Effect of Heat Treatment on the Content and Thermal Degradation Kinetics of Isoflavones in Soymilk
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Abstract:
Soy aglycone isoflavones can be assimilated directly by humans, whereas isoflavones in other forms can only be absorbed after the removal of the sugar moiety via hydrolysis. In this paper, the conversion of individual isoflavones during the heating of soymilk was illustrated by stimulating pasteurization (95 ℃), high-temperature short-time sterilization (121 ℃), and ultra-high temperature instantaneous sterilization (143 ℃), processes that are commonly used in industry. In addition, the kinetic analysis of the thermal degradation of major isoflavones was also conducted, in order to provide a theoretical basis for the production of a high proportion of aglycone isoflavones in soymilk. In raw soymilk, the content of malonylglucoside isoflavones was the highest, followed by glycoside and aglycone isoflavones. All three heat treatments resulted in the decrease of malonylglucoside isoflavones, which followed first-order reaction kinetics. In addition, the degradation rate constants increased with increasing temperature. The heat treatment at 95 ℃ resulted in the conversion of the malonylglucoside form to the glycoside form, whereas degradation, instead of conversion, of isoflavones occurred at 143 ℃. After the heat treatment at 121 ℃, the content of aglycone isoflavones in soymilk was significantly increased by 50% when compared with raw soymilk, thus improving the quality of soymilk. Compared with the other treatment temperatures, high-temperature short-time sterilization could reduce the degradation of isoflavones and increase the content of aglycone isoflavones, and was more suitable for the industrial production of soymilk.
