Milling is followed by mashing, which involves mixing the milled malt, now referred to as grist, with heated water in a brewing vessel known as the mash tun. Mashing converts more of the starches to sugar, and removes other solubles from the grist, forming a hot porridge-like mixture called mash. The mash is next transferred to the adjacent lauter tun, where, in a process known as lautering, the sweet liquid is separated from the grain. The grain is removed as a waste product and often ends up as livestock feed for local farmers. At this point, the brewers add hops.
Hops are the conical flower of the humulus lupulus vine, grown in several regions of the world.
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.