Condensed Tannin Accumulation and Composition in Skin of Shiraz and Cabernet Sauvignon Grapes during Berry Development

Abstract

Condensed tannins derived from the grape berry play a significant role in wine astringency, bitterness, color stability, and aging potential. Understanding the regional, varietal, and seasonal variability in winegrape tannin composition throughout berry development is essential for developing vineyard strategies to manage and manipulate winegrape tannin to meet winery specifications. Current knowledge of tannin accumulation and composition in grape berries is largely based on a single variety and geographical region, with most studies investigating seed tannin. To explore the variation in tannin accumulation and composition, Shiraz and Cabernet Sauvignon grape skin from two growing seasons in the Sunraysia region of southeast Australia were measured by acid-catalyzed cleavage in the presence of phloroglucinol. High-performance liquid chromatography was used to quantify the subunit composition, total tannin content, and average polymer length throughout berry development. The pattern of tannin accumulation and composition were similar for both varieties and seasons, with tannin content highest at fruit set before declining toward veraison. The pattern of accumulation and composition seen in grape skin from this study was different to that previously reported in Shiraz from the cooler growing region of McLaren Vale in South Australia where tannin content was highest at veraison. Results indicate that the differences between the pattern of tannin accumulation and composition in Sunraysia and McLaren Vale may be due to regional influences, which had greater impact on tannin composition than varietal or seasonal variability. Further research is required to determine the full extent of variation between regions and to determine the environmental factors that can be used to influence the pattern of tannin accumulation.