RELATION OF BACTERIA TO DAIRY INDUSTRY. 67 



ter of keeping the cow, consists largely in trying 

 to prevent bacteria from growing in milk or in 

 stimulating their growth in cream, butter, and 

 cheese. These chief products of the dairy will be 

 considered separately. 



SOURCES OF BACTERIA IN MILK. 



The first fact that claims our attention is, that 

 milk at the time it is secreted from the udder of 

 the healthy cow contains no bacteria. Although 

 bacteria are almost ubiquitous, they are not found 

 in the circulating fluids of healthy animals, and 

 are not secreted by their glands. Milk when 

 first secreted by the milk gland is therefore free 

 from bacteria. It has taken a long time to 

 demonstrate this fact, but it has been finally satis- 

 factorily proved. Secondly, it has been demon- 

 strated that practically all of the normal changes 

 which occur in milk after its secretion are caused 

 by the growth of bacteria. This, too, was long 

 denied, and for quite a number of years after 

 putrefactions and fermentations were generally 

 acknowledged to be caused by the growth of 

 micro-organisms, the changes which occurred in 

 milk were excepted from the rule. The uni- 

 formity with which milk will sour, and the diffi- 

 culty, or seeming impossibility, of preventing this 

 change, led to the belief that the souring of milk 

 was a normal change characteristic of milk, 

 just as clotting is characteristic of blood. This 

 was, however, eventually disproved, and it was 

 finally demonstrated that, beyond a few physi- 

 cal changes connected with evaporation and a 

 slight oxidation of the fat, milk, if kept free 

 from bacteria, will undergo no change. If bac- 



