Now the winemaker has his white juice, he may well decide to clarify it, to clear it of all the little grapey fragments still in suspension in it, particularly for protectively made wines. Centrifuges were once used for this but they can easily spin out too many solids. More usual today is to let solids settle to the bottom of a holding tank and then run off the clear juice into the fermentation vat to increase the tannin level, and some particularly traditional winemakers, especially in Burgundy, like to ferment whole bunches. This works only in climates with growing seasons long enough to ripen the stems fully as well as the fruit, otherwise the stems would make the wine taste horribly tough (most Australian red grapes, for example, would have unacceptably low acid levels if not picked until the stems were ripe). (A very particular exception to the destemming rule for reds is a fermentation technique called carbonic maceration,characteristic of Beaujolais,some Côtes du Rhône, and naturally tough Carignan in the Languedoc-Roussillon, where the aim is to produce fruity wines without much tannin. Here whole bunches are put into a saeled fermentation vat, the bottom layer of grapes is crushed by the weight of grapes above and starts fermenting naturally, giving off carbon dioxide . The whole vat becomes saturated in carbon dioxide under whose influence the whole grapes in the upper part of the vat undergo a special sort of internal fermentation whosw products have a characteristic soft,almostrubbery smell.)
The fermentation process
Now, and only now,can the winemaker consider conventional fermentation,the miraculous transformation of sweet grape juice into mucg drier, more complex-flavoured wine. If yeast is put into alcohol,heat, and carbon dioxide.The riper the grapes,the stronger the wine they are capable of making. Fermentation vats naturally warm up as the process gets under way, so in warmerclimates they may will need cooling jadkets to keep the “must”, as the pulpy mixture between grape juice and wine is know , below the temperature at which precious flavour compounds may be boiled off. The gas that is generated can make a winery a heady place at harvest time where the small is an intoxicating mixture of carbon dioxide, grapes and alcoholespecially if the fermentation vats are opentopped, as for some traditional red wines. White wines are made in realed vats so as ti protect the must from damaging oxidation and avoid any browning. A vat full of red must has its own protection, the thick “cap” of skins that float on the surface-which is why open-topped red wine fermenting vats are even a possibility.
Many aspects of winemakeing have been subjected to detailed scrutiny and improvement over the last few dacades, but yeast still presents some mystery and controversy. We will doubtless learn more about the exact effect of differebt species of yeast on different sorts of must. But for now the winemaker’s initial choice is whether or not to use specially selected and prepared yeast, so-called cultured yeast, as opposed to relying on the strains of yeast that are naturally in the atmosphere, called wild or ambient yeast.