Spectrophotometric Determination of Sulfhydryl Groups in Soymilk Using 2, 2'-Dithiobis- (5-nitropyridine)

Abstract

The measurement of sulfhydryl (SH) groups in soymilk is very important, because they are closely related to the firmness of tofu-gel. However, soymilk is not transparent and therefore the determination of its SH groups by spectrophotometric method is quite difficult. The present paper deals with the spectrophotometric method, by which the SH groups in soymilk can be determined directly and easily, using 2, 2'-dithiobis-(5-nitropyridine) (DTNP). First, the effect of perchloric acid (deproteinizing agent) on 5-nitro-2-thiopyridone (NTP), the reaction products between DTNP and SH groups, was investigated. As a result, NTP was stable in 5% perchloric acid and its peak of the absorption shifted from 386nm to 376nm accompaning with the increase of the absorption intensity. On the basis of these facts, we developed a new method to determine the SH groups in a turbid solution, such as soymilk. Using this method, the changes of the SH groups in soymilk during a heating were examined. The result showed that the SH content in soymilk increased during the first 2 or 3min by the heating at 100°C.