Impact of Postveraison Berry Exposure on the Skin Tannin Activity of Partial Extracts
James Campbell, Will Thomas, Andrew Waterhouse, and James Kennedy*
*Department of Viticulture and Enology, University of California, Davis, RMI North, room 3105; 595 Hilgard Lane, Davis, CA 95616
Previous work conducted on tannin maturation in Cabernet Sauvignon during the 2015 vintage suggested that pigmented tannin concentrations in partial extractions during this period influenced the thermodynamics of tannin interaction with a hydrophobic surface (tannin activity). Based upon these preliminary results, a shade cloth trial was implemented in 2016 to manipulate a variable tentatively identified to have a role in controlling tannin activity during maturation. Treatments were applied at veraison and consisted of 40% and 80% reduction in exposure relative to an ambient control. Quantum sensors were placed within the rows of each treatment to measure photon flux throughout fruit ripening and predawn leaf water potential was taken weekly to assure minimal variation in vine water status among replicates. Berries were sampled every two weeks, starting at veraison, and the skin tissue was separated from pulp and seed by hand, followed by extraction into 16% v/v ethanol for 72 hr. Extracts were analyzed for tannin concentration, average molecular mass, pigmentation, subunit composition, and activity. As in 2015, tannin activity and molecular size distribution declined from veraison until the berries were commercially ripe, while pigmentation of tannin increased during this time. The results suggested that anthocyanin/pigmentation had an effect on the tannin extracted into solution. Furthermore, they show that the treated sections of the block had higher tannin activity than fruit ripened under ambient conditions. The structure variables shown to have the greatest impact on tannin activity were identified as pigmentation and molecular size. Overall, this research provides insight into berry exposure during fruit ripening and its effects on tannin structure and corresponding activity.
Funding Support: AVF