ABSTRACT: The concentration of the reducing agent SO2 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 mM SO2 based on protein yields, purities, and functional properties. With no SO2, 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 mM SO2, 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 SO2 concentration increased. The functional properties of the 2 major fractions were greatly influenced by the addition of SO2. The thermal behaviors of the fractions were only slightly affected by using higher amounts of SO2. The solubility and hydrophobicity of the glycinin-rich fraction decreased with increasing SO2 concentration, whereas the solubility of the β-conglycinin-rich fraction increased. Emulsification properties of the glycinin-rich fraction were adversely affected by higher SO2 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 mM SO2.