Isoflavone Conversion of Black Soybean by Immobilized Rhizopus spp.

Isoflavones exhibit many bioactive benefits, such as antioxidation, antimutagenic effects, and reduction of symptoms of postmenopause conditions. Aglycone forms of isoflavone have shown more biological effects over their glycosylated ones. Most isoflavones in nature, however, exist as glycosylated forms and cannot easily be absorbed in the intestines. The purpose of this study was to develop a fermentation process of black soybean to produce functional food using a Luffa cylindrical fiber biofilm reactor. Three filamentous-fungi, including Rhizopus oligosporus (BCRC 31996), Rhizopus oligosporus (NTU-5), and Rhizopus oryzae (BCRC 30894), were cultivated in black soybean medium. The assay using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging effect of fermented broth and isoflavone aglycosylation rate of genistin and daidzin were evaluated as indicators to determine the most suitable strain. After a 2-day cultivation in flasks, DPPH radical scavenging effect of R. oligosporus NTU-5 reached 83%, which is the maximum value among three strains. R. oligosporus NTU-5 was subsequently chosen as target strain and used for the scale-up of fungal-based aerobic growth. The optimal cultivation condition obtained in a 5-L bioreactor with Luffa cylindrical fiber was 30°C, 0.1 vvm aeration, 100 rpm agitation, 10% inoculum volume, and 6% (w/v) black soybean as medium. After a 6-day cultivation, R. oligosporus NTU-5 converted isoflavones to their aglycones, daidzein and genistein, and these were 59.7 and 23.6 μM, respectively. Isoflavone aglycosylation rate of daidzin and genistin were 77.6 and 31.8%, respectively. The DPPH scavenging effect was 62%.