Preservation of Milk.—On account of the importance of preserving milk for use in large towns, in hospitals, and in the army during war, the question of its preservation has long been studied.
Chemical Processes.—The principle of preserving milk by chemical action consists in preventing, or at least retarding, the changes which inevitably follow exposure to the air. For this purpose, chemical substitutes are added which in themselves have no injurious action. Those most commonly employed are:—
| Carbonate of soda | 45 grs. | per quart. |
| Bicarbonate of soda | 45 grs. | „ |
| Boric acid | 15 to 30 grs. | „ |
| Salicylic acid | 12 grs. | „ |
| Borax | 60 grs. | „ |
| Lime | 20 grs. | „ |
The results obtained are of comparatively little value; the milk only keeps for a few hours, or at the most for three or four days.
Cold.—Refrigeration, which is so valuable in preserving all kinds of animal products for long periods, has also been used for preserving milk. Unfortunately, although cold impedes the development of bacteria, it also has the grave inconvenience of causing the cream to separate from the milk, and it being impossible to mix them again satisfactorily, milk preserved in this way is more or less unfit for consumption.
Heat.—The principle of preserving milk by heat is based on the destruction of the microorganisms at a high temperature. In this respect again, one meets with obstacles, for, if the heat be applied direct, some of the principles of the milk are converted into caramel, and if the temperature rises beyond 157° Fahr. (70° C.) the composition of the milk is changed.
Preservation by Oxygen.—Within the last few years the use of oxygen at a pressure of about two atmospheres has been recommended. When the milk is to be used it is only necessary slightly to relieve the pressure and allow the oxygen to escape, the liquor which remains having all the characters and qualities of fresh milk. The method appears excellent, but is too costly for every-day use.
Pasteurisation.—The pasteurisation of milk aims at destroying the greater proportion of the ferments above mentioned. The milk is heated at atmospheric pressure, and is kept for a time at a temperature of between 150° and 157° Fahr. (65° and 70° C.). It preserves its properties and composition, but sterilisation is not complete, and the milk cannot be kept indefinitely.
Concentrated Milk.—Concentrated milk is obtained by prolonged heating to 157° Fahr. (70° C.) in a vacuum, when it becomes syrupy by evaporation and its composition is not greatly modified. It is then drawn off into bottles, which are hermetically sealed and subjected to a higher temperature to complete the destruction of all the germs. Condensed milk keeps for a very long time. To prepare it for use it is mixed with a certain quantity of water, and then yields a liquid similar to normal milk.
Sterilisation.—Sterilisation necessitates the use of special apparatus in which the milk is heated in a water or steam bath sheltered from the action of the air, the temperature rising to 212° to 240° Fahr. (100° to 115° C.); all the ferments are destroyed, and the milk will keep indefinitely, but its composition is slightly modified.