Engineering Notes Index

Minnesota/Wisconsin Engineering Notes
Fall 2000

Reconditioning Overdry Soybeans

Bill Wilcke, Minnesota Extension Engineer
Vance Morey, Professor and Head, University of Minnesota Biosystems and Agricultural Engineering Department

Why consider reconditioning soybeans?

Large portions of the upper Midwest have experienced exceptionally good drying conditions this fall and many soybeans have been harvested at well under 13% moisture (wet basis). The moisture content normally used for trading soybeans is 13%. When soybeans are sold at less than 13% moisture, sellers lose potential income because they could have sold additional weight if the soybeans had been closer to 13%. This situation creates an economic incentive for sellers to recondition overdry soybeans to increase their moisture content.

Is it legal to recondition dry soybeans?

It is definitely illegal to add liquid water to any grain or oilseed to increase moisture content (and weight) prior to sale. But federal agencies have indicated that aerating crops during humid weather to maintain or improve crop quality is not illegal.

Will soybeans gain moisture if they are aerated during humid weather?

Yes, given enough time and a high enough airflow per bushel (cubic feet of air per minute per bushel of beans, or cfm/bu), it is possible to increase the moisture content of soybeans by aerating them with humid air. But here are some practical concerns and limitations:

• The process is quite slow--even with the high airflow per bushel (0.75 to 1.0 cfm/bu) available on bins that are normally used for drying. It is possible, but very time consuming, to recondition soybeans by using the low airflow aeration systems common on storage bins.
  
• Fan control is tricky and some beans could end up too wet for safe storage.
  
• You will end up with layers of wet beans and layers of dry beans unless you can find some way to remove the wet layers or to mix the wet and dry layers.
  
• Swelling that accompanies rewetting will increase stress on bin walls and could potentially damage bins.

Is reconditioning economically feasible?

If the fan is controlled so that it only operates during rewetting conditions and if systems with moderate fan power and moderate airflow per bushel are selected, the value of the weight gain should exceed the cost of the electricity to run the fan.

How does the rewetting process work?

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, for example to 13% moisture, during normal fall temperatures of 30 to 60°F, you would want to control the fan so that it operates during weather that has an average relative humidity of 65 to 70%.

Note that the recommended moisture for beans that will be stored through winter is 13%. Beans that will be stored into the following summer should be about 12% moisture and beans that will be stored a year or more should be about 11% moisture. Table 1 indicates that bean moisture increases sharply as relative humidity increases, which means that it is quite easy to recondition soybeans to a moisture content that is too high for safe storage.

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. To avoid problems with spoilage or marketing of beans that are wetter than 13% moisture:

• Mix the wet layers with the dry layers. 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.
  
• Remove wet layers from the bin as they reach 13% moisture. If reconditioning is done in a bin where air is pushed up through the beans, wet beans will be at the bottom of the bin and bean removal could be accomplished by using a tapered sweep auger that is permanently installed at the bottom of the bin. (This is the type of sweep auger used in continuous-flow bin dryers.) If reconditioning is done in a bin where air is pulled down through the beans, wet beans will be at the top of the bin and they could be removed from the top by using a vacuum-type grain conveyor, or they could be unloaded through the bin’s center unloading sump.
  
• You can also avoid excessive rewetting by using a fan controller that turns the fan off during very humid weather. This option, however, will greatly limit the amount of reconditioning that can be accomplished because the fan will turn off during the best rewetting conditions.

Table 1. Equilibrium moisture values (percent web basis) for soybeans

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

 How should the fan be controlled during reconditioning?

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 60% should result in rewetting. If you use a single humidistat to turn the fan on anytime humidity is greater than 60%, average humidity during the hours the fan operates should be well above 60% 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 just run the fan during nighttime hours.

If you aren’t equipped to mix or remove beans after reconditioning, you need to avoid rewetting them to moisture levels that are too high for safe storage. One approach is to add a second humidistat that stops the fan when relative humidity reaches very high levels. Or you could also install a more sophisticated aeration controller that monitors both temperature and humidity and only runs the fan when air conditions will bring the crop to the desired moisture content (for either drying or rewetting). The disadvantage of these approaches is that the fan doesn’t run as many hours as it would with a single humidistat control and you would recondition fewer beans. Running the fan at high humidities and then mixing the wet and dry beans or removing the wet beans would result in reconditioning of more beans.

How long will it take to recondition a bin of beans?

Reconditioning time depends primarily on airflow per bushel and on humidity of the air moving through the beans. Reconditioning is fastest when airflow per bushel is high and humidity is high. Using a bin equipped with a full perforated floor, a good-sized fan that can deliver at least 0.75 cfm/bu, and a humidistat to prevent fan operation at low humidities will provide the quickest reconditioning. 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. Using storage bins equipped with low-airflow aeration systems to recondition soybeans will require many months to move a rewetting front all the way through the bin.

Is there any danger of damaging the bin during reconditioning?

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. And a Michigan researcher reports that an attempt to recondition pea beans resulted in bin damage. We don’t have enough information on reconditioning soybeans through use of aeration 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, or periodically removing wet layers of beans from a bin might be ways to reduce outward pressure generated during reconditioning.

Does reconditioning work best with upward airflow or downward airflow?

If it’s possible to use downward airflow, that would be preferred for the following reasons:

• The reconditioned beans would be at the top of the bin where it would be easier to check moisture, to check for signs of mold, and to remove the reconditioned beans.
  
• Binwall stresses are likely to be lower if the rewet beans are at the top of the bin instead of beneath layers of dry beans.
  
• The heat generated by the fan motor and impeller are not working against you. Typical crop drying fans heat the air 2 to 3F. This heat increases the drying potential of the air and is enough to lower the equilibrium moisture of the crop about 0.75 percentage points compared to the moisture you would get with air that is not heated by the fan. Contrary to popular belief, the heat generated by the fan is much greater than the amount of solar heat captured by the bin roof.

Most bins equipped for high airflow per bushel are designed for upward airflow and it is not easy to reverse the airflow direction. These upward-airflow systems can still be used for reconditioning, but recognize that you will have slightly greater stresses on the bin walls and you will need to probe to the bottom of the bin to monitor bean moisture changes. Also, if you are using a humidistat to control the fan, you might want to place the humidistat’s sensing element between the fan and the bin so that it senses the air actually reaching the beans. Or if that’s not convenient, increase the humidistat setting about five percentage points to compensate for the drying effect of the fan heat.

Caution: If you are able to reverse the airflow direction in a bin that was originally designed for upward airflow, make sure that roof vents are open anytime the fan is operating to avoid damaging the roof.

Summary

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

• Use timers, humidistats, programmable controllers, or some other type of automatic control to limit fan operation to weather conditions that will cause moisture increases. If the only control available is a single humidistat, set it to limit fan operation to weather conditions with relative humidity greater than 60%.
  
• If possible, use systems that pull air down through the beans to avoid the humidity reduction caused by fan heat, to limit stresses on bin walls, and to keep wet beans at the top of the bin where you can more easily monitor them and remove them from the bin.
  
• Either mix wet and dry beans by using stirring devices or by transferring beans from one bin to another, or periodically remove wet layers of beans from the bin. In systems that use downward airflow, remove wet layers of beans from the top of the bin. In systems that use upward airflow, consider using a tapered sweep auger to remove layers of wet beans from the bottom of the bin.
  
• Keep reconditioned beans cool to reduce chances of spoilage. Try to keep beans at 20 to 30F during winter months and no warmer than about 50F during other times of the year.
  
• Watch carefully for signs of moldy beans and for excessive stress on the bin.

The information in this article is based on ASAE Paper No. 99-6046, Aeration Strategies for Reconditioning Dry Soybeans by W. F. Wilcke, R. V. Morey, D. J. Hansen, and R. A. Meronuck.

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