Where the malt contributes color, sweetness and body to the finished beer, the hops contribute flavors perceived as bitter, as well as floral or spicy aromas. After boiling, the wort is transferred to another vessel known as the whirlpool, where centripetal force removes unwanted proteins, hop leaves and other particulate matter. The wort is rapidly cooled as it passes through a heat exchanger and is oxygenated. 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.