Distribution Profiles of Isoflavone Isomers in Black Bean Kojis Prepared with Various Filamentous Fungi

This study was conducted to compare the transformation of both isoflavone derivatives (aglycones, beta-glucosides, and acetyl and malonyl gluclucosides) and beta-glucosidase activity in kojis fermented with various generally recognized as safe filamentous fungi including Aspergillus awamori, Aspergillus oryzae, Aspergillus sojae, Rhizopus azygosporus, and Rhizopus sp. no. 2. Solid fermentation was performed to prepare the kojis by inoculating the steamed black beans with starter organism and culturing at 30 degrees C for 3 days. Results revealed that fermentation caused a marked increase in the content of aglycone (daidzein, glycitein, and genistein), the bioactive isoflavone, and a significant reduction in the content of beta-glucoside isoflavone (daidzin, glycitin, and genistin), compared with the unfermented steamed black bean. The extent of increased aglycone and reduced beta-glucoside isoflavone content varied with the starter organism used. Among the various black bean kojis prepared, the Rhizopus sp. no. 2 koji showed the highest level of enhancement in aglycone content. In the Rhizopus sp. no. 2 koji, the percentage of aglycone to total isoflavone increased from an initial approximately 2.9 to approximately 58.9% after fermentation. In comparison, the percentages found in kojis prepared with other starter organisms ranged from 18.9 to 38.9% after fermentation. Further preparations of black bean kojis with A. awamori at different cultivation temperatures (25, 30, and 35 degrees C) and various fermentation periods (1-5 days) revealed that koji prepared at 30 degrees C for 4 days showed the highest content of aglycones, with 7.7-, 5.7-, and 4.8-fold increases in the content of daidzein, genistein, and glycitein, respectively. In addition, the increase of aglycone content and the increase of beta-glucosidase activity during the fermentation of this koji showed a similar trend.