Grape seed ripening evaluation by ortho-diphenol quantification

Two millennia of viticulture recognize the seed browning importance on tannin ripening and, thus, on grape and wine quality. This color change was recently attributed to phenolic oxidations. However an objective chemical index able to quantify the oxidation status of seed tannins was missing, probably due to the heterogeneous oxidation polymerizations. This work suggests the adoption of the ortho-diphenol quantification as indication of the tannin ripening process, because ortho-dihydroxylated substitutions are highly susceptible to oxidation. The method proposed is based on the ortho-diphenol characteristic complexation with molybdenum. Different cultivars (Merlot, Pinot noir, Croatina, Aladasturi, Alexandrouli, Odjaleshi and Tavkveri) were studied during three vintages in Oltrepo pavese, Italy. The color darkening correlated with the ortho-diphenol decrease. We believe this index could find useful applications in viticulture, supporting harvesting time decisions.