UC Davis

Interactions between proanthocyanidin material and anthocyanins can lead to the formation of pigmented oligomers and polymers. Although it is visually important that pigmented polymer forms, for the stability and ability to age a red wine, it also can have impacts on the taste and smell of the wine. Given that the synthesis of both proanthocyanidins and anthocyanins occur during fruit development, understanding the factors to improve them in the vineyard seems logical. In 2015 the ‘tannin activity’ method was used to understand how proanthocyanidin material extracted from grape skins and seeds changed over the ripening period. In that work, which ran from veraison until commercial harvest, there were clear changes in the proanthocyanidin molecular mass, activity, and the formation of pigmented polymer. Based on the sample size (N=372) across the growing season, it was expected that a controlled experiment would increase the correlations seen in the 2015 work. Due to the inverse relationship between activity and pigmented polymer formation in 2015, an experiment was designed in 2016 to look at shade cloth application to decrease the concentration of anthocyanins in the skin. Results from this work showed that changes in anthocyanin concentrations in the berry can impact the activity and molecular mass of proanthocyanidin material. Both molecular mass and activity have been shown to be correlated with wine astringency. The 2018 study built upon both earlier experiments by using white grapes and adding purified anthocyanins into the extracts to look at the ramifications of proanthocyanidin and anthocyanin interactions. The results from this experiment clearly showed the decrease in proanthocyanidin material size as the addition rate of anthocyanins was increased. Thus, increased anthocyanin levels in grapes will change the tannin composition during the winemaking process, and that consequently changes tannin activity. The combined results from these experiments suggest that viticulture techniques can have broad ranging implications for wine quality when looking at flavonoids and mouthfeel. They also suggest that the formation of pigmented polymer through the acid-catalysis SN1 reaction is happening much faster than previously suggested and warrants further investigation.