vi] THE BACTERIA OF MILK 107 



present, therefore, it is impossible to effect the perfect 

 sterilisation of milk. We must be content, accord- 

 ingly, with partial sterilisation, such as is effected by 

 pasteurisation. Intermittent sterilisation, we may 

 mention, may be carried out by heating the milk for 

 two hours at a temperature of from 70 to 75 C. 

 (158 to 167 Fahr.), then keeping it at a temperature 

 suitable for germ development, viz. about 40 C. 

 (104 Fahr.), in order to permit of the spores which 

 are left behind to develop into adult bacteria. The 

 milk is again submitted for two hours to a similar 

 temperature, and then again allowed to stand for 

 several days at the same favourable temperature, 

 40 .C. (104 Fahr.). These consecutive changes of 

 temperature are repeated four or five times, and at 

 last the milk is brought to 100 C. (212 Fahr.). But, 

 while complete sterilisation of milk may be described 

 as hardly within the range of practical dairying, partial 

 sterilisation is easily effected. It is a well-ascertained 

 fact that bacteria commonly occurring in milk, and 

 which excite its common fermentation, as well as the 

 most dangerous pathogenic germs, can be easily and 

 surely destroyed by heating for an hour at a tem- 

 perature of from 68 to 75 C. (154 to 167 Fahr.), 

 or by heating for three-quarters of an hour at 100 C. 

 (212 Fahr.) with steam. It is not difficult, therefore, 

 to perfectly sterilise milk which only contains the 

 common bacteria, and at the same time not to affect 



