Stability of isoflavone phytoestrogens in fermented soymilk with Bifidobacterium animalis Bb12 during storage at different temperatures

Soy isoflavones in fermented soymilk with Bifidobacterium animalis Bb12 were stored at various temperatures (-80, 4, 24.8 and 37 °C) for 8 weeks and the concentration of isoflavones determined weekly using reverse-phase high-performance liquid chromatography (RP-HPLC). The first-order kinetic model was used to assess the degradation of each isomer at each storage temperature. Soymilk predominantly possesses high concentrations of isoflavone glucosides and very low concentration of bioactive aglycone component. During storage at various temperatures, concentrations of individual isoflavone isomers appeared to be significantly stable (P < 0.01). Interestingly, aglycones showed much smaller degradation constants compared with glucosides at all the storage temperatures studied. Genistein and daidzein were much more stable than glycitein and had almost similar degradation patterns, despite differences in their concentrations in the fermented soymilk. It was, however, observed that 4 °C was the most suitable storage temperature for the product in order to guarantee minimal degradation of bioactive isoflavone aglycones.