20051226

The skinny on skin colour

This is the flipside of my previous entry on the role of the grape skin, in which we learned that leaving the skins in contact with the must will allow them to impart characteristics like flavour (in addition to colour). Part of the reason that I got so motivated to start this topic was that I felt the need to expand the simple refrain I repeatedly hear -- that "red wine is only red because of its skin" -- and delve into what gives white wine its different shades. Last time I mentioned that sparkling wines are mostly produced from free run juice, meaning that skins have very little influence on the finished product. Whites, as well as reds, can be free run too but for the most part skin plays a larger role in white wine than in sparkling wines. But exactly what?

Routinely, white wine production processes do not enlist the skins as much as red wine's would. Why? Skins on all grapes have tannins which is a useful ingredient in some cases and one you opt to leave out in others. Tannins are beneficial components of red wine because they promote ageability while adding structure; they are detrimental to white wine where their astringency can frequently mask gentle aromatic nuances naturally occurring in white grapes. Temper that basic idea with the knowledge that white wines can achieve some extra zing when skins are exposed to the must. This exposure can be quite desirable as long as it happens before fermentation. Should you ferment the skins, you won't get any of the winning benefits that the skin-in red wine enjoys. So the skin isn't particularly the thing when it comes to white wine.

That said, I still am left to wonder. If we can say that red wine is red in colour because skin pigments seep through the must during winemaking, how do we tell whether a white wine exhibits a hue that is due to its grape skin? Maybe there is no way to say for sure. Maybe one has to ask. Does a white wine's light colour result from its skins' light pigment? Or rather does it simply mean the light colour you see is the absence of colour -- that the skin has had less time to mingle with the must and therefore has not offered much in the way of pigment? And what shade-shaping container was the wine stored in prior to bottling? And also, how has age affected the colour? I guess it's hard to make a general statement just by looking at the colour of the finished product. In many cases, it could be a little of everything -- that is, some colour extracted from the skins, some colour leached from oak barrels, and even some colour produced just from the light colour of the grape's flesh. In an extreme example, Pinot Gris (or Tokay) often creates a white wine varietal that possesses a light blush tone due to prolonged exposure to its pinkish skins during vinification. Then to this distinctive shade additional pigment from its greyish-pink flesh and potential colouring from barrel storage as well as the passage of time. In the end it can look nearly like a a rosé, but it is not since no red grapes are added (but I'll have to save rosé wines for an upcoming post).

Backwash
This is a Backwash backwash. I implied a connotation of noble rot that was misleading when I wrote about Sauternes. I may have suggested that the fungus-spawning vine disease called noble rot was noble because of its Bordeaux provenance. While noble rot is indeed essential in Bordeaux's production of Sauternes -- the world's finest sweet wines -- the use of the term "noble" really serves more of a reference to the Botrytis Bunch Rot than Bordeaux in general. Botrytis Bunch Rot is a non-benevolent form of the same disease that sets noble rot into motion. Noble rot was coined more to distinguish it from the bunch rot to lay praise on the Bordelais areas that champion their Sauternes.

1 comment:

Marcus said...

Found some definitve text on the subject online:

Experience has also shown that the formation of pigmented tannins, as well as conferring stability on the colour of a red wine (for decades, in favourable circumstances) also modulates the astringency of the very high concentration of phenolics of the wine, improving its texture and other taste properties. The desirable effects of pigmented tannins on mouthfeel are well illustrated by a comparison of the taste properties of a red wine with those of a white wine made, like a red wine, with extensive maceration. Such highly tannic white wines are not just unattractive, but crude and coarse on the palate; furthermore, they do not improve with age but remain tannic and undrinkable. In contrast, the pigmented tannins of the red wine make it palatable and soft with good ageing characteristics, and this despite the fact that their presence increases further the phenolic and tannin polymer content of the red wine relative to that of its macerated white wine counterpart.

One important operation during the fermentation of most red wines, therefore, is to transfer the anthocyanin pigments from the skin cells to the wine. Colour transfer is achieved by keeping the skins adequately mixed with the fermenting wine (see maceration).

One might reasonably expect that the pigments in the new wine would be identical to those found in the grape skin. This may be the case for a few hours, but once the anthocyanins are mixed with the acids of the wine and other phenolics as well as the many products of fermentation, they begin a series of reactions leading to more complicated molecules (see pigmented tannins), in turn leading to a great diversity of derived pigments and colourless molecules. Derived pigments are classically assimilated to pigmented tannins arising from the addition of tannins to anthocyanins. However, they also include rather small molecules formed by the reaction of anthocyanins with other wine constituents such as acetaldehyde or pyruvic acid. Within a few years, only traces of the relatively simple monomeric anthocyanins remain. With wine ageing, polymers containing anthocyanin molecules may become larger and form aggregates so that some of them exceed their solubility in the wine and are precipitated as sediment.