Composition of Proteins in Okara as a Byproduct in Hydrothermal Processing of Soy Milk Sladjana P. Stanojevic,* Miroljub B. Barac, Mirjana B. Pesic, and Biljana V. Vucelic-Radovic Institute of Food Technology and Biochemistry, Faculty of Agriculture, University of Belgrade, Nemanjina 6, P.O. Box 14, 11081 Belgrade-Zemun, Serbia ABSTRACT: Protein quality, based on its subunit composition, in okara obtained as a byproduct during hydrothermal cooking of soy milk was assessed. The composition of 7S and 11S protein fractions was correlated with the physicochemical properties of protein in okara produced from six soybean varieties. The basic 7S globulin (Bg7S) and 11S protein were two main proteins in okara. Investigated soybean genotypes produced okara with mainly acidic A 5 and basic B 1,2,4 polypeptides of 11S proteins. Soybean 11S content was not an indicator of okara protein recovery or extractability. Of all tested relationships, extractable soluble protein content of okara was influenced only by soybean Bg7S (r = 0.86; p < 0.05) and its light subunit contents (r = 0.93; p < 0.05). Okara protein recovery depended on Bg7S heavy subunit content in soybeans (r = 0.81; p < 0.05). The high quantity of vegetable protein in okara (around 35%) and very high protein extractability (around 85%) qualify this byproduct for potential application in food preparation as a functional ingredient. KEYWORDS: okara, basic 7S globulin, glycinin, hydrothermal cooking, soybean genotype ■ INTRODUCTION Among various soy foods, soy milk and tofu are becoming more popular as low-cost substitutes for traditional dairy products for consumers and an ideal nutritional supplement for lactose intolerants. Okara is a byproduct obtained during processing of soybean for soy milk, which is rarely utilized. The ratio of essential amino acids to total amino acids in okara is similar to those of soy milk and tofu. 1 The high quality of protein fraction of okara suggests that okara protein could be applied in food production. Toda et al. 2 speculated that the basic 7S globulin (Bg7S) is likely the main protein in extracts of okara in hot water. It is known that Bg7S is a cysteine-rich glycoprotein that is composed of two subunits linked by disulfide bonding. 3 Both subunits are synthesized as isoforms. They are designated as “heavy” (H I,II ) and “light” (L I,II ) subunits with molecular weights of 27000 and 16000, respectively. 4,5 Other authors 6,7 designated these subunits as α- and β-chains of Bg7S. Yoshizawa et al. 7 elucidated the crystal structure of the Bg7S molecule that contains 12 cysteines in positions to form 6 disulfide bonds (4 in the α- and 2 in the β-chain). Bg7S binds a 4000 protein from soybean seed, leginsulin; it is also reported under the name of 43000 protein or leginsulin- binding protein. 8 Ligand blotting experiments showed that Bg7S can bind both insulin and insulin-like growth factors I and II. 9 Although Bg7S has no amino acid sequence homology with the human insulin receptor and insulin-like growth factor receptors, there are structural similarities between Bg7S and the human insulin receptor. Both proteins are glycosylated and have a cysteine-rich domain, and both have disulfide-bonded α and β subunit structures. Also, Bg7S was shown to have protein kinase activity in the α-chain, as for the insulin receptor β subunit. In addition, the amino acid sequence between the 41st and 53rd residues in the Bg7S H I,II subunit is homologous to that in the human low molecular mass insulin-binding protein. 10 Omi et al. 3 described the specific protein release from soybean seeds induced by high-pressure treatment. They isolated the major component of the released proteins and identified it as Bg7S. They discussed possible mechanisms of the pressure-induced protein release on the basis of the results obtained on the Bg7S localization in seed dermal tissue and pressure-induced structural changes of the dermal tissue. These results suggest that a large amount of Bg7S was present in the epidermal tissue of soybean seed cotyledon. Participation of other proteins of soybean 7S and 11S protein fractions in okara has not been sufficiently studied. β- Conglycinin (7S) and glycinin (11S) are the major soy proteins, representing about 70% of the total protein in soybeans. 11 β-Conglycinin is a trimeric glycoprotein consisting of α′, α, and β subunits with different combinations and physicochemical properties. Glycinin is composed of acidic and basic polypeptides, linked by a disulfide bridge. The structural properties of glycinin (dimeric and monomeric form) are influenced by pH values. 12 The ratio of glycinin/β-conglycinin and their polypeptide composition are indicators of functionality and the nutritive value of soybean proteins. 13-15 Because okara is obtained as a byproduct of soy milk production, understanding its protein composition and respective functionality could be useful for application of okara as a functional food additive. Therefore, the aim of this study was to assess the protein composition of okara prepared from different soybean genotypes by high-pressure hydrothermal processing and to correlate their polypeptide structure with the resulting physicochemical properties. Received: February 1, 2012 Revised: August 20, 2012 Accepted: August 20, 2012 Published: August 20, 2012 Article pubs.acs.org/JAFC © 2012 American Chemical Society 9221 dx.doi.org/10.1021/jf3004459 | J. Agric. Food Chem. 2012, 60, 9221-9228