PASTEURIZATION 203 



55° C. Some occasionally survive until the milk reaches 

 60° C. 



The cholera vibrio is similar to the diphtheria bacillus so 

 far as its thermal death-point is concerned. It is usually 

 destroyed when the milk reaches 55° C; only once did 

 it survive to 60° C. under the conditions of the experi- 

 ments. 



The dysentery bacillus is somewhat more resistant to 

 heat than the typhoid bacillus. It sometimes withstands 

 heating at 60° C. for five minutes. All are killed at 60° C. 

 for ten minutes. However, the great majority of these 

 micro-organisms are killed by the time the milk reaches 

 60° C. 



So far as can be judged from the meagre evidence at 

 hand, 60° C. for twenty minutes is more than sufficient 

 to destroy the infective principle of Malta fever in milk. 

 The M. melitensis is not destroyed at 55° C. for a short 

 time; the great majority of these organisms die at 58°, and 

 at 60° all are killed. 



A temperature of 60° C. for twenty minutes is sufficient to 

 destroy the virus of scarlet fever, streptococci, and other 

 pathogenic organisms. 



Milk heated to 60° C and maintained at that tempera- 

 ture for twenty minutes may, therefore, be considered safe 

 so far as conveying infection with the micro-organisms 

 tested is concerned. 



Chemical changes in milk produced by heat 



The changes produced in milk by heat depend upon the 

 degree of heat and the length of exposure. The exposure 

 of milk to a temperature of 60° C. for a short time does not 

 appreciably affect its chemical and physical properties. 

 The boiling of milk, however, produces pronounced changes. 

 These changes consist mainly of the following: — 



Decomposition of the proteins, and other complex nitro- 

 genous derivatives. 



