Solid-state fermentation of coffee silverskin with Rhizopus oryzae: A sustainable strategy to produce antioxidant-rich functional ingredients with enhanced in vitro bioaccessibility

Coffee silverskin (CS), a major by-product of the coffee industry, remains underutilized despite its high content of antioxidant compounds. This study aimed to optimize solid-state fermentation (SSF) of CS, to enhance the aqueous extraction and in vitro bioaccessibility of antioxidant compounds. A central composite rotatory design was applied to optimize fermentation parameters: temperature (20–40°C) and time (0–120 h). Response variables were total phenolic content (TPC) and overall antioxidant capacity (OAC). Optimal conditions (37 °C, 101.9 h) yielded 4.54 mg GAE/g CS (TPC), 57.46 mM TE/g CS (ABTS), and 18.14 mM TE/g CS (DPPH), corresponding to increases of up to 1.9-fold compared to control. In vitro digestion showed a 70% increase in antioxidant bioaccessibility compared to raw CS. These findings demonstrated that SSF-assisted aqueous extraction improves extraction and bioaccessibility of phenolic compounds from CS. This integrated optimization–bioaccessibility approach provides a promising strategy for the functional valorisation of this coffee by-product.