Our earlier estimates of how much of the fruit tannin could be bound by the insoluble matrix of the berry were made using an analysis conducted in wine with alcohol present. Since we subsequently found that alcohol has the effect of reducing the tannin binding by berry cell walls, our previous work underestimated the magnitude of this phenomenon. The most important research accomplishment from work during the 2005 season was the successful development of a method to measure the capacity of grape berry cell walls to bind tannin in the absence of ethanol. A search for a suitable commercial tannin preparation that could be used for such an assay was ultimately unsuccessful, and we were required to purify tannin from grape seeds and skins for use in the analysis. The total binding ability of grape berry cell walls is the product of the binding capacity per mg of cell wall material and the total amount of cell wall present (mg/berry). We measured both of these parameters in Cabernet Sauvignon fruit grown at Davis during normal ripening and during extended hang time. The results indicate that the amount of cell wall material in the skins and pulp of the berry do not change dramatically during normal ripening and extended hang time, but that the tannin binding capacity of mesocarp cell walls increase during ripening and show a decline during extended hang time. The reduced tannin binding by cell walls late in the ripening period may favor better tannin extraction from more mature fruit. Further targeted work would be needed to characterize this phenomenon.
We also studied the effect of drying on the mass of cell wall material and its tannin binding capacity in Cabernet Sauvignon fruit severed from the vine and hung in the canopy. Brix increased dramatically after cutting the clusters from the vine concomitant with a drastic loss in fresh weight of the berries. Despite these radical changes in solute concentration and water content, the mass of cell wall material and its tannin binding capacity appeared to change very little and remained similar to control fruit left on the plant for extended ripening.
The importance of the phenomenon we are addressing in this project can be easily seen when we compare the magnitude of the total cell wall binding capacity (skin + mesocarp) with the amount of tannin we find in grape berries. At the point of normal harvest in this experiment the total tannin binding capacity of the fruit was calculated to be 0.98 mg/berry. Data for Syrah fruit typically show about 2 mg of tannin in the fruit whereas Pinot noir is usually slightly lower at 1.6 mg/berry and Cabernet Sauvignon is usually higher around 2.6 mg/berry. Of course this number varies with the average seed number per berry, so these values are taken from our previous work simply to illustrate the fact that the total binding capacity of the cell wall material in a berry can be well over half (61%) of the amount of tannin found in the fruit for a variety like Pinot noir. Taken together this work suggests that grape berry cell walls may play an important role in tannin extraction from fruit during fermentationgrape berry cell walls may play an important role in tannin extraction from fruit during fermentation