Identification of Factors that Influence the Level of Tannins and Polymeric Pigments in Grapes and Wines
The objectives during the 2010 season were to 1) Conduct a small-scale experiment to determine when during fermentation the increase in bound tannin occurs. 2) Conduct experiments using alcohol free tannin and cell wall material obtained from mesocarp tissue to study the mechanism by which tannins become tightly bound to the insoluble matrix during fermentation. 3) Measure free and bound tannin in fruit and pomace samples from larger scale fermentations to confirm our observation that there is a significant increase in bound tannin attached to the insoluble matrix after fermentation. With regard to objective one, our results show that there is a steady increase in the amount of tannin immobilized onto the insoluble matrix during red wine fermentation. This result is of practical interest because it suggests that interventions intended to alter the adsorption of tannins to cell wall material must begin early in the fermentation. In studying the effect of particle size on tannin adsorption to cell wall material we found that more tannin could be adsorbed to smaller particles than to large ones. This result is interesting because is seems to suggest that adsorption could be a surface phenomenon and that the size of the fragments into which the cell wall material is cleaved during winemaking can influence the amount of tannin bound. This could have practical implications for handling skins and gross lees during fermentation. We found that temperature has an inhibitory effect on tannin binding to cell wall material, the amount removed decreased as temperature increased. Interestingly, however, the amount of tannin immobilized, i.e. not extractable with acetone after incubation increased with increasing temperature. We studied the effect of sugar concentration on tannin binding to cell wall material and found that increasing the amount of sugar present during the incubation of tannin with the insoluble matrix reduced both the amount of tannin adsorbed to the cell wall material and the amount immobilized. This result may help explain our observation that bound tannin steadily increased during fermentation (see above). We studied the effect of tannin concentration on the binding and immobilization of tannin to cell wall material. We found that tannin adsorbed to the insoluble matrix increases along with increasing tannin concentration but that the level of tannin immobilized to the cell wall material reached a maximum around 750 mg/L. While this result is difficult to interpret without further experiments, it may suggest that at some level tannin binding actually inhibits tannin immobilization to the cell wall material. We also studied the effect of pH, ionic strength and bisulfite addition on the binding and immobilization of tannins to cell wall material. None of these were found to have any influence on tannin binding.