Recovery of Phenolic Compounds and Proteins from Spent Coffee Grounds Using Eutectic Solvents

Spent coffee grounds (SCGs) are an abundant agro-industrial residue with high potential as a source of phenolic compounds and proteins. This study evaluated the extraction of these value-added fractions using hydrophilic and hydrophobic eutectic solvents, applied either alone or combined with enzyme-assisted extraction. A total of 31 hydrophobic eutectic solvents (HESs), nine hydrophilic deep eutectic solvents (DESs), and five conventional solvents were screened for phenolic recovery. Extraction performance was strongly formulation-dependent, with hydrophobic systems showing the highest phenolic yields. HES 4 (camphor:oleic acid) was the best-performing solvent, reaching 1279.49 ± 2.31 mg GAE L−1, followed by borneol:oleic acid (1133.92 ± 5.29 mg GAE L−1). Enzyme addition did not enhance phenolic extraction; the highest values under enzymatic conditions were 896.12 ± 4.80 mg GAE L−1 for HES 4 + Cellic® CTec2 and 819.84 ± 2.66 mg GAE L−1 for HES 4 + Viscozyme®. In contrast, protein extraction increased remarkably with enzyme supplementation, particularly with Cellic® CTec2. The highest protein recovery was obtained with HES 4 + Cellic® CTec2 (1608.74 ± 3.32 mg·L−1), compared with 506.37 ± 5.20 mg·L−1 for neat HES 4. In general, neat HESs were more suitable for phenolic recovery, whereas HESs combined with Cellic® CTec2 were more effective for protein extraction.