T h e "quick pro cess" is based on the facts previously noted,— namely, the rapid oxidation at the optimum temperature of 34° C. and neces sity for large amount of air. In the " quick process" large tanks, technically known as "generators," are em ployed. These are in the form of truncated cones, six to twenty feet high, with a false bottom near the lower end and a perforated horizontal disk or false head near the upper end. The space between is filled with some substance which is without action on either solution or bacteria and which will supply a large amount of surface to the air. This surface is usually supplied by use of shavings, blocks of wood, cobs, strips of rattan, coal and the like.
The generator must first be charged or infected with the proper kind of bacteria. This is generally done by pouring through it a culture of some desirable species. The organisms are deposited on the surfaces of the substratum employed and devolpe their zoiiglcea masses, so that the whole is covered with a layer of the slimy mother.
In the perforated disk or false head are a large number of small holes, each generally provided with a piece of wicking or string, down which the alcoholic solution can trickle and thus be brought, in a thin layer, in con tact with the bacteria. The alcoholic solu tion is introduced into the space above the false head, either by a spout, tilting trough or "sparger," a set of revolving arms per forated with holes from which the alco holic solution is forced into the top of the generator.
Below the false bottom is a row of holes through which air is admitted, and at the bottom a receptacle for the liquid which has passed through the generator. The oxidation of the alcohol within produces heat, and there is a constant updraught of air inside the generator from the holes below. Thus the solution which has been added is constantly coming in contact with fresh organisms and fresh air and oxidation is rapid. It is found practically that it requires about 1,000 liters of air to oxidize each 100 grams of alcohol.
Great care has to be taken with the heating as well as the ventilation of a vinegar factory. Since so much heat of oxidation is produced within the generators where the action is taking place, it is necessary to regulate the surrounding tempera ture so as not to get too high heat for the best bacterial activity. As the oxidation is usually not complete in a single generator, a vinegar factory is generally so arranged that the solution has to be pumped but once, and then flows by gravity from one generator to another until all the alcohol has been oxidized.
It is manifest that any substance which can be fermented to alcohol may be used as a starting point in vinegar-making. Thus, sugar, starch, and the like, may be used, but in such cases a prelimi nary alcoholic fermentation, by means of yeast, is necessary.
The product of the fermentation by the acetic bacteria, while mainly acetic acid and water, also contains acetal, aldehyde, and acetic and formic ethers, all of which combine to give the typical fruity refreshing odor and the characteristic taste.
The yield based on theory.
Knowing the alcoholic strength of the solution fermented, the chemist can easily calculate what the theoretical yield should be from the equation given. In practice it is found that the yield is about 80 to 90 per cent of the amount theoretically possible, and may even fall to 70 per cent.
The character of the organisms may be of importance here in addition to the other fac tors which have been indirectly suggested above (evaporation and insufficient oxidation). Some forms of acetic bacteria are so powerful in their oxidizing abilities that they even attack the acetic acid itself, oxidizing it to carbon dioxid and water.
Special vinegars.
Special kinds of vinegars are sometimes pre pared, having peculiar or characteristic tastes and odors. These are generally due to the addition of essential oils of certain plants, or maceration of the plants themselves with some of the vinegar. Tarragon, anise or herb vinegar may be cited as belonging to this class.
Home-making of cider vinegar.
The following instructions for making cider vinegar at home are from Bulletin No. 258 of the New York Agricultural Experiment Station (1904): "Among the conditions which may produce vine gar below standard are these: (1) The juice may be poor to start with because made from varieties of apples low in sugar, from green apples or from overripe or decayed apples; or the juice may be watered either directly or by watering the pomace and pressing a second time. (2) The fermentation processes may be delayed or disturbed by using dirty fruit or unclean barrels, thus affording entrance to undesirable organisms and causing the wrong kind of fermentation; the temperature may be too low to insure the necessary activity of favorable organisms; or air may be excluded by filling the barrels too full or putting the bung in too tight so that the bacteria can not live and work. (3) The acetic acid may disappear after its formation, destructive fermentation being encour aged by leaving the bung-hole of the barrel open or the barrel only partially full.