5o6 



MICROBIOLOGY OF MILK AND MILK PRODUCTS 



milk. The milk is first heated and then air under pressure is forced 

 through it. By this process the milk is heated to a temperature of 

 60 (i4oF.), and this temperature maintained for two hours, during 

 which time air is forced through the milk causing violent agitation and 

 the removal of the moisture. At the end of this time the milk is re- 

 duced to one-fourth its original volume.* The result of this process is a 

 pasteurized milk, with a marked reduction of the original germ content. 

 Investigations by Conn failed to show the presence of B. coli in milk 

 prepared by this process. The reduction in the bacterial content of the 

 milk is similar to that secured by other methods of pasteurization. No 

 additional sugar is added to this milk so the product is, therefore, a pas- 

 teurized milk containing a small amount of moisture. Because of the 

 small amount of moisture and the concentration of the milk solids, the 

 bacteria which survive the heating process do not grow rapidly at low 

 temperatures. The following figures will serve to illustrate the effect of 

 this process upon the bacterial content of milk: - 



Bacterial count per c.c. in original milk 



Bacterial count per c.c. in finished 

 product 



1,250,000 

 3,000,000 

 518,000 

 894,000 

 796,000 

 150,000 



15,000 



21,000 



26,000 



9,950 



10,000 



5,000 



The rate at which the bacteria develop in this milk is shown by the 

 following counts: 



Bacterial count per c.c. 



The lack of moisture and concentration of milk solids prevents the 

 rapid growth of these organisms so that bacterial changes do not take 



Data furnished by H. W. Conn. 



