Grape marc phenolics: extraction kinetics, quality and stability of extracts

The solid-liquid extraction of phenolic compounds from different vintage marcs with 60% ethanol was studied and experimental data were elaborated according to the different mathematical models applied in literature to recovery of bioactive constituents from plant materials. A first-order kinetics model was finally selected and the total phenolics diffusivities were calculated (1.23-1.50×10-10 m2s-1). The effects of several storage and usage conditions on extract stability (total phenolic content and antioxidant activity) were investigated. The extract was stable for up to one year if stored in darkness as a hydro-alcoholic solution at 4°C or as a freeze-dried powder at 25°C. Total phenolic content was stable at different pH values (3, 5, 7 and 9) for up to 400 days, while the antioxidant power decreased at pH > 5. Thermal treatment (121°C for 15 min at varied pH values) did not influence the ABTS radical-scavenging activity, while it increased total phenolic content and the Folin-Ciocalteu reducing power.