FERMENTATION ( from Lat. fermentarc, to ferment, from fermentum, yeast, from to boil). A term applied to the decomposition of certain carbon compounds by the action of living organisms, either directly or by means of enzymes which they secrete. When the sub stances decomposed contain nitrogen and sulphur (e.g. proteids), ill-smelling gases may be evolved. Such fermentations are usually distinguished as putrefaction. A great variety of carbon com pounds are fermentable, but carbohydrates and proteids are most notably so. The term was first applied to the decomposition of dissolved sugars into alcohol and carbon dioxide, the latter rising in bubbles through the liquid; the production of gases, though characteristic, is not, however, es sential to fermentation.
Fermentations are often named from the prin cipal product, e.g. alcoholic, lactic, butyric, acet ic, etc. The products of fermentations are very and diverse. From carbohydrates are derived various alcohols (ethyl, propyl, butyl, and amyl alcohol, marmite, glycerin) and organic acids (butyric, lactic, acetic, propionic, citric, snecillie, valerianie, formic. carbonic). From proteids arise ammonia, amido-acids, lett ein, tyrosin, skatol, and aromatic substances, mercaptans, hydrogen sulphide, marsh gas, car bon monoxide, nitrogen, hydrogen, etc. Not all of these substances are produced by any one fermentation or from any one substance; indeed, the products vary from time to tune in the course of fermentation, since the substances already formed, or the partial exhaustion of fermentable substances of a certain kind. affect the process.
The protoplasm of all plants is capable of causing fermentation. In the higher fornis, how ever, this power is eontined ordinarily to diges tive prooesses, but extraordinary changes occur when the plant is deprived of oxygen for respira tion. Then the protoplasm scents of composing itself and thus setting free energy sullieient to tide over the period of oxygen-starva I ion. provided this does not last more than a few hours. This process has been inaptly called in tramidecular respiration. Noteworthy alcoholic ferment al ion ensues when the protoplasm of ripening frnits and seeds and the dying tissues of cut and crushed plants are deprived of oxygen, as in the making of ensilage. It has lately been shown that fermentation is in these cases often due to an enzyme. zymase, secreted by the proto plasm. The most active fermentations are pro duced by the simpler organisms, the bacteria (q.v.) and the lower forms of fungi (q.v.), espe cially the yeasts and motors. The fermenting power of these organisms is so conspicuous that they were formerly called 'organized ferments' to distinguish them from enzymes. But the causes of the fermenting power of these lower plants are really the same as those above described for the higher plants, though in the latter it is less striking, because the conditions for its exercise occur more rarely. Some of the ferment organisms are adapted to live wholly without oxygen for respira tion, securing the necessary energy by fermenting the medium in which they live; such, for example, are the butyric ferments (Bacillus amylobaeter).
Others are able to utilize free oxygen, during periods of sluggish fermentation, or to do ont it by setting up active fermentation; such are the yeasts.
Fermentations may be grouped as (1) split ting; (2) oxidative; (3) eompound.
(1) Splitting fermentations are most common; they consist in the separation of the fermentable substance into two or more products. The best known of these is the alcoholic fermentation of sugar, produced chiefly by yeasts (Saecharo myces) and muco•s. In this process, which is employed commercially on a large scale in the manufacture of beer, wine, and spirits, and in the making of bread, most of the sugar (95 per cent.) is split up into alcohol and carbon dioxide thus: The other 5 per cent. is differently fermented, the products being glycerin, succinic acid. etc. It has been recently shown that this action is due to zymase, an enzyme formed by the yeast. in which several others had previously been found. Only sugars, the number of carbon atoms in whose molecules is 3 or a multiple of 3, can be fermented in this way. Of the hexoses only font•, and those dextro rotatory, are fermentable. Cane-sugar. a disac charide, is first broken up by invertase into glu cose and fructose, which are then fermented. Different yeasts attack the sugars differently; some ferment maltose and not saccharose; some ferment fructose better than glucose, others act on it less readily. Alcoholic fermentation is stopped by the accumulation of alcohol in the fluid ; 12 per cent. retards and 14 per cent. stops action. Lactic acid fermentation is well known from causing the 'souring' of milk and fruit juices. Sauerkraut and ensilage depend for their acidity upon it. Lactic acid bacteria (especially the Bartel-him acidi•lactici) are the agents. They attack the glucose direct 1Y, and the saecharose and lactose after `inversion..' converting the former into glucose and fructose and the latter into glucose and galactose. About 83 per eent. of the sugar is converted into ethylidene lactic acid in its isomeric forms; the rest is transformed into various by-products. The presenee of 8 per cent., or even less, of free acid stops the fermenta tion. lIntyric fermentation is responsible for the aroma of butter. The products (among which are butyric arid) are numerous and diverse; the process is complex and varied in its details.
(2) Oxidative fermentations cause the forma tlou of 111•W compounds by oxidation of the old.
The most important case is presented by the transformation of ordinary alcohol into vinegar. In making malt vinegar, dilute alcohol is allowed to trickle slowly over beech shavings slimy with the acetic organisms, Bacterium acet i. By tho time the liquid reaches the bottom of the cask, the alcohol has become converted into acetic acid. The oxidation takes place in two steps repre sented, respectively, according to the following chemical equations.
1. + 0 = C11,C110 + 11,0 Alcohol Oxygen Aldehyde Water