Valorization of Vine Shoot Waste into Phenolic-Rich Liquors for Laccase-Mediated Functionalization of Starch

Vine shoot residues represent an abundant lignocellulosic by-product of the wine industry and a promising source of phenolic compounds with potential functional applications. In this work, a biocatalytic strategy combining aqueous citric acid treatment and subsequent laccase-mediated oxidation was developed for the valorization of vine shoot-derived phenolic liquors. The pretreatment was optimized by response surface methodology, and the selected conditions, 190 °C, 75 min, and 0.82% citric acid, yielded a pretreated solid containing 2.9 ± 0.02% hemicellulose, 47.5 ± 0.20% cellulose, and 51.8 ± 1.87% lignin, together with a phenolic-rich liquor containing 27.66 ± 0.39 mg GAE g−1 dry solid. Chemical characterization by UHPLC-timsTOF-MS revealed a complex mixture of phenolic acids, lignin-derived compounds, carbohydrate derivatives, and secondary metabolites. Laccase-catalyzed oxidation was first used as a reactivity assessment step, showing that the phenolic compounds present in the liquor were susceptible to enzymatic transformation. This treatment decreased the total phenolic content, antioxidant capacity, and antimicrobial activity of the liquor. Subsequently, enzymatic oxidation was carried out in the presence of starch, yielding washed starch solids with retained Folin-reactive phenolic content of approximately 4 mg GAE g−1 starch and measurable antioxidant capacity. Overall, this study demonstrates an integrated valorization route in which citric acid-assisted fractionation of vine shoot residues generates phenolic-rich liquors that can be chemically characterized, enzymatically activated, and directly used for starch functionalization, providing a sustainable strategy to convert agro-industrial residues into bio-based functional systems.