The first alcoholic beverage humans made probably wasn't beer. The first fermented drink of which chemical traces have been found was made from a mixture of ingredients that included rice, honey, and wild grapes. Beer has, however, become the most consumed alcoholic beverage on Earth. To get a sense of the sheer scale of brewing today, consider that annual production volumes of major breweries are reported in millions of hectoliters (100 liters each), or millions of beer barrels (117 liters each).
Most other fermented beverages—such as wine, cider, or mead—simply cannot be produced on such a scale. Barley's crop yield is unmatched among ingredients well-suited for producing alcoholic drinks. Understanding why other high-yield ingredients—such as wheat, corn, or rice—are not generally used in beer, except in combination with barley, requires understanding the basic chemistry of the malting process.
Malted barley is composed of barley grains which have been steeped in water to initiate the germination process, in which the barley seed prepares to sprout. The grains are then rapidly dried with hot air to halt the germination process before the grains actually sprout. The goal of this process is to activate certain enzymes, namely alpha- and beta-amylase, which can break down the starches—which yeast cannot ferment into alcohol—into sugars, which yeast can ferment. Barley has a much higher enzyme content (or diastatic power) than other cereal grains like corn and rice, and is therefore much more capable of converting its starch into sugar in the mashing process.
Mashing is the first stage of brewing—brewers usually leaving the malting up to the maltsters. In the mash, malted barley is introduced to water between 140 and 160 degrees farenheit, and held at this temperature for a period of time, often 60 minutes. The heat and moisture allow the enzymes present in the barley to convert its starches to sugars. Mashing is performed in a large insulated container known as a mash tun.
After draining the hot liquor from the mash tun, the grains are often rinsed, or sparged, with more hot water. The sugary malt solution, which is now called wort ("wert"), is transferred to a large kettle and brought to a boil. At this point, beer's third ingredient (after barley and water) is added.
Hops. In the western United States, they've become something of a fetish, but hops weren't a common ingredient in beer until the Middle Ages. Before then, a variety of different plants were used to flavor and preserve beer, such as sweet gale, wild rosemary and yarrow. Recently, some American brewers have again begun to produce beers using these herbs. These ales are known as gruits, but they're still quite rare—for now the hop remains king.
Hops serve two functions. First of all, they make beer taste better (i.e., bitter). The bitterness they provide balances the sweetness of the malted barley, and the spicy, floral, or fruity flavors they contribute are often pleasant, sometimes divine. Secondly, hops contain alpha acids that are toxic to gram-positive bacteria such as Lactobacillus and Pediococcus, which often spoil beer, so hopped beer is less prone to infection and the off-flavors resulting from infection.
Hops are usually added to beer during the boil. Beer—or, more precisely, wort—is boiled for a long period of time, usually 60-90 minutes, which, among other things, isomerizes the alpha acids in the hops. This is important because alpha acids do not taste bitter as they are found in nature—only isomerized alpha acids provide bitterness. Hops boiled for a longer period of time will therefore contribute more bitterness, but the long boil will also destroy much of the hops' other flavors. Brewers often distinguish between bittering hops, added early in the boil, and flavor or aroma hops, which are boiled for a shorter period of time. Adding hops anytime after the wort has cooled, a technique known as dry hopping, contributes negligible bitterness but lots of hop flavor.
Once the beer has cooled, the wort is transferred into a fermentation vessel. The brewer usually pitches a single strain of ale yeast, Saccharomyces cerevisiae, either from a lab or harvested from a previous batch of beer. Notable exceptions include lagers, which require lager yeast, Saccharomyces pastorianus, and wild beers, which make use of a variety of wild yeasts and bacteria.
Even within S. cerevisiae, there are as many different yeast strains as there are breweries, due to the speed at which yeast evolves. Yeast strains crucially differ in how much they attenuate, that is, what percentage of the malt sugars they can convert into alcohol, as well as in what flavor compounds they produce as byproducts of fermentation. Belgian ale strains, for example, tend to be highly attenuative and produce pungent flavor compounds.
The yeast convert the malt sugars into alcohol over the course of a week or two in a stage known as primary fermentation. At the end of this stage the yeast begin to flocculate, or clump together, and drop to the bottom of the fermenter. Most brewers employ at least a short conditioning period following primary fermentation to allow the beer to clear of yeast and the yeast to clean up some less pleasant byproducts of fermentation, such as diacetyl and acetaldehyde.
After conditioning, the beer is ready to be carbonated. Most commercial breweries carbonate their beers mechanically, while most homebrewers and Belgians use a process known as bottle conditioning. In this process, the brewer adds a precisely measured amount of sugar to the beer immediately before bottling. The yeast ferment the sugar in the sealed bottle, producing only a very small amount of alcohol, but enough carbon dioxide to make a crisp, fizzy beverage. Bottle conditioned beers often have a small but significant amount of yeast at the bottom, so decanting the beer carefully into a glass, leaving the yeast with the last 1/4 inch of beer, is generally recommended to preserve clarity of appearance and flavor.