MICRO-ORGANISMS IN MILK 



261 



a screw which operates so as to hold the cream for a longer or 

 shorter period in the separator. The cream becomes more com- 

 pact and richer in fat the longer it is held subject to the centrif- 

 ugal force, so that with increasing richness more bacteria are 

 removed. 



The reason for the progressive decrease of bacteria in the 

 skimmed milk as the cream becomes richer is not clear. Wilkens 

 thinks that a large number of bacteria are destroyed by the violent 

 rotation of the centrifuge, but this does not explain the facts. 

 While the period during which the cream is held in the separator 

 increases, it decreases for the skimmed milk, so that fewer bacteria 



p.*' 



too 



2.0 



\ 



\ 



\ 



\ 



100 



19 * 



feo 



Fig. 84. Showing relative number of colonies in separator cream and 

 milk. Ordinates represent relative numbers of colonies, abscissae represent 

 percentage of fat in cream. Solid line = skimmed milk; broken line = cream. 

 The 100 line represents the relative number of bacteria in the original milk. 



are thrown into the slime. Since, however, the skimmed milk is 

 held for progressively shorter periods, as the cream is held longer, 

 the clumps of bacteria are probably broken up in smaller degree, 

 and thus the progressive decrease of the germ content in separator 

 milk is partially explained. 



Market milk, it is readily understood, may contain large num- 

 bers of bacteria, enormously greater than the number contained 

 in the udder. As a matter of fact, unless great precautions are 

 taken by producers, shippers, and distributors alike, heavy bac- 

 terial pollution of milk cannot be avoided. In most cases the 

 entrance of bacteria to milk is due to filth which drops into the 



