330 MILK AND MILK PRODUCTS. XIV. 



Cream. The fat globules of milk, being lighter than the 

 liquid in which they are suspended, tend to separate and 

 collect near the surface of the milk when the latter is allowed 

 to remain motionless under the action of gravity. The rapidity 

 with which the separation of the milk into two layers occurs 

 one rich in fat globules and the other almost devoid of 

 them depends upon many conditions. One of the most 

 important is the size of the globules. The milk of Guernsey 

 or Jersey cows quickly throws up its cream owing to the large 

 size of the fat globules, while that of Ayrshire cows is slow in 

 yielding cream. 



Another factor of importance is the difference in the magni- 

 tudes of the forces acting upon the aqueous and fatty portions 

 of the milk. Under ordinary conditions this (depending upon 

 gravitation) is practically constant (though differing to a small 

 extent according to the latitude) ; but, by imparting rapid rota- 

 tion, centrifugal force of far greater magnitude than the force 

 of gravitation, can be brought to bear upon the milk. The 

 separation of the lighter fat from the heavier aqueous portion 

 then becomes very rapid. This is the principle of the milk 

 separators which are coming so rapidly into use. For details 

 the reader is referred to any modern treatise on dairy work. 



Another method of facilitating the separation of cream is 

 known as "deep setting," in which the milk, while yet warm, 

 is placed in cans about 18 inches deep, which are then sur- 

 rounded with cold water or, better, ice. In this case the 

 whole of the fat will be found on the surface after about 

 12 hours. 



It is not quite easy to understand why the fat globules 

 should collect at the surface more quickly when the milk is 

 thus cooled than under ordinary conditions. As fat contracts 

 and expands with changes of temperature more rapidly than 

 water, a low temperature would tend to lessen the buoyancy of 

 the fat globules, and on that account tend to lengthen the 

 time necessary for their coming to the surface. 



The writer is of opinion that the explanation of the action 

 observed is to be found in the gentle convection currents 

 which are set up by the cooling action of ice or water on the 

 walls of the can. The milk in contact with the walls of the 



