The milled malt, called grist, is mixed with water, providing conditions in which starch, other molecules, and enzymes are dissolved and rapid enzyme action takes place. The solute-rich liquid produced in mashing is called the wort. Traditionally, mashing may be one of two distinct types. The simplest process, infusion mashing, uses a well-modified malt, two to three volumes of water per volume of grist, a single vessel (called a mash tun), and a single temperature in the range of 62 to 67 °C (144 to 153 °F). With well-modified malt, breakdown of proteins and glucans has already occurred at the malting stage, and at 65 °C (149 °F) the starch readily gelatinizes and the amylases become very active. Less-well-modified malt, however, benefits from a period of mashing at lower temperatures to permit the breakdown of proteins and glucans. This requires some form of temperature programming, which is achieved by decoction mashing. After grist is mashed in at 35 to 40 °C (95 to 105 °F), a proportion is removed, boiled, and added back. Mashing with two or three of these decoctions raises the temperature in stages to 65 °C (149 °F). The decoction process, traditional in lager brewing, uses four to six volumes of water per volume of grist and requires a second vessel called the mash cooker.

Other sources of starch that gelatinize at 55 to 65 °C (131 to 149 °F) can be mashed along with malt. Wheat flour and corn (maize) flakes may be added directly to the mash, whereas corn grits and rice grits must first be boiled in order to gelatinize. Their use requires a third vessel, the cereal cooker.

Modern mashing systems use mixed grists and mash mixers, which are efficiently stirred and temperature-programmed mashing vessels. Enzymes of bacterial and fungal origin may be added as aids. Ale and lager are mashed in the same equipment, but they require different temperature programs and grist composition. Modern breweries often practice high-gravity brewing, in which highly concentrated worts are made, fermented, and then diluted, allowing more beer to be brewed on the same equipment.

Separating the wort

The mash tun used in infusion mashing is fitted with a false base containing precisely machined slots through which the husk, preserved during milling, cannot pass. The trapped husk thus forms a filter bed that removes solids from the wort as it is drained, leaving a residue of spent grains. Wort separation takes 4 to 16 hours. For thorough extraction, the solids are sprayed, or sparged, with water at 70 °C (160 °F).

The decoction brewer transfers the mash to a separation vessel called the lauter tun, where a shallow filter bed is formed, allowing a more rapid runoff time of about 2.5 hours. Large modern breweries use either lauter tuns or special mash filters to speed up the runoff and conduct 10 or 12 mashes a day. As much as 97 percent of the soluble material is obtained, and 75 percent of this is fermentable. Wort is approximately 10 percent sugar (mainly maltose and maltotriose), and it contains amino acids, salts, vitamins, carbohydrates, and small amounts of protein.

Boiling

After separation, the wort is transferred to a vessel called the kettle or copper for boiling, which is necessary to arrest enzyme activity and to obtain the bitterness value of added hops.

Hops

Several varieties of hops (Humulus lupulus) are selected and bred for the bitter and aromatic qualities that they lend to brewing. The female flowers, or cones, produce tiny glands that contain the chemicals of value in brewing. Humulones are the chemical constituents extracted during wort boiling. One fraction of these, the α-acids, is isomerized by heat to form the related iso-α-acids, which are responsible for the characteristic bitter flavour of beer.

Traditionally, the dried hop cones are added whole to the boiling wort, but powdered compressed hops are often used because they are more efficiently extracted. In addition, the hop components may be extracted by solvents such as liquid carbon dioxide and added in this form to the wort or, after isomerization, to the finished beer.

Fermentation

In this most important stage of the brewing process, the simple sugars in wort are converted to alcohol and carbon dioxide, and green (young) beer is produced. Fermentation is carried out by yeast, which is added, or pitched, to the wort at 0.3 kilogram per hectolitre (about 0.4 ounce per gallon), yielding 10,000,000 cells per millilitre of wort.