On-Farm Soybean Drying and Storage

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 60^oF 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 80^oF at the same RH level will decrease EMC to 13.2%.

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

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

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 drying.           

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.

Low-temperature drying

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:

• Harvest soybean at low moisture content as possible. 
• Load beans into clean bins immediately after harvest. Bins should be cleaned and sanitized prior to harvest to minimize insect problems. Move soybeans from the field to grain bins as soon as possible. The amount of time before spoilage begins depends on grain moisture content and air temperature. A safe rule of thumb is to hold freshly harvested beans in carts or trucks no longer than 12 hours. Warm air temperatures > 80oF, and higher grain moisture levels are the most critical factors for decreasing the time required for the grain to spoil.
• Check the moisture content of each load of grain as it is placed in the drying bin. There can be some variation in moisture content, but you need to know the average moisture content of the bin to determine the minimum necessary air flow needed and the allowable depth of grain in the bin.
• Open air exits and start the fan as soon as the grain depth is about 1 foot deep on the perforated floor. Be sure to use spreading devices or some other means to keep the grain leveled as the bin is being filled. If the grain is allowed to cone, there will be an increase of small particles in the center of the cone/ or central portion of the bin resulting in the air not being able to reach this grain because of increased resistance to flow. This makes it very hard to dry and control moisture uniformly in the grain bin and may cause spoilage.
• Add soybean to drying bin in shallow layers until the moisture content decreases. High moisture soybean can be added in 4 feet layers on top of dry grain if the fan can provide at least 3 to 4 cfm/bu through the total depth in the bin. 
• Level beans inside each drying bin continuously – never allow coning to occur. Some manual work may be required to maintain a level surface on the top when the maximum depth is reached. This will ensure uniform airflow through all the grain assuming it has been placed in the bin with a good spreader.
• Monitor the moisture content of soybeans daily. Beans must be cooled to avoid nighttime condensation on the inner walls. If the heat has been on long enough for the complete mass of wheat to be warmed and the weather is clear and dry with humidity below 60%, turn the heat off when the moisture content of the grain drops to within 1% of the target moisture content. Continue running the fans, and the residual heat in the grain will finish the drying process.
• Probe the bin periodically to check for insect infestation and grain temperature increase. Beans temperature increase usually means moisture migration. Aerate whenever this is detected. If the problem is in the center of the bin and aeration is not effective, move the grain to another bin to solve this problem. Problems in the center of the bin usually indicate that a lot of fines and/or trash accumulated in this area during filling.
• Never add more heat than necessary to adjust the humidity of the drying air down to about 55%. The maximum heat needed, even in rain or 100% humidity will be about 15-17oF above the outdoor temperature. 

• Cool the grain off as soon as possible in the fall. Target temperatures should be initially around 60°F.
• Continue to aerate and uniformly cool grain to between 30°F to 40°F if possible. This will help avoid internal moisture migration and insect activity.
• Monitor grain and aerate monthly to maintain uniform temperature and moisture levels throughout. Aerate more often if moisture or temperatures increase.
• Keep the grain cool as long as possible into the early spring.
• Do not aerate in early summer unless problems develop.
• Cover fans and openings when not in use to help avoid air, moisture and potential insect movement.
• Monitor carefully and fumigate if needed.
• Inspect soybean surface at least every week throughout the storage period.

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.

• Soybean producers should always think safety first around drying and storage bins because grain suffocation accidents happen all too often. Wear an effective dust mask when exposed to grain dust when working in dusty conditions, particularly these resulting from moldy or spoiled grain. Exposure and inhalation of mold can cause severe allergic reactions.     
• Good safety practices are necessary for producers and workers who operate in soybean drying, storage and handling. Grain drying and handling can be dangerous. A deadly hazard exists for anyone in a grain bin as deaths occur every year from suffocation and injuries caused by unloading augers. Power to the unloading auger should be disconnected before entering bins. A knotted safety rope hanging near the center of the bin offers greater protection, and a second person should be standing by who can offer assistance. Air pockets sometimes form when grain bridges over unloading augers due to spoiled grain and moisture. This crusted surface should not be walked over because the pocket can collapse. Transport augers can hit power lines, unguarded augers can catch hands or feet, and fans and shafts can catch unsuspecting victims.