PRESERVATION OF MILK 375 



cedure, therefore, which makes the milk bacterially safe to 

 consume. 



There is, however, another and a very important side of 

 the problem which is still a very debatable one. This is a 

 chemical and physical matter, and may be put in the form of 

 the question, Is cooked milk inferior to raw milk as regards 

 digestibility and nutritive properties, and is it liable to cause 

 or lead to human disease because it has been boiled ? 



The Changes which occur when Milk is heated. The actual 

 changes which occur are influenced by two factors the dura- 

 tion of the exposure to heating and the actual temperature 

 attained. When milk is boiled the following are the essential 

 changes which result. A skin forms on the surface when the 

 heating is done in an open vessel, the pellicle consisting mainly 

 of lact-albumen. Its formation commences at a temperature 

 between 60 and 70 C. This surface pellicle does not form 

 if the milk is heated in a closed vessel. Carbonic acid gas is 

 expelled, and the calcium and magnesium salts are precipitated. 

 The precipitation of the calcium salts makes the casein in the 

 milk less easily coagulated by rennet. The lecithin and 

 nuclein are also said to be decomposed. The normal emulsion 

 of the fat globules is disturbed so that the cream does not rise. 

 The lactose is partially burnt (caramelisation), the milk becom- 

 ing brownish in colour. The milk acquires a peculiar taste, 

 which is marked in boiled milk, especially if carried out in an 

 open vessel, but which is also noticeable in milk heated to 

 70 C. Boiling also kills the ferments in the milk. 



According to Kastle the ferments in milk, or most of them, 

 can withstand a temperature ranging from 60 to 65 C. for 

 some time without material injury. Between 65 and 70 C. 

 most of them are weakened in their activity, and between 70 

 to 80 C. all of them are destroyed even after relatively short 

 exposure. In addition to these changes the germicidal power 

 of the milk is lost, or almost completely so, by boiling. The 

 essential bacterial change is that almost all the bacteria are 

 destroyed, those left consisting entirely of sporing forms and 

 certain highly resistant varieties. 



The effects of these by no means inconsiderable changes 

 produced by heat have been much discussed as regards their 

 influence upon the milk as a food for infants. 



