Developmental Changes in Phenolic Composition and Condensed Tannin Structure in the Skins of Three Table Grape Cultivars: ‘Rosetta’, ‘Shooting Star’, and ‘Hongju Seedless’

This study investigated developmental changes in phenolic compounds and condensed tannin structure in the berry skins of three table grape cultivars, ‘Rosetta’ (RT), ‘Shooting Star’ (SS), and ‘Hongju Seedless’ (HS), from flowering to harvest. Spectrophotometric analyses were used to quantify total phenolic, proanthocyanidin, polymeric tannin, and anthocyanin contents, while high-performance liquid chromatography coupled with thioacidolysis was applied to characterize flavan-3-ol composition and determine the mean degree of polymerization (mDP) of condensed tannins. Phenolic accumulation exhibited distinct cultivar-dependent developmental patterns. Total phenolics and flavan-3-ol monomers generally decreased as berries matured, whereas anthocyanins accumulated after veraison in RT and HS. Polymeric tannins increased during the early developmental stage and subsequently declined, while mDP showed an overall tendency to increase toward maturity, particularly in RT, although the effects of cultivar, developmental time, and their interaction on mDP were not statistically significant. HS maintained high flavan-3-ol concentrations during early development, whereas SS exhibited comparatively low phenolic accumulation together with relatively limited variation in mDP. These findings provide the first comprehensive characterization of developmental changes in flavan-3-ol composition, polymeric tannin structure, and mDP in newly developed Korean table grape cultivars and a structural basis for understanding cultivar-dependent phenolic changes during berry development.