Sustainable Valorization of Spent Coffee Grounds: Phenolic Compound Extraction Using Hydrophobic Eutectic Solvents

Spent coffee grounds (SCG) are the main by-product generated by the coffee industry, with an estimated annual production of approximately 7 million tons. Although commonly treated as waste, SCG constitute a valuable source of phenolic compounds, particularly chlorogenic acid, which has been associated with antimicrobial, antioxidant, antimutagenic, anti-inflammatory, and cardioprotective properties. These bioactive compounds are of interest as functional ingredients for food, cosmetic, and pharmaceutical applications. However, their recovery by conventional extraction methods often depends on volatile, flammable, or toxic organic solvents. In this context, hydrophobic eutectic solvents (HES) have emerged as a greener and more sustainable alternative. In the present study, phenolic compounds were extracted from SCG using HES combined with microwave-assisted extraction (MAE). Sixteen terpene-based HES formulated with fatty acids and fatty alcohols were evaluated. Among them, camphor:dodecanoic acid and borneol:dodecanoic acid gave the highest total phenolic contents. Process optimization showed that the borneol:dodecanoic acid system, under 12% water content, a 1:10 solid-to-liquid ratio, 57 °C, and 120 min, reached 80.94 ± 4.44 mg GAE g−1 by MAE. HPLC analysis revealed chlorogenic, caffeic, and ferulic acids as the main phenolic compounds, while the extracts also displayed high antioxidant activity. Overall, these findings demonstrate that HES-MAE is a promising and sustainable strategy for the recovery of value-added phenolics from SCG.