82 Dairy Bacteriology. 



CHAPTER V. 



FERMENTATIONS OF MILK. 



Milk, under normal conditions, is always contam- 

 inated with bacteria coming from the most varied 

 sources. If it is produced under clean conditions, the 

 number of bacteria will be small, but in any case, the 

 number of kinds of bacteria that find their way into 

 milk will be large. Many of them find in milk at ordi- 

 nary temperatures suitable conditions for growth ; they 

 use a portion of some of the constituents of the milk as 

 food, producing certain other compounds that are 

 known as "by-products." These by-products impart to 

 milk a taste and odor that is not found in fresh milk. 

 The effect of the action of bacteria may also be made 

 evident by the change in the appearance of the milk. 

 When these various changes become evident to the 

 senses, either by taste, smell or sight, the milk usually 

 is so modified as to be unfit for many ordinary purposes. 

 The preservation of milk, a subject to be treated later, 

 is a study of the ways of preventing or retarding the 

 growth of bacteria in milk, and thus delaying the time 

 when evidences of their action first become apparent. 



Each class of bacteria produces more or less specific 

 changes in the milk as a result of their growth. Certain 

 bacteria are of the greatest benefit to the butter and 

 cheese maker, while others are distinctly harmful to the 

 manufacturer of dairy products. The changes pro- 

 duced by the different bacteria are called "fermenta- 



