Fermentation is the transformation of carbohydrates into alcohol, usually with the aid of yeast. It happens during bread making, pickling and beer and wine making. The characteristics of fermentation described here refer to wine making, however they are common in all types of fermentation, the conversion of the sugars, a rise in temperature, bubbling and the removal of sediment.
During fermentation, the two types of sugar present in the grapes, fructose and glucose, transform into alcohol. Yeast is responsible for this conversion, and it is naturally present in grapes, however winemakers will usually add wine yeast to the liquid to help the process along. During the conversion to alcohol, the liquid develops its aroma and flavor as well as producing carbon dioxide. This part of the process is called primary fermentation.
Fermentation causes the temperature of the liquid and its container to rise, therefore the temperature of the room it is kept in needs to be monitored. White wine fermentation needs a temperature range of 55 to 65 degrees Fahrenheit, while red wine requires roughly 80 degrees Fahrenheit. Higher temperatures will result in loss of aroma.
As carbon dioxide is a byproduct of fermentation, the liquid will "bubble" during the process. This is particularly evident during racking, when the wine is transferred from one recipient to another with a clear hose pipe to remove sediment. When the bubbling stops, secondary fermentation is complete.
Sediment, also called "lees," is produced during wine fermentation, which can leave behind unpleasant flavors. Racking the wine allows air to agitate the wine and remove sediment, which tastes a bit like yeast. After racking the wine should be left for at least six months to a year before bottling and consuming.