Fermentation is a process of energy production in a cell in an anaerobic environment (with no oxygen present). In common usage fermentation is a type of anaerobic respiration, however a more strict definition exists which defines fermentation as respiration in an anaerobic environment with no external electron acceptor.
Sugars are the common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Yeast famously carries out fermentation in the production of ethanol in beers, wines and other alcoholic drinks, along with the production of large quantities of carbon dioxide. Anaerobic respiration in mammalian muscle under periods of intense exercise (which has no external electron acceptor) is, under the strict definition, a type of fermentation.
French chemist Louis Pasteur was the first zymologist, when in 1857 he connected yeast to fermentation. Pasteur originally defined fermentation as respiration without air.
Pasteur performed careful research and concluded, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells.... If asked, in what consists the chemical act whereby the sugar is decomposed ... I am completely ignorant of it.".
The German Eduard Buchner, winner of the 1907 Nobel Prize in chemistry, later determined that fermentation was actually caused by a yeast secretion that he termed zymase.
The research efforts undertaken by the Danish Carlsberg scientists greatly accelerated the gain of knowledge about yeast and brewing. The Carlsberg scientists are generally acknowledged with jump-starting the entire field of molecular biology
The reaction of fermentation differs according to the sugar being used and the product produced. Below the sugar will be glucose (C6H12O6) the simplest sugar, and the product will be ethanol (2C2H5OH). This is one of the fermentation reactions carried out by yeast, and is used in food production.
C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ/mol)