Xanthylium salts formation involved in wine colour changes

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 f urther incubation was found to yield two types of yellowish pigments showin g visible absorption maxima at 440 and 460 nm, respectively. Mass spectrosc opy (MS) spectral analysis showed that the first type were xanthylium salt pigments formed by dehydration of the colourless compounds followed by an o xidation process. The loss of a water molecule was shown to take place betw een two A ring hydroxyl groups of the colourless dimers. The second type we re shown to be ester derivatives of the first ones. Thus ethylester of xant hylium salt was obtained and fully characterized by mass and nuclear magnet ic resonance (NMR) spectroscopy. Esterification was found to involve the co lourless compound before dehydration and thus a general scheme for xanthyli um salt formation was postulated The proposed scheme constitutes a new xant hylium formation pathway as up to now only anthocyanin-flavanol reactions w ere supposed to form xanthylium salt derivatives during wine ageing. This w ork also provides new support to the contribution of xanthylium salt in col our evolution observed during wine ageing which is generally expressed in a n increase of absorption in the 400-500 nm, region of xanthylium salt absor ption maxima.