232 THE CHEMISTRY OP THE FARM 



a part of the albuminoids and fats, the production of 

 lactic acid, and the partial curdling of the milk. The 

 cream obtained in this way is contaminated with curd, 

 and contains various strongly-flavoured products of 

 decomposition, which deteriorate the quality of the 

 butter. 



A much better plan is to place the milk in metal 

 pails, 16 inches deep, surrounded by cold water or ice. 

 The cream rises quickly, and can all be obtained in 

 twelve to twenty-four hours from the time of setting. 

 Cream thus prepared is perfectly sweet and free from 

 curd, the low temperature at which the milk has been 

 kept having reduced chemical change to a minimum. 

 It occasionally happens that milk will not yield its 

 cream at low temperatures ; this is sometimes the case 

 with the milk of cows several months after calving 

 (when, consequently, the fat globules are small), and 

 especially when receiving a winter diet. 



A third plan of separating cream is by subjecting 

 the milk to extremely rapid horizontal revolution in a 

 centrifugal machine. Under these circumstances the 

 serum, being the constituent of highest specific gravity, 

 is thrown to the outer side of the revolving vessel, 

 while the fat globules rise into the centre of the mass. 

 In Laval's machine the new milk, at a temperature of 

 84°, enters in a continuous stream, and is immediately 

 separated into cream and skim milk, the former leaving 

 the apparatus by a pipe from the middle of the top, the 

 latter by another pipe at the side. Both the cream and 

 skim milk thus obtained are, of course, perfectly sweet. 

 The separation of cream in the centrifugal machine is 

 far more complete than in either of the other pro- 

 cesses. A^out 80 pp-'^ ''■ent. of the milk fat is removed 



