Description

       Tofu is a traditional soybean food. As a staple food, tofu has been playing an important role in people’s daily diet in most countries of Asia. There are many types of fresh tofu and tofu derivatives. Filled tofu and soft tofu are two of the most popular tofu products, in which calcium sulfate and glucono-d-lactone are used as coagulants.
　　Soymilk heating is a prerequisite for tofu gel structure formation from soybean protein. One of the purposes of soymilk heating is to denature the soybean proteins; that is, to expose the hydrophobic region and sulfhydryl groups to facilitate soymilk solidification in order to finally obtain a unique tofu texture. In the commercial tofu manufactures, soymilk is usually heated by steam injection. 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 could liquid food be heated evenly, but heating temperature and time could be accurately adjusted or controlled. Obviously, two-stage heating could be implemented with ohmic heating.
　　The denaturation temperatures for b-conglycinin and glycinin, which are major protein components in soybean milk, are around 70 °C and around 95 °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, in which glucono-d-lactone and calcium sulfate were used as coagulants, respectively. Tofu’s textural properties and syneresis rate, and soft tofu’s yield and solids recovery were measured.
　　Table 1 shows how 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 increased apparent breaking strength by 12.2% and apparent Young’s modulus by 16.2%, reduced the syneresis rate by 21.8% (Table 2), and increased the yield and the solid recovery by 4.5% and 5.4%, respectively (Table 3). Scanning electron microscopic observation shows that the tofu gel network prepared by two-stage heating was finer and more homogenous than that by one-stage heating. It was concluded that two-stage ohmic heating is a potential method for heating soymilk in tofu manufactures.

Figure, table

1. Table 1. Effect of heating method on filled tofu’s physical properties

   

2. Table 2. Effect of heating method on soft tofu’s physical properties

   

3. Table 3. Effect of heating method on soft tofu’s yield and solids recovery

   

4.