La fermentation (document en anglais)

It is through alcoholic fermentation that grape juice becomes wine. This fermentation is the conversion of natural sugars (glucose and fructose) into alcohol, under the action of yeasts (tiny microorganisms). Carbon dioxide is also produced. A second (malolactic) fermentation, which occurs later, gives the wine softness.

Alcoholic fermentation

Fermentation is both a complex chemical reaction and a totally natural process. The grapes ferment once the skin of the berries is broken: the sugars inside the ripe fruit then come into contact with yeasts present on the thin film that covers each berry, and fermentation begins. The wine producer simply provides the container (a vat) that contains the juice and crushes the grapes.

The process. Under the action of the yeasts, which consist of very small organisms, fermentation produces carbon dioxide initially and then ethanol, which is the alcohol in wine. Other substances result from this process: there is glycerol, which makes the wine rich; esters or aromatic compounds; higher alcohols used as flavor carriers; aldehydes; and acids. By-products from the fermentation contribute significantly to the flavor of the wine and are particularly responsible for so-called “secondary” aromas

Young wine

When the yeast has converted all of the sugar into alcohol, fermentation stops. Sometimes the sugar content is so high that the alcohol reaches a level that inhibits the action of the yeast: it produces a powerful yet sweet wine, still containing residual (unfermented) sugar. Also, if the ambient temperature is insufficient, the yeast may stop working before the sugar has been transformed: the wine will thus have an alcohol content lower than would have been possible in terms of the maturity of the grapes. Alcoholic fermentation usually ends after one to three weeks. The young wine is then very cloudy, because of the lees (dead yeast cells), pulp, and seeds in the vat.

Yeasts

Fermentation is caused by the action of yeasts present on the skin of the grapes or in the air, or added to the vat. Among the former two, only the most widespread variety, Saccharomyces cerevisia, can react with the grape sugars to produce alcohol. Each kind of yeast has special features as regards the production of alcohol, functional temperature range, the formation of flavoring substances, the rate of fermentation, etc.

Fermentation is different for red and white wines. For red wines, the crushed grapes and juice are put into a vat and we wait for fermentation to start. The length of time this takes is variable. For white wine, the grapes are first pressed to extract the juice, which is then fermented on its own in vats or barrels. Hence, the difference in flavor and structure between reds and whites. Only red wines are fermented with their skins and seeds; these pass on not only color but also tannins and additional ingredients that provide flavor. After fermentation comes the maturation phase.

Chaptalization and reverse osmosis.

Described in 1801 by Jean-Antoine Chaptal in his Treatise on making wines, chaptalization consists of adding sugar to a must to increase the alcoholic strength. By adding 2.27 oz of sugar per gallon of must (17g/l), an extra percent of alcohol is produced. Enrichment with sugar is carefully controlled in

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