126 



Dairy Bacteriology. 



may exert the same effect on the bacteria as a higher tem- 

 perature for a shorter time. In practice, two types of 

 pasteurizing machines are employed, depending on the 

 temperature at which the milk is to be treated. The dis- 

 continuous machines or intermittently operated pasteur- 

 izers are those in which the milk is heated for any desired 

 lime at any temperature. Such machines eonsist of 

 jacketed containers the inner receptacle being filled with 



PIG. 28. THE POTT'S DISCONTINUOUS PASTEURIZER. 



The milk is placed in the inner compartment. For heating and 

 cooling, hot or cold water is passed between the jackets. 



milk, while the outer space between the walls is filled 

 with circulating hot water or steam. The milk is kept 

 agitated by the rotation of the machine. After it is 

 heated, it is. cooled in the same container by replacing 

 the hot water first with cold water, then ice water. The 

 disadvantage of this process is that the capacity of the 

 machine is limited which precludes its use in places where 

 large quantities of milk or cream are handled; for the 

 pasteurization of limited quantities, it is very successful, 

 as every particle of milk or cream is under the direct con- 



