Red or White?
I hope I haven’t lost you already for here comes the interesting part. What are the main differences between red and white wine? Colour, and …? Tannins of course. So, red grapes contain anthocyanins and tannins, whilst white grapes don’t? No, the only thing that differs between red and white grapes is the lack of anthocyanins in whites, apart from varietal differences such as those between a Cabernet Sauvignon and a Shiraz grape. Some nifty research done by Simon Robinson and co-workers at CSIRO Horticulture (Adelaide) has suggested that the the difference can be traced to the presence or absence of a single gene product, namely a cyanidin-3-O-glucosyltransferase (1). The lack of this enzyme expression in white grapes results in anthocyanidins without the stabilizing sugars, which are unstable.., with a resultant absence of anthocyanin accumulation.
Thus, in theory, “white” grapes contain just as many tannins and/or tannin-precursors as red grapes. Aha, the difference must lie in the different processing (romantically termed vinification or wine making) of white vs. red grapes? Yes, and no! Not many nowdays “make” white wines by macerating crushed grapes until the end of fermentation, like we do with red grapes. Probably, it would result in horribly bitter and astringent white wines with rather different aromatic profiles compared to wines made with more modern techniques. However, a beautiful paper by Vernon Singleton and his co-worker (2) back in 1992, provided an alternative answer. They added varying amounts of purified anthocyanins and purified polymeric phenols extracted from grape seeds (very vague term, since none really knows what it consists of…) to white wine and allowed the mixture to stand for 10 days and mingle. Analyses of the composition later revealed that the simultaneous addition of anthocyanins and the poorly soluble polymeric phenols had resulted in the formation of anthocyanin-tannin complexes . Notably, the presence of anthocyanins significantly enhanced the retention of polymeric phenols in the wine. Only retained polymeric phenols end up in the bottle and notably, these polymeric anthocyanin-tannin complexes were more resistant towards sulphur bleaching.,
(The addition of sulphur dioxide (a common addition to all wines in most wineries) will bind to anthocyanins and result in a loss of apparent colour!)
This was only an experiment and it is quite feasible that such tannin-anthocyanin complexes are retained to a much higher degree than uncomplexed tannins and tannin-precursors during fermentation, pressing and storage. A significant proportion of wine phenolics bind to grape proteins and yeast present during the initial stages of wine making and Vernon suggested that such binding may differ between tannin-colour complexes and “free” tannin-precursors. Differing degrees of solubilities – the anthocyanin-tannin complexes may be more soluble due to the presence of sugar-groups on the anthocyanins – may also explain why anthocyanin-tannin complexes are retained to a greater degree. This and the fact that completely different varieties are used for white wine making, would explain why Vernon previously found white wines fermented in the presence of skins to be very white wine-like (3), ie. lacking in tannins..
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