Xanthylium salts formation involved in wine colour changes

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Abstract
Summary The reaction of (+)‐catechin in wine‐like model solution was investigated. First appearance of colourless dimeric compounds consisting of two flavanol units linked by carboxy‐methine bridge was observed. Their isolation and further incubation was found to yield two types of yellowish pigments showing visible absorption maxima at 440 and 460 nm, respectively. Mass spectroscopy (MS) spectral analysis showed that the first type were xanthylium salt pigments formed by dehydration of the colourless compounds followed by an oxidation process. The loss of a water molecule was shown to take place between two A ring hydroxyl groups of the colourless dimers. The second type were shown to be ester derivatives of the first ones. Thus ethylester of xanthylium salt was obtained and fully characterized by mass and nuclear magnetic resonance (NMR) spectroscopy. Esterification was found to involve the colourless compound before dehydration and thus a general scheme for xanthylium salt formation was postulated. The proposed scheme constitutes a new xanthylium formation pathway as up to now only anthocyanin‐flavanol reactions were supposed to form xanthylium salt derivatives during wine ageing. This work also provides new support to the contribution of xanthylium salt in colour evolution observed during wine ageing which is generally expressed in an increase of absorption in the 400–500 nm, region of xanthylium salt absorption maxima.