Isolation of Glycinin (11S) from Lipid-Reduced Soybean Flour: Effect of Processing Conditions on Yields and Purity

Defatted soybean flour was treated with hexane and ethanol to reduce lipid content and heated to inactivate lipoxygenase (LOX, linoleate: oxygen reductase; EC 1.13.11.12) to obtain lipid-reduced soybean flour (LRSF). The effects of processing conditions such as pH, reducing agent and storage time on yields and purity of glycinin (11S) were evaluated in the fractionation of soybean glycinin isolated from LRSF. Adjusting the pH of protein extract from 6.2 to 6.6, the yield of glycinin decreased by 16.71%, while the purity of the protein increased by 4.60%. Sulfhydryl and disulfide content of proteins increased by degrees with increasing pH. Compared with dithiothreitol (DTT) or beta-mercaptoethanol (ME) as reducing agent, the yield of glycinin was the highest when sodium bisulfite (SBS) was added to the protein extract at pH 6.4. The effect of DTT on yields of glycinin was the lowest of the three kinds of reducing agent. The purity of glycinin was similar when the three kinds of reducing agent were used. These results showed that SBS was the best choice for the isolation of 11S-rich fraction. Prolonging storage time in the precipitation stage, 10 h was the best for yields and purity of glycinin in the experiment, while there was no significant difference at P >= 0.05 for total sulfhydryl and disulfide content. The decreased free sulfhydryl content of glycinin indicated that the oxidation of free sulfhydryls and the formation of disulfide bonds occurred when the extraction time was prolonged.