Solid-state fermentation with Rhizopus oryzae HC-1 improves the metabolites profiling, antioxidant activity and gut microbiota modulation effect of soybeans

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In this study, Rhizopus oryzae HC-1, a food-grade fungal isolated from sufu, was utilized for solid-state fermentation (SSF) of soybeans, to investigate its effect on metabolites composition and antioxidant activity. Results revealed that SSF notably enhanced the total phenolics content (TPC), total flavonoids content (TFC), aglycone isoflavones, and antioxidant capacity of soybeans. The increase in aglycone isoflavones was attributed to the action of secreted β-glucosidase. The fermented soybean (FSB) with strongest antioxidant activity was further investigated by UPLC-MS/MS-based untargeted metabolomics analysis and in vitro human gut microbiota modulation effect by colonic fermentation. A total of 176 and 114 metabolites differed significantly between non-fermented soybean (NSB) and FSB in positive- and negative-ion modes, respectively. FSB and NSB had a beneficial modulation effect on the microbiota community structure. Additionally, colonic fermentation of both FSB and NSB for 24 h resulted in a higher relative abundance of beneficial bacteria Bacteroides compared to the control group, and FSB showed a greater modulation effect. Thus, SSF with R. oryzae HC-1 was found to significantly improve the metabolite profile, antioxidant activity and human intestinal microbiota modulation effect of soybeans, potentially offering novel functional or nutraceutical ingredients for the food and pharmaceutical industry.