Fermentation

The fermentation process, in simple definition, is a microbiological conversion of the sugars in grape juice into ethanol and carbon dioxide by the means of glycolysis. This conversion is carried out by the yeast Saccharomyces cerevisiae. In the glycolysis reaction, one molecule of glucose or fructose is converted into two molecules of ethanol and two molecules of carbon dioxide. These series of reactions nets two molecules of adenosine triphosphate (ATP) for the yeast cell, which is the molecule of high energy potential it uses to perform the work required to maintain life functions.

Fermentation, from the winemaker’s point of view, is the period of processing when the raw materials of grapes are turned into young wine. The process requires managing the logistics of harvesting the grapes, processing the grapes by sorting, destemming, and crushing, and making the required decisions of style in how the wine will be produced.

Winemaker’s must choose what type of container in which they want the fermentation to occur. Red wines are typically fermented in closed or open-top tanks. White wines are fermented in closed-top tanks or barrels. The juice is analyzed for its key physical properties, and a determination is made on what additions the juice might require, then the yeast that will perform the fermentation is chosen. Red wines will also require a cap management strategy to regularly break-up the layer of skins that forms on the surface and submerge is back into the juice.

Daily monitoring of the fermentation is performed to determine the utilization of sugar by the yeast, and as quality control to make sure no problematic fermentations or spoilage develop.