Handling and Storage Of Soybeans and Soybean Meal: Part 5 - Maintenance Of Quality

Maintenance Of Quality

Once soybeans reach maturity and are harvested, their inherent quality is fixed and can only be maintained by proper drying, cleaning, and conditioning. Quality maintenance involves various operations that will provide environmental conditions that minimize the combined deteriorative effects of ambient temperature and moisture on both soybeans and soybean meal. It involves slowing down undesirable chemical changes from occurring, inhibiting mold and insect activity. The major elements in achieving this goal consist of a reliable temperature monitoring system, periodic drawing of product samples and accurately testing them, and having a reliable cooling or aeration system.

Temperature Monitoring
The most convenient property of stored grains to measure is temperature. In small farm storage silos, one can use temperature probes to monitor grain temperature and can draw representative samples simultaneously. However, in large commercial silos, the use of thermocouple wires and multi-point potentiometer for remote temperature sensing is standard and essential. Modern grain storage silos are all equipped with temperature monitoring system. Without a remote temperature monitoring system, a storage manager will have to rely on grain probes to measure grain temperatures at various points. This is a very inefficient and cumbersome system that often discourages many storage managers from doing it on a regular basis, and stand the risk of incurring economic loss.

Product Sampling, Inspection, And Testing
The systematic monitoring of both the environmental and product condition is another essential element in maintaining the quality of stored soybean and soybean meal. This may be accomplished by regularly taking samples of the stored products and analyzing them in the laboratory for signs of deterioration. Signs of deterioration include off-odors, moldy condition, heating, discoloration, and presence of live insects. During the sampling operation, one also should inspect the storage facilities for signs of moisture accumulation or roof leaks and other physical damages to the storage facilities.

Cooling By Aeration
The primary purpose of aeration is to make the temperature of the grain bulk uniform. This prevents moisture migration in the grain mass due to natural convection currents induced by temperature gradients. Aeration may also be used to hold partially dried soybeans for a few days to prevent spoilage before proper drying. It should be noted that aeration is not intended to dry grains but a small degree of drying may occur when the ambient air relative humidity is 40% or much lower. Aerating soybeans in silos can be done either by pushing the air (positive pressure) or pulling it (negative pressure) through the grain mass. Each system has its inherent advantages and disadvantages over the other. For example, a positive pressure system may result in moisture condensation on the underside surface of the silo roof unless sufficient vents are provided. On the other hand, a negative aeration system may initially be pulling hot air from the open space beneath a hot metal roof during the day, thus adding more heat to the grain. A compromise method of aerating soybeans in tall silos would be to use a cross-flow air distribution system. This system eliminates the two problems mentioned above while reducing the aeration time and less fan power requirement. The schematic diagrams in Figure 12 illustrate the three systems of aeration discussed above. An effective aeration system should be able to cool the bulk grain with minimum amount of airflow at the shortest possible time. This can be achieved by selecting the right kind and size of fan, air distribution system, direction of the air, and airflow rate. The relationship between airflow rate, grain depth, horsepower requirement and air pressure are shown graphically in Figure 13.

For example, soybeans stored at a depth of 12.2 m (40 feet) with an airflow rate of 80 liters/min/m3 (1/10 ft3/min or CFM per bushel) will require a fan horsepower of 0.04 per 35.32 m3 (1,000 bushels) of soybeans at a static pressure of 3 cm (1.1 inches) of water. If the same soybeans are aerated at 40 liters/min/m3 (1/20 CFM per bushel), the fan horsepower will be 0.013 per 35.32 m3 (1,000 bushels) at a static pressure of 1.85 cm (0.73 inch) of water. This illustrates that reducing the airflow rate by half will result in the reduction of fan horsepower to 1/3 of previous condition.

A general guide in selecting the aeration rate of stored soybeans under warm and temperate climates is given in Table 4.