BACTERIA OF MILK 63 



of peptonizing bacteria is not objectionable when the 

 total number of bacteria is small, it is always objection- 

 able when the bacterial count is high. 



The number of bacteria present in market milk de- 

 pends upon (1) the original contamination, (2) the 

 temperature at which the milk has been kept, and (3) 

 the age of the milk, i.e., the time which has elapsed since 

 the milk was drawn from the cow. 



1. By original contamination is meant the bacteria 

 which get into the milk during milking and the subse- 

 quent handling of the milk. The extent of this depends 

 upon the cleanliness and health of the cows, stable prac- 

 tices, method of milking, cleanliness of the milk vessels 

 and utensils, etc. 



2. The temperature at which milk is kept affects not 

 only the total number of bacteria but also influences the 

 relative rate of increase of the different kinds or species. 

 As a rule, the higher the temperature the more rapidly 

 the bacteria multiply. For example, Conn found that 

 when fresh milk contained 6525 bacteria per c.c., 

 after 25 hours at 10 C. (50 F.) it contained 6425 bac- 

 teria per c.c., while after 25 hours at 21 C. (70 F.) it 

 contained 6,275,000 bacteria per c.c. 



When milk is promptly cooled to 10 C. (50 F.) 

 and held at that temperature, little or no increase of 

 bacteria will occur for twenty-four to thirty-six hours, 

 and even at 15 C. (59 F.) the increase will not be 

 very great. At temperatures above 20 C. (68 F.), 

 however, the bacteria increase very rapidly. 



As stated above, the temperature affects not only the 

 number of bacteria but also the relative development of 

 the different species. In other words, it determines the 

 type of fermentation or decomposition which the milk 



