The winemaker’s nightmare is a “stuck fermentation”. When fermentation stops before all thesugar has been converted into alcohol and the winemaker is left with a dangerously vulnerable mixture, which can all too easily fall prey to oxidation and nasty bacteria. The level of alcohol in a finished wine is an effective weapon against many bacteria.
The exact pace-time and temperature-of a red wine fermentation us critical to sort of wine that results. The warmer the fermentation(up to the dangerous flavour-evaporation limit) , the more flavour and colour will be extracted.Long , cool fermentations tend to result in light, fruity wines, but if a fermentation is too short and hot, the wine will also be low in body and flavour. The temperature rises as fermentation gets under way but is generally between about 72 and 86 ̊ F (22 and 30 ̊ C) even for full-bodied red wines, and cooler for aromatic white wines.
To extract tannins, flavour, and colour from the grape skins, the cap and the must need to be encouraged to commune with each other. This is generally done by either pumping the must over the cap or by physically punching it down into the liquid, although there is an increasing array of mechanical ways of submerging the cap. The science of this process, and post-fermentation maceration designed to extract and soften tannins, has become extremely exact.and a key factor in how much more palatable many young red wines are today. Vinomatics, sealed fermentation vats that mechanically churn the skins and must, may be useful for paler varieties such as Pinot Noir,or less ripe vintages.
Improving on nature
It is at the fermentation stage, red or white, thet the winemaker decides whether or not to add acid or sugar. To many wine drinkers this may sound like cheating. Many winemakers, on the other habnd, maintain it is essential to achieving a well-balanced wine. French winemakers, apart from those in the far south, have been adding sugar to fermentation vats to increase the alcohol content (not sweetnes) of the final wine for 200 years ever since this process, now called chaptalization, was proposed by the agriculture minister Jean-Antoine Chaptal.