Fermentation is what turns an ordinary grape juice into wine. It is a natural process in which yeasts (single–cell organisms that are naturally present on the skins of grapes) convert grape sugars into alcohol and carbon dioxide. The process may last from a few days to some months, a time during which carbon dioxide and heat are released as by–products. For dry wines fermentation ends when all the sugar is converted into alcohol. For sweeter wines, instead, it is the oenologist who decides when to end the process, adding alcohol in the form of neutral grape brandy, which kills the yeasts and allows part of the sugars to remain unfermented. Fermentation was traditionally done in wooden vats, barrels or neutral tanks. Today, many wineries use huge tanks made of stainless steel, which allow a precise control of the temperature.
