Fermentation

Fermentation is the stage of winemaking where grape juice is transformed into wine. It happens with the help of yeast, tiny single-celled organisms that live off the sugar in the grapes. The yeast converts the grapes' sugar (glucose and fructose) into alcohol and carbon dioxide, and it is this process that turns sweet grape juice into a dry or off-dry wine with alcohol. Along the way the yeast also produces a wide range of by-products in small amounts, which help give the wine its aroma and flavour, among them various aromatic compounds and glycerol, which contributes to a softer feel in the mouth.

Fermentation matters a great deal for what you experience in the glass. Temperature plays a big role: a cooler fermentation proceeds more slowly and often preserves fresh, fruity aromas, while a warmer fermentation goes faster. The type of yeast also affects how dry the wine becomes and its aroma. The bubbling carbon dioxide that is released is the reason a fermenting must hisses and foams.

A widespread misunderstanding is that all fermentation gives bubbles in the finished wine. The carbon dioxide normally escapes during fermentation, so still wines end up without fizz. In addition to alcoholic fermentation there is also malolactic fermentation, where bacteria convert malic acid into the milder lactic acid and make the wine rounder.