Acetic acid is produced either by the partial dehydrogenation and subsequent oxidation of bodies containing its elements, or by their destructive distillation. The first is effected—by their exposure, in a finely divided state, to the action of air or atmospheric oxygen, as in the quick process of making vinegar; or—by submitting them, in combination with ferments, to contact with a free supply of atmospheric air, as in the old field process of making vinegar; or—by exposure to the direct action of chemical or mechanical oxidizing agents, as condensed air (platinum-black process), chromic and nitric acid, &c. In general, it is alcohol more or less dilute, particularly as it exists in fermented liquors, which is thus converted into acetic acid. In the second process (destructive distillation), wood is the substance usually employed, and heat is the agent which develops the acid.
The conversion of alcohol into acetic acid is not immediate and direct. The atmospheric oxygen first oxidises two atoms of its hydrogen, aldehyd and water being formed; and this aldehyd uniting with one atom of oxygen produces one molecule of ACETIC ACID. The changes are represented in the following equations:—
| Alcohol. | Oxygen. | Aldehyd. | Water. | |
| 1. | C2H6O | + O | = C2H4O | + H2O |
| Aldehyd. | Oxygen. | Acetic Acid. | |
| 2. | C2H4O | + O | = HC2H3O2 |
After the first formation of aldehyd, the two processes, unless artificially checked, go on simultaneously, as long as any undecomposed alcohol is present.
The conversion of alcohol into acetic acid, although greatly accelerated by the presence of nitrogenised organic matter (according to
Mulder, of a fungus—the Mycoderma Vini or Vinegar Plant), is rather a case of eremacausis (slow combustion) than of fermentation. Acetification effects combination, as shown by the foregoing equations, whereas fermentation resolves complex bodies into simpler ones, e.g. sugar into alcohol and carbonic anhydride. Moreover, the presence of ferments is not essential to the change, since pure alcohol becomes acetified when exposed to the oxidising agents already named.
Another remarkable distinction between acetification and fermentation is, that the former requires the continued presence of atmospheric oxygen; whilst the vinous fermentation after being once established, proceeds perfectly without it.
During the oxidation of the alcohol of vegetable solutions, some of the other organic matters present also suffer change. A white gelatinous mass (mother of vinegar)[5] is commonly deposited; but this is a secondary result of the process, and not, as formerly supposed, one essential to it. In ordinary cases acetification occurs only at or near the surface of the liquid; which accounts for the length of time required for the operation under the old process of ‘fielding,’ and the shorter time in which it is accomplished by the improved process of Mr Ham. It proceeds favorably at temperatures ranging from 60° to 90° Fahr.; and most rapidly at 95° Fahr. (Liebig). In the ‘quick process’ of making vinegar a temperature of 90° to 92° is generally aimed at; but it often rises to 100°, or even to 105°, Fahr. As the temperature falls acetification proceeds more slowly, and at 46 to 50° Fahr. it ceases altogether (Liebig).
[5] It has generally been asserted that this substance contains vibriones, and other low forms of organised life; but Mulder describes it, under the name mycoderma aceti, as a plant of the order ‘fungi.’ It is formed at the expense of the constituents of the vinegar, and often causes whole vats of it to pass into water.