Beer: an alcoholic beverage made by the fermentation of cereals; traditionally barley, but also maize, rice, and sorghum. The first step is the malting of barley: it is allowed to sprout, when the enzyme amylase hydrolyses some of the starch to dextrins and maltose. The sprouted (malted) barley is dried, then extracted with hot water (the process of mashing) to produce wort. After the addition of hops for flavour, the wort is allowed to ferment. Two types of yeast are used in brewing: top-fermenting yeasts, which float on the surface of the wort, and bottom or deep fermenters. Most traditional British beers (ale, bitter, stout, and porter) are brewed with top-fermenting yeasts.
(Source: Oxford Reference)