CHAPTER VII. 

 BACTERIA AND BUTTER-MAKING. 



IN making butter from the butter fat in milk, it is neces- 

 sary to concentrate the fat globules into cream, preliminary 

 to the churning process. The cream may be raised by the 

 gravity process or separated from the milk by centrifugal 

 action. In either case the bacteria that are normally pres- 

 ent in the milk differentiate themselves in varying numbers 

 in the cream and the skim-milk. The cream always con- 

 tains per cc. a great many more than the skim-milk, the 

 reason for this being that the bacteria are caught and held 

 in the masses of fat globules, which, on account of their 

 lighter specific gravity, move toward the surface of the 

 milk or toward the interior of the separator bowl. This 

 filtering action of the fat globules is similar to what happens 

 in muddy water upon standing. As the suspended particles 

 fall to the bottom they carry with them a large number of 

 the organisms that are in the liquid. 



Various creaming methods. The creaming method has 

 an important bearing on the kind as well as the number 

 of the bacteria that are to be found in the cream. The 

 difference in species is largely determined by the difference 

 in ripening temperature, while the varying number is gov- 

 erned more by the age of the milk. 



1. Primitive gravity methods. In the old shallow-pan 

 process, the temperature of the milk is relatively high, as 

 the milk is allowed to cool naturally. This comparatively 

 high temperature favors especially the development of 

 those forms whose optimum growing-point is near the air 



