Polysaccharides Influence the Results of Polymeric Pigment Analysis in Red Wines
Julia Graves, Stephan Sommer
Abstract
The reliable quantification of phenolic compounds and pigments in red wines is an essential part of quality assessment since the size and chemical nature of these compounds determine long-term color stability and organoleptic properties. Several photometric assays have been developed in the past to address the differentiation of polymeric pigments on the basis of size, sulfite bleaching behavior, and reactivity with proteins. Polysaccharides like pectin or mannoproteins are known to interfere with polyphenol–protein interactions and could influence the accuracy of the assay results. In these experiments, different factors like single polysaccharide concentration, combinations of polysaccharides, charge-based and hydrophobic interactions, pH, ionic strength, and chaotropic disruption were studied with the goal to modify the most common polymeric pigment assay (Harbertson–Adams assay) for polysaccharide-rich wines. Pure polysaccharides in several red wines and wines with naturally extracted polysaccharides were analyzed, and the readings for polymeric pigments were compared to a control without additions. Charged compounds like pectin have a concentration-dependent effect on the analysis of polymeric pigments that is enhanced by the presence of uncharged polysaccharides, decreasing their apparent concentration. The change in assay efficiency is based on a combination of charge and hydrophobic interactions that need to be disrupted to achieve reproducible readings. The best results could be achieved with a combination of pH adjustment and chaotropic disruption. This maximizes the efficiency of the precipitating protein and disrupts hydrogen bonds between macromolecules, neutralizing the effect of polysaccharides.