Enrichment of Functional Characteristics in the Okara by the Fermentation of Rhizopus azygosporus

Purpose Okara is a solid waste of soybean-processed foods; it is normally used as a low-priced product, such as fertiliser and feed. An alternative method for the convertion of okara to valorized products is required. Methods This research employed Rhizopus azygosporus with fast-growing property to ferment bean dregs using solid-state fermentation technology and examined the functional composition and the physiologically effective bioactivities for comparison with unfermented okara. Results Rhizopus oligosporus BCRC 33956 and R . azygosporus BCRC31158 were cultivated on potato dextrose agar plates for 72 h after which they were used for comparison, and R . azygosporus exhibited approximately 1.3 times of growth rate than R . oligosporus . The contents of γ-aminobutyric acid (GABA) and crude polysaccharide in two days ferment accumulated to 696 mg/kg and 40.6%, respectively. During fermentation by R . azygosporus for 2 days, 74.6% genistin, a type of isoflavone glucoside, was depleted and 106.7 mg/L genistein was detected in the extracts. The results also showed that the proliferation of R . azygosporus enhanced several bioactivities in the okara fermentation. The relative angiotensin-converting enzyme inhibitory effects in the extracts of unfermented and fermented okara were 74.9% and 132.3%, respectively, compared with those of 10 mM captopril. The antioxidant capacity, including free radical scavenging activity and reducing power, was also significantly increased in okara accompany the growth of R . azygosporus. An animal study showed that GABA and functional characteristics and contents enriched by R . azygosporus in the okara fermentation were likely to have an obvious synergistic effect on the reduction of cortisol concentration in stimulated zebrafish, revealing the anti-stress function of fermented okara. Our work provides the process for development as a functional product, especially in anti-stress, which will further increase the value of okara and reduce waste, enabling sustainable consumption and production patterns. Graphical Abstract