Cereals, grains, or cereal grains are the seed products of grasses (members of the monocotyledonous families Poaceae or Gramineae) that are widely cultivated globally. The word “cereal” is derived from Ceres, the name of the Roman goddess of harvest and agriculture. The major cereal crops in order of annual production (average 2005–2007, FAO) are maize (733 MT), rice (642 MT), wheat (611 MT), barley (138 MT), sorghum (61 MT), millet (32 MT), oats (24 MT), rye (14.6 MT), triticale (12.5 MT), and fonio (0.38 MT).

The general morphology of a cereal grain consists of the husk (present in barley, absent from wheat) that covers the pericarp layer, beneath which is the aleurone layer, the embryo and the endosperm. See aleurone layer, endosperm modification, and pericarp. It is from the embryo that the roots and cotyledon of the barley plant would emerge if the barley seed were planted in the field. The husk and pericarp (to a lesser extent) protect the growing embryo during malting so as to enable easy handling during the malting process. The husk is utilized during lautering to form a filter bed to enable efficient wort separation. The role of the endosperm is to provide a source of nutrients and energy that are released by hydrolytic enzymes. In the mature barley grain the endosperm cells are essentially large sacs of starch granules and are not living. The hydrolytic enzymes are secreted initially by the scutellum and then more substantially by the aleurone layer, the outermost layer of endosperm cells that are living. The scutellum is a thin layer (from the Latin scutella meaning “small shield”) that also acts as nutrient-absorbing cells for the embryo.

Whole cereal grains are a rich source of vitamins, minerals, carbohydrates, fats and oils, and protein. A distinguishing characteristic of cereal grains is their energy storage component, starch, which makes up around 60% of grain weight in barley. See starch. It is the degradation of this starch during brewing, by the diastase enzymes (α-amylase, β-amylase, limit dextrinase, and α-glucosidase), that produces sugars that are converted by yeast into alcohol to make beer. The aleurone layer and initially the scutellum are the source tissues for many of the hydrolytic enzymes produced during germination. A notable exception is β-amylase, which is synthesized and accumulated in the grain before harvest.

Cereals can be used for brewing beer either after being malted or as unmalted starch adjunct. There are a number of reasons why malted barley is the cereal of choice for brewing beer. First, barley, after malting, has an optimal balance of enzymes to hydrolyze its polymers (i.e., starch and protein, cell-wall polysaccharide) and those from adjuncts into lower molecular weight substances (i.e., maltose, glucose, amino acids, etc.) assimilatable by yeast. Like other members of the tribe Triticeae (which includes wheat and rye), barley has very high levels of β-amylase (up to 2% of total protein) that assist with the production of wort with very high levels of the sugar maltose. Barley malt also produces cell-wall-degrading enzymes such as β-glucanase and xylanases that efficiently hydrolyze non-starch polysaccharides into smaller moieties that do not hinder brewing solid-liquid separation processes (e.g., lautering and beer filtration) and prevent the formation of undesirable hazes in beer that are related to these components. Malt also contains proteases that hydrolyze proteins into amino acids for yeast nutrition, as well as releasing and modifying other proteins so they can fulfil their enzymatic roles or fully enable their participation in the foam formation. Finally, the above combination of enzymatic processes along with the kilning regime produces a number of the desirable malty flavors and aromas typically associated with beer.

Cereals other than barley are widely used for making beer, but they are typically used in combination with barley malt, either in malted form or as a starch providing unmalted adjunct. Wheat, both malted and unmalted, is a frequently used adjunct. Wheat malt comprises approximately 50% of the grist bill in beer styles such as German Weizens; Belgian Wit or white beers use unmalted wheat. As well as impacting the flavor of these beers, the wheat also imparts the characteristic fine white protein haze that is expected in many of these beers. The addition of wheat, either as malt or unmalted adjunct, also has the reputation for improving foam stability, but this conclusion is questionable and may be based on the generally higher protein contents of wheat grain rather than any specific foam-promoting component contained in wheat. Other cereals widely used for beer production are maize (corn) and rice, which are extensively used in Asia and America. The use of rice adjunct in particular imparts a lighter color and more delicate flavor to the beers produced with it. See corn and rice.

In the last decade or so, it has been recognized that barley, like wheat, contains the gluten-like protein segments that aggravate the celiac condition in what is estimated to be up to 1% of the world’s population. To provide beer to celiac suffers, brewers have investigated the use of grists that do not contain any barley malt. This has focused attention on malting and brewing with the non-Triticeae cereals such as sorghum and millet and pseudo-cereals such as buckwheat. Although the brewing process is not as efficient as when using barley malt, acceptable commercial beers that are celiac-friendly are now increasingly becoming available. See gluten-free beer.