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The soybean [Glycine max (L.) Merrill, family Leguminosae, subfamily Papilionoidae] originated in Eastern Asia, probably in north and central China. It is believed that cultivated varieties were introduced into Korea and later into Japan some 2000 years ago. Soybeans have been grown as a food crop for thousands of years in China and other countries of East and South East Asia and constitute to this day, an important component of the traditional popular diet in these regions.
Although the U.S.A. and Brazil account today for most of the soybean production of the world (see Table 1-1), the introduction of this crop to Western agriculture is quite recent. Soybeans are, primarily, an industrial crop, cultivated for oil and protein. Despite the relatively low oil content of the seed (about 20% on moisture-free basis), soybeans are the largest single source of edible oil and account for roughly 50% of the total oilseed production of the world.
With each ton of crude soybean oil, approximately 4.5 tons of soybean oil meal with a protein content of about 44% are produced. For each ton of soybeans processed, the commercial value of the meal obtained usually exceeds that of the oil . Thus, soybean oil meal cannot be considered a by-product of the oil manufacture. The soybean is, in this respect, an exception among oilseeds.
It can be calculated that, the quantity of protein in the yearly world production of soybeans, if it could be totally and directly utilized for human consumption, would be sufficient for providing roughly one third of the global need for food protein. This makes the soybean one of the largest potential sources of dietary protein. However, the bulk of soybean oil meal is used in animal feeds for the production of meat and eggs. Despite considerable public and commercial interest in soybean products as food,the proportion of soybean protein consumed directly in human nutrition is still relatively small.
World production of soybeans has increased by a factor of eight in the last half century to reach its present level of over 100 million metric tons per year (Table 1-1, Fig.1). The leading producers are the U.S.A. (45%), Brazil (20%) and China (12%). Much of this phenomenal growth was due to the sharp increase in the U.S.A. production between 1950 and 1970, and to the introduction of the soybean to Brazilian agriculture in the sixties.
An important factor in this development was the considerable improvement in the yields, through plant breeding and advanced agrotechnical practice. Consideration of the economic advantages of soybeans has led many countries to start large scale production of this crop. The consequences of these efforts are now beginning to be seen. The share of the "rest of the world" in the production scene has been growing steadily to reach the present level of 23%.(Figure 2).
Table 1.1 World production of soybeans
Source: FAO Production Yearbook
Soybeans are marketed as most other major bulk commodities. Spot and future prices are governed by offer and demand. With the exception of periods of disastrous drought in the major producing areas, supplies have been able to keep abreast of the increasing demand. Consequently, the price of soybeans on the international market has remained remarkably stable, despite inflation. (See Fig.3).
Over 25% of the world production of soybeans is traded, unprocessed, on the international market ( Table 1-2 ). Most of the trading is done by a small number of large companies. The U.S.A. is the leading exporter, with approximately 75% of the traded volume.The leading importer is Japan . In addition, very considerable quantities of soybeans are processed in the countries of production, for export as meal or oil. In fact, some countries favour the export of meal and/or oil over the export of unprocessed beans, as a matter of foreign commerce policy. As an example, exports of soybean meal from Brazil far exceed the quantity of raw soybeans exported by that country.
The peculiar meal/oil ratio of soybeans, as mentioned before, may create an exportable surplus of one of the two products. This type of imbalance between the local demands for oil and protein explains part of the international commerce of soybean meal and oil.
Figure 1: World Production of Soybeans Based on data from FAO
Soybeans are sold by grade and the price is adjusted accordingly. In the U.S.A., soybeans are classified as grains and as such, their grading is regulated by the U.S. Grain Standards Act. The criteria for grading are test weight (weight per unit volume, lb./bushel), damaged seeds and calor (proportion of green,brown or black beans). The purchaser may include additional quality parameters according to the end use.
Moisture content is an absolute requirement and it is always specified in the contracts and certificates, regardless of grade.
Quality standards and guidelines for grading soybeans are given in Appendix 1.
Soybean production and trade quantities are often expressed in bushels. Although the bushel is a unit of volume, it can be converted to weight, assuming a standard weight-per-bushel value. One metric ton of soybeans is normally equivalent to 36.7 bushels. Conversely, one bushel of soybeans weighs 60 pounds or 27.24 kilograms.
Table 1.2 1987 Exports of soybeans and soybean products by principal exporting countries (1000 tons)
|COUNTRY||SOYBEANS||SOYBEAN MEAL||SOYBEAN OIL|
Source: FAO Commerce Yearbook.
Figure 3: Soybean Prices , 1974 to 1988 Based on data from FAO Production Yearbooks)
1.4 Agricultural Characteristics
Soybeans grow well on almost all types of soil, with the exception of deep sands with poor water retention. The optimal soil pH is 6.0 to 6.5, therefore liming may be required. With respect to climate, the soybean grows best in temperate zones. The soybean is a so-called short-day plant, meaning that flowering occurs when the nights begin to lengthen. The breeding of varieties with different maturation periods (maturity groups) has permitted optimal production in a wide range of latitudes. Recently, a worldwide program, known as the International Soybean Variety Experiment (ISVEX) and headed by the International Soybean Program (INTSOY) of the University of Illinois at Urbana-Champaign, demonstrated the feasibility of growing soybeans in subtropical and tropical regions as well. It was found that, given adequate variety selection and under experimental conditions, the yields obtained at tropical and subtropical locations were comparable to those observed under temperate climate conditions (about 1950 kg. per hectare). Although the yields obtained in actual production by farmers are much lower, the results of this remarkable experiment expand considerably the limits of the potential soybean growing areas of the world.
Rainfall in the range of 500 to 700 mm. is required for good yields. Adequate water supply is especially important during the period of pod and seed development ( pod filling stage ). Irrigation is now considered an essential factor for increased profit and security to the farmer.
An important characteristic of the soybean plant is its nitrogen fixation capability through symbiosis with nodulating bacteria in the soil. It has been estimated that up to 50% of the total nitrogen of the plant may be supplied by the nitrogen fixing mechanism.
Soybeans are planted in late spring to early summer. Full maturity is reached in early-to-mid-autumn. At this point, the leaves start to yellow and drop and the seeds begin to lose moisture. The decision when to harvest is important. Ideally, soybeans should be harvested when the water content of the seed is 13%, the maximum safe moisture level for long-range storage. If the moisture content at harvest is higher, forced-air drying of the seeds will be required prior to storage. On the other hand, if the seeds are too dry, extensive splitting and cracking of the beans may occur in the course of mechanical harvesting. Another factor to be considered is the respiration losses of the seeds between maturation and harvesting. Respiration rate is strongly moisture-dependent, being higher at high moisture content. Therefore, respiration losses may be considerable if harvesting is delayed too long when, for example, the rate of natural drying of the seeds is low, due to humid weather.
The use of heated-air dryers provides extra flexibility with respect to harvesting time and rate of harvesting, independently of weather conditions.
1.5 Physical characterisitcs and morphology of the soybean
The shape of the soybean seed varies from almost spherical to elongated and flat. The industrial varieties grown for oil are nearly spherical while the elongated varieties are the ones used as a vegetable. The colour of the seed may be yellow, green, brown or black. Industrial varieties are yellow and the presence of seeds of other colours in a lot is considered a defect. Seed size is expressed as the number of seeds per unit volume or weight. Industrial soybeans weigh 18-20 grams per 100 beans. The seeds of "vegetable" varieties are considerably larger.
Seed structure consists of the seed coat (hull) and two cotyledons, plus two additional structures of lesser weight: the hypocotyl and plumule. The cotyledon represent 90% of the seed weight and contains practically all the oil and protein in its palisade-like cells. Microscopic examination of these cells reveals the presence of protein bodies (also known as aleuron grains) and lipid bodies (or spherosomes) which constitute storage bodies for proteins and oil, respectively. Protein bodies measure, on the average, 10 microns while the lipid bodies have, typically, 0.2 to 0.5 microns in diameter.
The hull, which accounts for roughly 8% of the seed weight, holds the two cotyledons together and provides an effective protective layer. It can be removed from the seed by cracking followed by aspiration, as in the process of mechanical dehulling prior to solvent extraction.
1.6 Chemical composition
The composition of soybeans may vary somewhat according to variety and growing conditions.Through plant breeding it has been possible to obtain protein levels between 40% and 45%, and lipid levels between 18 and 20%. Usually, an increase of 1% in protein content is accompanied by a decrease of 0.5% in oil. Incidentally, this negative correlation between protein and oil is one of the reasons for the lack of interest in high-protein varieties, since the production of these varieties does not result in increased income per hectare cultivated.
The proximate composition of soybeans, in fairly representative average figures, is shown in Table 1-3. The effect of variety and growing conditions on the protein and oil contents of soybeans, within narrow limits, was also observed in the ISVEX experiment mentioned above.
Table 1.3 Representative proximate composition
|Seed part||% of whole seed weight||% (moisture-free basis)|
Source: Cheftel et al. (1985).1-6-1 Moisture:
As the water content of soybeans may vary according to storage conditions, composition data are expressed on moisture-free basis. For good storage stability and good viability as a seed, soybeans should have a moisture content of about 12% to 13%. Above this level, serious danger of mould attack exists, especially in hot weather. Below 12%, the beans tend to crack and split extensively in the course of handling. Too large a proportion of split beans is considered a defect as this may induce increased rancidity during storage.
Figure 4: Soybean Proteins; Effect of pH on Solubility
a- Characterization: The simplest criterion used for the characterization of proteins is their solubility in various media. As in all legumes, the bulk of soybean proteins are globulins, characterized by their solubility in salt solutions. The solubility of soybean proteins in water is strongly affected by the pH,as shown in Fig.4. Close to 80 % of the protein in raw seeds or unheated meal can be extracted at neutral or alkaline pH. As the acidity is increased, solubility drops rapidly and a minimum is observed at pH 4.2 to 4.6. This is the isoelectric region of soybean proteins taken as a whole.
The pH dependence of solubility is used in the manufacture of isolated soybean protein, whereby defatted, unheated meal is extracted with water at neutral or slightly alkaline pH, and the protein is then precipitated from the filtered extract by acidification to the isoelectric region.
More precise and detailed fractionation of the proteins can be carried-out by techniques such as ultracentrifugation, gel filtration and electrophoresis.
Since the classical work of W. Wolff, it has become customary to characterize the soybean protein fractions by their sedimentation constants.
Four major fractions, known as 2 S, 7 S, 11 S and 15 S have been studied extensively. (S stands for Svedberg units. The numerical coefficient is the characteristic sedimentation constant in water at 20oC. The figures are not exact but nominal. Thus the 11 S globulin has a sedimentation constant of 12.3). The 11 S and 7 S fractions constitute about 70% of the total protein in soybeans. The ratio 11 S/7 S is a varietal characteristic and may vary from 0.5 to 3.
The 2 S fraction consists of low molecular weight polypeptides (in the range of 8000 to 20000 daltons) and comprises the soybean trypsin inhibitors (see below). The 7 S fraction is highly heterogeneous. Its principal component is beta-conglycinin, a sugar containing globulin with a molecular weight in the order of 150000. The fraction also comprises enzymes (beta-amylase and lipoxygenase) and hemagglutinins (see below ). The 11 S fraction consists of glycinin, the principal protein of soybeans. Glycinin has a molecular weight of 320000-350000 and is built of 12 sub-units, associated through hydrogen bonding and disulfide bonds. The ability of soy proteins to undergo association-dissociation reactions under known conditions, is related to their functional properties and particularly to their texturization. The 15 S protein is probably a dimer of glycinin. Conglycinin and glycinin are storage proteins and they are found in the protein bodies within the cells of the cotyledons.
b- Nutritional quality: One way of evaluating the nutritional quality of a protein is by its chemical score, obtained by comparing its essential amino acid composition to that of a standard reference protein (e.g. whole egg protein).
The essential amino acid composition of soybean protein is shown in Table 1-4.
Table 1.4 Amino acid composition
of soybeans and wheat
|I- ESSENTIAL AMINO ACIDS|
|II- NON-ESSENTIAL AMINO ACIDS|
A: g/16 g Nitrogen
B: Percentage of Provisional Amino Acid Scoring Pattern
(*): First limiting (**): Second limiting
Source: Computed from data in FAO (1970) and FAO/WHO (1973).
The limiting amino acids are the ones containing sulphur (methionine and cystine). Their percentage in soybean protein is about 70% of that of whole egg protein. Hence the "chemical score" of soybean protein would be approximately 70%. On the other hand, for a plant protein,soybean protein is exceptionally rich in lysine and can serve as a valuable supplement to cereal foods where lysine is a limiting factor. Chemical score alone is not a satisfactory measure of protein quality since it does not account for protein digestibility and the biological availability of the amino acids. Other indicators such as biological value (BV), protein efficiency ratio (PER) and net protein utilization (NPU), determined by means of rat feeding tests, are better predictors but suffer from the fact that they assess the nutritional performance of the protein under certain experimental conditions which are ,usually, very different from real-life dietary situations.
The amino acid composition of soybean protein does not differ considerably from one variety to another. Attempts do develop, genetically, soybean varieties with a higher content of sulphur containing amino acids have not been successful.
c- Enzymes: Soybeans, as all seeds, contain the enzyme systems necessary for germination. Technologically, the most important enzyme in soybeans is lipoxygenase, also known as lipoxydase. This enzyme catalyses the oxidation of poly-unsaturated fatty acids by molecular oxygen, leading to the development of rancidity and beany flavour. The importance of lipoxygenase activity in the development of the characteristic "green bean" or "paint" flavour in many soybean products will be discussed in the chapters dealing with these products. Lipoxygenase activity is also the reason for the occasional use of small amounts of unheated soybean flour as a bleaching (carotenoid oxidizing) agent in wheat flour.
The enzyme urease is frequently mentioned in connection with soybean protein products. With no technological importance to itself, this enzymes has served as an indicator for the adequacy of the heat treatment given to soybean meal. Although better tests now exist for this purpose, residual urease activity is still sometimes used as an evidence of insufficient heat treatment.
d- Antinutritional factors: Several of the soybean proteins have been found to exert specific physiological effects. These are the trypsin inhibitors and the hemagglutinins (lectins).
Protease inhibiting proteins are widespread in nature, but the trypsin inhibitors of soybeans are the best known and most thoroughly studied. Inhibition of trypsin by raw soybeans has been reported more than 50 years ago and the first soybean trypsin inhibitor was isolated and crystallized in the early forties.
Soybeans contain two types of trypsin inhibitors. Both bear the names of scientists who first isolated and characterized them. They are respectively known as the Kunitz inhibitor with a molecular weight in the range of 20000, and the Bowman-Birk inhibitor which is a much smaller polypeptide in the 8000 dalton range. Both types consist of a number of differentiable proteins.The amino acid sequence and spatial structure of these proteins have been elucidated.
It has been known for a long time that raw soybeans or unheated soybean meal will impair growth when fed to young rats or chicks. This effect is completely eliminated when the soybean component is properly heated. Since trypsin inhibitors are also heat labile, it was concluded that their presence in the diet is responsible for the suppression of growth. In fact, growth is retarded if the inhibitors are added to diets containing heat-treated soybean meal.
A logical explanation for the harmful effect of the inhibitors could be that the inhibition of trypsin in the digestive track of the animal impairs protein digestibility and utilization. This hypothesis had to be abandoned, however, when it was observed that trypsin inhibitor preparations did impair growth when fed with diets containing completely pre-digested proteins. Inhibition of trypsin is not the only physiological effect of the trypsin inhibitors. It has been observed that their ingestion can result in increased pancreatic secretion and hypertrophy of the pancreas. Increased secretion of enzymes into the digestive tube represents an internal loss of protein. Since the proteins excreted by the pancreas are particularly rich in sulphur containing amino acids, this internal loss could be specially important if the diet is marginal in methionine/cystine.
Are soybean trypsin inhibitors toxic to humans? The bulk of the available information on their biochemical, physiological and nutritional properties stems from experimentation with animals or from in vitro investigations. There is no direct evidence as to the physiological effect of the inhibitors on humans. Nevertheless, it has become customary to take the necessary precautions for the removal or inactivation of trypsin inhibitors from soybean products intended for human consumption.
The lectins, formerly known as hemagglutinins, are proteins which possess the ability to agglutinate red blood cells. They are widely distributed in plants and some, such as the castor bean lectin ricin, are highly toxic. The lectin found in raw soybeans has, apparently, no observable dietary effect, good or bad. Furthermore, it too is easily inactivated by heat .
e: Functional properties: Many of the food uses of soybean products are based on the functional properties of soybean proteins. The functional characteristics include the ability of the proteins to thicken (viscosity), emulsify, form gels, foam, produce films and sulphur, absorb water and/or fat and create meat-like texturized structures.
Functional properties are related to the amino acid composition and sequence ( primary structure) as well as the spatial configuration of the protein molecule and the inter-molecular forces (secondary and tertiary structures). Soybean protein products with unique functional properties are available and constitute important tools in the formulation of the so-called "fabricated foods".1-6-3 Lipids:
The lipids of soybeans (crude soybean oil) consist typically of 96% triglycerides, 2% phospholipids, 1.6% unsaponifiables, 0.5% free fatty acids and minute amounts of carotenoid pigments.
The phospholipids are surface-active substances located on the surface of the oil bodies. The relatively high content of phospholipids in soybean oil ( two to three times higher than other common vegetable oils) is explained by the small size of the oil bodies, resulting in a larger surface per unit weight of lipids. Although the phospholipid fraction of soybeans contains a number of distinct substances, the technical term lecithin is used to name the entire fraction. Lecithin is a valuable emulsifier and has many food, medical and industrial uses. Because of their emulsifying power, the bulk of phospholipids must be removed from the crude oil before refining. This is done through a process known as degumming, because the phospholipids are separated as hydrated gums.
The unsaponifiables contain mainly tocopherols and sterols.They are partially removed in the course of deodorization.The free fatty acids and pigments are removed in the process of refining and bleaching. Thus the concentration of non-triglycerides is reduced in the refined oil to less than one percent.
The fatty acid composition of soybean oil depends on the variety and growing conditions. A typical composition, based on recently published data is given in Table 1-5. The unsaturated portion accounts for over 80%.
Table 1.5 Fatty acid composition of soybean oil
|FATTY ACID||SYMBOL||WEIGHT PERCENTAGE|
Source: Wahnon et al. (1988)
Soybean oil is classified as a semi-drying oil in view of its high linoleic and linolenic acid content. The presence of linolenic acid is responsible, in great part, for the stronger tendency of soybean oil to undergo oxidative deterioration. Advanced technologies are available for overcoming this problem and making soybean oil a good, multi-purpose and versatile edible oil. An overview of some of these techniques is included in Chapter 2.1-6-4 Carbohydrates:
Soybeans contain about 30% carbohydrates. These can be divided into two groups: soluble sugars ( sucrose 5%, stachyose 4%, raffinose 1% ) and insoluble "fibre" (20%). Raffinose is a trisaccharide composed of galactose,glucose and fructose linked in that order. Stachyose is a tetrasaccharide with the following structure: galactose-galactose-glucose-fructose. Raffinose and stachyose are not broken down by the enzymes of the digestive track but are fermented by the microorganisms present in the intestine, with the formation of intestinal gas. Flatulence, an inconvenience associated with the ingestion of pulses in general, is a factor which must be considered, sometimes, in the use of soybean products in human nutrition.
The insoluble fraction is a complex mixture of polysaccharides and their derivatives. The major part of this fraction consists of cell wall carbohydrates:cellulose, hemicelluloses and pectic substances. The insoluble carbohydrates are not digested by the enzymes of the gastro-intestinal track and can be characterized as "dietary fibre". They absorb water and swell considerably. Unlike other legumes, soybeans contain very little starch ( less than 1% ).1-6-5 Minerals:
The mineral content of soybeans, determined as ash, is about five percent. When soybeans are processed, most of the mineral constituents go with the meal and few with the oil. The major mineral constituents are potassium, calcium and magnesium. The minor constituents comprise trace elements of nutritional importance, such as iron, zinc, copper etc.
The biological availability of minerals may be impaired somewhat as a result of the presence of phytates in soybeans and soybean products. The mineral composition of soybeans is affected by the composition of the soil. Thus, the contamination of soils with undesirable elements such as heavy metals, as a result of irrigation with poorly treated waste water, may be reflected in the composition of the soybeans.
Cheftel, J.C., J.L. Cuq and D. Lorient (1985)
Tec & Doc Lavoisier, Paris
Food and Agriculture Organization of the United Nations ( FAO
Production Yearbooks (1974 to 1988).
Food and Agriculture Organization of the United Nations ( FAO
Commerce Yearbooks (1974 to 1987).
Food and Agriculture Organization of the United Nations ( FAO
"Amino-acid Content of Foods and Biological Data on Proteins"
Kinsella J.E., S. Damodaran and B. German (1985)
Physicochemical and Functional Properties of Oilseed Proteins with Emphasis on Soy Proteins in: "New Protein Foods", Vol. V, A.M.Altschull and H.L.Wilcke, Eds.
Academic Press, Orlando, Florida.
Liener, E.I. (1989)
Antinutritional Factors , in "Legumes", p.339-382, R.H.Matthews, Ed. Marcel Dekker Inc. New York
Nielsen, N.C. (1985)
Structure of Soy Proteins in "New Protein Foods", Vol.V, A.M.Altschul and H.L. Wilcke, Eds.Academic Press, Orlando, Florida
Smith, A.K. and S.J. Circle (1972)
"Soybeans: Chemistry and Technology" Vol.1 Avi Publishing Co. Inc. Westport, Conn.
Wahnon, R., S. Mokady and U. Cogan (1988)
Proc. 19th. World Congress I.S.F. Internat. Soc. for Fat Research, Tokyo
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