flavonols, a group of phenolic yellow pigments belonging to the flavonoid family found mostly as glycosides such as glucosides and glucuronides. quercetin glycosides, the most common flavonols in grapes, are abundant in vine leaves and present in skins and stems. Their concentration can be enhanced in grapes (and therefore in wine too) by exposing the berry cluster to the sun (see canopy management). Indeed, Price and colleagues argue that the quercetin level in grape berries can be used as an index of the sun exposure they have experienced. Like catechin, quercetin may enhance pigment stability in young wines through co-pigmentation and has anti-oxidant properties similar to those of resveratrol (see health). The release of flavonol aglycones from the corresponding glycosides may also result in a hazy wine as these molecules are poorly soluble in wine.
