Grape juice and skins, or must, fermenting in an open-topped steel tank. The layer of skins on the surface, called the cap, must be broken up and submerged at least three times a day.
During alcohol, or primary, fermentation, yeasts convert grape sugars into alcohol to produce wine. Three important considerations in this process are the selection of yeast, the temperature and the fermentation vessel (oak barrel versus stainless steel tank).
Yeast Selection: Yeasts naturally occurring in vineyards and wineries are sometimes relied on to ferment the wine, since they can impart exciting, exotic nuances. The phrase "natural yeast" may be found on the wine label. But natural yeasts are not used on all wines because they sometimes expire before fermentation is complete, and can also impart off flavors. The safer alternative is to purchase specific strains of yeast cultured in a laboratory to ferment all the sugars and produce completely dry wines.
Temperature: As yeasts convert grape sugars into alcohol, they also produce heat. Excessively high temperatures can kill the yeasts and make white wine's delicate fruit flavors taste stewed or dull, whereas cooler temperatures maintain the freshness of fruit. On the other hand, just a touch of warmth can add a richer, rounder mouthfeel. The average temperature range for fermenting white wines is approximately 45 to 60 degrees Fahrenheit in the New World, and nearly 10 degrees warmer in the Old World. Fermentation temperature is frequently mentioned on more detailed back labels.
Since increased temperatures increase the release from the skins of colors and tannins necessary for red wines, they are generally fermented at warmer temperatures than white wines. The average range is 75 to 90 degrees Fahrenheit.
Container: Red wines are generally fermented in large open-topped stainless steel tanks or neutral wood vats that don't add flavor to the wines. Small oak barrels are not used for two reasons. First, red wines have so many big flavors that the additional flavors gained by fermenting in small oak barrels wouldn't justify the extra work or expense. Second, small oak barrels have small bung holes, and fitting the skins through the opening would be very difficult.
With red wines, the carbon dioxide produced during fermentation pushes the skins to the surface of the must, creating a firm cap. In order to extract color and tannins from the skins, the cap needs to be broken up and submerged at least three times a day.
The cap can be punched down in a back-breaking manual process with wooden batons or, more romantically, by stomping on the cap barefoot and bare-legged. Mechanical punching machines also do the trick, though some purists believe machines are too rough with the cap.
The fermenting wine can also be pumped up from the bottom of the tank and splashed over the cap to break it up and submerge the skins.
Rotary fermenters are a more recent innovation. These horizontal tanks can be programmed to rotate at various intervals during fermentation, thoroughly mixing the skins and juice.