Abstract
The process of degradation of sulfhydryl (SH) groups in soymilk was investigated by using Glycine max var. Suzuyutaka (wild type) and the following seven mutant lines lacking lipoxygenase(s): L-1-, L-2-, L-3-, L-1, -2-, L-2, -3-, L-1, -3-, and L-1, -2, -3-null. The soymilk prepared from the L-1, -2, -3-null line of all the mutants had the highest SH content. The content of SH groups was the lowest with the L-1, -3-null line of the three double-null lines and the highest with the L-2-null line of the three single-null lines. These results show that lipoxygenases strongly participated in the degradation of SH groups in soymilk and that the L-2 isozyme had the greatest SH-degrading capability. When these soybean samples were ground under low-temperature conditions and in a nitrogen (N2) atmosphere to inhibit the degradation of SH groups caused by lipoxygenases, SH degradation of the L-2, -3- and L-1, -3-null lines was strongly inhibited at low temperature, while that of the L-1, -2-null line was strongly inhibited in the N2 atmosphere. In view of the strong inhibition of SH degradation in an N2 atmosphere with Suzuyutaka (wild type), which has three L-1, -2, -3 isozymes, these results suggest that not only the L-2 isozyme but also that the L-3 isozyme of the three in Suzuyutaka played an important role in SH degradation during soybean grinding.
