From waste to function: Bioaccessible spent coffee ground phenolic compounds retain antioxidant activity and protect Caco-2 cells after simulated digestion

Spent coffee grounds (SCGs) are an abundant agro-industrial by-product rich in phenolic compounds, yet their bioaccessibility and intestinal relevance remain insufficiently explored. This study optimized ultrasound-assisted extraction (UAE) of SCGs using response surface methodology (RSM) and evaluated the bioaccessibility, cytocompatibility, and intestinal antioxidant potential of the resulting extracts. RSM revealed distinct optimal regions for phenolic yield, antioxidant activity, and extract quality. Maximum total phenolic content reached approximately 28.91± 0.36 mg GAE/g SCG, maximum DPPH reached 39.22% ± 3.48, while digested extracts improved Caco-2 cell viability by up to 36% under oxidative stress conditions. Digested extracts were fully cytocompatible and significantly protected Caco-2 cells in vitro against hydrogen peroxide-induced oxidative stress, with extracts optimized for antioxidant activity providing equal or superior protection compared with those optimized for TPC. These findings demonstrate that functional efficacy at the intestinal level depends more on bioaccessibility and antioxidant reactivity than on absolute phenolic content, supporting the valorization of SCGs as a sustainable source of bioaccessible antioxidants for functional food applications.