Changes during ripening in the content of the principal bioactive polyphenols in five Greek native Vitis vinifera cultivars
The polyphenolic content of grapes and vinification products is subject of intense research, due to their antioxidant and health beneficial properties. In this regard, the assay of concentration and compositional changes of bioactive polyphenols during various cultivation stages is also of great interest and the subject of our study which focused on five Greek native Vitis vinifera species, cultivated under water stress, from veraison until maturation. More specifically, the seasonal variation of grape berry, seed and skin samples and their respective wines in (+)-catechin, (-)-epicatechin, epicatechin gallate, procyanidins B3 & B2, quercetin glycosides, phenolic acids and the stilbenes trans-resveratrol and ε-viniferin content was assayed. Results indicated that both total polyphenol and low molecular-weight polyphenols contents were dramatically decreased over the course of maturation process, from veraison to harvest. Mandilaria and Assyrtiko cultivars exhibited particularly high polyphenolic content at veraison, which by the harvest time was decreased. The observed decrease of polyphenolic content in berry samples was reversely proportional to the increase of dry matter and sugar accumulation. The polyphenolic content differences observed during maturation were rather quantitative, than compositional. Grape seeds were particularly rich in proanthocyanidins and their content fluctuated during ripening, with a strong reducing trend at harvest time. Mandilaria and Assyrtiko seeds exhibited a high polyphenolic potential at all maturation levels, reflecting their grape and wine increased polyphenolic content. All skins were characterized by the presence of quercetin glycosides, with quercetin-galactoside being the most abundant, while their concentrations were steadily increased over maturation, following the accumulation of sugars in grape berries. All samples were also assessed for their antioxidant activity to delineate the effect of ripeness on the overall antioxidant potential, revealing a reducing trend over maturation.