2004 Agronomy Meeting Poster #2

4/26/05 - Soil Compaction: What Can You Do? MN Crop News

Soybean Drying, Handling, and Storage

Bill Wilcke and Vance Morey
University of Minnesota Biosystems and Agricultural Engineering Department

Ken Hellevang
North Dakota State University Agricultural and Biosystems Engineering Department

Soybeans are usually traded on a 13% moisture basis, so it is to the farmer's advantage to harvest, store, and sell soybeans as close to 13% moisture (wet basis) as possible. Soybeans that are wetter than 13% moisture are likely to mold under warm conditions and buyers usually apply shrink factors and drying charges when wet beans are delivered. On the other hand, soybeans that are drier than 13% moisture are more likely to split during handling and since they weigh less, fewer bushels are available for sale. If the storage temperature is kept below about 60°F, soybeans can usually be held for at least six months at 13% moisture without mold problems. For storage under warmer temperatures or for storage times longer than six months, however, the recommended moisture content is 11%.

Storage Management for 11 to 13% Moisture Soybeans

Soybeans that are harvested at 11 to 13% moisture can be placed directly into ordinary storage bins equipped with simple aeration systems (perforated ducts or pads and relatively small fans). The suggested winter storage temperature for grains and oilseeds in the upper Midwest is 20 to 30°F. Since soybeans are usually harvested at temperatures well above 30°F, it is necessary to cool them by operating aeration fans during cool weather. Rather than waiting until outdoor temperatures drop to 20 to 30°F before cooling stored beans, it is best to cool them in 10 to 20 degree stages as average temperatures drop in the fall. For example, if beans are harvested at 55°F, you could wait a few weeks until average outdoor temperatures drop to 40°F and run the fans long enough to cool all the beans in the bin to 40°F. Then shut the fan off for a few more weeks and repeat the cycle when average outdoor temperatures fall to about 25°F.

The airflow provided by aeration fans is usually expressed as cubic feet of air per minute per bushel of beans, or cfm/bu. You can estimate the amount of fan operation time to cool an entire bin of beans by dividing the number 15 by the airflow in cfm/bu. For example, many on-farm storage bins have an airflow rate of about 0.1 cfm/bu, so cooling time would be about 15 divided by 0.1 or 150 hours, which is about six days. You can use this formula to estimate cooling time, but you should actually measure bean temperature at several different points in the bin to make sure that cooling is complete.

When you are operating aeration fans to cool beans that are 11 to 13% moisture, you don't need to worry too much about relative humidity. Beans near the point where air enters the bin will rewet during very humid weather and some overdrying will occur during very dry weather, but if fans are operated no longer than necessary to cool the bin, overall moisture change will be quite small. It takes about 50 times as long to change the moisture of a crop as it does to change its temperature, which means you can move a temperature front through 50 ft of beans by the time you've changed the moisture of a 1-ft layer. If you are concerned about operating the fan during weather that is too humid or too dry, however, it is possible to install controls that will operate the fan only during weather conditions that do not cause drying or rewetting. Keep in mind that these types of controls will keep the fan from operating a high percentage of the time and it will take much longer to cool the entire bin than it would without the controls.

Once soybeans have been cooled to 20 to 30°F, check them every two to four weeks during winter months to make sure that the temperature is stable and that no mold, insect, and crusting problems are developing. If you find problems, or if bean temperature has moved above or below the desired range, operate the aeration fan during 20 to 30°F weather to run a temperature front through the bin. If you need to hold the beans into spring and summer, increase your frequency of checking the bins to once a week, but unless a problem develops, it is not necessary to operate the aeration fans. If you do need to aerate during spring or summer, do so during the coolest weather available and make sure that you keep bean temperature less than 60°F.

When spoilage problems develop in stored beans, they often start in pockets of accumulated fines (small pieces of broken seeds, weed seeds, and stem material) and foreign material. This material is difficult to aerate and it is often wetter and more susceptible to mold growth than are whole seeds. Try to keep fines and foreign material out of the bin by setting combines for maximum cleaning or by running beans through a grain cleaner on the way into the bin. Or at least prevent the fines and material from accumulating in one spot by using grain spreaders to fill bins, by frequently moving spouts during bin filling, or by coring bins (removing some beans through the center unloading sump) after they are full.

For more information about grain and oilseed storage, obtain Management of Stored Grain with Aeration, AG-FO-1327, from the University of Minnesota (UofM) Distribution Center, or Crop Storage Management, AE-791, from the North Dakota State University (NDSU) Distribution Center.

Soybean Handling

Soybeans are subject to splitting during handling, so handle them gently. Belt conveyors, bucket elevators, and drag or mass conveyors provide the gentlest handling. But normal grain augers can be used if they are operated slow and full, and pneumatic or air-type conveyors can be used if the air to grain ratio is set properly and if lines are laid out with a minimum number of very gradual curves.

Avoid long drop heights in bean handling by frequently adjusting the position of conveyors or by using bean ladders or other devices that break long drops into a series of shorter drops. One handler of food- grade soybeans recommends 10 ft as the maximum height for any single drop.

Artificial Drying

Most years, fall weather conditions in the upper Midwest will dry soybeans to 11 to 13% moisture in the field. But some years, weather conditions prevent soybeans from drying to 13% moisture, and sometimes, growers harvest at moistures greater than 13% to avoid the harvest losses that can occur at lower moisture contents. (Soybeans can be harvested without too much damage up to about 18% moisture.) If soybeans are harvested at a moisture content much above 13%, artificial drying is necessary.

There is not much published research on soybean drying. Most of our drying recommendations are based on limited experience or are extrapolated from corn drying recommendations. In most cases, dryers that were designed for corn can be adapted for use with soybeans.

Natural-air drying

Using unheated air to dry soybeans usually works well, but it is a slow process (two to six weeks, depending on initial moisture, airflow, and weather). Bins used for natural-air drying should have full-perforated floors and fairly large drying fans. Fan power requirements depend on desired airflow and depth of beans. For example, delivery of 1.0 cfm/bu (cubic feet of air per minute per bushel of beans in the bin) through an 18-ft depth of soybeans would require about 0.6 hp (horsepower) per 1000 bushels of beans in the bin, while delivery of 1.5 cfm/bu through 18 ft of beans would take about 1.6 hp/1000 bu.

Management of natural-air soybean dryers is similar to that for natural-air corn dryers, except that soybean moisture values need to be about two percentage points lower than those recommended for corn. In southern Minnesota, use an airflow of 1 cfm/bu to dry 17 to 18% moisture beans, 0.75 cfm/bu for 15 to 17% moisture beans, and 0.5 cfm/bu for 13 to 15% moisture beans. In North Dakota and northern Minnesota, higher airflow is needed since fewer days are available for drying in the fall. In northern areas, use 1.0 cfm/bu to dry soybeans that are 16% moisture or less, 1.25 cfm/bu for 17% moisture beans, and 1.5 cfm/bu for 18% moisture beans. See Natural-Air Corn Drying in the Upper Midwest, BU-6577, available from the UofM Distribution Center or Natural-Air/Low-Temperature Crop Drying, EB-35, from the NDSU Distribution Center for information on equipping and managing natural-air dryers.

Because natural-air drying is a slow process, it will be difficult to use one bin to dry both beans and corn in the same year. Don't plan on having the beans dry before corn harvest unless the soybeans are only slightly wetter than 13%, or unless you use a shallow drying depth.

Low-temperature drying

Early in the fall, especially in years with warm, dry weather, it is possible to dry soybeans to less than 13% moisture with no supplemental heat. (See previous section on natural-air drying.) However, late in the fall, or in years with cool, damp weather, soybeans might not dry to 13% and it might be helpful to add a small amount of supplemental heat to the air in natural-air dryers. Do not heat the air more than 3 to 5 degrees F, though, or you will overdry the beans and you might cause an increase in splitting. Research has shown that exposing soybeans to relative humidity values of less than 40% can cause excessive splitting. For every 20 degrees F that you heat air, you cut its relative humidity approximately in half, so it doesn't take very much heat to produce relative humidity values less than 40%.

Some alternatives to adding supplemental heat to natural-air drying bins include:

High-temperature drying

Many kinds of gas-fired corn dryers can be used to dry soybeans, but be careful. Soybeans split easily if they are dried too fast or are handled roughly. Set the drying air temperature lower than you would for corn and avoid dryers that recirculate the crop during drying. Column-type dryers can often be operated at 120 to 140?F without causing too much soybean damage, although some trial and error might be required to set dryers properly. Examine beans leaving the dryer carefully and reduce the temperature if you're getting too many splits. If the soybeans will be saved for seed, keep drying temperatures under 110?F to avoid killing the embryo.

Don't forget that crops dried in gas-fired dryers must be cooled within a day or so to remove dryer heat. This can be done in the dryer or in aerated storage bins. Stored beans should be aerated again later in the fall to cool them to 20 to 30?F for winter storage.

Immature, frosted, or green-colored beans

In years when frost kills soybean plants before the seeds are fully mature, make sure you remove as much chaff and green plant material as possible before binning the beans. Immature beans can be stored without significant molding, but concentrations of green chaff can lead to heating in storage. Although it is commonly stated that green soybeans will eventually turn yellow in storage, the color change observed in a UofM laboratory study was minimal. It might still be worthwhile to store green soybeans for a few months after harvest though, to avoid the high discounts that are applied in years when large quantities of green beans are delivered during harvest. Just make sure that any green beans going into storage are clean, evenly distributed throughout the bin, and cooled as soon as possible after harvest.

Reconditioning Overdry Soybeans

In years with exceptionally warm, dry falls, soybeans are sometimes harvested at moisture contents well under 13% moisture. Although it is illegal to add liquid water to increase soybean moisture, it is possible, given enough time and a high enough airflow per bushel, to increase the moisture content of soybeans by aerating them with humid air. But here are some practical concerns and limitations:

Table 1 shows the moisture content that soybeans would come to if exposed to different combinations of temperature and relative humidity for long periods of time. If you continuously aerated a bin of beans, they would tend to lose moisture during periods of low humidity and tend to gain moisture during periods of high humidity. To recondition soybeans to 13% moisture during normal fall temperatures of 30 to 60°F, you would need to control the fan so that it operates during weather that has an average relative humidity of 65 to 70%. Table 1 indicates that bean moisture increases sharply as relative humidity increases, which means that it is quite easy to rewet a layer of soybeans to a moisture content that is too high for safe storage.

Table 1. Equilibrium moisture values (percent wet basis) for soybeans. Relative Humidity Temperature

Temperature
Relative Humidity (%)
(F)
50
60
70
80
90
32
10.0
11.8
13.7
16.2
19.8
40
9.8
11.5
13.5
16.0
19.6
50
9.5
11.2
13.2
15.7
19.4
60
9.2
11.0
13.0
15.4
19.1
70
8.9
10.7
12.7
15.2
18.9
80
8.6
10.4
12.5
15.0
18.7

During reconditioning, the moisture of the whole bin doesn't change at once. A rewetting zone develops and moves slowly through the bin in the direction that the airflow is moving. This is similar to the way a drying zone moves through a drying bin. In most cases, there are not enough high humidity hours available in the fall to move a rewetting zone all the way through the bin. And in many cases, depending on how the fan is controlled, the parts of the bin that have been rewet will be too wet for safe storage. It would be best to mix the wet layers with the dry layers to reduce spoilage risk and to avoid drying charges for the wet layers when the beans are sold. Mixing can be accomplished to a limited extent by emptying the bin and moving the beans through a grain handling system. The most effective way to mix the beans, though, would be to use an in-bin stirring system. In fact, bin dryers equipped with stirring augers are a good choice for reconditioning soybeans.

If the initial moisture content of the beans is 10% or less, controlling the fan so that it only runs when relative humidity of the air reaching the beans is greater than about 55% should result in rewetting. If you use a single humidistat to turn the fan on anytime humidity is greater than 55%, average humidity during the hours the fan operates should be well above 55% and the beans are likely to rewet to at least 13%. Since humidity is almost always higher at night than it is during the day, an alternative to a humidistat would be a timer set to run the fan only during nighttime hours. If you aren't equipped to mix beans after reconditioning, you need to avoid rewetting them to moisture levels that are too high for safe storage. Approaches to prevent excessive rewetting include:

The disadvantage of the last two approaches is that the fan doesn't run as many hours as it would with a single-humidistat control and less total moisture would be added. Running the fan at high humidity values and then mixing the wet and dry beans would result in greater average moisture content.

Reconditioning time depends primarily on airflow per bushel and weather conditions. It is fastest when airflow per bushel is high and air is warm and humid. Reconditioning will be most successful in a bin equipped as a drying bin - one that has a full perforated floor and a fan that can deliver at least 0.75 cfm/bu. Even with this airflow, it would probably take at least a month of fan operation to move a rewetting front all the way through the bin. And keep in mind that you can't run the fan continuously because in a typical fall, continuous fan operation would result in drying rather than rewetting. Attempts to use storage bins equipped with low-airflow aeration systems to recondition crops are usually not very successful - mainly because it just takes too long to move the rewetting front very far into the bin.

Soybeans swell when they absorb moisture, and experience during floods indicates that soaking the bottom few feet of beans in a bin can result in enough pressure to rupture bin walls. We don't have enough information on reconditioning soybeans through use of airflow to know whether this procedure can damage bins, but the process will definitely increase stress on the walls. Using a vertical stirring auger to mix layers of dry and wet beans might be one way to reduce outward pressure generated during rewetting.

To increase chances of success in using airflow to recondition soybeans:

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URL: http://www.soybeans.umn.edu/crop/harvest/index.htm

Last Modified 3/25/11 2:10 PM
efans@umn.edu

 

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