Effects of Removing Seed Coat and Hypocotyl and Squeezing without Heating on the Quality of Tofu

Abstract

Soybean glycosides have been shown to have undesirable taste such as bitterness and astringency. In the undesirable taste of isolated glycoside components, the order of their intensities was as follows; soybean saponin A group, B group, daizin, and genistin. As A group was found only in the hypocotyl of soybean seed and daizin was also rich in the hypocotyl, it is effective to remove seed coat and hypocotyl before soaking, to decrease undesirable tasting glycosides of these foods. Moreover, these glycosides were hardly extracted from soybean meal at lower temperature, therefore, squeezing rare soymilk without heating is also important for reduction of the glycosides in soymilk and tofu processing. In this paper, effects of removing seed coat and hypocotyl and squeezing without heating on quality of tofu were studied by texture profile analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and sensory evaluation. Cotyledon tofu which was made from cotyledon only was firmer than whole soybean tofu. From the structure of each tofu observed by electron microscopy, we found that oil droplets are evenly dispersed in cotyledon tofu, while oil droplets cohered in whole soybean tofu. The phenomenon of the latter tofu seemed to be responsible for the oily taste. The saponin fractions prepared from tofu were analyzed by TLC and HPLC. The most undesirable component group (A group) was less in cotyledon tofu than in whole soybean tofu. Sensory evaluation of tofu was designated as color, taste and texture, and those of cotyledon tofu were highly evaluated than those of whole soybean tofu.