Processing

Virtually all soybeans processed in the United States are solvent extracted (Fig. 4). Beans arriving at the plant are cleaned and dried if necessary before storage. When the beans move from storage to processing, they are cleaned further, cracked to loosen the seed coat or hulls, dehulled, and then conditioned to 10-11% moisture. The conditioned meats are flaked and extracted with hexane to remove the oil. Hexane and oil in the miscella are separated by evaporation and the hexane is recovered. Residual hexane in the flakes is removed by steam treatment in a desolventizer-toaster. The heat treatment inactivates anti-nutritional factors (trypsin inhibitors) in the raw flakes and increases protein digestibility. A metric ton of soybeans yields ca 180 kg oil and 790 kg meal.

NUTRITION AND TOXICITY Oil

Because of their high linoleic acid contents, unhydrogen-ated and partially hydrogenated soybean oils are good sources of this essential fatty acid. Soybean oil is the prin cipal vegetable oil consumed 4.66 x 1061 in 1985), and ca 75% is partially hydrogenated to impart high temperature stability to cooking oil, extend shelf life, and improve flavor stability and physical and plastic properties. Linoleic and linolenic acid contents of soybean oil are reduced by hydrogénation, but more important from a nutritional viewpoint are the migration of double bonds up and down the carbon chain and the conversion of cis to trans isomers, ie, positional and geometrical isomerization. Although long-term studies with rats and short-term tests with humans have failed to reveal toxic effects on ingestion of partially hydrogenated soybean oil, this complex problem is under active investigation.

Heating and oxidation of fats, especially under severe conditions, results in the formation of a variety of compounds including hydrocarbons, cyclic hydrocarbons, alcohols, cyclic dimeric acids, and polymeric fatty acids. Some of these compounds are toxic, but the present consensus is that an oil such as soybean oil is safe and nontoxic when used under normal cooking conditions.

Proteins and Meals

Nutritional properties of the oilseed protein meals and their derived products are determined by the amino acid compositions, content of biologically active proteins, and various nonprotein constituents found in the defatted meals. Phytic acid, which is common to soybean meals, is believed to interfere with dietary absorption of minerals such as zinc, calcium, and iron. Numerous studies have demonstrated that methionine is the first limiting amino acid in soybean protein, ie, it is in greatest deficit for meeting the nutritional requirements of a given species. Although it is common practice to add synthetic methionine to broiler feed to compensate for this deficiency, there is growing evidence that this is not necessary when soy proteins are fed to humans, with the possible exception of infants. The presence of trypsin inhibitors in soybeans is well documented, and when ingested, their primary physiological effect is to enlarge the pancreas. They are largely inactivated by moist heat, and there are no documented cases where ingestion of soybean proteins by humans has affected the pancreas. Long-term effects of ingestion of soy products by rats are under study.

PRODUCTS AND USES Oil

Most crude oil obtained from oilseeds is processed further and converted into edible products. Only a small fraction of the total oils is utilized for industrial (nonedible) purposes. For edible purposes, oilseed oils are processed into salad and cooking oils, shortenings, and margarines. These products are prepared by a series of steps as shown for soybean oil in Figure 5.

Degumming removes the phosphatides and gums, which are refined into commercial lecithin or returned to the defatted flakes just before the solvent removal and toasting step. Next, free fatty acids, color bodies, and metallic prooxidants are removed with aqueous alkali. Some processors omit the water-degumming step and remove the phosphatides and free fatty acids with alkali in a single

Flakes

Bearis

Figure 4. Schematic outline for processing soybean into oil and meal by solvent extraction. Source: Courtesy of Dravo Corp.

Bearis

Figure 4. Schematic outline for processing soybean into oil and meal by solvent extraction. Source: Courtesy of Dravo Corp.

Liquid hexane rv Liquia ne

Hexane Hexane vaP°r vapor and steam

Liquid hexane

Hexane Hexane vaP°r vapor and steam

To meal grinder
Figure 5. Schematic outline for manufacture of edible soybean oil products. D, deodorization; W, winterization; S, solidification. Source: Courtesy of the American Soybean Association and the American Oil Chemists' Society.

operation. High, vacuum steam distillation in the deodorization step removes undesirable flavors to yield a product suited for salad oil. Partial hydrogenation, under conditions where linolenate is selectively hydrogenated, results in an oil with greater stability to oxidation and flavor deterioration. After winterization (cooling and removal of solids that crystallize in the cold), the product is suited for use as salad and cooking oils. Alternatively, soybean oil can be hydrogenated under selective or nonselective conditions to increase its melting point and produce hardened fats. Such a partially hydrogenated soybean oil, by itself or in a blend with other vegetable oils or animal fats, is used for shortening and margarine. Blends of soybean or other oils of varying melting point ranges are utilized to obtain desired physical characteristics, eg, mouthfeel and plastic melting ranges, and the least expensive formulation.

Polyunsaturated fatty acids in vegetable oils are subject to oxidative deterioration. Linolenic esters in soybean oil are particularly sensitive to oxidation; even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants (iron and copper), addition of metal inactivators (citric acid), minimum-exposure to air, protection from light, and selective hydrogenation to decrease the linolenate content to ca 3%.

Nonfood Uses

Vegetable oils are utilized in a variety of nonedible applications, but only 2-3% of U.S. soybean oil production is used for such products. Soybean oil can be converted into alkyd resins for protective coatings, plasticizers, dimer acids, surfactants, and a number of other products.

Protein Products

Most of the meal obtained in processing of soybeans is used as protein supplements in animal feeds. Only in the last two decades have appreciable amounts been converted into products for human consumption, and these have been almost exclusively derived from defatted soybean flakes.

Feeds

Because of their high content of protein, protein meals are essential ingredients of poultry and livestock feeds. Proximate compositions are shown in Table 4. In contrast, dehulled soybean meal is low in crude fiber and high in protein. Although limiting in methionine, soybean meal is high in lysine is a key ingredient for blending with corn in formulating feeds for nonruminats, eg, poultry and swine. The two proteins complement each other; soy supplies the lysine and corn the methionine necessary to provide a balanced ration at relatively low cost.

Edible Products

At present, only defatted soybean flakes are converted into edible-grade products. Defatted soybean flakes for edible purposes are prepared essentially as outlined in Figure 4, except that more attention is paid to sanitation than in processing for feed use, and the solvent is removed in a vapor desolventizer—deodorizer or flash desolventizer to permit preparation of flakes ranging from raw to fully cooked. Desolventizer-toasters are used to prepare fully cooked (toasted) flakes to give maximum nutritive value. Degree of cooking is determined by estimating the amount of water-soluble protein remaining after a moist heat treatment with the protein dispersibility index (PDI) or the nitrogen solubility index (NSI). A raw, uncooked flake has a PDI or NSI of ca 90, whereas a fully cooked flake has values of 5—15.

Defatted soybean flakes give three classes of products differing in minimum protein content (expressed on a dry basis) flours and grits (50% protein); protein concentrates (70% protein); and protein isolates (90% protein). Typical analyses are shown in Table 5. Flours and grits are made by grinding and sieving flakes. Concentrates are prepared by extracting and removing the soluble sugars from defatted flakes by leaching with dilute acid at pH 4.5 or leaching with aqueous ethanol. Isolated soy proteins are obtained by extracting the soluble proteins with water at pH 8-9, precipitating at pH 4.5, centrifuging the resulting protein curd, washing, redispersing in water (with or without adjusting the pH to ca 7), and finally spray drying (Fig. 6). Flours and concentrates are also specially processed into textured products that are used as meat extenders and substitutes. Typical composition of soybean protein products and their uses are summarized in Table 5.

Oilseed proteins are used in foods at concentrations of 1-2 to nearly 100%. Because of their high protein contents, textured soy flours and concentrates serve as meat substitutes. At low concentrations, the proteins are added primarily for their functional properties, eg, emulsification,

Table 5. Typical Compositions of Soybean Protein Products and Their Uses," wt %

Protein

Constituents

Defatted flours _

and grits6 Concentrates0 Isolated"*

Table 5. Typical Compositions of Soybean Protein Products and Their Uses," wt %

Protein

Constituents

Defatted flours _

and grits6 Concentrates0 Isolated"*

Protein6

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