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The goal of soybean drying is to reduce its moisture content to meet the recommended
levels for safe, long-term storage. When placed in storage, soybean should be dried
quickly to a moisture level of about 12% to minimize any quality deterioration. Soybean
drying can be accomplished in bins by blowing large volumes of dry air through the
grain. This website will explore the challenges of soybean drying and storage.
Arkansas farmers harvested more than 3.1 million acres of soybeans in 2013. With a
state average of 43 bushels/acre, Arkansas soybean production reached 133 million
Soybean producers are increasingly interested in on-farm drying and storage due to
the various advantages they offer. For example, producers can afford to harvest soybean
early at higher moisture contents than normal to reduce the possibility of harvest
losses. Additionally, they may harvest soybean at faster rates if daily harvesting
hours can be extended. Moreover, on-farm storage may provide marketing flexibility,
and advantage, during soybean selling process.
Since soybean quality is highest at harvest, soybean producers should promptly dry
newly harvested beans to safe moisture levels in order to maintain their quality.
In most cases, producers are able to adapt dryers that were designed for other grains
for use with soybeans. However, dryers that recirculate or stir grain constantly should
be avoided. Drying fans sized for rice or corn will produce greater airflow through
soybeans, resulting in a higher drying rate.
As mentioned earlier, in order to dry soybeans, a large quantity of high-quality drying
air is passed through the bean pile deposited in a drying bin. At a given air temperature
and relative humidity, there is a corresponding grain moisture content that the seed
will achieve and will not gain or lose water beyond. This moisture level is known
as the equilibrium moisture content (EMC).
The following table shows the EMC of soybeans at different values of air temperature
and relative humidity (RH). For example, if the air temperature and relative humidity
are 60oF and 70%, respectively, the beans will be dried to 13.7% moisture content, assuming
the air is allowed to pass through the soybean pile under the same conditions for
a sufficient time. Increasing air temperature to 80oF at the same RH level will decrease EMC to 13.2%.
To determine the value of EMC in your area now, click the link Determination of the current temperature and relative humidity based on your zip code.
To learn more about Equilibrium Moisture Content, see the fact sheet titled: "Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts"To determine the values of EMC for various grains, download the Excel sheet by clicking
the link: "Equilibrium Moisture Content"
Air is the means by which grain is dried, serving as the medium that transports moisture
away from the grain. Air contains some energy and humidity. Air quantity and quality
are important factors that determine the final moisture content of kernels. Its quantity
is the volume of air that the drying fan can deliver More specifically, it should
be divided by the number of bushels the drying air passes through, presented as cubic
feet per minute per bushel (CFM/bu). On the other hand, air quality is related to
its temperature, and relative humidity. Air properties are determined graphically
using a psychrometric chart.
It should be noted that a specific volume of air (say, 1 cubic foot) at a certain
temperature has the capability to hold a specific amount of moisture. Increasing the
temperature of that volume of air increases its capacity to carry more moisture. This
means that the air-drying capabilities could be increased by adding energy to the
drying air. As a rule, the drying time is reduced by passing larger volumes of air
over soybean, or by increasing the air temperature, or both. However, it should be
mentioned that soybeans are sensitive to temperature fluctuations and can be easily
damaged by air that is too hot or too dry. Accordingly, the recommended minimum airflow
rates for drying soybeans are shown in the following table.
Minimum airflow rates for drying soybeans
Measured moisture content
Minimum airflow rate
18% to 20%
15% to 18%
13% to 15%
11% to 13%
To select and maintain your fan, please see the fact sheet titled "Selection, Performance and Maintenance of Grain Bin Fans"
Each soybean kernel contains dry matter and oil, which represent the grain’s primary
value, in addition to water. Most buyers use the moisture content (MC) of 13.0% as
the base moisture for soybeans. When grain is delivered to the elevator above its
base MC, buyers use a factor called “the shrink factor” in order to adjust the quantity
for the excess moisture. This is because grain buyers will not pay for the cost of
removing the excess water. Applying the shrink factor approximates the equivalent
number of bushels that would be in the load if the grain were dried to the base MC.
Conversely, some farmers often deliver grain to the elevator at moisture levels below
the base MC. This case is also less profitable to the producer since the buyer will
not apply an expansion factor. These factors clearly demonstrate how sensitive the
soybean production economics are to the moisture content of the soybean kernels sold.
A good example to demonstrate the potential loss due to soybean shrinkage using 200
tons of dry matter as basis for calculations is shown in following table. It is clear
from column 6 that the penalty due to shrinkage increases with the increase in the
soybean moisture. The buyer determines the penalty due to shrinkage as follows: Shrinkage
penalty bu=total weight(bu)×0.013×[MC decimal−0.13]. It is clear that marketing soybean
at any moisture level greater than 13.0% will decrease the total profit. Table 3. Effects of shrinkage factor on total loss.
On-farm drying methods
As mentioned earlier, there are different factors that should be taken into account
while drying soybeans. Therefore, drying systems that were designed for other grain
can be used to dry soybeans but only after careful selection of the appropriate air
temperature and relative humidity levels. Drying systems that facilitate extreme high
temperatures for air-drying (130oF-150oF) should be avoided when drying soybeans to
minimize seedcoat cracks. Batch and continuous-flow drying systems are less desirable
in drying soybeans because the heat input is difficult to reduce, not to mention they
require more handling than is required for in-bin drying systems. Consequently, bin-drying
systems; i.e., natural-air drying and low-temperature drying, are usually the best
options for drying soybeans.
Natural-air drying is a technique used to dry soybeans by passing unheated (natural)
air through the soybean mass until its moisture content reaches the EMC level. Since
soybeans are hygroscopic (susceptible to moisture absorption), their moisture content
will adjust according to the quality of air used. Therefore, drying soybean with natural
air can be accomplished only if the air temperature and relative humidity conditions
allow a net moisture transfer from soybean to the air. The drying speed under natural-air
drying depends on the moisture content of the soybeans as well as the temperature
and relative humidity of the drying air. Natural air bin drying systems are very efficient
for drying soybeans but can only be used under favorable weather conditions. As a
rule, air temperature should be above 60oF and the humidity below 75% to achieve natural-air
A natural-air drying system typically consists of a bin with perforated floor equipped
with a drying fan, a grain spreader, a sweep auger and an unloading auger. Stirring
devices may also be added. An external energy source, typically from fossil fuels,
is required to supply the electricity for the drying fan and the various augers. On
the other hand, the energy required for evaporating the moisture from the soybeans
comes from the energy already present in the ambient air. Successful soybean drying
with natural air is usually the most energy-efficient method of drying. However, it
is also the slowest drying method and has the greatest potential for grain spoilage.
Furthermore, natural-air drying is extremely sensitive to weather conditions. Consequently,
it requires the highest level of management if spoilage problems are to be prevented.
Special attention should be paid to soybean management practices in natural air when
excessive moisture or adverse weather is encountered. Unheated air-drying will not
be feasible in extended periods of damp weather.
In low-temperature drying of soybean, the drying air is heated 10°F above ambient
conditions. Similar to natural-air drying, low-temperature drying also requires a
perforated-floor bin, a grain spreader, under-floor unloading auger and a sweep auger.
A stirring device may also be added. The low-temperature drying technique has a higher
potential to dry soybean to the accepted long-term storage moisture contents when
compared to natural-air drying. Soybeans could be dried using the low-temperature
drying technique then stored in the same bin, thus minimizing handling and labor costs.
Generally, the comparative total cost for drying decreases as less energy is used
to heat the drying air. Thus, successful low-temperature drying is relatively economical
in terms of energy cost when compared to higher temperature techniques. Attention
should be paid also to the fact that more energy is required to operate the drying
fans than is needed to heat the air.
Corn Dryers Under Specific Environment
Soybean producers could use their corn dryers under a specific environment. They should
set the drying air temperature lower than they use for corn and they also should avoid
dryers that recirculate the crop during drying. This is because the soybeans are susceptible
to fracture if they are dried too fast or handled roughly. Some researchers reported
that it might be possible to use the corn dryer for soybean drying. Producers should
limit drying air temperature to 130°F-140°F for commercial beans and 100°F for seed
beans. Retention time in the heated section of dryers should be less than 30 minutes.
They also recommended that the relative humidity of the drying air should be greater
than 40% to help prevent skin cracks. Studies have shown that it is possible to develop
50-100% splits in less than 5 minutes of exposure time if incorrect drying procedures
are followed. Cracked soybeans will not keep well in storage and will break easily
during handling. Therefore, in some cases, it is recommended to use a simple shield
to recirculate some of the moist drying air back to increase the humidity of the drying
air. This approach facilitates a safe and gradual increase in drying temperatures.
Alternatively, filling the bin halfway will double the drying air volume per bushel
of grain, thus decreasing the time need to accomplish the desired drying levels.
The following are some tips that may help soybean producers achieve better grain quality
while minimizing the drying cost:
It is quite important to maintain the drying cost in the minimum level in order to
maximize the profits (returns on investment). As mentioned earlier, in order to dry
soybeans, producers need to determine the total pounds of water they will remove from
one bushel of grain. The number of BTUs to extract 1 pound of water will vary from
1,100 to 1,400, depending on how easily moisture is given up by the kernel. As the
kernel begins to dry, more energy is needed to extract the last bit of moisture. A
good estimate is to use an average of 1,200 BTU/pound of water to calculate the energy
needed to get rid of 1 pound of moisture. Table 3 summarizes the BTU/unit of fuel
as well as the burning efficiency.