Vinegar, diluted impure acetic acid, in the form in which it is usually produced by the acetification of the fermented juices of fruits and other vegetable substances. The acetification of alcohol may be effected by the action of spongy platinum, by which the alcohol is dehydrated and at the same time oxidized, aldehyde being supposed to be an intermediate product. (See Fermentation, vol. vii., p. 143.) The theory of acetous fermentation is not established beyond question. Liebig regarded it as a species of eremacausis or slow oxidation; but Pasteur and others maintain that it is produced by the development of a minute fungus (see Vinegar Plant), and the latter view is the one now most generally adopted. - The manufacture of vinegar is conducted according to different processes in different countries. In Great Britain it is manufactured on a large scale by the fermentation of malt, while on the continent it is usually made from soured wines, and also, as well as in England, from soured beer and ale. The production from malt in England is by the German rapid process, in which the fermented wort is made to fall in a shower upon birch twigs or shavings, the great exposure to the air facilitating the oxidation of the alcohol.
The liquor which is drawn off at the bottom of the cask is repeatedly turned in at the top until the acetification is completed. This method has the advantage of rendering insoluble glutinous and albuminous principles, which if retained are liable to produce putrefaction. - Wine vinegar is made on a large scale at Orleans in France. Wines which are unfit for drinking are sent here from all parts of the country. In a vinegar manufactory a great number of casks are set on end in a large room, where the temperature is maintained at about 86° F. The upper head of each cask is perforated with several holes about two inches in diameter for the circulation of air, and is placed at a more than usual distance below the edge, so that the wine may be turned in. A few gallons of boiling vinegar are first turned into the cask, and in a week or eight days about one tenth as much sour wine is added, and this process is repeated every seven or eight days till the cask is full. In a fortnight or three weeks the vinegar is ready for the market.
In France and also in other countries vinegar is made by mixing wine lees with. sour wine in perforated casks, sometimes placing the mixture in coarse sacks, drawing off the liquor, and subjecting it to further acetification in other casks perforated at the top; these are exposed to the heat of the sun in summer and to that of the stove in winter. - Wine vinegar is of two kinds, white or red, according as it is prepared from white or red wine; it contains, besides acetic acid and water, some ethyle acetate, and, according to Magnes Lahens, always a little aldehyde. It also contains some salts, principally bitartrate of potash. Its specific gravity varies from 1.014 to 1.022. The malt vinegar made in Great. Britain is of four different strengths, distinguished as Ncs. 18, 20, 22, and 24. The last is the strongest, containing 5 per cent, of real acetic acid; its specific gravity is 1.019, and it is called proof vinegar. Vinegar is liable to putrefactive fermentation, which was believed by the makers to be prevented by the addition of sulphuric acid, of which they are allowed by law in England to add 1/1000 part by weight.
It is now known that this is unnecessary, but the practice is continued. - In the United States vinegar is usually made from cider, and when this is made from sweet apples and is of good strength the vinegar is often preferred to all other kinds. The cider is exposed in barrels with their bung holes open to the action of the sun in summer, or it is kept in a warm cellar. Cider vinegar usually contains, besides ethyle acetate and other ethers, more or less malic acid. - When vinegar is subjected to distillation it is deprived of its coloring and other non-volatile matters, and is known in commerce as distilled vinegar, which is always weaker than the vinegar from which it is obtained, because the boiling point of strong acetic acid is higher than that of water. Vinegar is liable to be adulterated with sulphuric and sulphurous acids, and to contain more or less of metallic acetates derived from the vessels in which it is kept. Chloride of calcium will show the presence of free sulphuric acid when boiled with vinegar, without causing the least precipitate with the minute portions of sulphates usually present.