The Effect Of Soybean Endogenous Enzymes On Soy Yogurt Quality

Based on previous studies of our laboratory, the soybean protein was degraded bysoybean endogenous protease in the way of germinating soybean or insulating soymilk. Thensoymilks including degraded soy protein were fermented at42℃for approx.4h to producesoy yogurt (sogurt) with the combined probiotics of Lactobacillus helveticus B02,Streptococcus thermophilus IFFI6038and Lactobacillus bulgaricus AS1.1482. Thephysicochemical, rheological characteristics, microstructure, sensory properties andanti-nutrients activity of the fermented products were subsequently analyzed. In addition, theimprovment mechanism of endogenous enzymes on soy yogurt quality was expounded fromthe perspective of soy protein isolate.The changes of protein subunit composition and trypsin inhibitors activity in soymilkand physicochemical, rheological properties and microstructure characterizes of soy yogurtwere analyzed. The results showed that in the process of germination, the subunit of α, α’ andlipoxidase in β-conglycinin and acid subunit in glycinin were gradually degraded and trypsininhibitors activity (TIA) was decreased and became sensitive to heat. By comparison with theungerminated soy yogurt, the comprehensive qualities of germinated soy yogurt wereimproved. The titratable acidity increased and water-holding power changed. Moreover, therheological properties of soy yogurt were improved; viscoelasticity, yield stress and apparentviscosity significantly decreased. The microstructure of soy yogurt gel became exquisite andsoft.The effects of endogenous protease during germination on the molecular properties,conformational and functional properties as well as the in vitro trypsin digestibility of soyprotein isolate (SPI) were investigated. Lipoxygenase, α, α’ subunits of β-conglycinin (7S)and acidic chains of glycinin (11S) of soybean were degraded during germination.Concomitantly, the loss of tertiary structures of SPI occurred and molecular structure of SPIbecame stretched due to the germination. The data suggested that some selected functionalproperties and the in vitro digestibility of soy proteins could be improved by means of thegermination.The trypsin inhibitor activity (TIA) of germinated SPI presented a decreasingtrend and the in vitro trypsin digestibility of germinated SPI exhibited a consistent trend. Theprotein solubility (PS) and emulsifying properties of SPI were also improved by thegermination.The influence of endogenous enzyme activity during the heat-insulating process onsoybean yogurt was evaluated. Results showed that the protease activity was still exist. Under the action of soybean endogenous protease, the soy protein subunit contents began to changeafter soymilk heated4h at55℃, and after6h, the amino nitrogen content of soybean proteinincreased by14.04%. With soy milk heating time extended, the physichemical propertiesincluding acidity and pH of soy yogurt were improved, viscidity and elasticity increased, andthe characteristics of shear thinning got weak. After insulation treatment of soymilk, the tasteof soy yogurt also had very big improvement, particle feeling and beany disappeared, thetexture became exquisite and soft, viscosity increases, and its overall acceptance significantlyenhanced.