Brewing is the production of beer by steeping malt (most commonly, 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 by a variety of traditional methods, such as communally by the indigenous peoples in Brazil when making cauim. Brewing has taken place since around the sixth millennium BC, and archaeological evidence suggests that emerging civilizations, including ancient Egypt and Mesopotamia, brewed beer. Since the nineteenth century, the brewing industry has been part of most western economies.
Steps in the brewing process include milling, mashing, lautering, boiling, fermenting, conditioning, and filtering, each of which is detailed below. The final step is packaging, which is putting the beer into the containers in which it will leave the brewery, such as bottles, cans, kegs, or casks.
Milling is the act of crushing the grain kernels to separate the husk from the seed, which contains the majority of the carbohydrates and sugars. This makes it easier to extract the sugars during mashing. Care must be taken when milling to ensure that the starch reserves are sufficiently milled without damaging the husk and providing coarse enough grist that a good filter bed can be formed during lautering.
Mashing is the process of combining a mix of milled grain, known as the "grain bill", and water, then heating this mixture in a vessel called a "mash tun". Mashing is a form of steeping and defines the act of brewing, such as with making beer, tea, sake, and soy sauce. Technically, wine, cider, and mead are not brewed but rather vinified, as there is no steeping process involving solids. Mashing allows the enzymes in the malt to break down the grain's starch into sugars, resulting in a malty liquid called wort. There are two main methods: infusion mashing, in which the grains are heated in one vessel, and decoction mashing, in which a portion of the grains are boiled and returned to the mash, which raises the temperature.
Lautering is the separation of the wort from the grains. This is done either in a mash tun outfitted with a false bottom, in a "lauter tun", or in a mash filter. Most separation processes have two stages: first wort run-off, during which the extract is separated in an undiluted state from the spent grains, and sparging, in which extract that remains with the grains is rinsed off with hot water. The lauter tun is a tank with holes in the bottom that are small enough to hold back the large bits of grist and husks. The bed of grist that settles on it is the actual filter. Some lauter tuns have rotating rakes or knives to cut into the bed of grist to maintain good flow. The knives can be turned so that they push the grain, a feature used to drive the spent grain out of the vessel. The mash filter is a plate-and-frame filter. The empty frames contain the mash, including the spent grains, and have a capacity of around one hectoliter. The plates contain a support structure for the filter cloth. The plates, frames, and filter cloths are arranged in a carrier frame, like so: frame, cloth, plate, cloth, with plates at each end of the structure. Newer mash filters have bladders that can press the liquid out of the grains between spargings.
After mashing, the wort is boiled with hops (and other flavorings, if used) in a large tank known as a copper or brew kettle. Historically, the mash vessel was used and is still used in some small breweries. The boiling process is where chemical and technical reactions take place, including sterilization of the wort to remove unwanted bacteria, the release of hop flavors, bitterness, and aroma compounds, the stopping of enzymatic processes, precipitation of proteins, and concentration of the wort. Finally, the vapors produced during the boil volatilize off-flavors, including dimethyl-sulfide precursors. The boil is conducted so that it is even and intense, a continuous "rolling boil". On average, the boil lasts between 45 and 90 minutes, depending on its intensity, the schedule of hop addition, and volume of water the brewer expects to evaporate.
At the end of the boil, solid particles in the hopped wort are separated, usually in a vessel called a whirlpool or settling tank. This is done by swirling the wort, causing the centripetal force to push the denser solids, known as "trub", into a cone at the center of the bottom of the tank, where it can be easily removed. After removing the trub, the wort must be brought down to fermentation temperatures (68–78°F or 20–26°C) before yeast is added. In modern breweries, this is achieved with a plate heat exchanger. A plate heat exchanger has many ridged plates, which form two separate paths. The wort is pumped into the heat exchanger and goes through every other gap between the plates. The cooling medium, usually water, goes through the other gaps. The ridges in the plates ensure turbulent flow. The last few plates often use a cooling medium that can be cooled to below the freezing point, which allows a finer control over the wort-out temperature and enables cooling to around 50°F (10°C). After cooling, oxygen is often dissolved into the wort to revitalize the yeast and aid its reproduction.
Fermentation takes place in vessels which come in various forms, from enormous, cylindroconical vessels, to open-stone vessels, to wooden vats. After the wort is cooled and aerated, yeast is added, and it begins to ferment. During this stage, sugars from the malt are converted into alcohol and carbon dioxide, and the product can be called beer for the first time.
There are three main fermentation methods: warm (ales), cool (lagers), and wild or spontaneous (lambics). Fermentation may take place in open or closed vessels. There may be a secondary fermentation which can take place in the brewery, in the cask, or in the bottle.
After primary fermentation, beer is conditioned, matured, or aged in one of several ways, which can take from 2 to 4 weeks, several months, or several years, depending on the brewer's intention for the beer. The beer is usually transferred into a second container, so that it is no longer exposed to the dead yeast and other debris that have settled to the bottom of the primary fermenter. This prevents the formation of unwanted flavors.
The various methods of conditioning include:
- Kräusening: a conditioning method in which fermenting wort is added so that the active yeast will restart fermentation in the finished beer, thus introducing fresh carbon dioxide
- Lagering: a process of storing, conditioning, maturing, or aging a beer at a low temperature for a long period of time while still on the yeast
- Secondary fermentation: most of the remaining yeast will settle to the bottom of the second fermenter, yielding a less hazy product
- Bottle conditioning: a method in which a fermentation happens in the bottle, which generates CO2 that is trapped in the bottle, giving natural carbonation
- Cask conditioning: unfiltered and unpasteurised beer that is conditioned (including secondary fermentation) and served from a cask, either from a cellar by a pump or from a tap by gravity.
Not all beer is filtered, but for those that are, filtering stabilizes the flavor and gives beer its polished shine and brilliance. Filters range from rough filters that remove much of the yeast and any solids (hops, grain particles, etc.) left in the beer, to filters tight enough to strain color and body from the beer. Filtration ratings are divided into rough, fine, and sterile. Rough filtration leaves some cloudiness in the beer, but it is noticeably clearer than unfiltered beer. Fine filtration removes almost all cloudiness. Sterile filtration removes almost all microorganisms.