Flavonol Profiles of<i>Vitis vinifera</i>Red Grapes and Their Single-Cultivar Wines

The main flavonols found in seven widespread Vitis vinifera red grape cultivars include the 3-glucosides and 3-glucuronides of myricetin and quercetin and the 3-glucosides of kaempferol and isorhamnetin. In addition, the methoxylated trisubstituted flavonols, laricitrin and syringetin, were predominantly found as 3-glucosides. As minority flavonols, the results suggest the detection of the 3-galactosides of kaempferol and laricitrin, the 3-glucuronide of kaempferol, and the 3-(6' '-acetyl)glucosides of quercetin and syringetin. The flavonol profiles based on the eight above-mentioned flavonols allowed the cultivar differentiation of the grape samples. With regard to flavonol biosynthesis in the berry skin, quercetin 3-glucuronide predominated at véraison, followed by quercetin 3-glucoside, and only trace amounts of trisubstituted flavonols were detected. The proportion of quercetin 3-glucoside remained almost constant during berry ripening, whereas the proportion of quercetin 3-glucuronide decreased and the other flavonols, especially myricetin 3-glucoside, increased their importance. In wines, flavonol 3-glycosides coexisted with their corresponding free aglycones released by hydrolysis. The presence of laricitrin, syringetin, and laricitrin 3-glucoside in red wines is reported here for the first time. The extent of hydrolysis was widely variable among wines made from the same grape cultivar, and the results suggest the influence of the type of aglycone and glycoside on the rate of hydrolysis. Due to hydrolysis, the differentiation of single-cultivar wines gave acceptable results only when aglycone-type flavonol profiles were used.