The effects of soybean soaking on grain properties and isoflavones loss

•Lengthy soaking does not promote additional softening of the soybean.•A low ratio of soybeans to soaking medium may reduce protein leaching.•Isoflavone loss and degradation were affected by the soaking temperature and time.•Soybeans soaked at 55 °C had the highest conversion of β-glucosides to aglycones.•Soybean soaking at 55 °C for 5 h before subsequent processing is recommended.

The aim of this study was to investigate the effects of soybean soaking on grain properties and isoflavone loss in order to find suitable conditions for this step of soybean processing. A moisture content of 120% (dry basis) was achieved by soaking the soybeans at 55 °C and 70 °C for 3 and 2 h, respectively. Soybeans soaked at temperatures above 25 °C showed no significant difference (p > 0.05) in hardness after 1 h of soaking. The contents of total isoflavones and soluble proteins were better preserved at soaking temperatures of 25 °C and 40 °C. The β-glucosidase activity and contents of aglycone and β-glucoside isoflavones were closely related. Soybeans soaked at 55 °C for 5 h had a 6-fold higher aglycone isoflavone content than did whole soybeans (81.4 μg aglycones g−1) without considerable impairment of the aforementioned characteristics; soaking under this condition is therefore recommended before subsequent processing. This work helps to estimate the loss in soluble protein content and the loss and degradation of isoflavones during soybean soaking at different temperatures.