The four standard beer ingredients are malted barley (although other grains are frequently used in mass-produced American beers), hops, yeast, and water.
Malted barley is produced by germinating barley kernels, which causes enzymes in the seeds to start breaking down the hard starch molecules into smaller, soluble starch molecules, then kilning (drying). Diatases, enzymes that are capable of breaking down these soluble starches, are also produced. Kilning time and temperature (120 to 220 F) are varied to produce malts ranging from a light brown to black, with a corresponding variation in toasted flavor.
The malted barley is then crushed or milled. Heating it in a temperature-controlled water solution (140 to 160 F) causes the diatases to further break down the starch into sugars and dextrins during the process known as mashing. Homebrewers can perform mashing themselves or skip this step by purchasing malt extract, which is a thick syrup or powder.
(Carbohydrates can be thought of as chains of simple sugars. Simple sugars, or monosaccharides, have the chemical composition C6H12O6 with various arrangements of the atoms. Glucose and frucose are simple sugars. Disaccharides are two simple sugars put together, with the loss of a H20 molecule where the two are joined. Examples are sucrose, lactose, and maltose, which is the main constituent of malt extract. Trisaccharides are chains of three simple sugars, such as maltotriose. Higher sugars contain are few more simples sugars, while short chains are referred to as dextrins and more complex carbohydrates are starches. Malt extract contains approximately the following carbohydrates: 1% fructose, 10% glucose, 4% sucrose, 50% maltose, 15% maltotriose, and 20% higher sugars and dextrins.)
The malt extract is boiled for about an hour. The boil destroys remaining enzymes and microoganisms present, clarifies the final beer by coagulating proteins so that they settle out, and, most importantly, extracts flavors from the hops. Hops are added at various points during the boil: Hops added early in the boil, known as bittering hops, will have most of their volatile aroma molecules driven off, but more of their "alpha acids" will be dissolved, giving the beer bitterness. Hops added late in the boil instead contribute more aroma than bitterness and are known as flavoring hops. In some beers in which even more hop flavor is desired, hops are added as the beer is fermenting in the act of "dry hopping." Different strains of hops, which vary in both alpha acid concentration and aroma, may be used for bittering and flavoring.
Specialty malts can added before the boil or near the end of the boil. This is crushed barley that has had various types of preparation, such as roasting and various degrees of mashing. These malts can be used to affect desired properties in the beer, such as color, flavor, body, "mouth feel," and head retention.
The boiled malt-hop mixture is known as "wort" (a German word that is usually pronounced in the U.S. as "wert"). When it has cooled, the wort is "sparged", or stirred virgorously to dissolve oxygen in it. Yeast in then added, or "pitched," and will use this oxygen at the beginning of its life cycle.
Brewing yeasts are broadly divided into two categories, ale yeasts and lager yeast. (Wild yeasts also exist.) Each has many strains available commercially. Ale yeasts have been in use longer, can be used at higher temperatures (in the 55-70 F range), and sit at the topr of the wort during fermentation. They also produce organic fermentation byproducts other than ethanol, such as higher alcohols and esters, that give ales their flavor. In contrast, lager yeasts were developed to give lagers a cleaner taste. The yeast cells usually do their work at the bottom of the wort, and work best at cooler temperatures (40-55 F range).
Before the yeast is pitched, the wort is placed in a container with an airlock on top, which allows gases to escape, but doesn't allow contaminants to get in. The yeast cells multiply exponentially, then start eating away at the sugars in the wort, creating ethyl alchohol and carbon dioxide. This is fermentation. As sugars are used up and alcohol is created, the density of the wort drops. This is monitored by making hydrometer readings daily. Depending on the temperature, the initial "gravity" reading, the yeast strain, etc., the wort will be done fermenting in about seven days. What now remains is water, ethyl alcohol, unfermentable carbohydrates (carbos more complex than trisaccarides are generally not fermentable by brewing yeasts), hop acids and aromatics, a few proteins, and many other organic chemicals. This is basically flat beer.
Commercial breweries usually inject carbon dioxide into this flat beer, then bottle it. Homebrewers, lacking such equipment, typically add a controlled amount of corn sugar (dextrose), bottle immediately, then wait about two weeks for the small amount of yeast remaining in solution to ferment this sugar and create the desired amount of carbonation.
Then it's time for tasting. Click here for "A Brief Treatise on Beer Flavor."