THE GROWTH OF BACTERIA IN MILK 85 



the milk becomes very vile and unfit for use. These facts are 

 of much more importance in cheese-making than anywhere else. 

 When the milk is used for drinking it is almost sure to be 

 consumed before this protective action becomes noticeable; 

 but cheese, the casein of milk, must be kept many weeks for 

 ripening. To a less extent the facts are true of the ripening 

 of cream in butter-making. We thus see the reason why lactic 

 bacteria are necessary in butter and cheese-making, and why 

 starters have become so widely used. It has even been seriously 

 suggested that milk to be used for drinking should be inoculated 

 with lactic bacteria to insure their presence and aid in the 

 exclusion of other species. Here, too, we find an explanation of 

 the fact that sour milk and butter milk are frequently recom- 

 mended for drinking, these products being practically sure to 

 contain large numbers of the lactic acid bacteria. 



From all of these facts it will be evident that the question 

 of number in the bacterial content of any sample of milk is a 

 very complicated one. The original contamination is only one 

 factor and a small one. The age of the milk, the temperature 

 at which it is kept, the relative abundance of the different 

 species at the outset, the extent to which the different species 

 are favored by the temperature, the influence of one species 

 upon another, all these, and other factors as well, combine to 

 determine the bacterial content of any sample of milk. It is 

 manifestly impossible to make any general statements as to 

 what numbers or kinds of bacteria should be found in any 

 sample of milk. 



