CHARGE PROPERTIES OF SOME GRAPE AND WINE POLYSACCHARIDE AND POLYPHENOLIC FRACTIONS

Charge densities of the major wine polysaccharides and of grape seed a nd wine polyphenols have been determined with a method developed by Mu tek (Germany), relying on titration with a counter-charged polyelectro lyte associated to streaming potential measurements. Charge densities were determined in a model hydroalcoholic solution at pH 3.5 and in 10 mM KNO3 solutions, from pH 2 to 9. The polysaccharides tested - yeast mannoproteins, grape arabinogalactan-proteins (AGPs), and rhamnogalac turonan RG-II - carried negative charges in the wine pH range. The net charge density of yeast mannoproteins was shown to be related to thei r phosphorus content and to be pH-independent in the range tested (2 t o 9). Absolute charge densities of AGPs and RG-II were related to the dissociation of the carboxylic functions of their uronic acids and inc reased dramatically in the pH range 2 to 5. Polyphenolic fractions, in cluding native tannins isolated from grape seeds and polyphenolic frac tions recovered from wine, carried no charges or negligible charges at pH 3.5. As a consequence, wine polysaccharides may establish electros tatic and ionic interactions, but such interactions will play little, if any, in the association of polyphenols with other components.