Few investigations have been conducted to evaluate pressurized liquid extraction (PLE) technology as a sustainable method for the recovery of phenolic compounds of grape seed by-products. In this study, PLE combined with an experimental design was evaluated for optimizing the sustainable extraction of phenolic compounds from grape seed by-products. The solvent ethanol content (X1, 0–100%), temperature (X2, 20–100 °C) and time (X3, 1–11 min) were studied as independent experimental factors. Yield, TPC, antioxidant activity and phenolic composition were analyzed as optimized dependent variables. Two optimal extraction conditions at different temperatures (20 °C and 100 °C) were found, but thermal degradations at 100 °C allowed for selecting the optimal condition as 75% ethanol, 11 min and 20 °C. The optimal extracts showed high phenolic content (TPC = 350.80 ± 3.97 mg GAE/g extract) and antioxidant activity (ABTS, 9.31 ± 0.33 mmol Trolox/g extract), mainly composed of polymeric and mono-oligomeric flavan-3-ols. The digestion process reduced the TPC and antioxidant activity due to the low bioaccessibility of the flavan-3-ols, mainly as catechin, epicatechin and polymeric proanthocyanidin losses during the digestion process. However, increases in the antioxidant activity of the basolateral side (DDPH, 0.061 ± 0.000 mmol Trolox/g extract) were determined after in vitro transepithelial transport, which is a consequence of bioavailable catechin and epicatechin and reduced amounts of dimer B2, dimer B1, epicatechin gallate and gallic acid. Consequently, PLE combined with hydroalcoholic solvents at a low temperature resulted in a valuable methodology to obtain sustainable extracts from grape seed by-products (contributing to the circular economy), containing bioavailable phenolic compounds, which are able to increase the antioxidant status.
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