Identification of Factors that Influence the Level of Large and SmallPolymeric Pigments in Grapes and Wines
We have conducted a study of tannin binding to several yeast strains and found that different yeast strains have different abilities to remove tannin from a standard wine. Some strains have a greater capacity for tannin binding than grape berry cell walls, others have nearly the same and others still show less capacity compared to berry cell walls. This shows that yeast might be responsible for removing some of the tannin from wine that is extracted from fruit during winemaking. However, using model controlled temperature fermentations of Syrah, Pinot noir and Cabernet Sauvignon we found that differences in tannin and total iron-reactive phenolics were small but statistically significant. Work on small scale controlled fermentations have led us to concluded that while yeast may indeed remove some of the tannin from wine, the difference among yeast strains is likely to be small.
We studied two Pinot noir vineyards only a few hundred meters apart having the same clone on the same rootstock. Historical wines from the two vineyards showed a consistent difference in tannin levels between them from year to year over a period of several years. However, when we measured tannin in the skins and seeds we found that tannin in the fruit at harvest from the two vineyards was not different. We also measured the tannin binding capacity of cell walls from fruit from both vineyards and found that the insoluble matrix in fruit from the vineyard with low wine tannin had a greater ability to bind tannin than fruit from the vineyard with high wine tannin. Thus, in the two vineyards we studied, differences in wine tannin were attributable to differences in the capacity of cell walls to bind tannin. This result suggests that in addition to measuring tannin in fruit at harvest we need to also measure the ability of the cell walls to bind tannin as part of our experiments of maturity if we wish to know how extractable the tannin will be during fermentation.
We have found that ethanol decreases tannin binding to grape berry cell walls. This suggests that our previous estimates of the significance of tannin binding to the insoluble matrix of grape skins and mesocarp tissue actually underestimated the importance of this phenomenon. We have made progress toward developing a cell wall tannin-binding assay that takes place in a buffer system without ethanol. Our work indicates that tannin binding to berry cell walls plays a significant role in determining tannin extractability during fermentation.