Fermentation begins as soon as yeast is added to the cooled wort. This is also the point at which the product is first called beer. It is during this stage that fermentable sugars won from the malt (maltose, maltotriose, glucose, fructose and sucrose) are metabolized into alcohol and carbon dioxide. Fermentation tanks come in many shapes and sizes, from enormous cylindroconical vessels that can look like storage silos, to five-gallon glass carboys used by homebrewers. Most breweries today use cylindroconical vessels (CCVs), which have a conical bottom and a cylindrical top. The cone's aperture is typically around 70°, an angle that will allow the yeast to flow smoothly out through the cone's apex at the end of fermentation, but is not so steep as to take up too much vertical space. CCVs can handle both fermenting and conditioning in the same tank. At the end of fermentation, the yeast and other solids have fallen to the cone's apex can be simply flushed out through a port at the apex. Open fermentation vessels are also used, often for show in brewpubs, and in Europe in wheat beer fermentation. These vessels have no tops, making it easy to harvest top-fermenting yeasts. The open tops of the vessels increase the risk of contamination, but proper cleaning procedures help to control the risk.
Fermentation tanks are typically made of stainless steel. Simple cylindrical tanks with beveled ends are arranged vertically, and conditioning tanks are usually laid out horizontally. A very few breweries still use wooden vats for fermentation but wood is difficult to keep clean and infection-free and must be repitched often, perhaps yearly. After high kräusen, the point at which fermentation is most active and copious foam is produced, a valve known in German as the spundapparat may be put on the tanks to allow the carbon dioxide produced by the yeast to naturally carbonate the beer. This bung device can regulate the pressure to produce different types of beer; greater pressure produces a more carbonated beer.