Changes in Isoflavone Compositions of Soybean during Soaking in Water

Abstract

We examined changes in isoflavone compositions of 4 varieties of soybean (Toyomusume, Tamahomare, Murayutaka, Toyokomachi) during soaking in water, which is one of the important food processing procedures. There were greater amounts of 6″-O-malonylated isoflavone glucosides than those of any other components in all the varieties. This proportion was followed by isoflavone glucosides, whereas the proportions of free isoflavones and 6″-O-acetylated isoflavone glucosides were small. After soaking in water for 20 h at 15°C, the proportion of free isoflavones in all 4 varieties increased significantly, whereas that of isoflavone glucosides decreased. No significant changes were observed in the proportions of 6″-O-malonylated isoflavone glucosides or 6″-O-acetylated isoflavone glucosides. The largest in-crease in free isoflavone was seen in Tamahomare compared to the other 3 varieties. We measured β-glucosidase activities for each variety using p-nitrophenyl-β-D-glucoside as the substrate and found no difference among them. However, the water absorption rate of Tamahomare was significantly higher than that of other varieties at the initial stage of soaking in water, as was the turbidity of the water it was soaked in. The activity of β-glucosidase and ratio of free isoflavones to isoflavone glucosides in the soaking water of Tamahomare were also higher than those of other varieties. Given these findings, we determined that the changes in soybean isoflavones, from isoflavone glucosides to free forms, during the soaking was caused by β-glucosidase present in the soybeans, but that the differences among individual varieties were attributable to differences in water absorption rate and the quantity of eluted components including isoflavones and β-glucosidase.