Effect of selective thermal denaturation of soybean proteins on soymilk viscosity and tofu's physical properties

Thermal denaturation of soy proteins is a pre-requisite for tofu-gel formation. Differential scanning calorimetry of soymilk showed that the denaturation temperature of glycinin (92 degreesC) is about 20 degreesC higher than that of beta-conglycinin (71 degreesC). This makes it possible to denature soybean proteins selectively using two-step heating, that is, soymilk was heated at 75 degreesC for 5 min and then heated at 95 degreesC for another 5 min. The effects of selective thermal denaturation (STD) on soymilk viscosity and tofu's physical properties were investigated with three soybean samples and varied soymilk solid contents (10-12%). Comparing to one-step heating (95 degreesC, 5 min), STD increased soymilk viscosity by approximately 150% and tofu's apparent Young's modulus by approximately 20%, and reduced tofu's syneresis rate by approximately 10%. Change in tofu microstructure was also observed by scanning electronic microscopy. STD mechanism based on the interaction between glycinin and beta-conglycinin was discussed. (C) 2004 Elsevier Ltd. All rights reserved.