Evaluation of tannins interactions in grape (Vitis vinifera L.) skins

Tannins have a central role in grapevine berries both for their physiological and enological implications. In the skin tissue they can be in vacuolar solution, or associated to the cell walls through weak or strong physicochemical interactions. The present work aims to separate vacuolar, non-covalently and covalently bonded tannins fractions. A specific extraction procedure was developed. A first extraction in ethanol at low temperature allowed the quantification of vacuolar tannins. An urea treatment followed by an ethanol extraction at room temperature was able to separate non-covalently bonded compounds. Finally an acid catalysis was used to break down proanthocyanidin covalent bonds. The method was validated on ripe grape samples of three cultivars, on berries developed in two sun exposure conditions. The Ethephon treatment effect was also evaluated. Beside the method development, a preliminary evaluation of the cultivar, exposition and Ethephon treatment effects are discussed.