Repurposing grape pomace as a functional ingredient: in-depth analytical characterization and in vitro bioaccessibility evaluation of the phenolic fraction

Abstract

The study investigated the chemical profile of red grape pomace varieties and evaluated various extraction techniques for the recovery of polyphenols. Conventional maceration (MAC) and non-conventional extractions [ultrasound-assisted (UAE), ultraturrax-assisted (UTAE), enzyme-assisted (EAE)] were comparatively assessed on pomace from Merlot variety. UAE-based probe revealed as the most efficient, yielding higher concentrations of bioactives while preserving antioxidant activity. The optimal conditions were applied to pomaces from different grape varieties. Spectrophotometric and chromatographic analyses highlighted significant variation in polyphenol concentrations, with Merlot showing the highest while Sangiovese the lowest content of compounds. HPLC-PDA analysis indicated catechin, epicatechin and peonidin-3-O-glucoside as the most abundant (in the range 96.82–322.15, 115.81–361.16 and 111.72–402.36 µg g⁻¹, respectively), followed by quercetin (41.97–115.35 µg g⁻¹), gallic acid and procyanidin B2 (22.64–43.78 and 32.01–70.20 µg g⁻¹, respectively). Other polyphenols were present in lower concentrations (0.19 to 33.95 µg g⁻¹). Multivariate statistical analysis grouped the samples into three distinct clusters based on their bioactive profiles: cluster 1, comprising Merlot-3, Sagrantino-1 and Sangiovese-1; cluster 2 comprising Merlot-1, Merlot-2 and Cabernet-1; cluster 3 with Cabernet-2 as the sole member. A bioaccessibility study on both raw grape pomace (from Cabernet Sauvignon and Merlot) and their relative polyphenol extracts revealed a different pattern of bioactives during simulated gastrointestinal digestion. In some cases, gastric and intestinal digestion enhanced the concentration of specific compounds, resulting in bioaccessibility values exceeding 100%. The obtained findings suggest that the sample matrix may protect bioactives and promote their release during digestion, whereas isolated extracts may require tailored formulation strategies to preserve compound stability.