Fermentation is essentially anaerobic respiration found in microorganisms like certain Fungi and bacteria. In fermentation, the substrate for oxidation is extracellular (not present within the cell) since the organisms are basically saprophytic.
Fermentation as a process to produce alcohol was known to human beings since time immemorial, even though its scientific understanding dates back to only the middle of the 19th century. Experiments conducted by Louis Pasteur established that fermentation is carried out with the participation of living cells.
Buchner in 1897 found out that the extract of yeast cells (which does not contain living cells) can bring about fermentation of sugar solution. Subsequently it was discovered that the enzyme Zymase present in the yeast extract is capable of fermenting activity (break down of carbohydrates together with the formation of alcohol and C02).
The enzyme Zymase will be active only in the presence of phosphate. Sucrase and maltase are the other carbohydrate fermenting enzymes secreted by yeasts.
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In fermentation also, as in anaerobic respiration atmospheric oxygen does not participate. Oxidation is accomplished by the intermolecular atomic shifts, with the result the product will have less energy than the original substrate. Much of the energy released during fermentation is wasted as heat, while the remainder is used for the conduct of metabolic activities.
The end product of fermentation need not always be alcohol it may be various types of organic acids such as butyric acid, lactic acid etc. Depending on the end product, fermentation may be termed as alcoholic fermentation, lactic acid fermentation etc.
In the formula (iv) given above, for the formation of acetic acid from ethyl alcohol, 02 is necessary. Hence acetic acid fermentation is different from other types of fermentation.
Fermentation is carried out by microorganisms, a point which has been emphasized earlier. The following is a list of organisms which carry out the various types of fermentation.