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Soybean Processing…From Field to Consumer
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.
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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
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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.
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
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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
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.
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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).
Soy Processing, Products
and How They are Used
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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.
of Soybean Processing
Flow Diagram for Soybean Processing and Direct Solvent Extraction
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.,
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.
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