This is the first stage of winemaking, a complex biochemical phenomenon which involves transforming the sugars (glucose and fructose) into ethyl alcohol (ethanol), carbon dioxide and numerous products which are known as secondary products because they are present only in small quantities.
The agents for this process are the yeasts in the grapes which feed on the sugars and change them. During alcoholic fermentation heat is also produced, but the process may stop if the temperature reaches 35?-38?C; in this situation the must becomes vulnerable to bacteria which change the sugars into mannitol, producing an undrinkable liquid.
In order to prevent this danger (mannitolic fermentation) the cellar must be aired and cold water has to be run over the fermentation vessel. Other measures maybe adopted, depending on the situation in the place concerned. The most up-to-date vessels have a double skin with an internal cavity where a cooling or heating liquid circulates, thereby allowing the temperature of the process to be controlled. In northern areas there may be the opposite risk; fermentation may not start because the temperature is too low (10?C). In this case the cellar and the must would obviously have to be heated.
The start of alcoholic fermentation is signalled by gurgling caused by carbon dioxide gas being given off. Due to this typical noise fermentation is known in the trade as boiling.