Alcoholic fermentation

Primary fermentation transforms the natural grape sugars into alcohol. When complete, practically all of these sugars have been consumed.

Primary fermentation transforms grape juice into wine

The primary, or alcoholic, fermentation of Champagne wines is the process that transforms the grape musts into wine: the yeast consumes the natural grape sugars, producing alcohol and carbon dioxide (CO²) along with other by-products that contribute to the sensory characteristics of the wine.

Primary fermentation takes place immediately after pressing, usually in stainless tanks but occasionally in wood

A few producers still ferment their wines in oak (casks, tuns, etc) but most prefer thermostatically controlled stainless-steel vats. Capacity ranges from 50 to several hundred hectolitres and the content of each vat is carefully labelled by cru, pressing fraction, varietal and vintage.
The grape musts may also be chaptalized after racking (enriched with sugar) so as to obtain 11 per cent alcohol by volume by the end of fermentation.

Control of fermentation

Artificial yeasts (Saccharomyces cerevisiae, in dried or liquid form) are added to facilitate the control of fermentation. They work by consuming most of the sugar in the grapes, producing carbon dioxide and alcohol along with a large number of molecules (superior alcohols and esters) that have an important impact on the aromas and flavours in the wine. The entire process lasts about a fortnight, involving an exothermic (heat generating) chemical reaction that has to be carefully managed since temperatures higher than 18-20°C increase the risk of flavour evaporation and may cause the fermentation to ‘stick’ (grind to a halt).
Progress is monitored on a daily basis, checking the temperature and overall condition of the fermenting musts.