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UNDERSTANDING SOYBEAN PRODUCTS AND PROCESSING
TECHNICAL PAPER # 73
UNDERSTANDING SOYBEAN
PRODUCTS AND PROCESSING
By
Harry E. Snyder, Ph.D.
Technical Reviewers
Ellen Craft
Gordon L. Brockmueller
Joanne Hokes
Published By
VOLUNTEERS IN TECHNICAL ASSISTANCE
1600 Wilson Boulevard, Suite 500, Arlington, Virginia 22209 USA
Telephone: (703) 276-1800, Fax: (703) 243-1865
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Internet: vita@gmuvax.gmu.edu, Bitnet: vita@gmuvax
Understanding Soybean Products and Processing
ISBN: 0-86619-316-2
[C] 1990, Volunteers in Technical Assistance
PREFACE
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details. People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis. Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Patrice Matthews
and Suzanne Brooks handling typesetting and layout, and Margaret
Crouch as senior editor and project manager. VITA Volunteer Dr.
R. R. Ronkin, retired from the National Science Foundation, lent
his invaluable perspective, as a volunteer, to the compilation of
technical reviews, conversations with contributing writers, editing,
and in a variety of other ways.
VITA Volunteer Harry E. Snyder, who has a Ph.D. in microbiology
from the University of California at Davis, has taught and done
research in food science and technology for 30 years. Dr. Snyder
has also published a number of books and articles on soybeans and
other food related topics. Reviewers Ellen Craft, an agronomist,
and Gordon Brockmueller, a farmer, have extensive experience with
soybean production. Joanne Hokes' background is in the oilseed
processing industry, including both soybeans and peanuts. All
three reviewers are long-time VITA Volunteers.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries. VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations. VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
UNDERSTANDING SOYBEAN PRODUCTS AND PROCESSING
by VITA Volunteer Harry E. Snyder, Ph.D.
1. INTRODUCTION
Soybean Production
Since 1950, soybeans have become a valuable part of the world's
food supply and of the systems that produce and deliver food.
Production of soybeans has grown rapidly and in 1990 amounted to
approximately 100 million metric tons (MMT) annually. This compares
with about 500 MMT each for rice and wheat and 800 MMT for
coarse grains, predominantly maize.
Soybean production is widespread but is centered in temperate
climates. The United States produces about half of the total; the
other major producers are Brazil (15 MMT), China (10 MMT), and
Argentina (8 MMT). Soybeans contribute about 20 percent (13 MMT)
of the total vegetable oil and are the world's main, single
source of food oil. Palm oil accounts for 8 MMT and sunflower oil
6 MMT of the world's total.
The flowering of the soybean is sensitive to day length; therefore
cultivars (cultivated varieties) must be selected for the
latitude in which they will be grown. Poorly chosen cultivars may
flower before the plant has grown to sufficient size to maximize
yield, or the flowering may be so late that the beans freeze
before they are mature.
Types of Soybean Products
The main soybean products in international trade are beans, defatted
meal, and crude, degummed oil. The beans are usually purchased
for processing to crude oil and meal. The crude oil is
further refined to edible oil. The meal is used mainly as animal
feed, but can be processed into ingredients for human foods:
full-fat flours, concentrates (defatted meals with the soluble
sugars removed), and isolates (purified protein containing at
least 90 percent protein).
Soy products made for direct human consumption, for example soy
milk and soy curd, are normally not traded internationally because
of their susceptibility to spoilage, but soy sauce and
certain other fermented products are stable and can be shipped
internationally.
Composition of Soybeans
The soybean is particularly valuable because both oil and meal
are marketable products. About 20 percent of the weight of soybeans
is oil and 40 percent is protein. The rest is carbohydrate,
moisture, and ash. Properly stored soybeans contain less than 13
percent water.
The oil portion is evenly dispersed throughout the bean in structures
called lipid bodies, which are too small to be seen in a
light microscope. The oil is similar in composition to other
vegetable oils with a high concentration of polyunsaturated fatty
acids that are thought to be useful in the diet to protect
against coronary heart disease. Extracted oil usually contains 1
to 3 percent of phospholipid or gums, which tend to precipitate
on storage of the crude oil. For that reason they are usually
removed by washing the oil with water.
Other minor impurities in crude soybean oil that are removed in
refining steps are free fatty acids, pigments, and flavor compounds.
Since the oil is a liquid at room temperatures, hydrogen
is added to the polyunsaturated fatty acids to convert the oil
into margarines, shortenings, and other solids.
The defatted meal that remains after oil extraction contains
valuable protein that is useful in foods and feeds. The protein
products available as soy meal or flour contain 44 percent protein
if hulls are added back or 47.5 percent protein without
added hulls. For animal feed, the soy meal is normally mixed with
other ingredients to give a protein level of about 15 percent in
the final ration.
Soy meal is heated not only to remove the extracting solvent, but
also to inactivate proteins that may retard animal growth. Trypsin
inhibitor is the name of one such protein that has been widely
studied and is known to inhibit growth in young animals.
2. PROCESSING OF SOYBEANS
Oil Removal by Solvent Extraction
This discussion will emphasize the predominant products and processes
of commercial importance. The main oil removal process is
solvent extraction. It yields a complete oil removal (less than 1
percent oil remaining in the meal) and gives a meal that has not
been heat damaged. Solvent-extraction plants can process 500 to
4,000 tons per day.
The conditions under which beans are stored greatly influences
the quality of oil that may later be extracted from them. To
ensure oil quality the needed storage conditions are as follows:
1) Moisture content: 13 percent or less to prevent mold
growth. However, very dry beans tend to split when being transferred,
and the splitting lowers the oil quality.
2) Temperature: as low as feasible to minimize mold growth.
3) Cleanliness: insects or other contaminants can provide
moisture to start deterioration, which leads to increased temperature,
further increases in moisture, and spoilage.
To prepare soybeans for solvent extraction they are cracked into
several pieces and the hulls are removed by blowing air. Hulls,
which make up about 8 percent of the weight of beans, do not contain
oil and are separated to gain space in the extractors for
the oil-bearing tissue. The cracked pieces are conditioned by
steam to give a moisture content of about 10 percent at 170 [degrees] F
(77 [degrees] C). The conditioned pieces are turned into flakes at this
temperature by putting them between smooth rollers. A flake
thickness of 0.01 inch (0.025 cm) favors rapid solvent extraction.
Thinner flakes extract even more rapidly but also tend to
break into fine particles that clog the beds and cause solvent to
cut channels through the flakes instead of flowing smoothly
through them.
The flakes are conveyed to extractors. These exist in many different
forms, but all use beds of flakes 1 to 3 feet (30 cm to 90
cm) deep. The solvent, commercial hexane with a boiling point of
about 145 [degrees] F (64 [degrees] C), is pumped over the flake beds so that the
flakes entering the extractor are contacted by solvent that already
contains oil, while the flakes leaving the extractor are
contacted by fresh solvent.
A newer procedure for preparing flakes for extraction puts them
through an extruder (or enhancer) to form pellets. Pellets are
easier to extract and hold less solvent than flakes, making extraction
more efficient.
After extraction the hexane is recovered from the oil and from
the meal and reused. Since the hexane is extremely flammable,
solvent extraction plants must be designed to minimize chances of
sparks or open flames. Equipment is designed to minimize loss of
hexane for both safety and economic concerns. The solvent is recovered
in heat exchangers or flushed away by bubbling steam
through the product.
Solvent is removed from the defatted flakes by steam injection in
a device called a desolventizer-toaster, which also heats the
flakes to inactivate compounds such as trypsin inhibitor. The
flakes are then cooled and ground to the correct particle size
for feed mixing.
Oil Removal Without Solvent
The earliest techniques for recovering oil from oilseeds involved
pressing the seed with devices that used levers or screws. Later,
hydraulic presses replaced the mechanical presses. Today's most
efficient way to press oil uses an expeller, a screw-shaped device
rotating within a horizontal, heavy-steel, cylindrical cage.
As the oilseeds enter at one end of the cylinder, they are subjected
to high pressures between the rotating screw and the stationary
cage. The pressure forces oil through openings in the
cage, while the residual press cake is carried horizontally in
the direction of the shaft and is discharged at the other end of
the cylinder.
Expellers work best with oilseeds containing 40 percent oil or
more, but are less effective with soybeans, from which only
three-fourths of the oil is recovered by their use. Nevertheless,
expellers have great versatility and are the best method if many
different kinds of oilseeds are being crushed. Expellers are free
of the many safety problems involved in solvent extraction. Capacities
of individual expellers are much less than for solvent
extraction plants, with the biggest expellers handling about 60
tons/day. One can choose from a wide range of sizes of expellers
to fit the capacity of the crushing operation.
Soybeans need to be prepared for treatment by expellers much the
same as for treatment by solvent extraction. They should be
cleaned, cracked, and flaked for the greatest oil yield.
The meal obtained from expellers contains more residual oil than
from solvent extraction and therefore has a tendency to become
rancid. Highly rancid meal can be dangerous for animal feeding
because the hydroperoxide content makes the meal toxic. Another
problem with the meal is that considerable heat is generated
during expelling. If the meal is scorched by the heat, its nutritive
value may be decreased.
Oil Refining
Crude soybean oil, whether from solvent extraction or expellers,
is refined to convert it to a high quality, edible oil. The minor
components in crude soybean oil that are removed during refining
are gums (phospholipids or lecithin), free fatty acids, pigments,
and flavor compounds.
The gums are removed because they are insoluble in the oil and
gradually precipitate out of the oil during storage. The precipitated
material ("foots") is viscous and difficult to remove from
storage tanks or ship bottoms, and so it is often removed at the
crushing plant before the crude oil is shipped to a refinery. The
recovered gum or lecithin is a valuable by-product and is used by
the food industry as an emulsifier and anti-sticking agent.
The gums are removed by washing oil with water. About 1 to 2
percent water is added to the oil, and after a thorough mixing,
the oil and water are separated by centrifuging. The gums come
out with the water phase, but some oil is lost as well. Also, the
oil has to be dried after degumming to remove traces of water. If
the gums are not recovered for resale as lecithin, they may be
added to soybean meal to increase its caloric value.
Free fatty acids are removed because they lower the temperature
at which heated oil begins to smoke. Smoking oil is undesirable
for cooking.) To remove free fatty acids, the oil is washed with
a dilute lye (sodium hydroxide or potassium hydroxide) solution.
The lye changes the fatty acids to soaps, and they are removed in
the lye solution by centrifuging. The fatty acids may be recovered
for soap manufacture, or they may be added to meal. Sometimes
both gums and free fatty acids are removed in a single
washing with dilute lye.
Excessive pigments in the oil do no harm, but the oil darkens
with repeated heating. Dark oil is considered of low quality, and
manufacturers find that light colored oil sells better than dark
colored oil. Pigments (and remaining traces of gums, free fatty
acids, and minerals) can be removed by bleaching, which is done
by adding specially mined clays to the oil. The clays adsorb the
unwanted materials and are separated from the treated oil by
filtration. Valuable oil is adsorbed along with the unwanted
materials, but normally recovery of the oil is not cost effective.
The bleaching clay is discarded after one treatment.
The distinctive flavors of such oils as olive, peanut, or sesame
are desirable. The distinctive flavor of soybean oil is not desirable,
and so the flavors are removed to produce as bland an
oil as possible. Flavor compounds are difficult to remove, and
the only effective means is high temperature (500 [degrees] F/260 [degrees] C) steam
distillation under vacuum, a process is called deodorization.
Other processes for making soybean oil more useful as food include
hydrogenation to convert the oil to a solid for use as a
shortening or margarine, and winterization to prevent crystals of
fat from forming when the oil is chilled.
Soybean Concentrates and Isolates
Because the protein of soybeans is nutritious and easily available
in high concentrations, people have sought ways to incorporate
it into human diets. Full-fat or defatted flours as starting
materials contain the soluble carbohydrates that are naturally
present in soybeans. Some of the sugars (raffinose and stachyose)
are not digested and absorbed but are fermented by microorganisms
in the gut, a process that causes distressing intestinal upsets.
Consequently, processes have been developed to remove the soluble
sugars while concentrating the proteins. Removal of soluble sugars
from defatted flours gives a concentrate with 70 percent
soybean protein. Removal of all carbohydrate from defatted flours
gives a product with more than 90 percent soybean protein.
Concentrates are produced by making the protein portion of the
flour insoluble in water and then extracting the soluble carbohydrates
with water or water-alcohol mixtures. The protein can be
made insoluble in water by extracting flours that have been heated
in the desolventizer-toaster and using hot water--150 to 200 [degrees] F
(66 to 93 [degrees] C)--for the extraction. Alternatively, flours that have
been desolventized under vacuum to maintain protein solubility
can be extracted with water-alcohol (60 to 80 percent ethanol)
mixtures or at a pH of 4.5 to remove soluble sugars.
The resulting protein concentrates have varying degrees of protein
solubility. Protein solubility is measured by a nitrogen
solubility index (NSI) or a protein dispersibility index (PDI).
The higher the NSI or PDI the more soluble the protein. For example,
concentrates produced by hot-water leaching have low NSIs of
about 5, while concentrates produced by low-pH leaching have high
NSIs of about 70. High solubility would be useful if the concentrate
were to be used in a high protein drink, whereas use in
weaning food may not require high solubility.
Soy-protein isolates are produced by extracting defatted flours,
which have been desolventized under vacuum to maintain protein
solubility, with dilute alkali. The protein solution is then precipitated
by adding acid and the protein curd is recovered. If
the protein curd is washed with alkali to remove the acid, the
protein will become soluble, or the curd may be washed with water
and dried as an insoluble protein isolate.
Many uses have been found for soybean concentrates and isolates
in the human diet. They may be mixed with other foods to take
advantage of the protein that they contribute, for its nutritional
value or its improvement of the texture or solubility of the
mixture. They may be used in infant foods or formulas for their
nutritional value. Also, concentrates and isolates can be texturized
by putting a suspension of the protein through an extruder.
The extruded proteins have chewy textures that can simulate meats
and cheeses when modified with flavors and colors.
One problem with concentrates and isolates that has not been
solved is an off-flavor that resembles the raw soybean. Apparently,
lipid oxidation (rancidity) occurs during solvent extraction
of the oil. The oxidized compounds combine with the protein and
their flavors are very difficult to remove.
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