| Sugar. | Alcohol. | Carbonic dioxide. | ||
| C6H12O6 | = | 2C2H6O | + | 2CO2 |
The bubbles which appear when cider is "working" are carbonic dioxide. In the conversion of alcohol to acetic acid, a subtance called aldehyde is first formed. The oxygen for these changes is taken from the air:
| Alcohol. | Oxygen. | Aldehyde. | Water. | |||
| C2H6O | + | O | = | C2H4O | + | H2O |
| Acetic acid. | ||||||
| C2H4O | + | O | = | C2H4O2 |
By substantially the same slow process as that still employed in the household was all vinegar obtained from the time of Moses, or earlier, down to 1822. In 1814 Berzelius had found out the chemical composition of acetic acid, and De Saussure that of alcohol; so that, after Doebereiner had discovered that a weak solution of alcohol exposed to the air in contact with platinum-black was converted to acetic acid, he was enabled to set forth the theory on which depends the modern "quick process" of vinegar-making—the method now regularly employed in the vinegar-factories of Europe and America. The essential feature of this process consists in bringing the alcoholic solution into intimate contact with the air by causing it to trickle through a mass of loose material, which effects the acetification in from twenty-four to forty-eight hours. The operation is carried on in wooden tubs, six to ten or more feet high, called generators. Around the sides of the generator, a few inches above the bottom, is a ring of air-holes. Just above the air-holes is a perforated false bottom, and from this nearly to the top the generator is filled with beech-wood shavings, which are closely curled so that they will not crush and prevent the air circulating freely through them. A few inches above the shavings is a wooden head or sieve, perforated with small holes, which serves to distribute the alcoholic liquid, or "wash," evenly over the shavings. Several air-pipes are inserted in the sieve, extending a few inches above and below it. The generator has a cover with a hole in the middle through which the wash is poured in, and the ascending current of air passes out. The vinegar-room is kept at a temperature between 70° and 90° Fahr. A high temperature and large supply of air hasten the operation, but cause loss by the evaporation of the alcohol. If the temperature falls much below 60°, the acetification stops, and putrefaction sets in; while if too little fresh air is supplied, aldehyde, the half-way product mentioned above, instead of being promptly converted to acetic acid, is evaporated and lost. The presence of aldehyde in the air may be detected by its penetrating aroma and by the eyes smarting. The wash must be passed several times through the shavings, in order to effect its complete acetification.
Two kinds of vinegar are sold by grocers in the United States for domestic use—cider-vinegar and white-wine vinegar. Both kinds are
