Fermentation is the process that converts grape juice into wine. Through the interaction of yeast with the sugars in the juice, ethanol (alcohol) is produced and carbon dioxide is liberated. A number of critical variables have to be taken into account in producing good wine, including the temperature, sugar content, oxygen (air) in the must (fermenting juice) and yeast.
In order to make wine, fermentation can be carried out in different types of vat, eg stainless steel tanks, lined concrete vessels, barrels or even a glass bottle as is the case for producing sparkling wines.
Either natural (sometimes called ambient / wild) yeasts or cultured yeasts may be used to ferment grape juice. Natural yeasts are present in the wine cellars, vineyards and on the grapes themselves (which is why they sometimes have a ‘dusty’ appearance on the outside of the skins. Wild natural yeasts can contribute both positively to the taste of a wine and also negatively as they are unpredictable in their performance. Cultured yeasts, containing many yeast strains, are commonly used to provide more uniform fermentation and control adverse side reactions.
Secondary or malolactic fermentation also occurs in wines some time after the main primary process and is most commonly used in red wine production to convert tart malic acid to softer lactic acid.