156 PRACTICAL DAIRY BACTERIOLOGY 



PASTEURIZING ON A LARGE SCALE 



In the last few years the plan of pasteurizing the milk on a 

 large scale has come to be frequently adopted. This is done 

 sometimes in creameries in connection with butter-making, and 

 more frequently in some of our large cities for treatment of 

 the general milk supply. In some countries the pasteurization 

 of cream for butter-making is practically universal, but this 

 is not true in America. In the pasteurization of the public milk 

 supply the purpose has not been primarily to protect the public 

 but to keep the milk from souring. Milk distributers have found 

 it difficult to furnish milk that will keep without preservatives, 

 but have learned that the application of heat enables them to 

 do so. For this reason pasteurization has become widely adopted 

 by large milk companies. 



The pasteurization of milk on a large scale involves the use 

 of special machinery, which it is not the purpose of this book 

 to describe in detail. Two different principles are involved in 

 the different pasteurizing machines. 1 One class is called discon- 

 tinuous pasteurizers. (Fig. 57.) In these the milk is placed in a 

 large receptacle and then heated, usually by steam running 

 through coils of pipes of special devices. During the heating it 

 is constantly agitated and is retained at the desired temperature 

 for any length of time that may be wished ; after which it is 

 cooled by the running of cold water through the same pipes, or 

 by some other device. It is then removed as pasteurized, and 

 more milk takes its place. The second class is called continuous 

 pasteurizers. Here the milk flows into a machine in a constant 

 stream, and after passing through a series of conduits in which 

 it is heated to the desired temperature, it is passed through 

 others where it is once more cooled, and finally emerges from 

 the machine in a continuous stream of cool, pasteurized milk. 

 The efficiency of continuous pasteurizers depends naturally upon 



1 Weigmann. Milchztg., p. 417, 1901. 



