Qualitative composition and extractability of grape skin tannins during the ripening period. Role of the extraction solvent

Abstract

Aims: The changes that skin tannins undergo during ripening, which are crucial for their extractability and transfer to wine, were followed in Monastrell grape skins. To elucidate whether the commonly used analytical method involving extraction with 70 % acetone overestimates the quantity of tannins that could be extracted during the winemaking process, the extraction with aqueous 12.5 % ethanol was also monitored throughout the ripening period.

Methods and results: When 70 % acetone was used as extraction solvent, the results showed that skin tannin concentration decreased towards maturity. When tannins were extracted with 12.5 % ethanol, their concentration during the first part of maturation was much higher than that obtained with the acetone extraction. However, both methods gave similar quantitative results for the last sampling dates, although the mean degree of polymerization (mDP) of the extracted tannins was significantly lower using the ethanol method.

Conclusions: The acetone method provides, for ripe grapes, an accurate prediction of what may be quantitatively extracted by a fermenting medium, although it overestimates the tannin mDP.

Significance and impact of the study: The structural composition of tannins (and not only their concentration) might be essential for defining the sensorial characteristics of the wine and the differences observed in tannin mDP between the acetone and the ethanol method may be of importance when predicting wine organoleptic characteristics.