Rheological Properties of MgCl₂-coagulated Tofu and Conditions for Hard Tofu Production in China

Abstract

This study investigates the rheological properties and microstructure of MgCl₂-coagulated tofu (Mg-tofu) to obtain useful information for hard tofu production in China. The rheological properties of Mg-tofu are affected by soymilk concentrations and coagulant concentrations. A low MgCl₂ concentration (15mM) resulted in soft tofu gel; increasing the MgCl₂ content produced a hard tofu. Increasing the protein content of soymilk also commonly increases the rupture stress of tofu. However, an excessively high concentration of MgCl₂ caused a ductile rupture of tofu at low protein concentration. MgCl₂ exhibited a relatively narrow optimal range for coagulation of tofu. The stress relaxation curve of Mg-tofu, similar to that of CaSO₄ and glucono-delta-lactone (GDL), can be fitted with a four-element Maxwell model that includes two Maxwell models. The parameters obtained from the four-element Maxwell model, in particular the viscous parameters and stress relaxation times, increased with an increase of the protein content in soymilk. It is possible to produce Mg-tofu that has a similar rupture stress to that of commercial tofu in China. The narrow optimal range and its rapid reaction with soy protein may cause difficulties in handling during industrial hard tofu production using MgCl₂; this might be improved by mixing it with GDL or CaSO₄.