Potent antioxidative o-dihydroxyisoflavones in soybean pastes and their antioxidative activities

Relationship between o-dihydroxyisoflavone (ODI) content and antioxidative activity was studied by using the fermented soybean pastes and their Kojis, which supplied by Miso maker. 8-Hydroxydaidzein (8-OHD), 8-hydroxygenistein (8-OHG) and 6-hydroxydaidzein (6-OHD), which showed significantly stronger antioxidative activities than those of daidzein and genistein, have been found in fermented soybean balls incubated directly with Aspergillus oryzae, so called Misodama-koji. These ODI were also present in rich enough amounts to exert antioxidative activities in fermented soybean pastes, especially in all soybean Miso. However, any ODI was not detected in both rice/soybean Miso and barley/soybean Miso, which were used rice Koji and barley Koji, respectively. The antioxidative activities of various soybean pastes were also examined. The activities of soybean Miso which contained ODI were generally stronger than those of other type of Miso. There was a good correlation (r=0.812) between ODI contents and antioxidative activities. Any ODI was not decomposed during storing at 30°C for 9 months in the dark. These ODI were stable even if Miso soup was heated with boiling water for 30 minutes. From these results, it was suggested that these ODI in soybean pastes contribute to protecting from oxidative deterioration during the processing and storing of Miso.