Changes in soymilk during fermentation with kefir culture: oligosaccharides hydrolysis and isoflavone aglycone production

The objective of this study was to evaluate the changes in oligosaccharides and isoflavone aglycone content in soymilk during fermentation with commercial kefir culture. Soymilk was fermented with kefir culture at 25 °C for 30 h. The counts of lactic acid bacteria, Lactococcus lactis, Leuconostoc sp and yeasts; measurements of pH, acidity, α-galactosidase and β-glucosidase activity, sugar and isoflavone contents were performed at the intervals of time. In the fermented soymilk, the lactic acid bacteria counts increased from 7.6 log to 9.1 CFU g(-1), pH reached to 4.9 and lactic acid reached 0.34 g 100 g(- 1). The α-galactosidase was produced (0.016 AU g(-1)) with 100% raffinose and 92% stachyose hydrolysis being observed after the depletion of galactose, glucose and sucrose. Kefir culture produced β-glucosidase (0.0164 AU g(-1)), resulting in 100% bioconversion of glycitin and daidzin and 89% bioconversion of genistin into the corresponding aglycones. The fermented soymilk presented 1.67 μmol g(-1) of daidzein, 0.28 μmol g(-1) of glicitein and 1.67 μmol g (-1) of genistein.