Fermentation is the biochemical process in which energy is extracted from sugar without the use of oxygen. Fermentation can be performed by virtually all organisms, and some rely on it exclusively for their energy needs. Fermentation by yeast is the basis of the alcoholic beverage industry.

The chemistry of fermentation were first investigated by Louis Pasteur in 1860, who called the process la vie sans air, or life without air. Fermentation is indeed carried out without oxygen, and is therefore called an anaerobic process (an- meaning without; aero meanign air). Organisms that can either use air or not are called facultative aerobes, while those that must live without air are called obligate anaerobes. When oxygen is low, facultative aerobes, such as yeast, switch from cellular respiration, which requires oxygen, to fermentation.

In 1897, Hans and Eduard Beuchner discovered that fermentation could occur in a cell-free extract of yeast. This work...