Colourful Cues

We often use the intensity and quality of colour as an indicator of wine style, even before we’ve tasted the wine! It usually holds true for wines at either end of the spectrum, but can be less of a “value”in between. The tinge at the edge of the glass is most often related to the pH of young wine as stated previously. This alone is also a good cue for wine style. A purple/blue-tinged wine has a higher pH than a bright red one. This is explained by the different reversible equilibria that the different forms of free anthocyanins are involved in (4). I am not so sure if the same holds for anthocyanins in complexes, but please correct me if you have more up to date detail about that handy. High-pH wines are often, but not always, made from late harvest, over-ripe grapes, since grapes loose malic acid as the season progresses, and the concentration of potassium increases due to a loss of water near the end of maturity. Of course, in Australia we can just add a a couple of bags of acid to the must and correct it, but such additions, although often necessary in order to make well-balanced wines, can easily be overdone if we consider analytical results only! Thus, the purple-edged wines are most likely big and fat, whilst the bright red ones are acid and tart? Not always, but often towards those two alternatives. At least in countries where acid additions aren’t allowed!

The figure below illustrates clearly the effect of pH on the perceived colour of anthocyanins. All solutions contain the same concentration of the same anthocyanin, namely delphinidin-3-glucoside. The only difference is the pH (and the buffer used to achieve the pH differences). The concentration is not very high and at a high concentration of anthocyanins, all solutions will appear black, except at the edge!

Older wines have more of a brick-red/yellow hue than young wines. Phenolics oxidize and this irreversible oxidation is merely a function of time and environment. Such reactions are enhanced by the presence of light, high temperature and oxygen that has migrated through the cork (which is why Stelvin-capped bottles will appear fresher for a much longer time, they don’t allow as much oxygen to reach the wine!). Winemakers who gamble with the addition of low amounts of sulphur dioxide will also suffer, for sulphur dioxide does just that, protects the wine from the effects of oxygen by binding to it with great agility and speed before it can do any harm!

Until I get a chance to revise further, please don’t hesitate to ask any questions or to point out obvious faults! (Patrik, 17th of September, 2002). Unlike wine, old references seldom age gracefully. Only the best survive, and poor access to old ones in this modern internet-dominated age will make even old classics easily forgotten. My stock of anthocyanin-related references is clearly outdated, so if you find a nice, new, fresh and exciting paper, please send it to me!

(1) Boss PK, Davies C, Robinson SP (1996) Expression of anthocyanin biosynthesis pathway genes in red and white grapes. Plant Mol Biol 32: 565-569
(2) Singleton, VL.., & Trousdale, E.K. (1992). Am. J. Enol. Vitic. 43, pp. 63-70
(3) Singleton et al. (1975). Am. J. Enol. Vitic. 26, pp. 62-69
(4)Brouillard R (1982) Chemical strucuture of anthocyanins. In: P Markakis (Ed) ‘Anthocyanins as food colours’. Academic Press, New York. pp 1-40

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..

Structural Stability

Anthocyanins are almost always stored as glycosides (see glycoside section for further information) since the glycoside-lacking anthocyanidins are very unstable and can rapidly convert into other colourless forms, reversibly and irreversibly. Together with a range of different sugars and various acyl-groups (fancy name for acids) attached to different positions in a myriad of combinations, several hundred different anthocyanin forms are produced by plants. The structure of anthocyanins, the presence of various other compounds (particularly co-pigments, fancy name for compounds that can interact physically with anthocyanins) and the pH of the solution that they are in, all have an impact on colour absorption and hence colour perception. For example, wines with a pH closer to 3 than 4 have a bright red colour whilst wines with a pH close to or above 4 have an apparent colour that more closely resembles purple or even blue! (Late harvest monster Shirazes for example….).

What are Those Pigments?

Red wine, blueberries and petunia flowers all contain anthocyanins, the most common red plant pigments of all. Anthocyanins belong to a group of secondary plant metabolites that are called flavonoids, and are almost ubiquitously synthesized by plants in at least some tissues, particularly under stressful conditions. (Beetroot, by the way, owe their intense colour to betacyanins, a group of similar, but different, compounds). The flavonoids also include some of the pre-cursors of tannins, but more on that later…, and they are part of an even larger group of compounds thermed phenolics. All phenolics, including anthocyanins, contain at least one phenolic ring (benzene group or aromatic ring are alternative terms). Anthocyanins have three, and there are very very tiny differences in the structure of anthocyanins compared to other “neighbours” such as the yellow flavonols, that endow them with the ability to tickle our perceptions with different wavelengths (or colours).

The Colour of Wine

Wine colour, particularly red, is one of wine’s most outstanding features and a strong cue to wine style. On the surface it is quite simple, red wines contain red pigments, termed anthocyanins, and white wines don’t. Big wines have a lot of colour, and appear almost black with a purple tinge, whilst a light Beaujolais is quite easy to see through and clearly bright red. However, it is possible to go a bit more in depth into the subject and I will try to do so here, starting by explaining some basic terms and eventually venturing into the complex world of tannins!