Fermentation is a process whereby sugar is converted to acids, gases, and/or alcohol. Fermentation has long been used to produce alcoholic beverages such as beer and wine. Fermentation can be used to preserve food as well, such as pickles, yogurt, and kimchi. Fermentation can even occur in a human stomach!
Fermentation in wine making turns juice from grapes into an alcoholic beverage. Yeast changes sugars in the grape juice into ethanol and carbon dioxide. Temperature, oxygen levels, and speed of fermentation are critical variables to quality. Fermentation can be accomplished in stainless steel tanks, in a wood vat, or in the wine bottle itself.