Tannin Management

One of the keys to successful red winemaking is effective tannin management. This first occurs in the vineyard. Grapes, seeds and stems (if the stems are going to be used in macerations) can all contribute significant levels of polyphenols to the wine. Viticultural decisions can influence the extent and nature of the polyphenols that find their way into the must, although this is far from an exact science. While grapes were traditionally harvested on the basis of sugar levels, increasingly they are harvested with a view to achieving physiological or ‘phenolic’ maturity. Indeed, good viticulture can be summed up as encouraging a convergence of phenolic and sugar ripeness, with both at optimal levels at the same time. Shading of grapes is known to reduce the net quantity of skin tannins and also their nature. Unripe red grapes make nasty wines, not just because of high, herbaceous-tasting methoxypyrazine levels but also because of unripe or ‘green’ tannins. Seeds contribute a substantial amount of tannin to red wines and, if these are unripe and green, they can negatively affect wine quality. For this reason, one of the goals of current tannin research is to identify suitable markers of ‘phenolic’ maturity, which would give an indication of the best time to harvest. Another research objective is to identify specific grape tannins that can be used as markers of quality in viticulture.
Once the grapes have reached the winery, the way the polyphenolic substances (principally the tannins and anthocyanins) are extracted has a huge impact on the quality and character of the final wine. Winemakers have plenty of decisions to make about how to macerate red grapes so as to achieve the right level of polyphenol extraction. Some of the significant parameters that can be manipulated are the temperature of fermentation, pumping over or punching down the cap, the choice of fermentation vessel (small volume open-top fermenters, versus large tanks, versus rotary fermenters), the use of prefermentation cold maceration, and malolactic in barrel—and this list is far from complete. There are also new methods of extraction that are only just emerging, such as the flash d’etant system (that involves heating) and cross current extractors, but it’s too soon to say what sort of effect these will have and whether they will have wide take-up. The idea behind these techniques is that current extraction methods only pull out a proportion of the total phenolics present in grape skins, and it may be possible to enhance wine quality by removing more without also extracting unwanted polyphenolics from the seeds.
A subject of great current interest is microoxygenation, which has had a remarkably high take-up worldwide over the last decade considering that there are so far few experimental data backing up the claims of manufacturers of microxygenation devices and service providers who offer this technique to winemakers. It’s likely that oxygen applied at the right time and in the right quantities can have a beneficial effect on the mouthfeel and structure of red wines, but as yet there’s no clear evidence as to the sorts of tannin modifications that are taking place. It seems that microoxygenation is an important tool in tannin management, but winemakers currently have to ‘fly blind’, relying on guesswork and frequent tasting to judge when enough is enough for the particular wine they are working with.