Malted barley in the context of Α-amylase

Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and fermenting the resulting sweet liquid with yeast. It may be done in a brewery by a commercial brewer, at home by a homebrewer, or communally. Brewing has taken place since around the 6th millennium BC, and archaeological evidence suggests that emerging civilisations, including ancient Egypt, China, and Mesopotamia, brewed beer. Since the nineteenth century the brewing industry has been part of most western economies.

The basic ingredients of beer are water and a fermentable starch source such as malted barley. Most beer is fermented with a brewer's yeast and flavoured with hops. Less widely used starch sources include millet, sorghum and cassava. Secondary sources (adjuncts), such as maize (corn), rice, or sugar, may also be used, sometimes to reduce cost, or to add a feature, such as adding wheat to aid in retaining the foamy head of the beer. The most common starch source is ground cereal or "grist" – the proportion of the starch or cereal ingredients in a beer recipe may be called grist, grain bill, or simply mash ingredients.

In brewing, adjuncts are unmalted grains (such as barley, wheat, maize, rice, rye, and oats) or grain products used in brewing beer which supplement the main mash ingredient (such as malted barley). This is often done with the intention of cutting costs, but sometimes also to create an additional feature, such as better foam retention, flavours or nutritional value or additives. Both solid and liquid adjuncts are commonly used.