The exact origins of beer are unclear. It is apparent, however, that beer has existed for centuries. In the 18th century B.C., owners of brewpubs in Babylonia were fined for overcharging their customers. Humans the world-over developed their own versions of beer made from available grains. Sake was developed long ago in Asia and Pre-Columbian Native Americans made beer from corn. Beer may have been important because it provided an alternative to drinking contaminated water or because it provided a palatable use for grain that had gotten wet and would otherwise have spoiled. The December 19, 1620 entry into the Mayflower journal leaves no doubt that beer was important: "We could not now take time for further search or consideration, our victuals being much spent, especially our beer."

Beers are made from corn, wheat, rice, and barley but they are classified by the type of yeast used in their production. Beers brewed in Colonial America were all ales, made with top-fermenting yeast, like those found in England. In the mid 1800s, German emigrants brought bottom-fermenting yeast to the United States and produced German lagers. The mellow lager quickly replaced ale as the beer of choice. The majority of breweries in the world still produce lagers. During the 19th century, several American beers called common beers that do no fit the ale or lager styles developed. One such example, California steam beer which originated during the 1849 California Gold Rush, uses ale fermentation and lager yeast. "Steam" refers to the high concentration CO₂ produced during secondary fermentation.

In the United States, the number of breweries reached an all-time high of 4131 in 1870—at the same time as the national prohibition movement became organized. The Prohibition Amendment made beer and all alcoholic beverages illegal in 1920.

Breweries reopened following the repeal of Prohibition in 1933. Americans drank national beers produced by a few large breweries until the 1980s when states began to allow microbreweries and brewpubs. A microbrewery, as the name suggests, produces a small quantity (under 15,000 barrels per year) of beer for commercial sale. A brewpub is a microbrewery that sells its beer on site.

A variety of flavors have been used in beer to balance the sweetness of the grains. An ancient Peruvian recipe that is still used uses strawberries for flavor. Around 300 years ago, hops (Humulus lupulus) flowers became the flavor of choice because they also contributed antibacterial action and clarifying action. The a-acids (humulone, cohumulone, and adhumulone) in hops provide the bitter flavor and large proteins settle out with tannins to clarify the beer. Other molecules in hops including trans-isohumulone are responsible for the antibacterial properties.

BEER BIOLOGY

Barley is the source of the fermentable sugars in modern beer. Malt is formed by germinating, or malting, barley seeds. The plant cells convert starch in the endosperm to fermentable sugars. Additionally, plant proteases reduce the total protein and make amino acids available for the yeast. The malt is then dried (~50°C) and roasted (~75°C).

Fermentation
The alcohol in beer is produced by yeast fermentation. The final alcohol content is determined by comparing the initial and final specific gravities. The final alcohol content in commercial beer is controlled by tax laws rather than by the yeast. Normal mixing and transferring the wort to a fermentation vessel will aerate the wort. Initially, the yeast will grow quickly as they use aerobic respiration. When the oxygen is depleted the yeast will switch to fermentation and produce ethyl alcohol.

Carbonation is added to beer by bubbling in CO₂ or by a secondary fermentation in which sugar or unfermented wort is added to the bottled beer. Commercial beer is aged several weeks and pasteurized in the bottle. Some beers are filtered rather than pasteurized to remove contaminating microbes.

Spoilage
Chlorophenols form when phenols in the wort combine with chlorine in water to give a plasticlike taste to the beer. Visible light (400-520 nm) causes the production of the skunk-smelling mercaptan from humulone. After the initial transfer of wort, aeration can result in oxidation of a variety of chemicals in the beer resulting in off-tastes.

Off-tastes and odors are produced most often by wild yeast and lactic acid bacteria (Lactobacillus and Pediococcus). Micrococcus kristinae is the only aerobic bacterium reported in beer spoilage. During the 1990s, gram-negative strictly anaerobic bacteria including Pectinatus, Selenomonas lacticifex, Zymophilus, and Megasphaera were isolated from spoiled beer. Some yeast strains produce acetaldehyde which gives a rotten-apple taste to beer. Diacetyl, dimethyl sulfide, cis-3-hexanal, and organic acids are the most frequent products of contaminating bacteria.