Concerning flavonols, quercetin-3-O-glucuronide was the most abundant compound of this phenolic class in all varieties, representing from 63.1% to 83.2% of total flavonols. Additional flavonols showed strong variations in the separate cultivars evaluated ( Fig. 2B). Isorhamnetin-3-O-(6-O-feruloyl)-glucoside represented the 12.6%, on average, of total flavonols in white cultivars, whilst the red varieties ‘Sousão’, ‘Touriga Nacional’, and ‘Tinta Barroca’ displayed a concentration ranging from 3.3% to 8.8%. Kaempferol-3-O-rutinoside was more abundant in red varieties than in white cultivars, representing the 6.5% and 4.4%, on average, respectively ( Fig. 2B). The flavonols present in the lowest concentration in grape stems were kaempferol-3-O-glucoside (2.9%, on average) and kaempferol-3-O-rutinoside (4.4%, on average).