Determination and Consequences of Berry Size in Winegrape Production

A series of field experiments were conducted with Cabernet Sauvignon to investigate means of creating differences in berry size and the consequences thereof for winemaking. The primary focus was the hypothesis that irrigation alters wine composition primarily by changing berry size and the “dilution” of compounds of sensory significance. Additional questions included testing whether pruning to a high initial crop load and subsequent thinning could be used to manipulate berry size; attempting to make wine from berries of different size; and evaluating berry size distribution in clusters and on vines. Putting on a heavy crop load and thinning later was ineffective as a means of realizing smaller berries. Cluster thinning prior to veraison created four-fold differences in crop load, the highest being about 11 tons/acre. Yet the results clearly showed no differences in final berry size or color extracted from skins. This somewhat surprising result implies that even larger crop loads are required before berry growth becomes limited. It is in part for this reason that we propose to impose a wider range of crop loads and to investigate the consequences of crop load differences for wine chemical and sensory attributes in the coming season. When vines were differentially irrigated, mean berry volume was 17% greater with High irrigation (2x normal) compared to Low irrigation (minimal water applied). The predicted differences in must and wine composition would be of similar magnitude if there were no effects of vine water status on the amount of skin or seed solutes per berry. However, both anthocyanin and skin tannin per berry were greater in Low irrigated fruit than in the other treatments. Thus, irrigation regimes altered fruit composition by means other than simply berry expansion. Accordingly, the wine of Low irrigated fruit had 28%greater tannin concentration than the High irrigated wine (vs. the theoretical 17%). Seed tannin per berry (and seed mass) did not differ among irrigation treatments for any berry size class. In order to test the feasibility of sorting berry size for winemaking, hand harvested clusters were carefully destemmed, fruit sorted according to size, and made to wine. The tannin concentration was 13%greater in the small berry wine than in the other treatments; less than predicted on size alone. The results were promising in terms of the feasibility of developing technology to create small berry wines post harvest. However, it is not yet clear how important sorting might be to wine flavor, which is what we propose to address in the next funding cycle. Our informal bench tasting indicated treatment differences among irrigation and berry size treatments, the latter apparently differing in vegetative character. This report is limited to the data that have been analyzed to date with some limited interpretation of the implications of the data. However, the major tests were accomplished. We found that irrigation affected wine composition more than predicted by berry size alone and that sorting berry size had less than the hyothetical effect on wine tannin. Large differences in yield had no effect on berry size. We hope to pursue the sensory consequences of differences in yield, berry size, and irrigation.