Effect of water-to-bean ratio on the contents and compositions of isoflavones in tofu

The present study investigated the changes of the bioavailable isoflavones, including daidzin, genistin, daidzein, and genistein, during the making of tofu. The amount of extracted daidzin and genistin in soy milk increased with increasing water-to-bean ratios from 5 to 9 and reached the maximum level at the ratios of 9-11. On the other hand, the amount of extracted free isoflavones (daidzein and genistein) was not affected by the water-to-bean ratio at the range of 5-11, and their extracted amounts in soy milk were 2-4-fold those in raw soybean. It is suggested that these free isoflavones are mainly derived from daidzin, genistin, malonyldaidzin, and malonylgenistin through enzymatic hydrolysis during the making of soy milk. Tofu made with water-to-bean ratios of 9:1 and 10:1 had the maximal retentions of daidzin and genistin, which were due to the fine homogeneous network microstructure that is supposed to be more effectively retained through hydrophilic interaction with protein. On the contrary, the retained amount of free isoflavones decreased significantly as the water-to-bean ratio increased from 7 to 11, due to their weakening hydrophobic interaction with protein. In this study it was found that the homogeneous microstructure of tofu improved the retention of hydrophilic daidzin and genistin and that the increased amount of drained water does not significantly reduce their retention in the final tofu products as generally imagined.