Effect of grape maturity on the phenolic content and antioxidant capacity of pomace derived from unripe and ripe grapes

Grape pomace is a phenolic-rich by-product of grape processing with notable antioxidant properties, yet the valorization potential of pomace derived from unripe grapes (UG) used for verjuice production remains poorly characterized. Unlike ripe grape (RG) pomace, which has been extensively studied, there is a lack of fraction-specific data comparing the phenolic content and antioxidant capacity of UG and RG pomace. This study compared the phenolic content and antioxidant capacity of pomace fractions (seeds, skins, and stems) obtained from UG and RG of the white cultivar Bianca, processed under identical conditions. Ethanolic extracts were analyzed for total phenolic content (TPC), total flavones and flavonols content (TFFC), ferric reducing antioxidant power (FRAP), and DPPH radical scavenging activity. Pomace fractions were analyzed in triplicate, and differences between maturity stages were evaluated statistically. Seed fractions showed the highest values, with UG seeds reaching 104 ± 1.5 mg GAE/g DW (TPC) and 1720 ± 198 µmol AAE/g DW (FRAP), exceeding RG seeds by 38% and 78%, respectively. Overall, UG fractions consistently exhibited higher phenolic content and antioxidant activity than RG fractions across most measurements. In contrast, TFFC trends differed among fractions, reflecting the selectivity of the AlCl₃ method toward flavones and flavonols. These findings demonstrate that pomace generated during verjuice production possesses substantially higher antioxidant potential than corresponding pomace from ripe grapes. The results highlight verjuice-derived pomace as a promising source of natural antioxidant ingredients for food and nutraceutical applications, supporting its valorization within circular economy frameworks.