There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

Implications of antinutritional components in soybean foods

The nature of the variation of dielectric properties with frequency, temperature, and product density is discussed. Techniques for measurement of dielectric properties are briefly reviewed, and graphical data on the dielectric properties of grain and soybeans as functions of moisture content, frequency, temperature, and bulk of density are presented. Applications in the electrical measurement of the moisture content and in the dielectric heating of these materials are discussed. >

Dielectric properties of agricultural products-measurements and applications

The soybean is one of the most economical and valuable agricultural commodities because of its unique chemical composition. Among cereal and other legume species, it has the highest protein content (around 40%); other legumes have a protein content between 20% and 30%, whereas cereals have a protein content in the range of 8-15%. The soybean also contains about 20% oil, the second highest content among all food legumes. (The highest oil content is found in peanut, which is about 48% on dry matter basis. The third highest oil content is chickpea, which is about 5%. The remaining food legume species have oil contents in the range of 1-3.6%) (Salunkhe et al. 1983). Other valuable components found in soybeans include phospholipids, vitamins, and minerals. Furthermore, soybeans contain many minor substances, some of which, such as trypsin inhibitors, phytates, and oligosaccharides, are known to be biologically active. Others, such as isoflavones, are just being recognized for their powerful ability to prevent human cancers and other diseases (Messina et al. 1994, Chapter 10 of this book). In this chapter the chemical components of soybeans are discussed with respect to their occurrences, properties, nutritional value, physiological roles, and assay methodology.

Chemistry and Nutritional Value of Soybean Components

There are limitations to which one is justified in drawing broad generalizations concerning the diverse biological and physiological effects of soy protein products. Nevertheless, there appear to be two distinct situations: (A.) Proper heat treatment exerts a beneficial effect upon the nutritive value of whole soybeans, full-fat and defatted meal. Associated with proper heating is inactivation of trypsin inhibitor and other heat-labile factors and conversion of raw refractory proteins to forms that are more readily digested. (B.) Moist heat also has a beneficial effect upon the nutritive value of soy protein isolates. However, a deficiency of certain essential nutrients and the interaction of phytic acid with protein, vitamins, and minerals during processing are the primary factors responsible for the poor nutritive value of soy isolates. Occasionally mineral deficiency symptoms do occur in animals fed soybean meal. It is a misnomer to refer to the growth-inhibiting and pancreatic hypertrophic properties as a “toxic” effect since both properties are reversible. Modern analytical techniques should be used to reinvestigate the relationship between phytic acid and availability of minerals and vitamins in soy protein isolate diets. Research also is needed to determine more accurately vitamin and mineral contents of soy protein isolates and the availability of vitamins and minerals in soy protein concentrates. Breeding soybean varieties genetically deficient in antinutritional and nonflatulent factors does not appear promising. More research is needed to determine whether fermentation and enzyme processes can be used to prepare flatulent-free soy products. Minor factors to be considered in assessing the nutritive value of soy products include a weak goitrogen present in soybeans, and a very low estrogenic activity.

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Biological and physiological factors in soybeans.

The nutritional quality of soya products is determined not only by the quantity and availability of the amino acids which make up the protein of such products, but is also markedly affected by the processing conditions which are employed in their manufacture. The most important factor in this regard is the application of some form of heat treatment which serves to inactivate a number of naturally occurring constituents of the soybean. Although non-toxic in the truest sense of the word, these are substances which can nevertheless elicit adverse physiological responses in animals, and, unless destroyed, can detract from the full nutritional potential of soya protein. The best known and certainly the most studied of these factors are the inhibitors of trypsin and chymotrypsin, enzymes which play a key role in the digestion of proteins in animals. These will be discussed with respect to their possible model of action in vivo, and an attempt will be made to evaluate their nutritional significance in man. Also present in soybeans are several other heat-labile components whose physiological significance is less well understood. These include the phytohemagglutinins (lectins), goitrogens, antivitamins and phytates. Less sensitive to the destruction effects of heat are a number of factors which are capable of producing a wide variety of physiological responses in animals and include saponins, estrogens, oligosaccharides and allergens. Although present in readily detectable quantities, their effect on the nutritional quality of soya protein is questionable. Not to be overlooked is the fact that harsh processing conditions such as excessive heat treatment or extraction under alkaline conditions may lead to the destruction of economic importance at the present time, brief consideration will be given to the possible effects of germination and fermentation on the nutritive quality of the protein resulting from such treatments.

Factors affecting the nutritional quality of soya products

HEAT TREATMENT of soybeans to inactivate intrinsic growth inhibitors has been limited to a temperature times pressure times moisture interaction (Osborne and Mendel, 19 17; Hayward et al., 1936). Presumably, the pressure effect increases the possible temperature while moisture is necessary to mediate some as yet unidentified biochemical reaction accomplishing inactivation of the growth inhibitor. Dry oven heating was noted by Osborne and Mendel (1917) to be quite ineffective in improving the nutritive value of soybeans; an observation supported by recent unpublished data in our laboratory, even after increasing the initial moisture content of the beans to 30%. In conjunction with studies of radiofrequency (RF) dielectric heating on a variety of agricultural and related products, we have investigated the effect of this type of energy on the nutritive value of soybeans. A rapid inactivation of the growth inhibition effect in whole dry beans was observed yielding a product with little browning.

Lata D. Manage

Papers

Rapid improvement in nutritional quality of soybeans by dielectric heating

A Research Note RAPID IMPROVEMENT IN NUTRITIONAL QUALITY OF SOYBEANS