Proteins in white wine, I: Procyanidin occurrence in soluble proteins and insoluble protein hazes and its relationship to protein instability

The presence of procyanidins in wine protein fractions was established by a chemical assay and by pyrolysis DI-EI MS/MS of proteins. The chemical assay, involving acid-catalysed oxidative degradation of the procyanidins, gave cyanidin which was specifically quantified by HPLC. The assay was calibrated with purified grape seed tannin (GST); procyanidin was expressed in μg GST. Cyanidin yields were little altered by the presence of a large excess of protein. The limit of detection of the assay was 1 μg GST and the limit of quantification 3.5 μg GST. Both heat-induced and natural hazes isolated from various white wines were all shown to contain procyanidins, with a content ranging from < 0.02 to 4.9% (w/w). Although a crude soluble protein isolate from white wine contained a detectable amount of procyanidins, none were found in fractions separated chromatographically from this isolate. This observation, and the absence of procyanidins in resolubilised heat-induced haze, demonstrated that procyanidins are only weakly associated with both heat-induced hazes and soluble wine proteins. Nevertheless, procyanidins appear to be implicated in the formation of protein hazes in wine, because many soluble wine proteins that had been rendered free of procyanidins would precipitate to give haze in wine but not in model wine which was devoid of phenolic compounds.