Fermentation (Lat. fervere, 'to boil'), a change brought about in organic liquids by the agency of a ferment. Ferments are of two classes, soluble and organised. (1) Soluble ferments, or zymoses.—This class is of less importance than the other. When isolated, the ferments are white, amorphous substances, freely soluble in water and in glycerine, and usually insoluble in alcohol. They are found in grain which has germinated (malt), in saliva, gastric juices, &c., and also usually occur along with organised ferments like yeast. The commonest soluble ferment is Diastase (q.v.), occurring in malt. Others of considerable importance are ptyalin, found in saliva; pepsin, in gastric juice; synaptase, in bitter almonds; myrosin, in mustard, &c. These all resemble one another very closely in properties, and all have the power to cause substances like starch and cane-sugar to combine with water, forming glucose and analogous uncrystallisable sugars, thus:
Diastase converts the starch contained in barley first into dextrinose, then into glucose or grape-sugar, thus:
The same effects can be produced by boiling these bodies with dilute acids. Soluble ferments act most rapidly at a temperature of 75° C. (167° F.), and are destroyed by boiling. Their action is impeded or entirely stopped by many substances such as borax, citric and tartaric acids, and some antiseptics; on the other hand, prussic acid, mercurial salts, alcohol, chloroform, ether, and essential oils have little effect on soluble ferments.
(2) Organised Ferments.—The chemical changes induced by these always depend on the life-process of minute organisms (see GERM THEORY) which feed upon sugar or other substances in solution, and excrete the product of the fermentation. Fermentation nearly always consists of a process of breaking down of complicated organic substances like sugar into simpler ones like alcohol and carbonic acid. Economically, alcoholic fermentation is of far greater importance than any other. It is caused by the so-called yeast-plant (Saccharomyces cerevisiae) in solutions containing uncrystallisable sugar (glucose). The sugar may be originally in various forms. In malt extract for beer it exists as glucose or dextrinose; in grape-juice for wine manufacture, chiefly as glucose; in potatoes for making potato-spirit or 'marc,' as starch; or, finally, as cane-sugar itself. The latter two, when fermented, are first converted by the soluble ferment which accompanies yeast into glucose, and that is then transformed by the yeast itself into alcohol, carbonic acid gas, and small quantities of glycerine, succinic acid, and other substances.
Brewers' yeast contains an enormous number of one-celled organisms (Saccharomyces cerevisiae), on which its activity entirely depends. Under the microscope these are seen to be colourless, nearly transparent, roughly spherical bodies of a diameter of about 0.00035 inch. When the conditions are favourable—i.e. nourishment plentiful, and temperature about 80° F., the yeast-cells multiply very rapidly by budding. When food is deficient they may multiply by the formation of spores. Fermentation of grape-juice to form wine depends on organisms of the genus Saccharomyces, of which there are many species not yet very sharply defined. The various species are found in different kinds of wine must, or even in different positions in the same fermenting-vat.
Putrefaction and rotting are very much akin to fermentation, and all depend on the presence and growth of minute organisms.
The following are some of the varieties of fer- mentation which possess considerable importance or interest:
Acetous Fermentation.—It has long been known that wine under certain circumstances absorbed oxygen from the air and turned into vinegar. This is now known to be due to the conversion of the alcohol into acetic acid by an organism, the Mycoderma aceti. The wine is allowed to trickle slowly through barrels filled with beech-shavings, through which fresh vinegar has previously been poured in order to impregnate the shavings with the 'mother of vinegar,' which contains the Mycoderma. Under the microscope the M. aceti appears as very minute elongated rods (diameter about 0.00008 inch) united into chains.
Lactic fermentation is the cause of the souring of milk. It is caused by a definite rod-like organism or bacillus, which reaches the milk from the air. The life-conditions of this ferment are almost the same as those of the yeast ferment, but there are certain substances attacked by the one and not by the other, and vice versa. During lactic fermentation the sugar of milk is converted into lactic acid, which ultimately causes the curdling of the milk.
Butyric Fermentation.—This is one of a large number of processes of fermentation usually roughly classed as putrefaction, on account of the foul smell produced during the action. It is caused by an organism (Fermentum butyricum) which has the power of breaking down such substances as sugars, starches, tartaric and citric acids, and albumenoid substances, producing butyric acid and carbonic acid.
Ammoniacal Fermentation is also one of the processes which form part of putrefaction. Most organic substances containing nitrogen yield ammonia as one of the products of putrefaction. This process is of great importance in nature, as by it nitrogenous animal matter, contained in manure, is converted into salts of ammonia, which can be absorbed by earth. They are then acted on by the nitrifying organism which is found in all fertile soils, and by it slowly oxidised into nitrates, which can then be absorbed by the roots of plants.
Conditions of Fermentation.—As all fermentation, except that produced by soluble ferments, is caused by living organisms, the optimum conditions of fermentation are (1) sufficiency of nourishment and moisture; (2) temperature about blood-heat (98° F.); (3) absence of poisonous substances. Deficiency of nourishment or water may kill a ferment, or may cause it to form spores, which then resist drying, and may even exhibit vitality after boiling, which is instantly fatal to all active ferments. A low temperature renders ferments inactive, a temperature much above 100° F. is fatal to their life. All antiseptic or disinfectant substances (see ANTISEPTICS), such as corrosive sublimate, chlorine, fumes of burning sulphur, eucalyptus-oil, act by impeding or destroying the life of the ferments. Many of the products of fermentation are themselves antiseptics, as in the case of alcohol, so that alcoholic fermentation always arrests itself when the spirit has reached a certain strength. Some ferments cannot live in presence of air, others require it; while others again are indifferent to it. For further information, see Fermentation, by Schützenberger (Inter. Sc. Series), and the article 'Fermentation' in Thorpe's Dictionary of Applied Chemistry.
FERMENTED LIQUORS are alcoholic beverages made by fermentation of saccharine fluids and juices; the principal being the different kinds of ale or beer, made by fermentation of an infusion of malt—chiefly of barley, but also sometimes of other kinds of grain—and wine, made by fermentation of grape-juice. Cider is made by fermentation of the juice of apples; perry, of that of pears; palm-wine, by fermentation of the sap of different kinds of palm. Fermented liquors, commonly called wines, are also made from the juice of various kinds of fruit, as currant-wine from that of the red currant; and from the juice of some roots, as parsnip-wine from that of the parsnip, &c. The sap of the American Aloe, or Agave (q.v.), yields the fermented liquor called Pulque, much used in Mexico. A wine is made from the sap of the birch, and that of some other trees is used for a similar purpose. Mead is a fermented liquor made from honey. From every fermented liquor a kind of spirit may be obtained by distillation. See BEER, WINE, &c.