Definition - What does Fermentation mean?
Fermentation is a winemaking process that uses yeast to convert the sugars in grape juice to alcohol. In the winemaking process, fermentation starts during crushing and can last until after bottling. It is a necessary process in winemaking, in order to make the wine alcoholic.
Fermentation takes place in two stages - primary fermentation, which takes between 3 and 7 days and is completed in barrels; and secondary fermentation, which takes up to 2 weeks and is completed in barrels, oak or bottles depending on the style of the wine.
There are three man factors that winemakers must monitor during the fermentation process: temperature, speed and oxygen levels in the must. If any of these factors are off, it can risk stuck fermentation and potential wine faults could occur.
WineFrog explains Fermentation
Fermentation is the process that makes wine, wine. When the grapes are crushed, the natural yeasts in the grape skins are introduced to the natural sugars in the pulp, which launches fermentation. The must is then stored in large barrels to continue fermenting. For the first stage - Primary Fermentation - there is a lot of activity; the natural yeasts are busy eating the sugars and cultured yeast is added to the mix to help control the process so that a quality wine with the correct flavors is produced. The first stage creates up to 70% of the alcohol found in a wine.
The second stage - secondary fermentation - is a slower, calmer process. The sugars have almost all been consumed and the yeast is having trouble finding its food source. Over the two weeks, the remaining 30% of the alcohol is converted.
The style of wine the winemaker is hoping to create is influenced by the fermentation. If they want a still wine, they will keep fermentation in barrels, adding oak, yeast or bacteria to control the final characteristics. If they want a sparkling wine, they will induce secondary fermentation in the bottle, which traps the carbon dioxide in the wine, creating the bubbles we associate with sparkling wines.
It is important to note that "Secondary Fermentation" is not the same thing as "Second Fermentation." It is just the continuation of primary fermentation.
Winemakers also take into account temperature, which plays an important role. White wine is usually fermented at cooler temperatures (between 18-20°C/64-68°F); higher temperatures are used to bring out complexity. Red wines are fermented at higher temperatures (up to 29°C/85°F); cooler temperatures are used to bring out the fruit flavors. Any temperatures higher than 29°C/85°F could "boil off" some of the flavors or stun the yeast, making them inactivity and halting fermentation - a condition called stuck fermentation.
Other types of fermentation that do not follow the above format are Malolactic Fermentation - which uses bacteria; and Carbonic maceration - which encourages fermentation inside the grape.