Patterns of Variation for Sugar and Organic Acid Composition during Berry Ripening in Cold Climate Wine Grape Cultivars

The physical and chemical composition of wine grape berries at harvest is a key factor that determines the fruit quality characteristics, and ultimately, the quality of the wine produced. As berries ripen, their chemical composition changes with regards to sugars, organic acids, and phenolics etc. Knowledge of the developmental profiles of these compounds is important for determining optimal harvest times in making quality wines. While these profiles have been well characterized for Vitis vinifera cultivars, little is known about the changes of chemical composition in cold-hardy wine grape cultivars. These cold-hardy hybrid cultivars, which combine the excellent fruit quality of Eurasian species (V. vinifera) and the cold tolerance of American species (V. riparia, V. labrusca, V. rupestris, etc.) allow them to be grown in cold climate regions that experience harsh winter conditions. In this study, we report the changes observed in soluble solids (°Brix), titratable acidity (TA), and pH for eleven wine grape cultivars grown in Chaska, MN, that were sampled approximately every 10 days from August to October during the 2010, 2011, and 2012 growing seasons. Concentrations of sugars and acids were measured to predict optimum maturity for these cultivars. We are also determining profiles for organic acids in grape juice using ultra performance liquid chromatography (UPLC). This will enable us to track concentration changes for two predominant acids (tartaric and malic acids) during the berry ripening process. Understanding the varying proportions of these acid components in juice will help guide growers toward optimal harvest times for the intended wine style.