A new class of stable red pigments detected in grape pomace was analysed by electrospray ionisation mass spectrometry. They were shown to be pyruvic acid derivatives of genuine grape anthocyanins by synthesis experiments. The major product was identified by NMR (1H, NOE, HSQC, HMBC) experiments as the malvidin-3-monoglucoside pyruvic acid adduct. Its formation results from cyclisation between C-4 and the hydroxyl group at C-5 of the original flavylium moiety with the double bond of the enolic form of pyruvic acid, followed by dehydration and rearomatisation steps. This type of reaction leads to increased colour stability. Various yeast metabolites other than pyruvic acid were shown to react with grape anthocyanins following this mechanism, suggesting that it may be an important route of conversion into stable pigments during the maturation and ageing of wine.