v] CHANGES WHICH MILK UNDERGOES 55 



twenty-four hours, from 80 to 85 per cent of the fat 

 will be found in the surface cream layer. The rate at 

 which the fat globules find their way to the surface 

 is dependent on their size. The larger globules rise 

 first. The very small globules never rise at all, as 

 the amount of nitrogenous matter which adheres to 

 them is too great for them to assert their lighter 

 specific gravity. This accounts for the fact that all 

 the fat is not found in the cream layer, even after 

 milk has stood for a long time. No doubt, with the 

 help of centrifugal force, more complete separation 

 may be effected ; but even under such circumstances 

 complete separation is not obtained, although as 

 much as from 90 to 96 per cent of the fat may be 

 separated in this manner. 



Souring. After a time milk spontaneously co- 

 agulates and develops a sour taste. Before this 

 takes place, however, a careful inspection of the milk 

 would show that it had undergone very considerable 

 changes. From the moment of its leaving the udder 

 it is taken possession of by a class of bacteria which 

 are known as lactic bacteria, and of which no less 

 than one hundred different kinds have been identified 

 already. These bacteria produce lactic acid in milk. 

 This they do by decomposing the milk-sugar, which 

 is present to the extent of about 4J per cent, and 

 which is a very easily decomposable substance. Now 

 these bacteria, like all other bacteria, are very much 



