CHAPTER IV. 



GROWTH OF BACTERIA IN MILK. 



THE number of bacteria in any sample of milk is at first 

 dependent upon the extent of the original contamination, but 

 the number found at later hours is more dependent upon the 

 rapidity of growth. The original contamination may be a 

 few thousands or perhaps many thousands per c.c., but the 

 milk distributed in our cities often contains them by hun- 

 dreds of thousands and frequently by millions. These larger 

 numbers are the result of the multiplication of the original 

 bacteria, and their size depends upon two factors : ( I ) The 

 temperature of the milk, (2) the age of the milk. Of these 

 two factors the temperature is the more important. Indeed, 

 the temperature at which the milk is kept is of more impor- 

 tance in regulating the number of bacteria than the extent of 

 the original. contamination. 



GERMICIDAL PROPERTY OF MILK. 



Milk furnishes a most excellent food for dairy bacteria, 

 and common bacteria are capable of growing and multiply- 

 ing rapidly in milk. Milk, when drawn from the cow, is 

 at a high temperature, a temperature at which most dairy 

 bacteria grow very rapidly. Strange to say, however, the 

 bacteria that find entrance into the milk during the milking 

 do not at once begin to increase in numbers (102). On the 

 contrary, if the number of bacteria in fresh milk be deter- 

 mined and then analyses be made of the same milk at inter- 

 vals of two or three hours, it is found that the number 

 usually slowly declines, and for some time practically never 

 increases. The following three experiments may serve as 

 an illustration (101). 



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