**2. Key grape phenolic compounds**

The most important grape polyphenols comprise anthocyanins, flavan-3-ols, proanthocyanidins and flavonols (flavonoid family), and phenolic acids and stilbenes (non-flavonoid family). Each family can be present in their free or conjugated forms, differing by their hydroxylation level and by the substitution of the hydroxy groups (methylation, glycosylation, acylation) and even forming adducts between them (e.g., phenolic acids with anthocyanins; condensed tannins). This fact explains the great chemical diversity of polyphenols in grapes [6].

It is noteworthy that polyphenolic composition in grapes is highly affected by different factors such as viticulture practices, environmental conditions (soil, climate), and pathogen attacks [7]. Although, one of the most important factors is undoubtedly the varietal or genetic differences [8] as well as the winemaking process.
