Tannins are a group of polyphenols found in fruits, leaves, trees, etc., well known in the leather industry and in apples, persimmons and grapes, because of their capacity to interact with other polyphenols or other components either from the food product or from saliva. Prior to being able to interact with other compounds, tannins have to be extracted from the food matrix, which depends on their chemistry, as well as the chemical structure of other components, such as cell wall material and proteins. Vitis vinifera grapes are commonly grown around the world and are used in winemaking, providing good quality wines with different levels of tannins responsible for the final wine’s astringency. Many studies have focused on tannins extractability and retention with cell wall material, and the reactivity of tannins with proteins in Vitis vinifera grapes and wine, but there are very few reports for other Vitis species. However, depending on the environmental characteristics of certain regions, Vitis hybrid grapes are grown and used to produce wines more and more. This review focuses on the comparison of the chemistry of tannins, and their reactivity with other macromolecules in Vitis species.

Wines produced from red interspecific hybrid grape cultivars (Vitis spp.) typically have lower tannin than wines produced from vinifera cultivars, which can be attributed to the lower concentration of tannins and higher concentration of tannin binding proteins of interspecific cultivars. Tannin in wines produced from hybrid cultivars could potentially be increased by blending hybrids with vinifera. We hypothesized that blending of grapes prior to fermentation (cofermentation) should result in final wine tannin concentrations lower than wine tannin concentrations predicted from the individual components due to protein-tannin binding, but that this effect should be absent from monovarietal wines blended post-fermentation. To evaluate this hypothesis, over a two-year study, a high tannin V. vinifera cultivar (Cabernet Sauvignon) was blended with an interspecific hybrid (Marquette) at different ratios either before (cofermentation) or after fermentation. The tannin and protein concentrations of the wines were measured by methyl cellulose precipitation assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), respectively. Tannin and protein concentrations in blended wines were compared to values predicted from the linear combination of the two monovarietal wines. Co-fermented blends with a high proportion of Marquette had up to 25% lower tannin than predicted, but for most cofermentations and post-fermentation blends observed and predicted tannin concentrations did not differ. However, protein concentrations for many of the blends – especially from cofermentation - were lower than the predicted values, in some cases >50%. Loss of protein due to adsorption to tannin was well modeled by a Freundlich absorption isotherm.