CHAPTER VI 



RELATION OF BACTERIA TO BUTTER 



Effect of creaming on the distribution of the bacteria in 

 milk. When cream is separated from milk in a centrifugal 

 separator, by virtue of its lighter specific gravity, it is thrown 

 toward the center of the machine. The skim milk and the 

 heavier insoluble particles which constitute the sediment that 

 collects on the wall of the separator bowl are of course 

 thrown to the outside. This differentiation of the milk ele- 

 ments also exerts a material influence on the distribution of 

 bacterial life in the milk. If milk were wholly liquid, i.e. 

 contained none of its normal constituents in other than a 

 soluble condition, the distribution of the bacteria present in 

 the milk would depend entirely upon their specific gravity 

 in comparison to that of milk ; but as there are insoluble sub- 

 stances present which are materially lighter (fat), as well as 

 those that are heavier (casein and dirt), than the milk serum, 

 the effect of these materials modify profoundly the distribu- 

 tion of the bacteria. Mud and silt in a body of water carry 

 down the larger part of the microorganisms that may be 

 suspended in the same. So in a similar way the fat globules 

 and the separator slime will be found to act as differential 

 agents in the partial separation of organisms. The immense 

 number of fat globules passing rapidly toward the center of 

 the separator bowl carry with them mechanically a large part 

 of the bacteria in the milk. In gravity creaming the same 

 phenomenon occurs, but to a much less degree. 



