Studies on the processing and properties of soymilk. II. Effect of processing conditions on the trypsin inhibitor activity and the digestibility in vitro of proteins in various soymilk preparations

Soymilks prepared from beans which had been soaked either in water or 0.4 M sodium carbonate solution for 24 h as a pretreatment were subjected to heat treatment under different conditions for varying lengths of time, to destroy the trypsin inhibitor activity. The rate of inactivation oftrypsia inhibitor in soy milks prepared from carbonate presoaked beans was faster than that of the water presoaked preparation when processed at 98°C and this effect was primarily associated with the change that occurred in the pH of the former system; The effect of alkaline pH's at 98°C on the inactivation of trypsin inhibitor was examined and it was found that the rate of inactivation was changed from zero order at pff 6.8 to first-order kinetics at pH 9.9. Regression equation relating pH of the system and time of heating at 98°C for 100% destruction of the inhibitor activity is presented. This effect of pretreatment was eliminated when both milks were processed at 115°C in cans because of the constancy of pH under these conditions. The influence of heat processing conditions on the enzymic digestibility of proteins in both soymilks was also studied. The pepsin digestion showed no significant differences between milks prepared from the water and the carbonate presoaked soybeans, and was highest in milks which had not been heat treated. With trypsin the digestibility increased with the degree of heal treatment up to the point where the trypsin inhibitor was destroyed, after which further heating resulted in lower digestibilities. A 19% increase in digestibility by trypsin was observed in the milk prepared from carbonate presoaked beans when compared with that from water presoaked beans and after both milks had been heated at 98°C for just sufficient time to destroy the trypsin inhibitor. When the milk prepared from water presoaked beans was autoclaved at 115°C to the same end point, its digestibility increased but it was still about 6% lower than that of the milk prepared by alkali presoaking method and processed at 98°C. The digestibility of casein using pepsin under the conditions used was lower than that of both soymilks. When trypsin was used the digestibility of casein was approximately the same as that of the adequately processed soymilk from carbonate presoaked soybeans or the autoclavtd soymilk from water presoaked beans. Other effects of using a carbonate presoaking treatment far the production of soymilk have also been discussed.