Alcoholic fermentation

This natural microbiological process is the of fermentable sugars contained in grapes (glucose and fructuose) into :

• ethyl alcohol,
• carbon dioxide (CO2),
• other by-products (glycerol, etc.).

It is the result of the work of yeasts, unicellular fungi present on the grape bloom, on the material, or inoculated into the must. Alcoholic fermentation is a decisive phase in winemaking, whether we're talking about red, white, rosé, still, sparkling or semi-sparkling wines. It's the key to a cuvée's sanitary quality, as well as its aromas, taste, color, acidity, sweetness

The transformation of grape sugars into ethanol begins spontaneously when they come into contact with indigenous yeasts. This happens as soon as the grapes are harvested, when the bloom cracks under the weight of the bunches, provided the temperature is above 12°C. This is known as "spontaneous" fermentation, and is mainly due to non-Saccharomyces yeasts..

Ethyl fermentation continues in vats, where it is traditionally "guided" by the inoculation of exogenous Saccharomyces yeasts, also known as active dry yeasts (ADY). This addition takes place after vatting for red wines, and after settling for white and rosé wines.

Alcoholic fermentation lasts a total of two to three weeks. .It stops spontaneously when all the fermentable sugars have been converted into ethyl alcohol. However, it can also be stopped earlier by the winemaker, particularly for sweet or syrupy wines. This is followed by malolactic fermentation, essential for the vinification of red grape varieties. It gives the wine its suppleness, stability and roundness, thanks to the production of lactic acid from malic acid.

Successful alcoholic fermentation of a wine requires : it must remain optimal. Daily monitoring of this parameter takes several points into account:

• yeast life is permitted between 15 and 30°C,
• alcoholic fermentation is an exothermic reaction, i.e. it generates heat,
• each grape variety, and even each wine, has its own ideal fermentation temperature, which influences its organoleptic uniqueness.

La quality of winemaking equipment, such as temperature-controlled vats and cooling systems.Too high a temperature can bring fermentation to a premature halt, which is detrimental to the wine. Conversely, too low a temperature is detrimental to start-up and yeast development. It is therefore generally accepted that temperatures conducive to successful alcoholic fermentation are :

• between 25 and 30°C for red wine,
• between 15 and 22°C for white and rosé wines.

Another parameter to take into account is the proper oxygenation of the yeasts as they digest the sugars into alcohol. . This is achieved by pumping-over, the regular action of lifting the must from the bottom of the tank to the top while it ferments.

Alcoholic fermentation has been used in gastronomy (bread, wine, beer, etc.) for millennia, albeit for a long time on an empirical basis. It wasn't until the 17th century that the microbial agents involved were discovered and studied under the microscope, invented in 1677. The role of yeast and bacteria in fermentation was understood not only as a chemical reaction, but also as a biological one, thanks to the work of Pasteur in 1857.