Alcoholic fermentation of wine is a biological process, triggered with the addition of yeasts, in which the sugars in the must turn into ethyl alcohol, CO2, heat and other by-products.
Gaseous carbon dioxide (CO2) causes the solids, mainly the skins, to rise to the surface of the must. This phenomenon forms a compact layer at the top, called the cap. The cap protects the must from bacteria and oxidation.
The must can be pumped over to make the mixture in the tank consistent. This technique consists of transferring the fermenting must from the bottom to the top of the tank to soak the cap. Spraying devices can also be used to distribute it evenly over the surface and they also prevent the formation of channels that could affect the maceration process.