II THE SOURCES OF BACTERIA IN MILK 23 



milking. If not kept absolutely clean they may actually 

 add bacteria to the milk (see Chapter XVI.). 



Milk-coolers are sometimes a source of bacteria in milk. 

 They may be placed in dusty places, so that the milk passing 

 over them, exposing as it does a large surface to the air, takes 

 up a good many bacteria. Also they may not be kept efficiently 

 clean, traces of milk being left, in which bacteria multiply 

 enormously and are subsequently added to the milk. A third 

 but much less common way by which coolers may contaminate 

 the milk is by being leaky, the water being added in small 

 quantity to the milk. 



The following experiments by Orr (loc. cit.) illustrate that 

 milk-coolers may act as contaminators of the milk. The 

 mixed milk in each case was sampled before and after passing 

 over the cooler, with the following results : 



Orr explains the higher number of organisms at 20° C. 

 compared with at 37° C. as due to the bulk of the added 

 organisms being derived from the air. 



In the same way milk-separators usually add bacteria to 

 the milk passing through them. If the separator is not 

 perfectly clean, the increase may be due to added bacteria, 

 but in ordinary cases the increase is more apparent than real. 



If milk is separated, and the cream and separated milk 

 remixed, it will usually be found that the germ-content of 

 the reconstructed milk is higher than before separation. 



Severin^ carried out numerous experiments and always 



1 CcntraJhJ.f. Bakt., 190.5, Abt. 11. xiv. \<. 605. 



