During fermentation, yeasts convert the sugar from the grapes into alcohol(ethanol) and carbon dioxide; heat (energy) is generated in the process. The duration of this process depends on several factors, such as the temperature, the type of yeast used, the size of the fermentation container and the ripeness (i.e. sugar content) of the grapes. On average, alcoholic fermentation takes about ten days - sometimes just under a week, more often two to three weeks. In rare exceptional cases, fermentation can even last several months.
As mentioned, the winemaker can actively influence the course and duration of fermentation, for example by choosing the yeasts and controlling the temperature:
Yeasts are already present in the must or mash in natural form, provided they have not been killed by sulphurisation. If these natural yeasts are used to ferment the must or mash, this is called Spontaneous fermentation. However, this type of fermentation has uncertainties because the natural yeasts may not be strong enough to complete the fermentation completely or may produce undesirable chemical compounds that can affect the aroma of the wine or even pose health risks. Therefore, the natural yeasts are often killed by sulphurisation in order to subsequently carry out the fermentation in a controlled manner with pure-breeding yeasts.
Pure-breeding yeasts have certain properties which - because they have been specially bred - are already known in advance. This means that they can be specifically selected and used for the fermentation of the must or mash. With appropriately bred yeasts, the aromatic character of the wine can also be influenced to a great extent. The addition of fermentation aids and yeast nutrient salts can support the work of the pure-bred yeasts.
An important factor in fermentation is temperature. Traditionally, the fermentation temperature was more or less self-regulating and depended on the cellar temperature. In modern cellars, the fermentation tanks can be cooled and the fermentation temperature thus actively controlled: the lower the temperature, the slower the fermentation process(cold fermentation). The size of the fermentation tanks also affects the fermentation temperature: the smaller the tanks, the lower the fermentation temperature. The fermentation temperature and the associated fermentation time have a great influence on the style of the wine.
The yeasts usually work until either the sugar present in the grapes (or added) is completely converted into alcohol or the alcohol content becomes so high that the yeasts stop working. They then become sluggish and die off, as they cannot survive in an excessively alcoholic environment. Among the pure-breeding yeasts, however, there are particularly efficient types that can easily reach 16 percent by volume.
If the winemaker wants a residual sweetness Wine, he can stop fermentation prematurely. This can be done by cooling or by adding sulphur. A third variant is to leave the wine to ferment fully and then add sweet reserve to subsequently increase the sugar content of the wine again. The sweet reserve is unfermented (i.e. sweet) grape must that has been protected from undesired fermentation by sterilisation.