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Published October 23, 2009, 11:22 AM

Challenging fall harvest leads to questions about soybean drying and winter storage

A challenging soybean harvest is creating many questions related to storage and drying.

By: Jim Stordahl, DL-Online

A challenging soybean harvest is creating many questions related to storage and drying.

Soybeans at 11 percent moisture have similar storage characteristics to wheat or corn at 13 percent moisture, so an allowable storage time chart for cereal grains can be used to estimate allowable storage times for soybeans.

For example, soybeans at 18 percent moisture content would be similar to cereal grains at 20 percent moisture, so would be expected to have an allowable storage time of about 50 days at 50 degrees. The allowable storage time is reduced to 25 days at 60 degrees and extended to about 90 days at 40 degrees.

The amount of natural air-drying that will occur in late October and November is limited.

The equilibrium moisture content of soybeans for air at 40 degrees and 70 percent relative humidity is 13.5 percent. At airflow rate of 1.0 cubic foot per minute per bushel is expected to dry 18 percent soybeans in about 70 days.

Adding supplemental heat to raise the air temperature by five degrees will permit drying the soybeans to about 11 percent moisture in about 55 days.

Only about one-half of the beans would be expected to dry by mid-November when outdoor temperatures become too cold to efficiently dry. Adding additional heat would cause the beans on the bottom of the bin to be dried to a lower moisture content and it would only slightly increase drying speed.

Cool the soybeans to between 20 and 30 degrees for winter storage and complete drying in the spring. Start drying when outdoor temperatures are averaging about 40 degrees.

Increasing the airflow rate will increase the drying speed. However, the fan horsepower required to achieve the higher airflow rate becomes excessive unless the grain depth is very shallow.

For a soybean depth of 22 feet, the rule-of-thumb is that it will take about 1.0 horsepower of fan for each 1,000 bushels of soybeans. It will require about 2.5 horsepower to achieve an airflow rate of 1.5 cfm/bu and about 5 horsepower for 2.0 cfm/bu.

The type of fan greatly affects the airflow provided per horsepower, so use a fan selection program such as developed by the University of Minnesota, and available at the North Dakota State University grain drying and storage web site, or use the fan chart for your fan that shows the amount of airflow delivered at various static pressures.

Soybeans can be dried in a high temperature dryer, but the plenum temperature needs to be limited to minimize damage to the beans. Refer to the manufacturer’s recommendations for maximum drying temperature. Typically the maximum drying temperature for non-food soybeans is about 130 degrees. Even at that temperature some skins and beans will be cracked. One study found that with a dryer temperature of 130 degrees, 50 to 90 percent of the skins were cracked and 20 to 70 percent of the beans were cracked. Another study found that 30 percent of the seed coats were cracked if the drying air relative humidity was 30 percent and 50 percent of the skins and about 8 percent of the beans were cracked at 20 percent relative humidity. The relative humidity is reduced by one-half for each 20 degrees that the air is warmed. Therefore, if air at 40 degrees and 80 percent relative humidity is warmed to 60 degrees, the relative humidity is reduced to 40 percent, and if it is heated to 80 degrees the relative humidity is reduced to 20 percent. Monitor the amount of damage occurring during drying and regulate the plenum temperature to obtain the acceptable amount of damage.

Food soybeans and seed beans must not have damage to the seed coat, so natural air or low temperature drying is the preferred drying method.

For more information, contact me at 800-450-2465 or stordahl@umn.edu. Source: Dr. Kenneth Hellevang, NDSU Agricultural Engineer.

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