THE RELATION OF MICROORGANISMS TO MILK 



461 



species which occur in the udder of the cows, but under ordinary condi- 

 tions there will be in addition to the normal udder types such others as 

 may occur on the cow's body and in the dust and atmosphere of the 

 stables. Market milk, therefore, when first obtained from the cow 

 ordinarily contains a mixed flora, the different types present depending 

 upon the sanitary conditions under which the milk is produced. 



The future development of this initial flora is largely dependent 

 upon the temperature at which the milk is kept. If the milk is held at 

 temperatures between 10 and 21 there will result what may be con- 

 sidered as the normal development of milk fermentations. These 

 changes may be divided for convenience into four periods or stages. 



FIRST STAGE. GERMICIDAL PERIOD. It has been shown by a num- 

 ber of investigators that instead of an increase in the numbers of bacteria 

 in fresh milk there is normally a decrease in the number during the 

 first few hours after its production. The rapidity of this decrease and 

 the length of time over which it extends seem to be determined largely 

 by the temperature at which the milk is kept. The higher the tempera- 

 ture the more rapidly the number of organisms decreases and the more 

 quickly the end of the germicidal period is reached. If the tempera- 

 tures are kept fairly low the rate of decrease is much slower but the de- 

 cline will extend over a considerably longer period. This is shown by 

 the following examples given by Hunziker. 



TABLE SHOWING THE GERMICIDAL ACTION IN Cow's MILK 



The exact reason for this decline is at present not well understood. 

 Some investigators believe that milk possesses a certain germicidal ac- 

 tion or property which results in the destruction of a portion of the 

 organisms found in the milk at the outset. 



* Fahrenheit. 



