The wort is then sent directly to a fermenter where the yeast is pitched, or added. Fermentation, a process in which the singled-celled yeast convert sugars in the wort into carbon dioxide and alcohol, follows. Fermentation can last up to a week. This is most evident in the character of traditional styles such as Bavarian weiss bier or Belgian lambic. All beer is classified as either ale or lager, depending upon which of two types of yeast has been used in fermentation.
Ales trace their heritage to the British and Belgian brewing traditions.
Ale yeast ferments at relatively warm temperatures and at the top of the fermenting vessel. Lagers, which trace their heritage to German brewing culture, employ a yeast that works at cooler temperatures and at the bottom of the fermenting vessel. A period of conditioning follows fermentation. For lagers, this conditioning period takes place in a lager cellar, where the beer is stored at near-freezing temperatures for between four and 12 weeks, depending upon style. Ales also condition at near-freezing temperatures for one to two days.