THE COMMERCIAL CANNING INDUSTRY Canning is so called because the food material, either animal or vegetable, is "packed" in metal or glass containers, hermetically sealed and steril ized or "cooked" by the application of beat. The containers, commonly spoken of as "cans," are generally made of tin plate, although, for certain kind of foods, glass jars are sometimes used. The process is capable of very wide application, as all kinds of foods, except those eaten only in the raw condition, may be preserved in this way, and thus the abundance of one season or one locality may be made available at another place or time.
The genera] object of the process is apparent from the foregoing, but it may be stated that the main problem is to prevent decomposition or spoil kg. changes induced in foods by the activity of various kinds of micro organisms which ferment cr putrefy the foods, giving rise to products of harmful or undesirable character and rendering the food matt for use.
From a sanitary point of view, canned foods, if properly prepared, are of the highest value, as they are free from bacteria. This fact, combined with their convenience and the ease with which they may be transported, has led to an enormous manu facture and consumption of these very satisfactory f Jod products. In this article the canning of vege table foods only will be considered.
Methods of sterilization.
Sterilization of the can and its contents is effected by one of the following methods : (1) water bath, (2. chemical bath. (3) steam under pressure in strong chests or kettles "retorts." Figs. and 249 show sterilizing or cooking apparatus.
(11 The water bath. As its name implies, steri lizatiea by this means consists in boi+rig the cans or jars for a single period or discontinuously, a temperature of 100° Centigrade (212° F.) being thereby obtained.
(2) The chemical bath. This consists of a strong solutionof somesalt,gen orally calcium chlorid, because of its great sol ubility. The boiling point of the solution being much higher than that of water, higher temper atures may be reached by its use than with the ordinary water bath, and consequently a shorter time is required to bring about sterilization. This
method was first em ployed in this country about 1863, but was not a success because the cans of that time were not strong enough to withstand the pressure generated within. The method of use is the same as with the water bath, i. e., the filled cans are boiled for a certain definite period.
(3) Steam under pressure. This method of steril ization was introduced about 1870. The tempera ture in this case may be varied by control of steam pressure. The steam being confined in the retorts, of course the pressure is equal within and without the cans ; thus, unless the outside pres sure is removed suddenly, the strain on the cans is not great and loss from bursting is small. Most of the modern cans, however, are sufficiently strong to withstand sudden changes without injury.
There are two modifications of the retort, known as the " wet retort" and the " dry retort." In the former, the kettle is filled with water and steam under pressure blown in, so that the boiling-point of the water is much raised owing to the increased pressure. These kettles are generally cylindrical and are placed in a vertical position, with a heavy lid on the upper end. When in use, this lid is fast ened down by means of heavy bolts. The kettles are generally provided with three valves,—an intake valve for steam at the bottom, an outlet for water at the bottom and an exhaust valve for steam in the lid. Although spoken of as a " wet retort," it can be used without water in the same way as a "dry retort." In the "dry retort," the steam under pres sure is blown in, directly replacing the air and coming directly into contact with the cans.
The Portland type of retort consists of a heavy iron chest, about cubical in shape. One side of the cube is the door, which is hinged and fastens by bolts. With the exception of the door the retort is cast all in one piece, the door forming a separate casting.