Fermentation is used in the brewing of beer to give beer its foaminess and also to make it alcoholic. It is a very precise process, and different beers will go through different styles of fermentation. Generally, styles of fermenting beer can be divided into two broad types based on the two major kinds of yeast that are used in brewing: top-fermenting yeast and bottom-fermenting yeast. Ale is fermented with top-fermenting yeasts and is brewed at a higher temperature.

The acidic byproducts of fermentation give tartness to wine and many foods, such as pickled cucumbers or yogurt. Another byproduct of fermentation that is useful to people is carbon dioxide. Though the gas is poisonous to people, bakers use it to force bread to rise. It is also carbon dioxide that causes champagne to be bubbly and beer to have its distinctive foamy head. It's important to recognize that the alcohol is actually the waste product of fermentation, at least from the yeast's point of view. Generally, yeast won't produce a proper alcohol when exposed to oxygen, despite the fact that the yeast prefer anaerobic processes to aerobic ones as long as there is sufficient sugar present.

However, the processes performed by yeast during fermentation could not be fully understood until the experiments of Louis Pasteur in the 1850s and 1860s. In 1857, Pasteur released the results of his experiments on lactic acid fermentation, which is a type of fermentation that commonly occurs in human muscles when the blood is unable to supply enough oxygen to the muscles during exertion. His results in that experiment proved that this sort of fermentation is caused by living things. During the 1860's, Pasteur devised an experiment in which he filled a specially-crafted bottle full of a broth that should induce fermentation.

These beers are generally made from the traditional barley, hops, and water that have been used in Europe for centuries. Some of them incorporate wheat, oats, and other milled grains. Yet there are other classes of beer that are made from the fermentation of wholly different materials. One such beer is Amiad beer, produced in Israel. It is made from the fermentation of various fruits including kiwi, pomegranate, and blackberry.

An unusual method used to flavor beers is used by Mountain Brewing Company, a beer-making outfit in Arizona. They use a wort that includes roasted chili peppers, which have a distinctive taste that remains unchanged throughout the fermentation process. Some American malt liquors actually use dextrose as a cheap adjunct, coupled with specific enzymes, to cut the costs associated with fermentation and brewing.