Changes in sugars, organic acids and phenolics of grape berries of cultivar Cardinal during ripening

Abstract

The sugars, organic acids and phenolics were analysed by high-performance liquid chromatography in the grape berries of ‘Cardinal’ collected during the last month of ripening (four sampling dates in July). The contents of phenolic compounds were determined in skin and pulp separately, while sugars and organic acids in the whole grape berry. Besides, as a common practice for determination of harvest time, the sugar and total acid contents in grape juice were measured. The changes in mentioned compounds were confirmed. The ratio between °Brix and titratable acids rose rapidly during ripening. As expected, the concentrations of glucose and fructose increased, but sucrose and tartaric and malic acids decreased. Glucose was the most abundant sugar and tartaric acid was major organic acid. Among phenolics in skin, there were no significant differences in chlorogenic and p-coumaric acid and rutin. Generally, grape skin phenolics reached the highest value during fourth week of July when the skin had especially high content of quercetins, catechin and syringic acid. In comparison with skin, the pulp was very poor in phenolics and general trend in their changes was not observed. The proportion of phenolics in pulp compared to skin ranged for syringic acid 1.4-23.9%, chlorogenic acid 1.2-6.3%, catechin 2.4-12.1% and total phenolics 1.3-3.0%. Catechin or epicatechin were the major phenolics in grape pulp during ripening. The contents of total phenolics were not significantly different in grape pulp, during ripening. From the aspect of phenolics, the best time for harvest of table grape would be when skin reached the highest concentration of them.