Evaluation of Ripening Indicators for Harvest-time Decision Making in Cold Climate Grape Production

Currently, growers and winemakers of cold climate interspecific hybrid grape (CCIHG) cultivars rely solely on the technological maturity variables Brix, titratable acidity (TA), and pH as ripening indicators for harvest and winemaking decisions. In contrast, using additional chemical variables to determine optimal harvest times is a common practice in most regions of the world where the European grape Vitis vinifera is cultivated. As there are significant differences in berry chemistry between CCIHG and V. vinifera, the objective of the present study was to evaluate, by linear regression coupled with principal component analysis and cluster analysis, the suitability of six additional maturity variables (berry fresh weight, total protein, total phenolics, monomeric anthocyanins, percent polymeric color, and total tannins) as potential ripening indicators for the CCIHG cultivars Brianna, Frontenac, La Crescent, Léon Millot, Maréchal Foch, Marquette, MN 1220, and Petite Pearl. We observed that berry fresh weight, total phenolics, and percent polymeric color had inconsistent and difficult-to-model relationships from year to year and across distinct grape cluster microclimate treatments, making them unsuitable as ripening indicators. In addition to the standard technological maturity variables of Brix, TA, and pH, the variables of total protein concentration for white cultivars and of monomeric anthocyanin, total protein, and total tannin concentrations for red cultivars have consistent linear relationships from veraison to harvest, making them good candidates for ripening indicators.