Advancements in Green Extraction of Polyphenols from Fetească albă Grape Pomace Using Natural Deep Eutectic Solvents: Optimization and Applications as Inhibitors for Enzymatic Browning

Abstract In this study, white grape pomace (WGP) from Fetească albă variety was used as a valuable source of apple polyphenol oxidase (PPO) and peroxidase (POX) inhibitors. A natural deep eutectic solvent (NaDES), composed of choline chloride and lactic acid in a 1:2 molar ratio with 20% water, was prepared and used for ultrasound-assisted extraction (UAE), optimized by response surface methodology. The experimental data were fitted to a quadratic model for all studied dependent variables. The model allowed the validation of UAE parameters (60 °C, 30 min, liquid-to-solid ratio of 10 mL/g. At optimized parameters, the extract showed a total polyphenolic content of 68.89 ± 0.25 gallic acid equivalents (GAE)/g dry weight (DW), flavonoids of 4.93 ± 0.13 quercetin equivalents (QE)/g DW, and free-radical scavenging activity of 100.01 ± 0.29 mMol Trolox/g DW. The extract was analyzed for its polyphenolic profile, showing a predominance of phenolic acids (70%). The major compounds identified were vanillic acid (264.32 mg/kg DW), cis-piceid (51.51 mg/kg DW), phlorizin (50.59 mg/kg DW), azelaic acid (39.90 mg/kg DW), chlorogenic acid (28.40 mg/kg DW), and suberic acid (23.38 mg/kg DW). The extract was used for potential inhibitory effect on PPO and POX extracted from apple ( Ionatan variety). The kinetic parameters suggested a strong enzyme affinity for WPG extract. The IC 50 values were 0.30 mg GAE/mL for PPO and 33.24 µg/mL for POX. Molecular docking tests were performed using the enzymes as receptors and the major phytochemicals identified by UHPLC as ligands. The tested ligands preferentially attached to PPO cavities located on the opposite side in respect to the channel leading to the catalytic amino acids, suggesting no direct interference with the enzyme activity. On the other hand, POX activity might by directly affected by ligands attachment to the active site.