Traditional and Emerging Feedstocks for Food and Industrial Bioproduct Manufacturing - Food and Industrial Bioproducts and Bioprocessing - page 17

Soybeans contain about 40% protein (dry basis) and are rich in amino acids lysine and
threonine. However, low content of the essential amino acid L-methionine limits nutritive
value of soy protein. Globulins, referred to as glycinin and conglycinin, are the most
abundant protein types in soybeans. Health benefits of soybean proteins have been studied
extensively. Positive effects of soybean proteins on hypertension and heart disease,
osteoporosis, bone health and certain cancers have been reported (Friedman and Brandon,
2001). The US Food and Drug Administration (FDA) has authorized a health claim on the
role of soy protein in reducing risk of coronary heart disease. The FDA rule permits a
statement on the soybean containing product labels indicating that “Diets low in saturated
fat and cholesterol that include 25 g of soy protein a day may reduce the risk of heart disease”
(FDA, 1999 ).
Soybeans are believed to be responsible for 90% of the food-related allergenic reactions
(L'Hocine and Boye, 2007). Allergens are proteins. Although significant advances have
been made in the identification and characterization of soybean allergens, it is still not
entirely clear which soy proteins cause allergic reactions. At least 16 allergenic soy proteins
have been identified (L'Hocine and Boye, 2007). It might be possible to remove some of the
known soybean allergens through traditional breeding and genetic engineering. However,
considering that most proteins have metabolic, storage or protective functions in seeds,
removal or modification of some of the allergenic proteins may impair the nutritional and
functional properties of soybeans.
Soybeans contain compounds that impair activity of enzymes trypsin, chymotrypsin,
carboxypeptidase, elastase, and

-amylase. The presence of enzyme inhibitors, Bowman-
Birk inhibitor (BBI) (an inhibitor of chymotrypsin and trypsin) and Kunitz trypsin
inhibitor (KTI), in unheated soybeans hinders the activity of protease enzymes trypsin and
chymotrypsin and, consequently, limits protein hydrolysis during digestion. Rat feeding
studies with some of the soybean derived inhibitors led to development of pancreatic lesions
and caused neoplasia or tumor formation (McGuiness et al ., 1987 ). Interestingly, BBI has
the opposite effect, it has been shown to have cancer prevention properties. It has been
postulated that BBI may block the formation of oxygen free radicals by stimulated
neutrophils, inhibit tumor promotion and prevent the digestion of proteins to amino acids,
thus depriving rapidly growing cancer cells of essential amino acids (Troll et al ., 1986 ).
Soybeans contain other bioactive proteins, cytochrome-c, lectin, lipoxygenase and
urease. Lectins are glycoproteins and known as hemagglutinins because of their red blood
cell agglutinating properties. Although growth inhibiting properties of lectins have been
established in rat studies, removal of lectins from a soy extract did not result in significant
improvement in the weight gain of rats (Friedman and Brandon, 2001).
Even though the secondary plant metabolites, isoflavones, saponins, phytic acid, flatus-
producing oligosaccharides and goitrogen, are minor components of soybeans, they have an
impact on the nutritive value of soybeans. For example, high phytic acid content in soybeans
could impair mineral absorption (Friedman and Brandon, 2001). Significant variations in
total isoflavone content among soybean cultivars have been reported, 1161-2743 μg/g, a
2.5-fold variation from the lowest to highest isoflavone content (Wang et al ., 2000a ). There
has been great interest in the role of isoflavones in prevention and treatment of chronic
diseases. (Messina and Messina, 2000). However, there are many inconsistent research
studies on the health benefits of isoflavones (Tikkanen and Adlecreutz, 2000; Anderson
et al ., 1999 ; Washburn et al ., 1999). The disparities in the research results were attributed to
the different amounts and types as well as different ratios of the 12 isoflavones (3 aglycones,
3 glucosides, 3 acetyl glucosides, and 3 malonyl glucosides) used in the diets (Friedman and

α