1.5 inches static pressure. This is the
necessary airflow for 5 feet of shelled corn at 25% moisture content. The
depth of grain in a layer should be varied based on moisture content.
The higher the grain moisture content, the shallower the grain depth
should be. Consequently, the airflow rate will also vary.
A limitation of heat rise and a humidistat are used with in-storage
drying. The burner is normally set to give from 10 to 20 F temperature
rise over ambient, or outside, air temperature. The humidistat, located
in the air plenum, is set at 55 to 60% relative humidity for grain that is
to be stored or at a higher level for grain that will remain in the bin only
a short time before marketing.
In-storage drying is a slow drying process which might fit well into an
operation where less than 10,000 bushels are dried and stored for farm use
or later sale. Generally only about 4 feet can be dried per level layer, and
more grain should not be added until the preceding layer has begun to dry
on top. Most manufacturers give more specific guides on depth of layers,
based on grain moisture content as related to size of equipment used.
A batch dryer is usually a portable or stationary unit made specifi-
cally for drying grain (Figures 2 and 4). There is little or no storage
capacity associated with the dryer. Drying capacities may range from
70 to 750 bushels per hour. The capacity of the dryer is often rated at 5
percentage points of moisture removal. The airflow rate is high at up to
100 c.f.m. per bushel or more. The shallow drying columns are 12 to 24
inches thick. Operating temperatures generally are 140 to 1800F. Batch
Tilt Up Grain
Hopper to Load Dryer Fan and Heat Unit
Figure. PTO-powered portable batch dryer with self-contained
dryers are designed to remove about 10 percentage points of moisture
from an 18-inch thick layer of grain in approximately 3 hours when
operating at 1400
An additional 30 to 45 minutes is required to cool the grain if cooling
facilities are not provided. In recirculating batch dryers the grain is
constantly being turned, recirculated, or mixed during the drying
operation. Some batch dryers are equipped with electronic controls to
operate the loading and unloading equipment. Some dryers also re-
quire a surge bin to keep the dryer filled as the grain shrinks in volume
For large grain producers and commercial grain dryers the continuous-
flow dryer is the most popular method of drying. Approximately half of
all dryer models commercially available are continuous-flow. The drying
capacity of continuous-flow dryers generally ranges from 100 to 3000
bushels per hour for 5 percentage points of moisture removal.
As with some of the batch dryers, the grain is dried in relatively thin
columns of 12 to 24 inches. Dry grain is automatically discharged at the
bottom as wet grain is continuously added at the top of the columns. Air
of quite high temperature, up to 250 F, is forced through the lower sec-
tion. The grain remains in the dryer from 2 to 3 hours.
There are three basic designs for continuous-flow dryers (Figure 3).
These designs are based on the direction of airflow relative to the flow of
grain. The basic designs are cross flow, concurrent flow, and counter
flow. Anyone interested in a discussion of the advantages and disad-
vantages of these designs should contact a specialist on grain drying
Using fast-drying or high-temperature methods of drying generally
causes quality deterioration in grain. The damage is done when high
heat is used to dry the grain completely. Fast moisture removal (below
18%) can increase kernel breakage, impair milling properties, and in-
crease chances for mold development and insect damage in storage.
To overcome these problems and save energy a dryeration process was
developed which makes use of a combination of the drying procedures
already discussed (Figure 2). The dryeration process is described below.
1. Stop rapid- or high-temperature drying in batch, batch-in-bin, or
continuous-flow dryers without cooling sections when the grain
reaches a moisture level of 16 to 18%.
2. Transfer the hot grain to a bin for tempering 8 to 12 hours.
3. After the grain has tempered, cool very slowly using only 1/2 c.f.m.
per bushel for approximately 12 hours.
In the process the grain will give up from 2 to 3% of its moisture, since
nearly all the heat in it is used in evaporation. Theprocess will not work
if the grain is cooled too fast. Because of condensation problems, it can-
not be cooled where it is stored unless the bin is insulated. Using the
dryeration process helps gain additional drying capacity, in addition to
maintaining grain quality. With two supplemental tempering-cooling
bins, the capacity of a high-temperature batch or continuous-flow drier
can be increased about 50%.
Stirring and Recirculating Additions to Bin Drying
The problems associated with batch-in-bin and in-storage drying
have been somewhat alleviated by equipment to move the grain within
the bin (Figure 2). Two types of equipment are available to mix or move
the dry grain in the bin to prevent overdrying.
Grain-stirring devices are the more common of this in-bin equipment.
They simply consist of one or more vertical auger screws which lift the
grain from near the perforated floor upward toward the top of the grain
mass. The vertical-auger screw, powered from a horizontal radial arm
near the top of the bin, is moved from near the center of the bin to the
walls and then returned. As the auger screw moves outwards and
returns, it also revolves around the center of the bin.
This stirring device reduces overdrying problems and permits the
drying of deeper batches of grain at temperatures above those normally
used to minimize overdrying. Airflow in the bin may be increased by as
much as 10%, which in turn increases the rate of water removal.
Limited research seems to indicate little advantage until grain depth
reaches 3 feet or more. Stirring devices cause the fines in the bin to
move to the bottom. The fines are not appreciably scattered at the bot-
tom of the bin if the grain was placed in the bin without the aid of a
spreader. However, the stirrer does break up areas where wet grain,
fines, or trash have accumulated. The stirrer does not cause mechanical
damage to grain. Major field problems have been reported concerning
mechanical breakdown of equipment, but these problems should lessen
with time. Just above the bin floor, there is an approximately 4-inch
layer of grain which is overdried, and grain damage near the bin wall
has been reported where the grain was left in storage. Also, there has
been concern about bin wall failure, especially at the bolted joints.
Grain recirculators have been developed to overcome problems with
limited batch size and overdrying of lower layers in bins. A horizontal
revolving auger on the bin floor draws grain from the bottom of the bin
and delivers it to the boot of a vertical center auger. The vertical auger
elevates the grain to the top, where it is redistributed in the bin or
elevated by another auger to an adjoining cooling bin. Recirculators in
batch-in-bin drying work on the counter-flow principle, as discussed
above: air moves upward as the grain moves downward. The grain is ex-
posed to the hottest air while it is still losing appreciable moisture,
which results in less damage. As with stirrers, the same movement of
fines and mechanical problems might be expected.
Low-Temperature, Natural-Air, and Solar Drying
None of these methods are recommended for Florida conditions. Low-
temperature, natural-air, and solar drying are all slow drying processes
aimed primarily at saving energy and reducing drying cost. As men-
tioned earlier, these are not the only methods of reducing energy use.
Low-temperature drying is similar to in-bin layer drying except
very low amounts of heat are added, usually 5 to 70F. Airflow rates are
usually between 1 and 3 c.f.m. per bushel. Average outside air
temperature should be below 50F before heat is added, which limits
this process to late October for most of Florida. Drying time will
generally run over a month, depending on the moisture content of the
grain and other factors.
Unheated or natural air-drying is similar to low-temperature drying,
except that no heat is added other than approximately 20F from the fan
motor. Application of this slow drying process is limited, since the high
airflow rates required are almost economically prohibitive.
Solar heat drying involves some combination of supplemental heat,
low-temperature, and natural air-drying. The system consists of large
areas of flat black plates or surfaces covered by one or more transparent
cover plates of glass or plastic. Radiant energy from the sun is absorbed
directly by the uncovered surface or passes through the transparent
cover to be absorbed. Air is forced or drawn over and/or under the black
surface to pick up heat. Typical installations, which provide 3 c.f.m. of
air per bushel of grain (dried in layers 2 to 4 feet deep), will generate 9F
elevation in temperature, depending on weather conditions.
Greater temperature elevations can be achieved with commercial
solar panels, but larger panels may be prohibitively expensive. More
information about solar grain drying will be covered in an extension
publication entitled Solar Crop Drying.
High average humidities and temperatures are a cause for great con-
cern in the storage of grain crops in Florida. This combination is ideally
suited to the growth of molds and production of aflatoxin in stored
grains. Levels of aflatoxin above 20 parts per billion may cause serious
health problems in livestock and poultry, especially younger animals
bT keep grains in good condition over a period of time, steps must be
taken to control temperature and moisture content during storage. Tb
do this, the grain must be placed in storage at an acceptable moisture
content and should be free of trash and fines, as well as cracked kernels,
which can cause "hot spots" and/or "wet spots."
Storage facilities for grains should be weatherproof and free from in-
sect and disease bearing litter. These facilities should be equipped with
properly designed aeration systems capable of moving a sufficient
volume of air through the mass of grain to control temperature and
moisture levels within desired limits.
Grains stored in metal bins without aeration can spoil in storage even
though the grain was originally dried to the recommended level. This is
caused by moisture migration, insects, and molds. Problems associated
with all three can be significantly reduced through proper moisture
Stored grains or beans harvested in the summer or fall produce air
currents, within the bin, which results in moisture condensation. The
process can occur within a completely enclosed and sealed bin and is
caused by temperature differences in the grain. As outdoor air
temperature decreases, the bin walls cool, which in turn cools the grain
layer near the walls. As the air near the walls cools and becomes
heavier, it moves toward the bottom of the bin. The interior holds heat
and warms the air. This makes the air expand and become light, caus-
ing it to rise. As this warm, moist air rises, it passes through the cooler
top layer of grain. This causes the warm, moist air to cool and produce
condensation. As this warm, moist air continues to rise, it comes into
contact with the cold roof and further condensation occurs. This condi-
tion, known as moisture migration, creates a wet zone in the top of the
bin. This process is shown in Figure 5. Mold and insects thrive in the
warm, moist areas.
Moisture control of stored grain will be covered in an extension
publication entitled Moisture Migration in Stored Grains.
Moisture migration can be prevented in grain bins by forcing low
volumes of air through the bin contents, producing uniform
temperatures throughout the mass. This process is called "aeration."
The aeration fan should be operated when the outside temperature is
10 F or more below the temperature of the grain in the bins. If it is
desirable to lower the moisture content level, the fan should be
Cold Air Grain Surface
High Moisture Zone
Warm Grain Moisture on
CO t1 r
Figure 5. Convection air currents caused by differences in
temperature produce moisture condensation in top layer
operated when humidity of the outside air is 65% or below. Operation of
the fans on a clear day immediately following a cold front is highly
Aeration fans should have a capacity of 1/4 to 1/2 c.f.m. per bushel
capacity of the bin. Fans should deliver this volume of air at the max-
imum static pressure developed by the stored grain (normally 1 to 3
inches of static pressure, depending upon the size of the bin, moisture
content, and depth of the grain).
Aeration systems should direct the airflow downward (see Figure 6).
This will allow for warm, moisture-laden air to be exhausted to the out-
side from the bottom of the bin. Using a downward airflow will prevent
condensation from forming on the inside of the storage bin roof, from
where it could fall back on the grain and cause spoilage.
The best aeration system is one that incorporates a moisture tester
and a grain probe. Weekly checks of the bin should be made. The bin
should never go unchecked for over a month. A weekly check will detect
Roof Vent Hatch
Plenum Perforated Floor
Transition Duct A
1 Aeration Fan
Figure 6. Typical aeration system used with metal bin for long-term
storage of grains.
hot spots and moisture buildup before extensive harm can be caused to
the grain. Start the fans immediately if any moisture or heat buildup is
If a batch drying bin is used for grain storage, the high volume drying
fans may be used to aerate the crop. Fans should be operated 2 to 3
hours, or longer, 2 to 3 times a week, on clear days when the relative
humidity is below 65%. High volumes of air moving upward through
the stored grain generally do not cause moisture to accumulate in the
top layers of the stored product. Of course, no heat should be
added when aerating.
Adequate air space should be provided between the surface of the
stored grains and the roof or ceiling of the storage building. Storage of
seed grain in buildings where temperatures may become too high
should be avoided.
The latest recommendations on amount and type of insecticides to use
on stored grains should be checked and followed carefully. This is ex-
tremely important for grain intended for human consumption. Addi-
tional information on sanitation and insect control can be found in the
extension publication entitled Pest Management Strategies for Storing
Grain in Florida.
Machinery and handling methods must be carefully planned to main-
tain grain quality and to provide convenient, efficient, and economical
grain conveying. Conveying equipment for filling and emptying bins
and transferring grain from one bin to another may be either portable
or stationary. Portable equipment works better for systems with a few
small, individual bins. Stationary equipment is usually better for
larger storage systems.
For most systems, auger conveyors and elevators are the most prac-
tical choices. They are easily moved, reasonably priced, and relatively
trouble free. They can be bought in 10 to 40 foot lengths. Single-tube,
6-inch diameter elevators move up to 1500 bushels of grain per hour,
depending on the height of lift and auger speed. Portable auger con-
veyors in short lengths and small sizes can be moved about by hand.
When in operation, they may be leaned against a building or grain con-
veying truck or trailer. Larger sizes should be mounted on wheels and
raised with a windless or other device.
Improperly selected or operated conveyors and elevators may crack or
crush the grain, which invites insect infestation and lowers both grade
and germination. If pneumatic conveyors are used, they should be con-
structed to prevent cracking of the grain kernels by impact. Sharp
turns in conveyor ducts must be avoided and air speed must be carefully
controlled. Pneumatic conveyors and grain conveying in general will
be covered in the extension publications entitled Grain Conveying and
Employ preharvest preventive management practices to insure
proper grain bin sanitation and insect control.
Do not dry grain to a moisture content below that required for safe
storage. First, the extra drying takes longer and costs more.
Second, the extra water removed from the grain could have been
sold at the price of grain.
Aerate grain to help maintain quality in storage. Aeration is low-
volume ventilation (about 1/4 c.f.m. per bushel) to maintain
uniform temperature throughout a grain mass and to prevent
Monitor grain during storage to head off storage problems. Check
every week or two for musty or spoiled odors, crusting, moisture
condensation, elevated temperature, insects, rodents, etc.
Grain drying and handling can be dangerous. Transport augers can
hit power lines, unguarded augers can catch hands or feet, and fans and
shafts can catch unsuspecting victims.
A deadly hazard exists for anyone in a grain bin when the unloading
auger is started. Deaths occur every year from suffocation and injuries
caused by unloading augers. Many of these victims are children.
Disconnect power to the unloading auger before entering bins. A
knotted safety rope hanging near the center of the bin offers great pro-
tection. Have a second person standing by to offer assistance and sum-
Air pockets sometimes form when grain bridges over unloading
augers because of spoiled grain or moisture. Never walk on this crusted
surface; the pocket can collapse and leave a big hole.
Wear an effective dust mask when exposed to grain dust. Avoid
breathing dust from moldy or spoiled grain.
Fumigants are dangerous and should be applied only by trained, ex-
perienced operators working in pairs.
Insecticides are poisonous. They should be used only when needed,
and they should be handled with extreme care. Directions and precau-
tions on container labels should be carefully followed.
When there are children on the farm, never engage any machinery
before checking on the possible presence of a child.
This public document was promulgated at a cost of $1127.00, or 36 cents
per copy, to provide information on grain drying and storage on Florida
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORI-
DA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, K. R.
Tefertlller, director, In cooperation with the United States Department Il AS
of Agriculture, publishes this information to further the Purpose of the
May 8 and June 30, 1914 Acts of Congress; and Is authorized to pro-
vide research, educational Information and other services only to Indi-
viduals and Institutions that function without regard to race, color, sex or national ori-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulk
rates or copies for out-of-state purchasers Is available from C. M. Hinton, Publications
Distribution Center, IFAS Building 664, University of Florida, Gainesvllle, Florida
32611. Before publicizing this publication, editors should contact this address to deter-