FERMENTATION

A form of respiration which requires no oxygen. There is an incomplete breakdown of food; carbon dioxide and other products, such as alcohol, are formed. The word is commonly used to refer to the conversion of sugars (and sugars derived from starch) into ethyl alcohol by the enzymes of yeast.

The process of fermentation has been used from prehistoric times in the preparation of foods and beverages, but the causative agents of fermentation were not recognized until the middle of the nineteenth century.

The end-products resulting from the natural fermentation of glucose, namely, alcohol and carbon dioxide, were identified by Gay-Lussac in 1810, but it was thought that this process resulted from contact catalysis and the decay of animal or vegetable materials. This explanation was refuted by the work of Pasteur (1857) on the lactic acid fermentation. In the course of this investigation, Pasteur determined that fermentation was caused by living cells, that different microbial species caused different fermentations, that the nitrogenous materials present served only to support the growth of the cells, that lactic acid was produced when cells (removed from the fermentation mixture) were added to a sugar solution, and that the natural fermentation yielded both alcohol and lactic acid, but that the amount of each could be altered by changes in pH.

In later studies, Pasteur showed that the conversion of glucose to alcohol, C6 H 12 0 6 - 7 2C02 + 2C 2H50H, was caused by yeast cells growing under anaerobic conditions, thus leading to the definition that fermentation was "life without air." A more modern definition of fermentation would be those energy-yielding reactions in which organic compounds act as both oxidizable substrates and oxidizing agents. Anaerobic reactions in which inorganic compounds are utilized as electron acceptors may be termed "anaerobic respirations," whereas reactions in which oxygen serves as a terminal electron acceptor are respirations.

Almost any organic compound may be fermented provided it is neither too oxidized nor too reduced, since it must function as both electron donor and electron acceptor. In some fermentations, a compound is degraded via a series of reactions in which intermediates in the sequence act as electron donors and acceptors; in others, one molecule of the substrate may be oxidized while another molecule is reduced, or two different organic compounds may be degraded after a coupled oxidation-reduction reaction. These fermentations provide energy required for the growth of a variety of cells. In addition, many microorganisms can carry out, in appropriate conditions, a number of fermentative reactions (e.g., oxidations, reduction, cleavages) which do not yield useful energy, or do not yield sufficient energy for growth.

In view of the great variety of different compounds which may be fermented and the enzymatic capabilities of different microorganisms, it is not surprising that numerous compounds important in industry (e.g., ethyl alcohol, butyl alcohol, acetone, 2,3-butylene glycol), in the production, preservation, and seasoning of food (e.g., lactic, citric, and glutamic acids), and in medicine (e.g., vitamins are extracted from the yeast carrying out the alcoholic fermentation) may be produced most cheaply through microbial fermentations. In addition, fermentations continue to be important in the production offoods (e.g., the lactic and propionic acid fermentations in the making of cheeses), beverages (e.g., the alcoholic fermentations in the making of wine and beer), and in the leavening of breads (by the carbon dioxide produced in the equation previously given).

It should be pointed out that fermentation, while sometimes requiring the least expensive processing equipment to handle, and often fairly low-cost raw materials, is not always the most economic route. At one time, nearly all industrial alcohol (ethyl) was produced via fermentation. Currently, most ethanol is prepared by the direct hydration (vapor phase, catalytic) addition of water to ethylene; or by the indirect hydration of ethanol via the sulfation-hydrolysis process. However, with conservation measures possibly affecting the availability of hydrocarbons for chemicals (ethylene is produced by thermally cracking hydrocarbon feedstocks), interest in fermentation of agricultural feedstocks may return.

In addition to alcoholic fermentation, there are hundreds of other types offermentative processes. Some of these include:

Amolytic fermentation- the fermentation of starch, but specifically it is an incomplete fermentation of starch in which simple sugars are not produced.

Butyric Fermentation- in which butyric acid is produced. The organisms producing this type of fermentation are mainly anaerobic like Clostridium butyricum. Some organisms, such as Clostridium tetani, the organism causing tetanus, and Clostridium botulinum, the organism causing botulism, also produce this type of fermentation.

Lactic Fermentation- in which lactic acid is produced. This is an important fermentation for the preservation of food. Lactobacillus bulgaricus, L. casei, and Streptococcus lac tis are used for the manufacture of dairy products, such as sour cream. Lactobacillus plantarum is used in the preservation of certain vegetables, such as the production of pickles and kraut.

Controlled Oxidative Fermentations- by which a number of industrial chemicals are produced. Citromyces, for example, can be used for the production of citric acid from sugar. Aspergillus niger will yield oxalic acid by partial oxidative fermentation, but if the mold is permitted to remain in contact with the acid, it will convert it to carbon dioxide.

Some sugars such as glucose may be completely oxidized to carbon dioxide by certain bacteria, most molds, and some yeasts. Such microorganisms produce complete oxidation by fermentation. Many bacteria and yeasts are able to produce a gassy fermentation. The gaseous end product in the fermentation of vegetable products with Leuconostoc mesenteroides and Lactobacillus brevis is carbon dioxide. The gaseous end products of the coliform group are carbon dioxide and hydrogen.

Ropy Fermentation- which causes the spoilage of foods. Ropy milk is caused by Aerobacter aerogenes, Lactobacillus bulgaricus, L. casei, and Alcaligenes viscosus. Ropy bread is caused by members of the Bacillus mesentericus group which are identical with or are strains of B. subtilis or B. pumilus. Rope in maple syrup is produced by A. aerogenes.

A characteristic cultural reaction of Clostridium perfringens and many other clostridia is known as a stormy fermentation. When the organism is inoculated into milk the lactose is fermented and the casein is coagulated.

The anaerobic respiration which takes place in the muscles of higher animals, when insufficient oxygen is available for a complete breakdown of the food, is also called fermentation. Lactic acid and carbon dioxide are the products of this type of fermentation.

In "Van Nostrand's Scientific Encyclopedia", eight edition, editor Douglas M. Considine, P.E, Springer Science + Business Media, LLC, New York, USA, 1995,excerpt p. 1211. Digitalized, adapted and illustrated to be posted by Leopoldo Costa.
p.1211

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