Soybean handling and storage
June 01, 1998
by Teresa Acklin
Soybean and meal characteristics differ from those of cereal grains, requiring distinct considerations to preserve quality.
By Ulysses A. Acasio
Soybeans represent the world's most important oilseed as a source of vegetable oil and protein, as they can be processed into flour for food and meal for feeds. In China, Japan and Southeast Asian countries, soybeans are used primarily in the preparation of various food products, while in the U.S. and Europe, soybeans are used mainly for production of oil and meal.
Two kinds of soybean meal full-fat and defatted are used in the feed industry. Defatted soybean meal is a by-product of the oil extraction process and comes in either loose or pelleted form. Brazil and Argentina export their soybean meal in pelleted form, while the U.S. exports soybean meal mainly in loose form.
Because of the enormous worldwide economic and nutritional importance of soybean products, it is critical to recognize the various factors contributing to the deterioration of soybeans and soybean meal. Deterioration can occur during handling and storage from harvest until reaching the end-user.
Soybeans and soybean meal are normally handled in bulk form in developed countries, while bag handling is prevalent in developing countries. Some countries are now in a transitional period from bag to bulk handling systems. Regardless of the system used, it is important to recognize the advantages and disadvantages of each system to minimize product deterioration and economic losses.
The structure of a soybean seed makes it susceptible to splitting and breakage during mechanical handling. The extent of breakage in soybeans during conveying varies with the impact force imposed on each individual seed. The percentage of broken soybeans can be as high as 4.5% with a free fall drop of 30 meters. The least breakage occurs with bucket elevators.
Sound soybean kernels will store better than kernels damaged by freezing, cracking, splitting and storage fungi because a sound seed coat provides some degree of protection against insect and fungal attack. For this reason, a grain handling system that minimizes cracking and splitting of soybeans is preferred.
Belt conveyors are recommended as they handle grain gently and efficiently and can also move grains for great distances without damage. It is very common for soybeans to be conveyed as many as 15 to 19 times between harvest and processing or export, and the amount of broken and split kernels increases with every handling operation.
The storability of soybeans is affected by the degree of damage to the seed coat and by other factors such as mold or insect attack. It is therefore important to inspect soybeans for mechanical and other forms of damage prior to storage. If the amount of broken or split soybeans is very high, it may be prudent to separate the broken or split grains by sieving. This material can then be used first as opposed to long term storage with the original stock.
Soybean meal is difficult to handle because of poor flowability and bridging characteristics, and it tends to settle or consolidate over time. This phenomenon occurs in most granular materials and becomes more severe with increased moisture, time and small particle size. Because of this, bulk soybean meal is best stored in flat storage buildings instead of vertical silos.
Standard conveyors can be used for loading into the building, and front-end loading tractors can be used in combination with standard conveyors for reclamation. New sweep auger designs are now available that effectively reclaim meal products from silos.
Sweep augers range in model and capacity. One model is designed to reclaim meal products from flat-bottom silos and another for hopper-bottom silos. However, because of their very high initial cost, they have not yet become standard equipment in the industry. Addition of calcium carbonate, calcium bentonite or sodium bentonite at a level between 0.25% to 0.5% is effective in improving the flowability of soybean meal. Other innovations, such as electric vibrators and compressed air to fluidize the meal, are available to improve discharge of soybean meal from storage bins.
The three major factors affecting the storability of soybean and soybean meal are moisture content, temperature and duration of storage. The general condition of the product and amount of foreign material also affect their storability.
Moisture is perhaps the most important single factor affecting storage of soybeans and soybean meal. Soybeans with moisture above 13% should be dried to reduce the risk of deterioration from seed respiration, mold attack, spontaneous heating and reduced germination.
It is widely recognized that there are two forms of moisture in cereal grains and oilseeds:
free moisture, which is in vapor form and is removed during drying; and
bound moisture, which remains “locked” in the molecular structure of the grain and cannot be removed by drying. In general, soybeans can be kept for a given period of time with no appreciable degree of deterioration if the moisture content is relatively low. The safe moisture content of soybeans is presented in Table 1.
Soybeans and soybean meal are hygroscopic materials and will either lose (desorb) or gain (adsorb) moisture from the surrounding air. The rate of moisture adsorption or desorption is directly related to the manner by which a product is exposed to the atmosphere.
For example, soybeans kept in jute sacks tend to gain or lose moisture more readily than soybeans in bulk form. Hence, bag storage has self-ventilating and cooling characteristics that make it the preferred method over bulk systems in many developing countries.
On the other hand, bulk storage of soybeans allows the use of aeration or cooling techniques to maintain desirable quality in a controlled manner. Aerating soybeans in bags is possible, but is not practical because of the uneven airflow patterns that result in non-uniform grain temperature.
Temperature is another very important factor influencing soybean storage. Growth of fungi and chemical changes such as oxidation increase with temperature in both meal and whole beans.
Insects develop and reproduce best between 27° and 35°C. Below 16°C, insects become inactive and die of starvation. Exposure to temperature greater than 60°C kills most insect species in 10 minutes.
Temperature also affects mold development. Soybeans with a moisture content of 14% to 14.3% that are maintained at 5° to 8°C can be stored for more than two years without mold damage, while soybeans kept at 30°C can be invaded by molds in a few weeks and severely damaged in six months.
Recent studies indicate that soybeans can be stored at 10.5% moisture at any temperature without the danger of mold attack. However, at this moisture content, insect infestation may develop unless the temperature is maintained below 20°C. This is equivalent to a seed wet-bulb temperature of 15°C or corresponding relative humidity of 60%.
Temperature also influences moisture migration. The driving force in moisture migration in a grain mass is temperature gradient. This condition causes very small air movements and water vapor translocation in the grain mass.
It has been estimated that a grain mass temperature gradient of about 16.7°C can induce an inter-seed airflow of 0.06 meters per minute. Thus, even grains stored at moisture contents considered safe may spoil because of moisture migration associated with inter-seed air currents.
Air movements in the grain bin are influenced by outside air temperature, creating temperature gradients in the entire grain mass. This phenomenon affects all types of storage silos whether made of concrete or metal.
Because of the insulating property of grains, the effect of diurnal temperature changes on the stored grain is minimal, but cumulative. It takes a long time of relatively constant high temperature gradients to initiate significant air movements.
However, experience indicates that under hot and humid tropical conditions, grains stored in metal bins exhibit “sweating” next to the wall. This can be attributed to extreme high temperatures reached at the inner surface of the metal silo on a hot day. At night, rapid cooling of the metal results in moisture condensation as the dew point is reached. Caking and charring in metal silos can be attributed to this phenomenon.
Deterioration of soybeans and soybean meal in storage is a combined function of moisture content, temperature and duration of storage. Therefore, for safe storage, a combination of the three factors providing unfavorable conditions for mold development, such as low product moisture, low temperature and short storage period, is desirable.
Foreign material, defined in U.S. Standards as all materials that pass through a 3.2 mm round-hole sieve and all materials other than soybeans remaining on the sieve, also affects storability. Fine foreign materials tend to segregate during bin loading and occupy void spaces in the central region of the grain mass. Meanwhile, the large and lighter materials will accumulate close to the walls of the silo.
During aeration, air will flow around spots with higher concentration of fine foreign materials and through pockets of high concentration of large foreign materials. This condition will create a non-uniform flow of air during aeration, thus making it an ineffective operation.
Consequently, the non-aerated pockets in the grain mass are potential sites for hot spots that will provide an ideal environment for insects to grow and multiply. Cleaning soybeans prior to storage will minimize the risk of spoilage and economic loss. This should be part of an integrated approach to any quality maintenance program of grain processing companies.
Heating is the most common indicator of a problem in stored grains and oilseeds. High grain temperatures normally indicate either microbial or insect activity, and if left unchecked, they may lead to heat-damaged or charred grains as a result of the phenomenon of stack burning.
In cereal grains heating, temperature caused by microbes or insects peaks at about 58°C then declines to ambient levels. At the peak temperature, insects and molds are killed, thus making the heating process self-limiting.
But in soybean heating, as temperatures progress above 50°C, the oxidation of the oil in soybeans becomes a self-sustaining process, and temperatures above 150°C may occur. At this extreme temperature level, charring will definitely occur and spontaneous combustion or fire becomes a distinct possibility if sufficient oxygen is present at the hot spot.
Because of this danger, hot spots in stored soybeans must be cooled or dissipated before they reach the critical level. If no action is taken when heating in soybeans occurs, either the product will be lost by stack burning (charring) or at worst, the entire facility will be lost through fire.
Aerating soybeans when fire has already started makes the situation worse. A temperature monitoring system in soybean storage silos is essential, and immediate corrective measures for heating cannot be over-emphasized. Color and appearance also are indicators of condition. In general, sound soybeans are plump, with a bright uniform tan, not green, color. Discolored or shriveled soybeans usually indicate inferior quality and lower market value.
Color change is typically associated with mold invasion accompanied by microbial respiration and subsequent heating. This deterioration process can be detected by periodic drawing of samples from the stored soybeans as part of an integrated approach to quality maintenance.
Once detected, appropriate measures can then be taken such as cooling the grain either by aeration or use of a portable cooling unit. Another corrective measure is to transfer the grain to another silo, thus breaking any hot spots present and cooling the soybeans during the conveying process. However, this should be done only as a last resort since it is costly and will increase the amount of broken or split soybeans.
Musty odor usually indicates an advanced stage of insect or mold infestation and should be dealt with immediately. If an odor is detected, the soybeans should be aerated to remove the bad odor and cool the material. Beans should then be used at the earliest opportunity. The grain should be fumigated immediately if insects are present. A sharp odor may indicate rancidity that occurred because of chemical changes in the oil component.
The presence of a large population of weevils and small moths usually indicates an advanced stage of infestation. Granary weevil may infest whole soybeans but not soybean meal, while red flour beetle and khapra beetle will infest soybean meal at relative humidity above 75% and temperatures above 30°C. Almond moth can develop even at 8.8% moisture (wet basis) and a temperature of 25°C.
Lumping and caking indicate a very advanced stage of fungi invasion in soybeans and soybean meal. In metal bins, caking usually occurs on the bin walls as a result of “sweating” or moisture condensing on the inner surface of the cold bin wall. The condensing moisture is absorbed by the adjacent grains resulting in either sprouting or mold growth. Lumping may also occur in spots where grain moisture increased from a leaky roof, moisture migration or translocation by natural convection.
In bag storage systems, soybean and meal caking also may occur as a result of increased moisture content adsorbed from the atmosphere, a leaky roof or capillary moisture from the floor. Capillary moisture can be eliminated by putting the bags on pallets, and concrete floors can be made water-proof during construction by installing plastic sheets as moisture barriers before pouring in the floor slab.
Stored soybeans also may undergo physical, physiological and chemical changes even under ideal storage conditions. Some of the changes may or may not have a negative effect on the final use of soybeans and soybean meal, depending on the degree of change.
Important changes include declines in soybean seed viability, changes in grain color, increased or decreased moisture, decomposition of phospholipid and protein denaturation. Soybean grain is more resistant to deterioration during storage than soybean meal, and full-fat soybean meal deteriorates more rapidly than defatted meal because of its higher oil content.
One common indicator of chemical change in stored soybeans is the level of free fatty acid (FFA) present. An increase in FFA above 1% may translate into lower quality oil. The effect of temperature and moisture on FFA content in soybeans is shown in Figures 1, 2 and 3.
The graphs clearly show that high storage temperatures and soybean moisture contents above 10.5% result in increased FFA content of oil above acceptable levels of less than 1%. These findings indicate the need to keep the temperature and moisture content of stored soybeans as low as possible and to use soybeans before the FFA oil content exceeds acceptable levels.
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 providing 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 and inhibiting mold and insect activity. The major elements in achieving this goal consist of reliable systems for monitoring temperature, periodic drawing and accurate testing of product samples and cooling or aerating the product.
Temperature of stored grains is the most convenient property to measure, and modern grain storage silos are all equipped with temperature monitoring systems. In large commercial silos, the use of thermocouple wires and multi-point potentiometers for remote temperature sensing is standard and essential.
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 measuring on a regular basis; subsequently, they stand the risk of incurring economic loss.
The systematic monitoring of both the environmental and product condition is another essential element in maintaining the quality of stored soybeans 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. During the sampling operation, one also should inspect the storage facilities themselves for signs of moisture accumulation or roof leaks and other physical damage.
The primary purpose of aeration is to make the temperature of the grain bulk uniform. This prevents moisture migration in the grain mass from 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 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.
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 fan power requirements.
An effective aeration system should be able to cool bulk grain with minimum amount of airflow in 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.
For example, soybeans stored at a depth of 12.2 meters with an airflow rate of 80 liters per minute per cubic meter will require a fan horsepower of 0.04 per 35.32 cubic meters of soybeans at a static pressure of 3 cm of water. If the same soybeans are aerated at 40 liters/minute/cubic meter, the fan horsepower will be 0.013 per 35.32 cubic meters at a static pressure of 1.85 cm of water. This illustrates that reducing the airflow rate by half will result in a horsepower reduction of one-third.
A general guide in selecting the aeration rate of stored soybeans under warm and temperate climates is given in Table 2. Ultimately, success in the storage and handling of soybeans and soybean meal is dependent on understanding the combined effects of the three most important factors causing spoilage: temperature, moisture and time.
High ambient temperatures and relative humidity above 65% favor both fungi and insect development in stored products. An effective program of monitoring the condition of the stored products and means of maintaining uniform product moisture and temperature are indispensable elements in successful maintenance of soybeans and soybean meal quality.
The original quality of the soybeans and soybean meal cannot be improved upon, and quality will decline with time. However, deterioration can be reduced to an acceptable level by keeping the products cool and dry during the storage period to ensure a good quality product for processing or marketing.
Ulysses A. Acasio is with the Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas, U.S. This article is based on a technical bulletin prepared for and distributed by the American Soybean Association in Singapore.Table 1 Safe storage periods for soybeans at selected moisture levels
Table 2 Aeration rates for stored grains
|Moisture content (%), wet basis||Market Stock||Seed Stock|
|10%-11%||4 years||1 year|
|10%-12.5%||1-3 years||6 months|
|13%-14%||6-9 months||poor germination|
|14%-15%||6 months||poor germination|
|Source: H.J. Barre, 1976.|
|Airflow rate in liters/minute/cubic meter of grain|
|Type of Storage||Temperate climates||Warm climates|
|Flat structure||40 to 80||80 to 160|
|Upright structure||20 to 40||27 to 80|
|U.S. Department of Agriculture|