Dr. Richardson says that putrefactive changes in meat are due to the decomposition of the water contained in the tissues. The means which have been found to arrest this decomposition are - (1) a low temperature (2) a high state of desiccation; (3) the application of antiseptics; (4) the exclusion of air.

Refrigeration

Subjection to a low temperature is a thoroughly effective way of preserving meat, but it can be considered only as temporary, decomposition ensuing when the cold state is abandoned. Nevertheless, its effects are sufficiently lasting to serve practical ends, and the process seems most likely to solve the problem of conveying large quantities of fresh meat to this country. Numerous plans have been devised, all aiming at the production of a sufficiently low temperature at a remunerative cost. The principal are: -

(1) Harrison's

The meat is first frozen, and is then packed in a chamber on board ship, the air of which is maintained in a thoroughly dry state, so as to keep up a slow but constant evaporation from the surface of the meat. The meat is placed in tanks, which are kept cool by directing a stream of brine among ice, and regulating the strength of the brine so as to produce the de* sired degree of cold. The ice and brine are kept in tanks above the meat, and from them streams constantly trickle over and around the meat-tanks. The consumption of ice is less than 50 tons for 50 tons of meat, and the proportion decreases with larger quantities. The meat retains its full flavour, and will keep good in a temperature of 63° to 68° F. (17° to 20° C.) for 70 to 80 hours after removal from the tanks. The drawback is the bulk of ice required. (2) Tellier's. - The joints of meat are placed in a chamber, through which is passed a current of air charged with ether or other volatile substance, so as to reduce the temperature sufficiently low to preserve the meat, without freezing its juices.

(3) Mort And Nicolle's

In this process, the freezing agent is ammonia solution under a pressure of 50 to 70 lb. a sq. in. The freezing-room is kept below 32° F. (0° C), and the meat is frozen quite hard.

(4) Poggiale's

A low temperature is maintained by the evaporation of methylic ether, and circulation of chloride of calcium.

(5) Bell And Coleman's

This process is perhaps the most completely successful of all that have been introduced, and is equally applicable to the preservation of fresh meat during transport by land or sea, and while being stored. The meat is placed in a chamber made as nearly air-tight as possible, and of the best-known non-conducting materials. The air which is made to circulate in the meat-chamber is cooled so as to maintain a temperature never exceeding 50° F. (10° C), and never so low as to actually freeze the meat. The cold is obtained by the re-expansion of compressed and cooled air. Cold-producing machines on this principle are by no means new, but a great difficulty hitherto met with in applying this system has been the formation of particles of ice during the re-expansion. This is avoided by a more effectual cooling of the compressed air, and by subsequently treating the air so as to separate moisture from it, by subjecting it, before re-expansion, to an atmosphere cool enough to ensure the deposition of any remaining moisture that would be liable to freeze; moreover, care is taken that the air shall not be so highly dried as to have a desiccating effect upon the meat.

(6) Knott's And Kent's

In Knott's refngerating-car, air is cooled by passage over a freezing-mixture, or ice alone, and a constant circulation of it is kept up, the temperature being best maintained at a little above the freezing-point, say at 33° F. The air is both dried and cooled. Kent's well-known refrigerator-safes are made upon much the same principle, the great feature being a downward draught. Importations of meat from America have been made by this system, the meat being sewn up in bags and suspended in a chamber surrounded by a temperature of about 37° F. (3° C), the draught being produced by a steam-fan worked over the ice-tanks.

Desiccation

Animal matter, preserved by the absorption of its moisture, loses its flavour, and becomes tough and indigestible; the fat becomes rancid, and in damp weather the meat absorbs moisture, and turns mouldy and sour. These tendencies are corrected by adding absorbent substances with fat food - as sugar and spice, to form "pemmican," and farina, to produce "meat-biscuits." Altogether, the process seems ill-adapted for preserving meat in a fresh state, and two methods only need be mentioned.

(1) Tellier's

The meat is placed in vessels whose air is repeatedly exhausted, and replaced by carbonic acid gas, which latter is finally absorbed by a concentrated solution of potash. The meat loses 18 to 20 per cent, by weight, and is kept in vacuo.

(2) Sacc's

This process has been described under Fruit. When applied to meat, the brine produced furnishes an extract of meat on evaporation, the acetate of soda crystallizing out. This extract is added in the proportion of about 3 per cent, to the preserved meat. The latter, before use, requires to be steeped for 12 to 24 hours in water containing about 1/2 oz. sal-ammoniac to the pint.

Antiseptics

The use of chemical antiseptics has long been known, common salt being a very generally employed agent of this class. The difficulty seems to be to ensure the meat retaining its freshness, and to avoid its acquiring any unpleasant flavour. From among the very various processes devised, the following are selected as being most noteworthy.

(1) Hcrzen's

The quarter-carcases are soaked for 24 to 36 hours in a solution composed of 3 parts borax, 2 parts boracic acid, 3 saltpetre, and 1 salt, in 100 parts water; they are then packed with some of the same. Before use, they need 24 hours' soaking in fresh water.

(2) Reynoso's

The meat is subjected to the action of compressed nitrogen, carbonic oxide, etc. After being kept in this state for 40 days, the freshness has been so maintained that blood has flowed from the joints.

(3) Richardson's

Dr. Richardson made some test experiments with meat treated with various antiseptics, under a temperature varying from 45° F. (7° C.) to 110° F. (43° C), for a period of 75 days. The results may be summarized thus: - Methylene: preservation, good; colour, imperfect. Methylal: faint taint of decomposition. Cyanogen: preservation, excellent; colour, perfect; structure, firm. Sulphurous acid: some tainted; colour, dark. Sulphurous acid and lime-juice: some tainted; colour, indifferent. Sulphurous acid and glucose: some tainted; structure, dense. Nitrate of methyl: preservation, good; colour, yellowish; structure, firm. Formates: entirely fresh, and excellent in colour.

(4) Estor's

This consists in treatment with sulphurous acid and chlorine in succession.

(5) Gamgee's

The animals are killed by inhaling carbonic acid, etc, and the carcases are kept in an atmosphere of carbonic or sulphurous acid. This does not prevent decomposition where bruises exist.

(6) Medlock And Bailey's

The meat is immersed in a solution composed of equal pnrts of water and bisulphite of lime, of 1.05 sp. gr. It acquires no unpleasant flavour. This is one of the most successful of the antiseptic processes.

(7) Pelletier's

The meat is covered with a coating of gum, then immersed in acetate of alumina, then in solution of gelatine, allowing the whole to dry on the surface. The antiseptic acetate of alumina forms an insoluble compound with the gelatine.

(8) Pagliare's

Gum benzoin is boiled in a solution of alum. The meat is immersed in this compound, and excess moisture is driven off by a current of hot air, leaving the antiseptic on the meat.

(9) Jones And Trevethick's

The meat is put into tin canisters, which are hermetically closed, except two holes in the lid. These are plunged into a vessel containing water, and after the air has been exhausted by an air-pump through one hole, sulphurous acid gas is admitted through the second, and this alternation is continued till all the air is out. The sulphurous gas is then replaced by nitrogen, and the holes are closed.

Exclusion Of Air

As the presence of oxygen seems to be essential to the existence of decomposition, many plans for the preservation of meat have been based upon the exclusion of air from it. By far the most important are the numerous modifications of cooking in air-tight cans, called " canning," which have been conducted for years with great success. The heat of the cooking destroys any microscopic germs, if such be present, and at the same time expels all air from the receptacle and from the substance itself. The preservation is complete, but over-cooking is unavoidable, and the meat is rendered soft, fibrous, and insipid.

(1) " Canning." - There are three chief modifications of the canning process - (a) "Aberdeen;" (6) " steam-retort;" (c) "chloride calcium bath." The Aberdeen process probably originated with Appert, whose plan was brought into use during the Crimean war. The meat is placed in vessels nearly closed; these are then put into a close boiler, and the heat is raised to 234° F. (112° C). After about 3 hours' cooking, the vessels are hermetically sealed. McCall's improvement upon this consists in the addition of a little sulphite of soda. Jones' improvement lies in the fact that the water is first driven off at 230° F. (110° C.) in vacuo, and the heat is then raised to, and kept at, 270° F. (132° C). The special feature is the vacuum, all the oxygen being extracted by means of tubes connecting the tins with the vacuum-chamber; this greatly reduces the time. By the steam-retort plan, the meat is canned up, leaving a pinhole, and the cans are put into a retort under steam at 230° F. (110° C), and kept there for 1 1/2 to 2 hours; they are then taken out, and the pin-holes are soldered up while steam is issuing from them. The cans are again steamed at 240° F. (116° C), and cooled. The object of the chloride of calcium bath is to obtain a higher temperature.

The raw meat is put into cans having a pin-hole, as before. The cans are placed for half their depth in a solution of chloride of calcium, boiling at 260° to 270° F. (127° to 132° C). The heat is gradually raised from 180° F. (82° C.) to 230° F. (110° C.\ and the steam is allowed to blow off for 4 hours, during which time the meat is being cooked. The holes are then closed by a drop of solder, the heat is raised to 260° to 270° F. (127° to 132° C.) for 1/2 hour, and the cans are withdrawn and cooled. Ritchie's deviation from this consists chiefly in desiccating the meat first in an oven at 400° to 420° F. (204° to 216° C), and then packing it in cans, with the addition of meat jelly to create steam, before subjection to the chloride of calcium bath.

(2) Naylor's Process

The meat is cooked, and then packed in cases, and covered with stearine (tallow).

(3) Redwood's Process

The meat is immersed in melted paraffin at 240° F. (115° C), to concentrate the juices, and expel the air. Thus condensed, the meat is covered with a coating of paraffin. Before use, it is placed in boiling water, which removes the paraffin; it can only be used in its cold state, not bearing re-cooking.