Eizo Tatsumi1, Wang Lijun2, Liu Zhisheng2, Michiko Monmma3, Li Lite2
1 Japan International Research Center for Agricultural Sciences
2 College of Food Science and Nutritional Engineering, China Agricultural University
3National Food Research Institute (NFRI), National Agriculture and Food Research Organization

Thermal denaturation of soybean proteins is a prerequisite for tofu gel formation. Soymilk is conventionally heated with a one-stage steam heating. Since steam temperature is higher than 100 °C and steam condenses after injection, uneven heating and soymilk dilution are unavoidable. Ohmic heating, which utilizes the inherent electrical resistance of a liquid to generate heat, is becoming a promising method for food processing. With ohmic heating, not only liquid food could be heated evenly, but also heating temperature and time could be accurately adjusted or controlled. Obviously, two-stage heating could be implemented with ohmic heating.
The denaturation temperatures for -conglycinin and glycinin, which are major protein component in soybean milk, are around 70 °C and around 92 °C, respectively. Soymilk was heated by either one-stage heating (100 °C, 5 min) or two-stage heating (70 °C, 10 min and then 100 °C, 5 min). Filled-tofu and soft-tofu were prepared with the heated soymilk, which glucono-?-lactone and calcium sulfate were used as coagulants, respectively. Tofu’s textural properties and syneresis rate, and soft-tofu’s yield and solid recovery were measured. Compared to the one-stage heating, the two-stage heating increased the filled-tofu’s apparent Young’s modulus by 44% and reduced the tofu’s syneresis rate by 22%. In case of soft-tofu manufacture, the two-stage heating not only improved the physical properties significantly, but also increased soft tofu yield and solid recovery.