Identification of Factors that Influence the Level of Large and Small
We prepared a fine suspension of the insoluble cell wall material from Cabernet Sauvignon berries from vines grown in the U.C. D. vineyard and determined their capacity to bind tannins at different times during ripening. Mesocarp cell walls collected from fruit 40 days after veraison had more than five times the capacity for tannin binding as walls from veraison fruit. The binding capacity appeared to decline as the fruit approached harvest. The significance of this result is that it may help us understand why some fruit has easily extracted tannins while others exhibit lower extractability.
We also studied Cabernet Sauvignon at several commercial vineyards in Napa Valley and compared the amount of total cell wall material and the tannin binding capacity of cell walls derived from that fruit to those from Davis. We found considerable variation in the binding capacity from location to location. The results also show that the amount of cell wall material in skins and mesocarp varied among the different vineyards. Our experiments allowed us to calculate the total cell wall binding capacity of fruit on a per berry basis. This is an important value because by comparison with the amount of tannin per berry we were able to determine that the capacity of cell walls to capture tannin can amount to more than a third of the tannin present in the fruit. This result shows that tannin binding to cell walls may be an important factor in our ability to extract tannin from fruit during fermentation.
In order to see how cell walls from different varieties bind tannins we carried out a survey of different varieties for comparison to the Cabernet Sauvignon described above. We studied Grenache at three locations, Tempranillo, Pinot noir, Syrah and Tannat. The results show for the first time how cell walls from different varieties can bind different amounts of tannin. Just as Cabernet Sauvignon from different locations showed different binding capacities, Grenache from three different vineyards had remarkably different capacity to bind tannin. The tannin binding capacity was also found to be quite variable among the different varieties. We believe that our results may help explain why different varieties can exhibit such a range of tannins in finished wines when the amount measured in the fruit shows a much more narrow range of values. Likewise, our results may help explain how a single variety from different locations can have widely different amounts of tannin in the resulting wines when there is little difference in the tannin level in the fruit at harvest.
We used selective degradation with CDTA and NaOH to evaluate the contribution of various cell wall polysaccharide components to tannin binding. Both treatments removed about one third of the mass of the native cell wall material. Our results indicate that the CDTA-soluble pectins do not participate in tannin binding whereas cellulose and hemicellulose do. This is a very important result because it suggests that different cell wall components may behave differently with regard to tannin binding.
