Résumé
Phenolic compounds (anthocyanins, tannins), mainly present in the skin of the grape berry, play a decisive role in the sensory properties of red wine. They are partly extracted during the maceration step of the winemaking process and can undergo different structural changes., Their extraction is mostly related to their initial contents in skins, medium conditions and to their interactions with cell wall material, which modulate their diffusion. The aim of this work was to characterize the impact of grape variety, berry size and density on the extraction of skin polyphenols at maturity harvest, and to identify the main factors involved.Thus, grape heterogeneity was studied at technological maturity, for two contrasted varieties in terms of phenolic compositions (Grenache, Carignan). The berries were sorted according to their size (vol-, vol+) and then density (deg-, deg+). The grape skins were analyzed (polyphenol constituents, alcohol insoluble solids (AISs) polysaccharides and proteins). Wine-like maceration experiments were performed on fresh skins in model systems at constant parameters (22°C, under argon, without yeast), by increasing the percentage of ethanol from 0 to 15%. After the diffusion experiment, precipitates were recovered and skins were then successively washed with new 0 and 15% wine like solvents until total phenolic extraction. The Polyphenol concentration was determined by spectrophotometry and chromatography. Skin AISs differed between the two varieties by their carbohydrate composition and protein content. Fresh skins had similar contents in polymeric tannins, but strongly differed by their anthocyanin contents (higher in Carignan and in the ripest berries) and composition (higher proportions in coumaroylated anthocyanins in Carignan). During maceration, anthocyanin extraction differed between the two varieties and with the berry density, whereas berry size had no impact. The initial quantities of polyphenols in grape skins was not sufficient to predict their concentrations in wine. Both a selective precipitation of coumaroylated anthocyanins and chemical changes strongly modulated anthocyanin concentrations. Neither anthocyanin contents nor the AIS compositions had a decisive impact on tannin extraction. The analysis of residual polyphenols in skins evidenced a deficit in the mass balance due to different evolution mechanisms of anthocyanins and tannins. Further analyses are necessary to evaluate the composition of wines based on the composition of the skin and the pulp, which is a strong demand from the vine-wine sector, especially in the context of varietal selection in response to climate change.