Value-adding Okara via Monascus purpureus solid-state fermentation: Microstructural remodeling, metabolite enrichment, and bioactivity enhancement

Okara, the main by-product of soybean processing, is rich in bioactive compounds yet remains underutilized due to unfavorable flavor, gritty texture, poor digestibility, rapid spoilage, and disposal concerns. To valorize, this study explored upcycling okara via solid-state fermentation with M. purpureus over 25 days to improve its structural, chemical, and functional transformations. Comprehensive analyses by SEM, UPLC-MS, and GC-MS demonstrated a honeycomb-like porous surface with enhanced water-holding, water-swelling, oil-absorption, and cholesterol-binding capacities, metabolic shifts, and volatiles dominated by buttery 2,3-butanediol rather than off-flavors. Total phenolics, flavonoids, and saponins increased 2.27-, 3.08-, and 1.70-fold, respectively, while ergosterol, monacolin K, and l-carnitine accumulated to 688.85, 24.96, and 332.21 mg/kg, with no citrinin detected. β-glucosidase activity converted isoflavone glycosides into bioactive aglycones. Functional assays showed improved antioxidant activity, α-glucosidase inhibition, and reduced lipid accumulation in 3 T3-L1 cells. These findings highlight a sustainable, citrinin-free approach to upcycle okara into a multifunctional nutraceutical ingredient.