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son holds that even if bacteria could retain their activity at the temperature of frozen meat, they would be practically prevented from penetrating into the meat by these layers of ice which separate the muscle fibers, and that the histological changes which have sometimes been reported as occurring in frozen meats may be due to the mere physical effects of freezing, especially if followed by too rapid thawing, rather than to any bacterial change or other deterioration.
Richardson and Scherubel's examinations of frozen meat for bacteria both by direct microscopic and by cultural methods indicated that beef which had been kept frozen even so long as 600 days was free from bacteria at a depth of one centimeter or more from the surface. On the other hand, .in meat kept at 2°-4° C. bacteria had penetrated to a depth of about one centimeter in thirty days.
The principal result shown by chemical analysis of a large number of samples of beef which had been kept frozen from 33 to 554 days (in a room whose temperature varied from - 90 to - 120 C.) was that the exterior of the meat dried to a depth of from 2 to 4 millimeters in the course of a year in the open freezer, after which the progress of the drying was extremely slow. The moisture content of the portion thus dried was about 30 per cent; that of the frozen meat as a whole was about 76 per cent — the same as for corresponding cuts of fresh meat. There was no increase of ammoniacal nitrogen in the stored meats, which is considered by these investigators as strong evidence that there was no bacterial decomposition of the proteins. Neither was there any difference between the fresh and frozen meats as regards cold water extract, total nitrogen of cold water extract, or the coagulable proteins, the albumoses, or the nitrogenous extractives.
It is hardly necessary to point out that such good preservation over long periods of time is not to be expected of meat which is merely refrigerated without being hard frozen.
Other methods of preservation. Aside from cold storage, the principal means of preserving meats are drying, canning, and the application of preservative substances.
Drying is, when applicable, a very effective method and has been long used. In some climates it is only necessary to cut the meat into strips and hang it out of doors. The "jerked beef " of the West was prepared in this way, and a mixture of dried lean meat with fat known as " pemmican " is concentrated food largely used by explorers. Dried meat is, however, by most people considered less attractive than fresh meat, and as a commercial process, the drying is slow and troublesome.
Canned meat is now put up in large quantities. Often all of the meat of the iore quarter and the cheaper cuts of the hind quarter are canned. There is a tendency to use the leaner carcasses for canning, both because the fat beeves can be sold at better prices in the fresh state and because the leaner meats are more attractive than the fat meats when canned.
Sometimes the beef is cured with salt, and usually also a little saltpeter, and then canned and sold as " canned corned beef." When preserved by canning alone without salting the product is sometimes called " canned roast beef " and sometimes simply "canned beef." The following is an outline of the latter process.
The meat selected for canning is cut into pieces usually one to four pounds each, depending upon the size of cans to be filled. It is then parboiled by putting into a tank with water and cooking with steam. Or the meat may be parboiled in larger pieces, then trimmed free from gristle and superfluous fat, and cut by machinery into approximately uniform pieces of a size proportioned to the size of the cans.
The parboiling causes a shrinkage of the meat so that (while being cooked in water) its water content is diminished.
That part of the fat which is cooked out of the meat rises to the top and is skimmed off; the extractives, the salts, and the very small amounts of protein which are extracted remain in solution in the water in which the meat is cooked, which thus becomes of value for the making of soup stock and meat extract.
Wiley estimated that this cooking extracts a little over one part in one hundred of the protein of the meat, about one third of the " extractives," and up to one half of the salts.
After the parboiled meat has been packed in the cans, enough of the " soup liquor," made by concentrating the water in which the meat was cooked, is added to fill the spaces between the pieces and to restore so far as is practicable the flavoring constituents lost in parboiling. This added "soup liquor" may also contain salt, sugar, or molasses as a flavoring.
In canning tongue and in other cases in which the form of the product is to be preserved, the cans are filled by hand. In the case of corned beef and potted or deviled meat, the cans are filled by means of the " stuffing machine," which presses into the can approximately the required amount of meat, the weight being tested and adjusted as each can leaves the machine. The cap of the can is then soldered on by means of the "capping machine," which leaves the can completely sealed except for the small vent hole in the top. The cans are then tested for leaks and any leaks found are repaired by hand.
The cans are then sent to vacuum machines by means of which the air is exhausted from within the can and the vent holes sealed while the can is in the vacuum chamber. From the vacuum machines the cans are run out on tables and again inspected to make sure that they are free from leaks.1
The cans are now ready for " processing," which simply means the heating of the can and contents to a sufficient temperature to insure its keeping. The temperature and time of heating depend chiefly upon the size of the cans, but also to some extent upon other conditions. Probably the most usual temperature is between 2250 and 2500 F. (1o7°-i2i° C.), which is usually
1 Any can found leaky at this point is repaired by hand, the vent reopened, and the can returned to the vacuum machine.
attained by the use of superheated steam in large iron or steel boilers or "retorts." Sometimes an oil bath is employed as a means of maintaining the high temperature. In case the nature of the product makes it desirable to avoid a temperature above boiling, the processing may be accomplished by placing the cans for a sufficient length of time in large open kettles or tanks of water which are kept at the boiling point by means of steam coils.
As the cans come hot from processing, the ends are slightly bulged outward owing to the expansion of the contents by the heating. They are now subjected to a cold spray until the contents are thoroughly chilled, when the ends of the can should be slightly concave and should remain so until the can is opened for use.
Finally the cans are washed in alkali to remove any grease, then in water, dried, painted, and labeled. Many establishments maintain warm " test rooms" at a temperature of i00°i io° F. to which is sent a sample batch of each " run " of canned meats to make sure that no cans prove defective when kept for several days at this high temperature.
A sound can should have slightly concave ends and should give only a dull sound when struck on the top or bottom; a can which shows bulging ends and emits a hollow or drum-like sound when struck on the top or bottom is likely to be leaky, or improperly packed, or to contain material which has undergone decomposition with production of gas.
Application of preservative substances is another common and important method of preserving meats. The substances which have been used to any considerable extent for this purpose are salt, saltpeter, boric acid or borates, sulphites, vinegar, wood smoke, and sugar. Salt, sugar, vinegar, and wood smoke are condimental as well as preservative in their properties, and there is no restriction upon their use. Saltpeter in addition to its preservative action has the property of maintaining or even intensifying the red color of beef. Under the present laws it