Effects of Protein Composition on Gelation of Mixtures Containing Soybean 7S and 11S Globulins

Abstract

Soybean 7S and 11S globulins, which are two major components of soybean protein, were mixed in various ratios. Gelation of the protein blend after the addition of glucono-δ-lactone (GDL) or calcium sulfate was studied to analyze the gelation process of tofu (soybean curd). Dynamic viscoelasticity measurements at a constant temperature as a function of time were done. Faster gelation was observed in mixtures containing 11S in a higher proportion for both the GDL and calcium systems. The value of the storage modulus of the 5 : 5 blend system of 7S and 11S globulins was minimum when the GDL concentration was adjusted to 17mM, at which level the gels consisting of 7S globulin alone and 11S globulin alone had a similar storage modulus. Under the same conditions, although the gel of the soybean protein isolate showed almost the same value for the storage modulus as a mixture of 7S : 11S in the same ratio, the gelation was slower than that expected from the 7S and 11S blend. Since the use of calcium sulfate led to a large syneresis for 11S globulin, no homogeneous gels were obtained from this system. The 7S fraction in the mixture prevented the syneresis of 11S globulin gel induced by calcium. Gel networks observed by scanning electron microscopy for 7S globulin with GDL was finer than those for the 11S one. The gel formed by soybean protein isolate involved both the 7S- and 11S-types of networks. The coarser networks were mainly observed in the gel of the 5 : 5 blend. The 11S fraction in the mixed system mainly formed a gel matrix.