Optimisation of soybean hydrothermal treatment for the conversion of β-glucoside isoflavones to aglycones

The objective of this work was to optimise the conditions for the soaking process of soybean for maximal conversion of β-glucoside isoflavones into aglycones using response surface methodology. The investigated variables were the time (3, 4, 6, 8, and 9 h) and temperature (41, 45, 55, 65, and 69 °C ± 1 °C) for the soaking of soybean in 1:1.5 (g:g) ratio with a buffer solution pH 6. The maximum aglycones content (1.22 μmol g−1) in soaked soybeans was estimated and validated at 55 °C ± 1 °C and 6 h of soaking. In this condition, the hydrothermally treated soybean grains showed a minimum content of β-glucoside isoflavones and low β-glucosidase activity. A 30% reduction in total isoflavones content of the system was estimated at 69 °C ± 1 °C not depending on the soaking time, attributed to the thermal instability of malonylglucosides. This work may contribute to the preparation of soy products rich in aglycones from hydrothermally treated soybean.