74 A MANUAL OF BACTERIOLOGY 



inevitable, siiice commercial milk is exposed to more or less con- 

 tamination with bacteria before it reaches the consumer. 



One of the methods of determining the quality of milk is by 

 bacteriological examination. To determine the exact character 

 of the milk, physical and chemical exammations should also be 

 made. In general the bacterial content of milk depends upon 

 three factors : (a) the number and kinds of organisms gaining 

 access to the fresh milk ; (J) the temperature at which the milk 

 has been kept ; (c) the age of the milk at the time samples are 

 drawn for analysis. 



The student may consult the following works, among others : 



Barthel. Methoden zur TJntersuchung von Milch und Molkereiprodukten, 

 2te Aufl. Leipzig, 1911. 



Conn. Bacteria in Millt. New York, 1907. 



Fleischmann. The Book of the Dairy. Transl. by Aikman and Wright. Lon- 

 don, 1896. 



Jensen. Essentials of Milk Hygiene. Transl. by Pearson. Philadelphia, 1907. 



Lafae. Handbuoh der technischen Mykologie. Jena, 1905-1908. 



LoHNis. Handbuoh d. landw. Bakteriologie. Berlin, 1911. 



Marshall. Microbiology. Philadelphia, 1911. 



Milk and its Relation to Public Health (by various authors). Bulletin No. 41, 

 Hygienic Laboratory, U.S. Public Health and Marine Hospital Service, 1909. 



Russell and Hastings. Experimental Dairy Bacteriology. New York, 1909. 



SwiTHiNBANK and Newman. Bacteriology of Milk. London, 1903. 



Weigmann. Mykologie der Milch. Leipzig, 1911. 



Exercise 109. The Contamination of Milk with Bacteria from 

 Various Sources 



1. From atmospheric dust. The relation of bacteria to dust 

 has already been demonstrated in Exercise 60. Parts of this 

 experiment may, however, be repeated under different conditions 

 in the barn or in the milk rooms. 



2. From the coat of the cow. Carry four plates of sterile agar 

 to the barn. Wet the inner surface of a milk pail and place an 

 uncovered plate of agar in the bottom of the pail. Hold the 

 pail in position for milking and manipulate the cow's udder as 

 in milking for thirty seconds. 



Expose the other two plates under cows whose udders and 

 flanks have been thoroughly moistened with a clean moist cloth. 

 Incubate the plates in the laboratory for two days at 30° C. 



