Vinegar as used as an article of food is the pm duct of a process of fermentation in which a liquid of low alcoholic content is changed to a dilute solution of acetic acid, together with certain com pounds which give a fruity ethereal odor or "bou quet." This substance has been known for a very long time, as is not strange when it is noted that the change goes on in nature, entirely without man's intervention, if the juices of sweet fruits are exposed to the activity of numerous micro organisms which are abundant in the soil and on the surfaces of the fruits themselves.
For certain uses, or when only the acidity char acteristic of the acetic acid is desired, "vinegar essence," containing high percentages of acetic acid in a relatively pure state, may be made from certain kinds of wood by a process of distillation. Undoubtedly much of the cheaper grades of vinegar for table use has its origin in this way. It is cheaper than the production of the acetic acid by fermenta tion. By a proper admix ture of ethers and flavor giving bodies a solution may be made which simu lates the product of the fermentation process, but never has the "bouquet" and the fine quality which characterizes the latter kind.
Fermentation vinegar.
Fermentation vinegar, or that properly used as a condiment, may be prepared from numerous kinds of alcoholic solutions, but especially from cider, wine or beer, through the agency of a class of bacteria generally known as the acetic bacteria. These little organisms have the power, under proper conditions of temperature and aeration, of oxidizing the alcohol to acetic acid and water in accordance with the chemical equation Probably an intermediate substance, aldehyde, is formed sometimes, although it is not certain that this is always the case.
In order to have this reaction proceed it is necessary to have (1) a lively and suitable micro organism ; (2) solutions of relatively weak alcohol, as the organisms are poisoned by amounts much over 10 per cent, and, indeed, will not work rapidly in solutions approaching this concentration ; (3) an abundance of air ; and (4) a well-regulated and favorable temperature.
The acetic group of bacteria comprises a number of species, perhaps twenty of which have been iso lated and described, all characterized by their power of oxidizing alcohol to acetic acid almost in accor dance with the chemical equation given above. They are also to be recognized by the fact that they require air for development and form large masses or scums of gelatinous character (zooglcea), the so called "mother of vinegar." The formation of these
masses is progressive, and goes on so long as the food and other conditions remain suitable for the organisms. The cell wall of each individual swells to a large size and becomes practically fused with the cell wall of its neighbor, until huge masses of jelly-like consistency, and containing millions of bacterial cells, are produced.
The upper temperature limit of growth of the organisms is about 42° C., the lower limit about to 6° C., while the action is manifested most strongly at about 34° C., a fact that is of great importance in the production of vinegar.
Methods of making vinegar.
Two distinct methods of vinegar manufacture have been developed. One of these is practically an imitation of what might be called the natural acetic fermentation, while the other is a fermenta tion carried out under forced draught. The former is generally called the French or Orleans method because it was and still is used in making vinegar from wine ; while the latter is known as the "quick process" or the German process.
The custom prevailing among farmers in this country is, in many respects, similar to the Orleans method. It is well known that if a barrel of cider be freely opened so that air comes in intimate con tact with the cider it " turns," especially if kept at a moderately warm temperature. The explana tion of this is that the organisms, which were present in large numbers on the skins of the fruit, gain entrance to the cider, but so long as there is no free access of air they develop but slowly, if at all. Given access to air and a favorable tempera ture, they immediately begin the oxidation of the alcohol to acetic acid, and the cider turns slowly to vinegar.
In the Orleans method this process is varied somewhat. Vats or barrels having free access of air are filled about a quarter full of good vinegar. This supplies the " culture." An equal amount of wine is then added and the alcohol oxidized. At the end of a few days another quantity of wine is added and finally a third. The vat is now full, and after the oxidation of the alcohol has become essen tially complete, three-quarters of the vinegar is removed and the process re peated over and over. By this method ex cellent vine gar may be made, but with con siderable expenditure of time.