This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The process of refrigeration does not involve the actual freezing of meat or vegetables, but implies their preservation in chambers at a temperature which is maintained but a few degrees above the freezing point. This causes less alteration in flavour than freezing. The cold is artificially generated, and beef, fish, fruits, and vegetables are now successfully transported for thousands of miles in refrigerator cars and rooms fitted for the purpose on steamships.
The refrigerating processes applied to the preservation of meat, etc., are several. In most of them the actual reduction of temperature of the meat is maintained by cold air and not by contact with ice.
One method consists in the adaptation of the principle that compressed air on expanding derives the energy for its expansion from heat, which it abstracts from all surrounding bodies. The liberation of strongly compressed air, therefore, produces intense local cold in its immediate vicinity. The air is originally compressed by a force pump, and the heat which is developed by the compression is removed by a circulating stream of cold water. The cooled compressed air is then liberated with the effect described. Other apparatus is constructed on the principle of ice machines, which are operated by evaporating ammonia, which produces extreme cold.
The keeping of meat by refrigeration is rapidly superseding the canning process for this kind of food, over which it has many decided advantages. Between 15 and 20 per cent of all the mutton eaten in Great Britain is brought from New Zealand and the River Plate in a refrigerated condition.
Major Woodruff, U. S. A., writes: "The French Government is taking the initial step towards applying this new industry to the purposes of war. They have succeeded in keeping dressed beef in a perfect condition for three or four months with the present appliances.
"A moment's thought will show what a revolution this matter of cold storage can make in military practices. It will obviate all necessity of keeping live cattle near the army, a system that so often results in diseased animals and fatal epidemics among the soldiers. It will help to wipe out of existence all the salt meats formerly supplied, and will thus avoid that large list of diseases of stomach, bowels, and nutrition that salt meats have been accused of causing".
By sterilisation of food is meant the process of rendering it germ-free by heat, and it includes the preservation of such food in sterilised vessels. Practically all thoroughly cooked food is for the time being "sterilised," and overdone meat keeps longer than underdone meat, for if the outer layers are firmly coagulated and dried by the heat of boiling or roasting, they form an envelope which is less pervious to the atmospheric air and germs. (See Cooking, pp. 263, 264).
Canned food (p. 282) is also sterilised, but the latter term is applied chiefly to milk which has been treated by the method described on p. 85.