Influence of matter other than grapes on quercetin evolution and wine polyphenols

Matter other than grapes (MOG), such as leaves and stems, is increasingly present in mechanically harvested fruit and is known to affect wine quality. This study investigated how MOG levels below 3 % (w/v), which is the threshold considered acceptable for quality wine by industry standards, impact quercetin concentration and the broader phenolic composition of wine. Fermentations were conducted using both synthetic must and Merlot must, with the addition of 1 %, 2 %, and 3 % (w/v) grape leaves or stems. Results showed that grape leaves significantly increased the levels of quercetin derivatives, particularly quercetin-3-glucuronide and quercetin-3-glucoside, which may contribute to instability and precipitation during ageing. Grape stems, while contributing little to quercetin enrichment, markedly increased catechin and proanthocyanidin concentrations. Both leaves and stems affected fermentation kinetics and modified chemical parameters such as pH and total acidity. Colour measurements also varied depending on the type and amount of MOG added. This study provides quantitative data on how MOG levels below 3 % influence quercetin derivatives and other phenolic compounds during wine fermentation. The results highlight the importance of limiting MOG in harvested grapes to maintain wine composition and quality.