This past fall, the upper Midwest experienced exceptionally good drying conditions during soybean harvest and many soybeans were harvested at 8 to 10% moisture (wet basis). The moisture content normally used for trading soybeans is 13%. When soybeans are sold at lower moistures, sellers lose potential income because they could have sold additional weight if the soybeans had been wetter. This situation creates an economic incentive for sellers to increase the moisture content of beans that are less than 13% moisture.
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 recently ruled that it is not illegal to force humid air through a crop to increase moisture content.
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:
If the fan is controlled so that it only operates during rewetting conditions, the value of the weight gain should exceed the cost of the electricity to run the fan. In a study at Purdue University where they used relatively high airflow (0.5 to 1.0 cfm/bu) to rewet 10% moisture beans, they were able to increase the weight and value of the beans enough to more than compensate for the cost of electricity to run the fans.
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 accomplish rewetting, for example to 13% moisture, during normal fall temperatures of 30 to 60F, 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%. Table 1 indicates that bean moisture increases sharply as relative humidity increases, which means that it is quite easy to rewet soybeans to a moisture content that is too high for safe storage.
During rewetting, 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 rewetting 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 |
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 just run the fan during nighttime hours.
If you aren't equipped to mix beans after rewetting, 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 get less total weight gain. Running the fan at high humidities and then mixing the wet and dry beans would result in greater average moisture content and greater weight gain.
Rewetting 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. Rewetting will be most successful in a bin equipped as a drying bin--one that has a full perforated floor and a good-sized 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 rewet 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 rewetting 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 might be one way to reduce outward pressure generated during rewetting.
If it's possible to use downward airflow, that would be preferred for the following reasons:
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 rewetting, 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.
To increase chances of success in using aeration to rewet soybeans:
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/