When vegetables and animals are deprived of life, the elements of which they are composed exert an action on each other; some of them enter into new combinations, others become entirely undecompounded, and the identity of the original substance is destroyed. Fermentation is of three kinds: first, the vinous; second, the acetous; third, the putrid. The two first kinds are peculiar to vegetable substances; the last is common both to vegetable and animal substances, though the change it indicates is, in reference to animal substances, more usually called putrefaction. Moisture, and generally access of air, are necessary to fermentation; and a warm temperature materially promotes it, while by an excess either of heat or cold it is entirely checked.
The vinous fermentation never takes place except in substances containing sugar, and it is most remarkable in those which contain the most of the saccharine principle. If a decoction of a vegetable holding much sugar in solution, or saccharine vegetable juices, or simply a mixture of sugar and water, be exposed to a heat of 70° in a vessel either uncovered, or not entirely closed, in a short time the fluid becomes very turbid, bubbles rise to the surface and break; mucilage is at the same time disengaged, part of which sinks to the bottom, and the remainder rises to the top, where, with the bubbles entangled in it, a stratum is formed, called yeast. When the quantity of the fermented fluid is considerable, the operation goes on briskly for several days, afterwards it becomes gradually more languid, but it is a considerable time before it completely ceases. A. fluid which has undergone the vinous fermentation is entirely changed in its properties; its specific gravity is diminished; its sweet taste and viscidity is gone; it becomes brisk and transparent, and has acquired a pungent spirituous flavour.
It forms beer, cyder, wine, etc. according to the substance which has furnished the saccharine juice; and from whatever it has been prepared, it affords, by distillation, a light inflammable fluid, called alcohol. From the experiments of Lavoisier, it appears that sugar is converted into alcohol by the loss of a part of its oxygen The oxygen separated is employed to form carbonic acid gas, which produces the bubbles observed an the fermenting liquor. A small quantity of yeast is always added to liquors intended to be fermented, as it materially accelerates and renders uniform this process through the whole mass of fluid.
When liquors are fermented for the use of the table, they are put into casks while the fermentation is yet active; at first the bunghole is left open, and as yeast is discharged, the barrel is filled up with a part of the fluid or wort reserved for that purpose; afterwards the vessel is closed. But if the fluid be allowed to remain a sufficient time in open vessels, the acetous fermentation comes on, which changes its taste and smell, and converts the fluid into vinegar. This change takes place most rapidly at the temperature of about 90°, and is promoted by changing the surfaces of the liquor by stirring it, or pouring it from one vessel to another. During the acetous fermentation the alcohol imbibes oxygen to a degree that converts it into an acid; and if the liquor which has undergone this process be distilled, pure vinegar, instead of ardent spirit, comes over. Simple mucilage will pass to the acetous fermentation, without being preceded by the vinous, or at least the vinous fermentation is so transient as not to be discernible.
Wines deprived of mucilage cannot be converted into vinegar.
When dead vegetables contain much saccharine matter, and the other circumstances necessary to fermentation are combined, the vinous, the acetous, and the putrid fermentation, succeed each other in regular order. When mucilage is the predominant principle of the vegetable, the acetous fermentation, above described, is the first change discoverable, the putrid follows of course, as it is always the last, but the vinous does not appear. When albumen and gluten are predominant in the vegetable matter, the putrid fermentation only is apparent. We have observed the progress of a saccharine fluid, from the vinous to the acetous fermentation; let us now trace it to the putrid. When vinegar has been completely formed, and the warmth and exposure to the air in which it was formed are still continued, it gradually becomes viscid and turbid, an offensive gas is emitted, ammonia flies off, an earthy sediment is deposited, and the remaining fluid scarcely differs from water. Such is the change produced by putrefactive fermentation in a saccharine fluid.
When moist vegetables are heaped together in considerable quantities, their putrefaction is attended with the production of considerable heat, their whole texture becomes less coherent, their colour dark, and nitrogen, hydrogen, carbonic acid, and ammoniacal gases, begin to be evolved. When the putrefactive process has advanced to this stage, the vegetable matter affords excellent manure; for it is obvious that the principles of vegetables are liberated, and are ready to nourish the seed or the root to which the manure is applied, while the warmth with which the decomposition is attended enables the seed or root more readily to receive the food thus offered. The putrefaction of animal substances goes on under the same circumstances that promote the putrefaction of vegetables - humidity, a temperature neither hot nor cold, and the access of the atmosphere; but is distinguished by a far greater noisomeness. The presence of the air is the least essential particular, for putrefaction goes on in vacuo, the air required being supplied by the decomposition of water. A very small quantity of salt hastens putrefaction, while a considerable quantity remarkably retards it, and is therefore used in the preservation of animal food.
The first indication of putrefaction in animal substances is a cadaverous odour, their substance becomes soft, pale, then green, blue, and lastly, a blackish brown; the smell at the same time becomes more nauseous and penetrating, ammoniacal gas is perceived, other gases also escape, which are of an infectious and poisonous nature; in the end, the substance loses all traces of organization, becomes dry, soft, and reduced to a state resembling that of an earth. The worms and insects generally found among putrefying substances are not produced by putrefaction, and therefore not a necessary consequence of it; life never springs but from life, and the maggots are there because the insects from which they spring, directed by instinct, have deposited their eggs among matter suitable for their food.