394 



MICROBIOLOGY OF MILK AND MILK PRODUCTS 



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

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

 sidered as thie 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. 



The work of other investigators seems to show that the so-called 

 germicidal action is felt by certain species and not by others as is indi- 

 cated by the foUomng sample. 



* Fahrenheit. 



