Pasteurization. 57 



no item in the entire process left to haphazard or to 

 chance. 



We have previously .seen that, besides the acidity, 

 there are other causes for the curdling of milk, that 

 the latter may even curdle without being at all sour, 

 and that there exists a large number of bacteria 

 which possess the property of separating a rennet- 

 like ferment and which, consequently, if they be pre- 

 sent in sufficient numbers, are able to make milk 

 curdle. Milk in which such bacteria predominate 

 will curdle very easily at warming without any ab- 

 normal degree of acidity having previously been 

 observed. The reaction of milk is, therefore, not 

 always an unerring sign of probable curdling when 

 warmed, but the warming, itself, rather constitutes 

 the surest experiment towards the examination of 

 milk in this direction, more particularly of such milk 

 which is produced under conditions entirely remote 

 from our observation. This is also true of pasteur- 

 ized milk. All bacteriological investigations of 

 pasteurized and sterilized milk have shown that 

 it is more especially the group of rennet — or butter 

 acid bacteria — which in their endurate form of spores 

 resist the influence of heating better than other bac- 

 teria. For this reason well pasteurized milk contains, 

 when it becomes older, principally these bacteria, and 

 it may curdle in the course of time without percept- 

 ably increasing in acidity. 



The keeping quality of pasteurized milk can, there- 

 fore not be examined by the chemical reaction, but 



