Solid-State Fermentation Drives L-Carnitine Accumulation and Isoflavone Bioconversion with Functional Enhancement in Soybean.

Solid-state fermentation (SSF) is widely used to enhance the functional properties of plant-based substrates. In this study, soybean powder was used as a substrate for SSF with Rhizopus oligosporus for up to 20 days. SSF altered the physical properties of soybean, increasing its water-holding capacity by 1.4-fold, water-swelling capacity by 1.64-fold, oil-absorption capacity by 1.55-fold, and cholesterol-binding capacity by 1.49-fold at pH 2.0. Biochemical characterization of fermented soybean showed a 3.13-fold higher TPC, a 3.84-fold higher TFC, and a 2.52-fold higher TSC. L-carnitine content in soybean rose from 0.66 ± 0.15 mg/kg DM to 161.21 ± 3.78 mg/kg DM, representing a 244.26-fold increase over nonfermented soybean. Additionally, the isoflavone glucosides daidzin, genistin, and glycitin progressively decreased during fermentation and became undetectable by days 15-20, whereas their corresponding aglycones exhibited compound-specific and time-dependent changes. Daidzein rose from 37.26 ± 19.16 μmol/kg DM to 691.06 ± 36.98 μmol/kg DM after 15 days. Genistein, initially undetectable, reached 1212.44 ± 43.32 μmol/kg DM after 10 days. Meanwhile, glycitein increased from an initial 82.96 ± 2.48 μmol/kg DM to 238.70 ± 4.91 μmol/kg DM. The soybean's antioxidant activity, measured by ORAC, FRAP, and ABTS assays, increased by 1.93-, 2.93-, and 1.5-fold, respectively. Its inhibitory effects on α-glucosidase and mushroom tyrosinase improved by 2.31- and 1.5-fold, respectively. These results indicate that fermented soybean has potential as a functional ingredient in the nutraceutical, cosmeceutical, and pharmaceutical industries.