Differences in morphology and composition of skin and pulp cell walls from grapes (Vitis vinifera L.): technological implications

Abstract

The morphology and composition of skin and pulp cell walls of four premium red wine grape (Vitis vinifera L.) varieties were studied to determine whether the technological differences observed in previous studies between the four different varieties during the winemaking process (especially, the anthocyanin extractability) could be partly explained by their respective cell wall compositions. Monastrell grapes showed the highest amount of cell wall material in their skins (55–57 mg cell wall/g skin), while Syrah showed the largest amount of cell wall material in the pulp (5.1 mg cell wall/g pulp). The highest percentage of galactose and cellulosic glucose in skin and pulp cell walls was also found in Monastrell grapes. The cell wall composition of Monastrell grapes, together with their morphology, suggests that the firmer pulp and skin are responsible for the difficulties this variety shows for anthocyanin extraction (extractability index varying from 37 to 60). As cell wall composition may be modulated by the action of enzymes, the activity of pectinmethylesterase, polygalacturonase, cellulase and α- and β-galactosidase was studied. No polygalacturonase and cellulase activity was found. The two galactosidases showed higher activity than pectinmethylesterase (25–290 units/g tissue versus 0.3–0.4 units/g tissue), especially in Cabernet Sauvignon grapes, although in Monastrell grapes α- and β-galactosidase activity was low, which may explain the large amount of galactose in Monastrell grapes cell wall.