Usually different grape varieties are pressed separately and the must kept apart to see how it develops before blending takes place. In big wineries where there is volume production, everything often goes into the press at the same time. There is a red Burgundy called Passe-Tout-Grain, which means "processed all at the same time’.
Red wine and rosé get their colour from skin contact, though a rosé obviously spends less time in contact than a red. The slightest contact is enough to give the wine a faintly pinkish hue.
Once the juice has been extracted and has spent its time in contact with the skin, it is piped into fermentation vats, where it begins its conversion into wine. Wine is produced when yeast, naturally occurring or added, attacks the sugar in the grape juice, breaking it down to release energy in the form of heat The action of yeast on sugar produces a number of by-products which include carbon dioxide and, most important of all, alcohol.
The yeast goes on working until it has used up all the sugar in the juice, until the alcohol reaches a level above which it cannot function, or until the fermentation is stopped artificially. Yeasts can only operate within certain temperature levels. If the temperature falls they go into a state of suspension; if the temperature rises too high, they are killed. In the past, because of temperature fluctuations, the aim was to complete the fermentation process as quickly as possible, but this often led to high temperatures and the juice being ‘baked’. Much of the massive investment in modem wineries goes into fermentation tanks which can be temperature-controlled. White wine particularly benefits from slower fermentation at cooler temperatures, because it brings out much more flavour.
After fermentation, the white wine is usually allowed to settle for a few days and then is filtered. The new horizontal presses are very efficient at extracting juice but they also allow small particles of skin to enter the must. This is one reason why young fermented wine has such a cloudy, milky appearance, and the particules must be removed.
At this stage the wine-maker can ‘adjust’ the wine. In many countries sugar can be added, a process called chaptalisation. This reduces the acidity of the wine. In many regulated areas sugar is not allowed, and the wine-makers have to use concentrated must, which raises both the alcohol content and the acidity levels.
The wine-maker may also add small quantities of chalk to reduce acidity if necessary.
Concrete vats and stainless steel tanks have taken over from the traditional oak barrels in many places. The barrels may look fine in a cellar but they are generally not as efficient as the modem storage containers. Stainless steel tanks have the added advantage that they can be used for fermenting, storage and blending, while wood is difficult to clean and hugely expensive. Only occasionally is white wine allowed to ferment a second time - a process called malolactic fermentation - to reduce acidity levels. This technique is still used in parts of Chablis and the Loire, for instance. It usually happens in the spring when the warmer weather stimulates dormant yeasts.