Differences in the amount and structure of extractable skin and seed tannins amongst red grape varieties

Red wine is a very rich source of flavanols, consisting of catechins and oligomeric and polymeric proanthocyanidins (PAs). The aim of this study was to investigate the composition of the fraction of flavanols, which is extractable from the grape into the wine. Using HPLC-DAD-MS analysis carried out before and after thioacidolysis of wine-like extracts obtained from skins and seeds, the flavanol composition of 20 samples derived from Cabernet Sauvignon, Carmenere, Marzemino, Merlot, Pinot Noir, Syrah and Teroldego was determined. It was confirmed that grape extracts are mainly rich in monomers and small oligomers (mean degree of polymerization < 8). Grape seed extracts contained three monomers (catechin, epicatechin and epicatechin gallate) and procyanidin oligomers. Grape skin extracts contained four monomers (catechin, epicatechin, gallocatechin and epigallocatechin), procyanidins and prodelphinidins oligomers. Upper and extension units of PAs were constituted mainly of epicatechin units, with the co-presence of catechin and epicatechin gallate in the seed PAs, and of gallocatechin in the case of skin PAs. The terminal units of the grape PAs were a mixture of five compounds, the same found also as monomers. Catechin gallate, gallocatechin gallate and epigallocatechin gallate, which are found in other flavanol-rich, plant-derived foods and beverages, were not present in grape extracts. The grape variety determines both the amount and the structure of wine flavanols, leading to differences in the composition, which are likely to play an important role in the nutritional and sensorial properties of the wines.