592 DISTURBANCE .IN THE MILK SECRETION AND CHANGES IN THE MILK. 



inevitably follow exposure to the air. For this purpose, chemical sub- 

 stitutes are added which in themselves have no injurious action. Those 

 most commonly employed are : — 



Carbonate of soda 

 Bicarbonate of soda 

 Boric acid . . 

 Salicylic acid 

 Borax 

 Lime 



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. — Kefrigeration, 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 micro-organisms 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 IST'^ Fahr. (TO'^ C.) the com- 

 position 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 tempera- 

 ture 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 sub- 

 jected 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. 



