processing basics

Farmers dedicate much time and hard work to produce and harvest millions of tons of soybeans. That work is complemented by the considerable effort and care the soybeans receive after harvesting to ensure the high quality and continuous supply of soy products to processing industries and consumers.

Soybean processing involves a series of steps to produce commodities for food, industrial, and animal feed uses.

Threshing consists of separating the beans from the pods (portion of the plant fruit that encases the soybean seeds). Most soybeans are harvested and threshed simultaneously by modern combines. Threshing can also be done by hand using simple tools, the help of vehicles, or simple hand or motor-driven machines. Whatever the system used, it is very important that threshing be done with care to prevent breakage of the beans or hulls. Careless threshing can reduce the product's quality and foster subsequent losses from the action of insects and post harvest disease..

Soybeans have to be moved throughout the post harvest system. This includes soybean transport from the fields to the threshing or drying site, and then to storehouses or to collection centers and further transpost to processing industries or to bigger central storage buildings. Finally, the beans move from these industries or storage buildings to wholesalers or retailers for final marketing. The type of transport used to move soybeans depends on the amount of beans and distance traveled. It is especially important to transport the beans from the field to storage centers as soon as possible to avoid deterioration.

Losses during transport must be minimumized. Loss means the difference in weight between the quantity loaded before transport and the quantity unloaded after transport. In addition, there is a loss in quality when the beans undergo changes during transport. To avoid transport losses, bags must be checked before filled with beans since the bags are reused and can tear and leak during loading or unloading. Care must be taken to load and arrange bags properly in the truck to avoiding crushing the lower layers and to permit air to circulate around the bags. Also, soybeans must be protected while being transported in the rainy season.

" Drying" is a post harvest phase during which the beans are rapidly dried until they reach the "safe-moisture" level. After threshing, the moisture content of the beans is sometimes too high for good conservation (13 to 15 percent). The purpose of drying is to lower the moisture content in order to guarantee conditions favorable for storage or for further processing and handling of the product. Drying can be done by allowing warm, dry air to circulate around the f beans.

Essentially two methods of drying are utilized, either natural or artificial drying.

  • Natural drying. In dry climates and soybean producing regions that may not have access to mechanized drying equipment, the threshed soybeans are spread in thin layers on a drying-floor where they are exposed to the air and sunlight for about 1 to 2 weeks. The beans must be stirred frequently to encourage uniform drying. As a rule of thumb, the relative humidity of the ambient air must not exceed 70 percent for drying to be effective. When relying on natural drying methods, soybeans must not be exposed at night. The cold and moist night air fosters re-humidification of the beans which may be detrimental to bean quality. Natural drying methods should not be used in humid regions or during rainy seasons.
  • Artificial drying. In humid tropical and subtropical regions or areas with unfavorable weather conditions at harvest, artificial drying is necessary. In these regions, it is often difficult to safeguard the quality of newly harvested soybeans. With the introduction of high-yielding soybean varieties and the use of agriculture mechanization, it is possible to harvest large quantities of soybeans in a relative short time and quickly dry the beans for storage. Due to the length of the growing season, weather conditions, or subsequesnt crops to be planted shortly after soybean harvest, farmers are forced to harvest soybeans with high moisture content. Consequently, it is necessary to dry the beans artificially. This method of drying consists of exposing the beans to forced ventilation of air that is heated to certain degree in special equipment called "dryers".

Cleaning consists of eliminating impurities and debris from the harvested crop. Sometimes cleaning is done more than once through the post harvest system and may be accompanied by sorting the beans according to quality.

After threshing, soybeans are contaminated by soil, plant and insect waste, small pebbles, weed seeds, or broken soybean seeds. The broken seeds and other impurities hinder drying operations, make post harvest processing longer and more costly, lower the end-product quality, and serve as targets for post harvest disease.

The simplest cleaning method involves tossing the beans into the air and letting the wind carry off the lightest impurities. This cleaning method does not eliminate the heavier impurities. Cleaner-separator machines are used when large quantities of beans are cleaned. They are motor-driven and consist mainly of a reception hopper, a fan and set of vibrating sieves. Cleaning is done by repeated suction of the lightest impurities, followed by siftings of the beans.

Soybeans are generally packed in bags made of either jute, cotton fibers, or plastic. Bag packaging is seldom used in developed countries but it is widespread in developing countries because it is economical and well adapted to local grain-transport and marketing conditions. The type of bag determines the height of the stacks. Generally, the bags are stacked on wooden platforms called pallets, in order to prevent direct contact of bags with the floor. The free space between the top layer of the stacks and the top of the storehouse should be at least 1 meter. Sometimes, small-farmers keep small quantities of soybeans in sealed containers for self-consumption.

Storage is an important phase of the post harvest system. During this phase, the soybeans are stored in a manner to be readily available and high quality. The main objectives of soybean storage are to permit deferred soybean use, to ensure seed availability for the next crop cycle, to guarantee regular and continuous supplies of raw soybeans for processing industries and to balance the supply and demand of soybean, thereby stabilizing its market price.

Processed Soybean Uses
Soybeans are grown primarily for meal, and oil is a secondary product. During processing, the soybeans are cracked to remove the hull and then rolled into full-fat flakes. The rolling process disrupts the oil cells, facilitating solvent extraction of the oil. After the oil has been extracted, the solvent is removed, and the flakes are dried, creating defatted soy flakes. While most of the defatted soy flakes are further processed into soybean meal for animal feeding, the flakes can be ground to produce soy flour, sized to produce soy grits or texturized to produce textured vegetable protein (TVP) for food uses. Further processing can produce high protein food ingredients such as soy protein concentrates and isolated soy protein. These ingredients have functional and nutritional applications in various types of bakery, dairy and meat products, infant formulas and the so-called new generation soy foods. Due to this difference in soybean use, two different types of soybeans have emerged: food beans and oil beans (Liu et al. 1995, Orthoefer and Liu 1995; Wilson, 1995).

Meal and Oil Processing
Early oil mill processing of soybeans were typically small scale operations using hydraulic and screw presses (Goss, 1944). Gradually, the screw press replaced the less efficient hydraulic press. In 1934, the first solvent extraction process was introduced. Improvements in oil extraction are continuously evolving. Major changes during the last two decades have included introduction of the expander, installation of heat recovery systems and co-generation (making steam and electricity on site by burning waste by-products like hulls), improved working conditions for employees (dust and sound control), reduced contamination of the environment, automation of equipment, introduction of computer control of the processes, and reduction of manual labor (Lusas, 2000).

Direct solvent extraction, referred to as "full" pressing or prepress-solvent extraction, can separate oil from soybeans. Some crushing industries combine these extraction methods to maximize oil extraction and its quality. Solvent extraction is the most widely used method for oil extraction in the Western world. However, mechanical extraction is often preferred by small extraction plants throughout the world to remove the oil.

Soybean meal and oil also can be produced by the ExPress System, where the whole or de-hulled soybeans at field moisture are fed continuously to a dry extruder. Within the extruder barrel, the material is subjected to friction and pressure, and heat is generated. The temperature profile within the extruder barrel can be varied depending upon the intended use of the processed meal. This process does not require an external heat source. Typically, the top temperature at the exit of the extruder barrel is 150 °C. Lower temperature profiles are used when the meal is intended for use as a functional ingredient in food applications (Wijeratne, 1999).

(Excerpts taken form FAO Post Harvest Compendium)

Goss, W.H. 1944. Processing soybeans. Soybean Dig. 5(1): 6-9. 
Liu, K.S., Orthoefer, F. and Thompson, K. 1995. The case of food-grade soybean varieties. INFORM 6(5): 593-599.

Lusas, E.W. 2000. Oilseeds and oil-bearing materials. In Handbook of Cereal Science and Technology. K. Kulp and J. G. Ponte, Jr. (Ed.), Marcel Dekker, Inc., New York, pp. 297-362.

Orthoefer, F. and Liu, K.S. 1995. Soybeans for food uses. Int’l Food Marketing & Technol. 9(4):4-8.

Wijeratne, W.B. 1999. Alternative technology for primary processing of soybean. In World Soybean Research Conference VI: Proceedings, H.E. Kauffman (Ed.), Publisher Superior Printing, Champaign, Ill., pp. 368-370.

Wilson, L.A. 1995. Soy foods. In Practical Handbook of Soybean Processing and Utilization, D.R. Erickson (Ed.), AOAC Press, Champaign, IL., pp. 428-459.