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Grape juice is turned into alcohol by the process of "fermentation." Grapes on the vine are covered with yeast, mold and bacteria. By putting grape juice into a container at the right temperature, yeast will turn the sugar in the juice into alcohol and carbon dioxide. The grape juice will have fermented.

Yeast gives flavor to wine. However things on the outside of a grape are not necessarily so good for the production of good wine (for example, acetic bacteria on the grapes can cause the wine to turn to vinegar). The winemaker commonly eliminates unwanted contaminants by using the "universal disinfectant," sulfur dioxide. Unfortunately, the sulfiteswhich remain in the wine may cause a lot of discomfort to some wine drinkers (see the section on Allergic Reactions to Wine). Some winemakers prefer not to do this, and purposely create wines that are subject to the vagaries (and different flavors) of yeast that is "wild," that is not a commercial yeast strain used by the winemaker ("wild yeast fermentation"). By the way, some have said that these wild yeasts are found on the grape, but a number of people have commented that there is no documentation that any wild yeast living on the skins of grapes leads to alcoholic fermentation. They propose that these "spontaneous" fermentations occur due to commercial yeast populations that live in the winery and have become "wild" over several generations--and have not been cleaned away or otherwise eradicated.

The winemaker has many different yeast strains to choose from (and can use different strains at different times during the process). The most common wine yeast is Saccharomyces.

This is a good place to mention "Brett" or the Brettanomyces strain of yeast. But since it is a side-light and this is written as a hyper-text document, you can check it out now. Otherwise, you will find the discussion as the next section.

As yeast works, it causes grape juice ("must") to get hot. But if there's too much heat, the yeast won't work. One modern way to deal with this is to put the juice into large stainless steelcontainers that have refrigeration systems built around the sides. The winemaker can regulate temperature precisely.

A less modern, but still wide widely used way to ferment wine is to place it in small oak barrels. "Barrel fermentation" is usually done at a lower temperature in temperature controlled rooms and takes longer, perhaps around 6 weeks. The longer fermentation and use of wood contributes to the flavor (and usually expense) of the wine.

The skins and pulp which remain in a red wine vat will rise to and float on top of the juice. This causes problems (if it dries out, it's a perfect breeding ground for injurious bacteria), so the winemaker will push this "cap" back down into the juice, usually at least twice a day. In large vats, this is accomplished by pumping juice from the bottom of the vat over the top of the cap. Some winemakers use a screen to keep the cap submerged at all times.

Eventually the yeast is no longer changing sugar to alcohol (though different strains of yeast, which can survive in higher and higher levels of alcohol, can take over and contribute their own flavor to the wine--as well as converting a bit more sugar to alcohol).

After all this is completed, what you have left is the wine, "dead" yeast cells, known as "lees" and various other substances.