Fermentation Process of Wine Making

Definition of fermentation

The anaerobic conversion of sugar to carbon dioxide and alcohol by yeast. Any of a group of chemical reactions induced by living or nonliving ferments that split complex organic compounds into relatively simple substances.Unrest; agitation.

Natural fermentation begins immediately after grapes are crushed. If one squeezes a bunch of ripe grapes in his hand and lets the juice fall into a container, the juice will ferment into a kind of wine. Wild yeasts in the dust-like "bloom" covering the skins of ripe grapes begin the fermenting cycle.

Most winemakers in California and other wine growing areas prefer to aid the process with a pure starter yeast. These yeasts, added to the new "must," promote clean and uniform fermentation.

A small quantity of sulfur dioxide—perhaps 100 parts per million of juice—usually is added to the new "must" to kill or inhibit undesirable organisms. Sulfur dioxide, in proper amounts, promotes clean fermentation but does not kill the true wine yeasts.

Fermentation in wine making is simply the chemical change of natural grape sugars (mostly dextrose and levulose) into roughly equal parts of ethyl alcohol and carbon dioxide gas. The latter escapes during fermentation.

Wines owe much of their character to the fermenting process. Different grape varieties have individual traits, but several important characteristics are determined during fermentation.

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Alcoholic content of wine depends on the amount of natural grape sugar in the "must" and whether the wine is allowed to ferment (or convert) all of its sugar. Completely dry dinner wines—those in which all sugar is converted—usually contain 11 to 13 per cent alcohol in California, where the grapes reach full maturity every year. In many areas of the world, it frequently is necessary to add sugar to the "must" before fermentation to obtain-even in dry wines—the proper alcohol level.

If sweetness is desired in the wine, the winemaker will halt fermentation while some sugar remains. Alcohol content is adjusted by blending with another wine (in dinner wine types) or introducing wine spirits (in dessert wine types).

Wine color depends on whether and for how long juice is fermented with the pigment-bearing skins, since the juice of most grapes has little or no color. The skins and seeds also contain tannin, so contact during fermentation determines the "puckery" quality considered desirable in red wines.

Fermentation of appetizer and dessert-type wines is halted by addition of wine spirits (neutral grape brandy distilled from wine) to retain some sweetness from the natural sugar of the grapes and to make the wine more stable.

FERMENTING EQUIPMENT

Wine is fermented in vats, tanks or ovals, in to which the freshly-crushed "must" is pumped from a reservoir beneath the crusherstemmer. The traditional fermenting containers are vats — open structures of wood or concrete holding up to about 60,000 gallons of "must."

Fermenting tanks, which are covered and tend to be larger and deeper than vats, have become popular with some of the larger wineries in California. They can be made of wood or lined steel or concrete.

 Ovals—closed wooden casks of up to 3,000 gallons capacity —still are used to ferment.

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