190 FIRST PRINCIPLES OF AGRICULTURE. 



in fat depends upon the quality of the milk from which 

 it is derived, and upon the method used in creaming. It 

 is, therefore, not a product of uniform composition ; in 

 fact, it is much less uniform than milk, its content of 

 fat — the chief constituent of value in it — ranging from 

 as low as ten per cent to as high as forty per cent. 



Its purchase or sale, either as food for families or for 

 the production of butter at creameries, should be based on 

 the actual content of fat rather than its volume or measure. 



Systems of Creaming. — These are divided into two 

 classes, — first, the setting systems, in which the cream 

 rises under natural conditions ; and second, centrifugal 

 systems, in which mechanical force is used. 



The simplest setting system is the open-air shallow 

 pan; it is also the most common, but it does not give 

 the best results. In order to get the largest quantity of 

 cream by this method, the milk has to stand too long, 

 which endangers the quality of the butter; besides, the 

 long exposure to the air induces rapid changes and sour- 

 ing, which render the skim-milk less valuable as food for 

 animals, and make it unfit for human food. The deep- 

 setting systems permit of a better regulation of the tem- 

 perature, and of a more perfect protection from the air; 

 while the rapidity and completeness of the creaming is 

 not decreased. 



Mechanical separation is more economical of space, 

 time, and labor, and a larger percentage of the fat of 

 the milk is obtained by it than by any other method ; 

 besides, perfectly fresh cream and skim-milk can be im- 

 mediately obtained by this system. This system of cream- 

 ing has taken the place of the others to a great extent 



