This section is from the book "The London Medical Dictionary", by Bartholomew Parr. Also available from Amazon: London Medical Dictionary.
(From fermento, to ferment). Fermentation, ecbrasmus, brasmoa, is an intestine motion excited, with the assistance of proper heat and fluidity, between the integrant and constituent parts of farinaceous and saccharine substances, from which result new combinations of their principles. The heat required is about seventy degrees of Fahrenheit's thermometer.
The phenomena of fermentation are, however, now better understood than by the chemists of the old school; and this may, perhaps, excuse our enlarging on a subject not strictly medical, though the term is so common in medical authors; but employed without any scientific discrimination.
The subjects of fermentation, we have said, are fari-nacea and sugar; but the former are scarcely rich enough in the saccharine principle to ferment without some preparation. This consists in wetting, and in exposing the grain to a degree of warmth to excite the process of vegetation; but no new saccharine principle is added: it is merely more completely developed. Sugar must be employed in every fermentation; but it requires about four times its weight of water. With the sugar, mucilage is also requisite; an ingredient which the coarse sugar usually contains. But it is singular that a vegetable acid must also be previously contained in the substance to be fermented (Annales de Chimie, xxxvi. 23.); and this we shall find to be supplied, in beer, by the barm or yeast, and is contained naturally in all the fruits.
When these ingredients are in due proportion, and the temperature raised to nearly 70°, an intestine motion commences; the liquor becomes thick and muddy; an additional degree of heat is excited in proportion to the rapidity of the process, which sometimes rises so high as 95°, and carbonic acid gas arises. In this process the sugar disappears, and the fluid becomes clear, as well as of a less specific gravity; and, as it is styled, of a vinous taste, owing to the formation of alcohol. The other ingredients seem merely to have assisted the process, and to remain unchanged; for we still find the mucilage both in wine and beer, and the Vegetable acid in the former; though the small portion employed as a ferment in the latter seems to have escaped with the carbonic acid gas. Thus the sugar appears to be in part decomposed, and to have separated in the form of carbonic acid gas; and the other part, with a large excess of hydrogen, forms the alcohol, combined with the colouring matter, and the vegetable acid. The superfluous extractive matter, which the vinous liquor cannot dissolve, rises to the top, or sinks to the bottom, in proportion to the quantity of air entangled with it.
In this process a portion of malic acid is formed, perhaps from the tartarous; and some oxygen is seemingly evolved. It was supposed that the latter was derived from the open air; but Fabroni has informed us that fermentation proceeds with equal rapidity and success in close and in open vessels. Annales de Chi-mie, xxxi. 302.
After this active process of fermentation is at an end, it still proceeds in a more slow, often in an imperceptible, way. If wine or beer be kept in a heat, from 70 to 90 degrees, it gradually thickens, grows hot with a gently hissing noise, and filaments are observed to move in it, though previously fine. The heat and noise lessen, the filaments subside, and the liquor is again clear; but it is no longer vinous: it is acid. The result of the acetous fermentation seems not to be connected with the alcohol. Yet if the assertions of some of the older chemists, that the strongest wines, when rendered acid, afford the strongest vinegars, be true, alcohol may probably have some effect. These vinegars have not, however, been examined; and we strongly suspect that they would appear to have been impregnated with acetous ether. The extractive matter seems to be the substance which first experiences the change; for when it is carefully separated, wine will not become sour, though found by Chaptal to become acid, when vine leaves were added. (Annales de Chimie, xxxvi. 245.) It is said also, with some truth, that fermented liquors do not become acid, unless they are exposed to the atmosphere, from whence the oxygen, essential to the acidity, is absorbed. There seem, however, to be some cases, in which this exposure to the atmosphere is not necessary; for wine will become sour in well corked bottles. In general, however, no cork is sufficiently tight to prevent, after some time, the escape of alcohol; and the atmospheric air finds access by the same course; in bottles which contain acid wine, some space will always be found empty, and the acidity is in proportion to this space. The flakes are owing to the extractive matter which commenced the process; but some portion of this still remains, and the malic acid is the last to experience the change.
The last stage of this important process is putrefac-tion, in which the extractive matter is still more completely separated, and organisation wholly destroyed.
This is the common and regular process; but it is sometimes varied in almost every step. In many instances, no traces of a vinous spirit are found, but the fluid hastens rapidly to the state of vinegar. At other times putrefaction as quickly comc3 on; and, in some of the stronger wines and cyders, no art can apparently convert them to vinegar. We have exposed some of the strong Devonshire cyder for a whole summer under a sunny wall without producing vinegar. The appearance of vinegar, though it presupposes the existence of a vinous state, therefore by no means confirms it; for the vegetable acid may be produced in a variety of ways, by distilling gum, sugar, wood, or tartars, by the action of sulphuric acid in a concentrated state on these bodies, or by the spontaneous decomposition of some animal substances, particularly urine.
It has been supposed, that if gluten forms a large portion of the fermenting bodies, ammonia will appear in the product; but this has little foundation. The only varieties in the process are those mentioned, with the greater quantity or perfection of each of the results, which are subjects rather economical than chemical or medical.
We have remarked that a vegetable acid is necessary to excite fermentation; and that in the process of making beer from malt, a ferment of this kind is necessary. In different places, where the wine is of different qualities, the ferments are of course various. Thus on the Rhine, where the grapes are peculiarly-acid, they add fresh meal; the Chinese add a decoction of barley and oats; and we sometimes assist the more insensible process, after the active period is at an end, by a little wheat or barley.
The ferment usually employed for beer is barm or yeast, which Mr. Henry has imitated by the union of carbonic acid gas, with some mucilaginous substances capable of entangling the air. Good barm is, however, a very complicated mixture, containing the carbonic, mucous, acetous, 'and malic acids, alcohol, extractive matter, mucilage,sugar, gluten, and water, besides some lime and potash, with traces of silica and phosphoric acid. Mr. Westrumb, however, in Crell's Annals, has found that the only essential part is the gluten, with a vegetable acid; and it is seemingly indifferent of what kind this acid be, if it is of vegetable origin.
In the human body, by a loose analogy, fermentation has been said to take place, and all its various phenomena have been supposed to produce corresponding effects. We have thought it probable that a similar process takes place in the stomach; but there is not the slightest evidence of any fermentation in the circulating fluids. What former physicians intended by this term is rather an assimilatory process, by which, at least, the poison introduced is increased in quantity; but there is no foundation for supposing that all the fluids are thus changed. In the small pox, for instance, some change occurs; and all the matter which passes through the skin, as well as that which is stopped in its passage, and produces the pustules, will excite the disease in a person liable to it; but the effects soon cease, and all the matter thus changed is at once carried off; for neither in small pox nor measles will the blood convey the disease. In all assimilatory processes, however, there is a strange mystery which we cannot develop; and there is no little probability that the whole depends on the state of the capillaries produced by the fever. We purposely eluded this consideration under the head of Exanthemata, as we had not then matured the ideas that then occurred to us. We may resume the subject when speaking of the different individual diseases, if we can reduce our suspicions to a probable shape.