Potential Improvement of Soymilk Gelation Properties by Using Soybeans with Modified Protein Subunit Compositions.

To examine the effects of protein subunit compositions on tofu gel formation, we prepared soymilk from a low-β-conglycinin soybean line lacking two β-conglycinin subunits and from a low-glycinin soybean line lacking three glycinin subunit groups, the proportion of glycinin to β-conglycinin (11S/7S ratio) of which was 3.76 and 0.12, respectively. The two kinds of soymilk were mixed to give various 11S/7S ratio, and heated at 80°C with 0.4% glucono-δ-lactone (GDL) in a water bath to make tofu gel or in a cell for measuring dynamic viscoelasticity. The breaking stress of the GDL-induced tofu gel made from the low-β-conglycinin soymilk was nearly four times higher than that made from the low-glycinin soymilk. The tofu breaking stress and the soymilk storage modulus both increased with increase of 11S/7S ratio. The storage modulus correlated well with the tofu gel breaking stress (r=0.932**). The decrease of glycinin content by lacking the subunit(s) resulted in decrease of 11S/7S ratio. The heated soymilk made from soybean seeds having different glycinin subunit compositions showed a close association between 11S/7S ratio and storage modulus (r=0.946**). Therefore, 11S/7S ratio is considered to have an important effect on the physical properties of tofu gel. The soymilk from low-β-conglycinin soybean and low-glycinin soybean showed a similar pH change after the addition of MgCl2. Soymilk from the low-β-conglycinin soybean coagulated immediately after addition of 0.3% MgCl2 was added. Soymilk from the low-glycinin soybean, in contrast, remained liquid even at 1°C. These results suggest that soymilk gelation properties can be improved by using soybeans with modified protein subunit compositions.