Formations and Changes of o-Dihydroxyisoflavones during Production of Soybean Pastes

Abstract

8-Hydroxydaidzein (8-OHD), 8-hydroxygenistein (8-OHG) and 6-hydroxydaidzein (6-OHD), which show significantly stronger antioxidative activities than those of daidzein and genistein, have been found in fermented soybean (mame) Miso, including their Kojis. Formations and changes of these potent antioxidative o-dihydroxyisoflavones (ODI) in the production of soybean pastes were studied. The antioxidative activity and the content of each isoflavone analog in soybean balls (Miso-dama) with different Koji making periods were first determined. Daidzin and genistin were hydrolyzed into the corresponding aglycones, daidzein and genistein, respectively, by β-glucosidase produced from Aspergillus oryzae. The resulting daidzein and genistein were then hydroxylated to produce 8-OHD, 6-OHD and 8-OHG, by the hydroxylase produced from the Koji fermentation during the stage of sporulation. These ODI increased during 4 days of fermentation. Soybean Miso were prepared by using Miso-dama Kojis fermented for 2 and 4 days, and matured at 30°C for ca. 6 months. Both 8-OHD and 6-OHD were hardly decomposed and their contents were held at almost constant levels throughout these ripening periods. 8-OHG increased at the initial stages of maturing. The antioxidative activities gradually increased during ca. 6 months of ripening, and their remarkable enhancements were observed at the initial stages. It is concluded from these results that these ODI are greatly responsible for protecting fermented soybean Miso from oxidative deterioration during Koji making followed by maturing.