MICRO-ORGANISMS IN MILK 259 



pies from the same bottle of milk, even though well shaken, will 

 almost invariably give different results, sometimes varying within 

 wide limits. This is due in part to the fact that bacteria occur 

 largely in clumps, and these clumps are not always easily broken 

 up. Furthermore, bacteria chng to suspended matter and, when 

 milk stands, many bacteria settle with suspended particles, while 

 large numbers adhere to the fat globules and rise to the surface. 

 Gravity cream, therefore, contains more bacteria than the skimmed 

 milk below the cream line. 



The distribution of bacteria in fresh cream depends in a measure 

 upon the method by which the cream is separated. Since bacteria 

 cling to the fat globules it is to be expected that a large number 

 of the bacteria in the milk rise with the cream. Furthermore, 

 bacteria multiply rapidly during the time required for the cream 

 to rise, and the temperature best adapted to the process is also 

 favorable for bacterial growth. In centrifugal cream conditions 

 are somewhat different. The cream is separated from the milk 

 immediately after milking, when the temperature is still rela- 

 tively high, or the cream is separated within a short time, if the 

 milk is delivered at a creamery. There is, therefore, no appre- 

 ciable chance for bacterial growth, although clumps of bacteria 

 are broken up during centrifugation. 



The germ content of cream has received the attention of some 

 investigators. Russell and Hastings state that gravity cream is 

 usually richer in bacteria than separator cream, but that both 

 gravity and separator cream have a larger germ content than the 

 milk from which they were obtained. This is not surprising in 

 gravity cream, since some time must elapse before the cream is 

 gathered, but in separator cream the only explanation that is 

 offered is the fact that bacteria cUng to the fat globules and thus 

 escape from the skimmed milk. This view is held by some 

 authorities. Scheurler, working with 20 per cent, cream, agrees 

 that both gravity and centrifugal cream contain more bacteria 

 than the milk before centrifugation, and Niederstadt goes so far 

 as to state that the cream contains 75 per cent, and the separator 

 milk 25 per cent, of the germ content of the milk. Rolet, after 

 centrifugating five samples of milk, found a sUghtly higher num- 

 ber of bacteria in the cream than in the milk. Anderson and also 

 Wilkens substantially agree with this view. 



Other workers have obtained wholly different results. Back- 

 haus and Cronheim's experiments indicate that by centrifugation 

 the majority of bacteria go into the skimmed milk. Eckles and 

 Barnes state that 47 per cent, of the bacteria are thrown into the 

 bowl sediment; 29 per cent, are in the skimmed milk, and 24 per 

 cent, in the cream, Swithinbank and Newman state that 



