Grape skin tannins

Fournand, D., Vicens, A., Sidhoum, L., Souquet, J.-M., Moutounet, M., Cheynier, V. (2006). Accumulation and extractability of grape skin tannins and anthocyanins at different advanced physiological stages. J. Agric. Food Chem., 54, 7331-7338. 

Anthocyanin Measurements. Quantitative analysis of concentration by direct colorimetry in the visible range is not practical with red wines because of substances (tannins in particular) which absorb at the maximum for anthocyanins (from 500 to 550 nm). Such a simple method may be used if need be for plant parts especially rich in anthocyanins, such as grape skins. 

Seed tannins give the wine structure and body, while skin tannins provide fullness, roundness and color. However, there is a high risk of excessive astringency if seed tannins dominate, while bitterness and a herbaceous character are typical of too much extract from the skins, especially if the grapes are insufficiently ripe. 

Fig. 6.37. Cross-section of grape skin (Chardonnay variety). Observation under an optical microscope after coloring with toluidine blue (x 207). Condensation of tannins at different levels in the skin. The darker-colored cells located near the surface of the skin correspond to the maximum condensation of tannins (Amrani-Jontei, 1993)...

Common dimers such as procyanidin B1 (PBl) and procyanidin B2 (PB2) are also present in grape skins. Colorless flavanols include C and EC, the monomeric units of proanthocyanidins. Proanthocyanidins, also known as condensed tannins, are characterized by polymerization degree (PD) ranging mainly between 3 and 11, but varying up to 17 or more (Iriti and Faoro 2006). 

The following phenolic compounds are present in the grape skin at maturity benzoic and cinnamic acid, flavonols and tannins. They are distribnted in the cells of the epidermis and the first snb-epidermal layers in both white and red grapes. In addition, the red grape skin contains anthocyanins, essentially located in the hypodermal cell layers. Exceptionally, in certain years, the cells adjacent to the pulp can be colored. The pulp itself is colored in the case of Tenturier varieties and some American vines or direct producer hybrids. Antho-cyanin composition varies from cultivar to cultivar, depending on the anthocyanidin substitution and heterosidic nature of the cultivar (see Volume 2, Section 6.2.3). 

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