Effects of Reducing Agent Concentration on Soy Protein Fractionation and Functionality

The concentration of the reducing agent SO 2 significantly affected fraction yields, purities, and compositions during soy protein fractionation, especially the purity of the glycinin‐rich fraction. The optimum amount of reducing agent was 5 m M SO 2 based on protein yields, purities, and functional properties. With no SO 2 , the glycinin‐rich fraction contained 29% of the total protein with only 63% glycinin, and the β‐conglycinin‐rich fraction contained 10% of the total protein with 94%β‐conglycinin; whereas, by adding 5 m M SO 2 , the glycinin‐rich fraction contained 23.4% of total protein with 81.5% glycinin, and the β‐conglycinin‐rich fraction contained 16.8% of the total protein with 83.7%β‐conglycinin. Increasing amounts of storage proteins were lost in the whey fraction as SO 2 concentration increased. The functional properties of the 2 major fractions were greatly influenced by the addition of SO 2 . The thermal behaviors of the fractions were only slightly affected by using higher amounts of SO 2 . The solubility and hydrophobicity of the glycinin‐rich fraction decreased with increasing SO 2 concentration, whereas the solubility of the β‐conglycinin‐rich fraction increased. Emulsification properties of the glycinin‐rich fraction were adversely affected by higher SO 2 concentrations, whereas, those of the β‐conglycinin‐rich fraction were improved. The β‐conglycinin‐rich fractions had better emulsification properties than did the glycinin‐rich fractions. The best foaming properties were achieved at 5 m M SO 2 .