During the alcoholic fermentation, the sugar contained in the must (grape juice) or  in the berries is consumed by yeasts and turned into alcohol. 

The sugar in the grapes is called Glucose: 

The most commonly found yeasts in wine are micro-organisms scientifically known as Saccharomyces Cerevisiae:

The alcohol produced is Ethanol:

The chemical reaction can be written as follow: C6H12O6 → 2C2H5OH + 2CO2.

It means: a molecule of glucose, when transformed by the yeasts during alcoholic fermentation produces 2 molecules of ethanol and 2 molecules of carbon dioxide. 

This simplified equation has a concrete application:  since it is simple to measure the amount of sugar in the must, it is as simple to project the percentage of alcohol in the wine after the alcoholic fermentation. This is how you can actually determine the maturity of the grapes. 

HOW DOES IT WORK IN THE WINERY?

Although fermentation is a natural process, the intervention of the winemaker during this phase is crucial. 

Where do the yeasts come from? 

• Yeasts are very sturdy micro-organism. They can be found pretty much everywhere: on the grapes in the vineyards, on the cellar equipment (press, pipes, tanks, pump…). They survive easily from one year to the other. As soon as they get in contact with water and the sugar of the grapes, they wake up, multiply and begin fermenting. 
• Yeasts are not very different from fungi, and therefore are destroyed by chemical treatments used to protect vineyards from deadly fungi: mildew and odium. This is why, vine growers who use chemical pesticides rely on Dry Selected Yeast to Supplement (yeasts that are reproduced and grown in a lab and that can be reactivated with the addition of sugar and water) to ensure the completion of the alcoholic fermentation. 

How do you trigger the alcoholic fermentation? 

The yeast start fermenting on their own, as soon as they are in contact with the sugar of the grapes. Yet, several factors increase their activity. Temperature is the key factor: most yeasts develop in warm temperatures. In general, wine makers want to postpone the beginning of the alcoholic fermentation of red wines, to allow a maceration (antocyanins that bring color in the wine are easily extracted from the skins before fermentation) and the easiest way to do it is simply to refrigerate the grapes.