182 



AGRICULTURAL BACTERIOLOGY 



The milk in contact with the walls flows less rapidly than 

 that in the middle of the machine; consequently, while 

 some of it may be heated sufficiently long to permit of the 

 thorough destruction of all disease-producing bacteria, if 



they are present, other 

 portions may be insuffi- 

 ciently heated. When 

 the possibility exists 

 that such organisms as 

 the typhoid and tubercle 

 bacteria may be present, 

 it is apparent that no 

 method can be regarded 

 as entirely satisfactory 

 that will not insure the 

 destruction of these 

 types. 



While this flash 

 method of pasteuriza- 

 tion was earlier adopted 

 by many milk dealers 

 for the treatment of 

 milk supplies, it has not 

 been with the full ap- 

 proval of health authori- 

 ties, and gradually it has been displaced by the more effi- 

 cient holding method. The fundamental principle of the 

 holding process is that a given quantity of milk can be held 

 at any desired temperature for any given length of time. 

 This makes it possible to treat the milk in such a way as 

 to insure perfect safety, through the complete destruction 

 of disease-producing organisms. . 



In the methods described the destruction of all bacteria 

 is not attained. To prevent the rapid development, it is 



Fig. 39. A Home ; Made Pasteurizer 

 Unless the milk is known to be safe, it 

 should be pasteurized in the home. A 

 tumbler is a convenient cover for the milk 

 bottle during pasteurization and storage. 



A floating dairy thermometer is desirable 



