Acetic Fermentation is to the wine maker and wine dealer, after alcoholic fermentation, the most important.
Fermentable matter and ferment are also concerned in it, but oxygen also is necessary.
It has long been known that the alcohol contained in fermented liquids, such as wine, beer, etc., will disappear under certain circumstances, and give place to vinegar or acetic acid, and that the air, or rather its oxygen, plays a part in this reaction.
To the chemist the reaction is simple, and is formulated thus:
| Alcohol. | Water. | Acetic Acid. | |
| C₂H₆O + | O₂ = | H₂O + | C₂H₄O₂, |
or the oxidation may take place by two reactions, with the production of an intermediate product, aldehyde:
| Alcohol. | Aldehyde. | ||
| C₂H₆O + | O = | H₂O + | C₂H₄O, |
| Aldehyde. | Acetic Acid. | ||
| C₂H₄O + | O = | C₂H₄O₂ | |
According to Pasteur, the oxidation of alcohol is the consequence of the action of a ferment or cryptogam, Mycoderma aceti, and it makes its appearance on the surface of liquids, while in acetic fermentation, in the form of a continuous membrane, mother of vinegar, either wrinkled or smooth, which is generally formed of very minute elongated cells, whose greater diameter varies from .000059 to .000118 of an inch; these cells are united in chains, or in the form of curved rods. Multiplication seems to be effected by the transverse division of the fully developed cells. The conditions of nutrition are similar to those suitable to the alcoholic ferment, the hydro-carbon matter being supplied by dilute alcohol. It may, however, be supplemented by the acetic acid itself; for if the process is left too long to itself, the vinegar loses its strength by being consumed. The most favorable temperature is between 76° and 82° F.
Antiseptic agents, which arrest the development of beer yeast, act in the same manner on the Mycoderma aceti. Sulphurous acid is especially active in this manner; hence the use of the sulphur match in sulphuring wine casks.
There is another ferment, Mycoderma vini, or flowers of wine, which is found in wine and other alcoholic liquids exposed to the air when fermentation is over or has become languid, which resembles in many respects the acetic ferment. It has the power of producing alcoholic fermentation, and is supposed by some to be derived from the Saccharomyces. Like the Mycoderma aceti, it is developed on the surface of fermented alcoholic liquors, in the form of smooth or wrinkled films or membranes, but thicker and more compact. It grows with great rapidity, and it has been calculated that one cell would, in forty-eight hours, produce about 35,378 cells. These cells are of various forms, ovoid, ellipsoidal, and cylindrical, with rounded extremities. The ovoid cells have their greater diameter about .000236, and their smaller one, .000157 of an inch. The cylinders have their diameters .00047 × .000118 in. The nutritive principles are the same as those of the mother of vinegar: alcohol, salts and nitrogenous compounds. It also appears capable of utilizing for nutrition the secondary products of alcoholic fermentation, such as succinic acid and glycerine. Its development is most active between 61° and 86° F. (See [Sherry].)