108 SCIENCE AND PRACTICE OF DAIRYING. 



hours, even although the surrounding conditions are exactly the same. 

 It is further known that, with an increasing percentage of lactic 

 acid in milk up to the point of spontaneous coagulation, the precipi- 

 tation of the caseous matter becomes more and more imperfect. 

 It is also known that it is not the same in samples of milk of 

 different origin, and that it is sometimes more perfect and some- 

 times less perfect, according to the exact composition of the mineral 

 salts of the milk. The fatty globules, in their motion, have to push 

 aside or push through the coagulated masses of serum. It follows 

 from the nature of the molecular forces coming into play in this 

 connection, that the resistance offered by the different causes 

 mentioned diminishes with the rise of temperature and increases 

 with the lowering of temperature, and also that the condition of 

 the precipitation of the caseous matter is more perfect the higher 

 the temperature. The resistance above referred to is only to be 

 reckoned with in the case when the milk-serum is at perfect rest 

 during creaming, or when, at any rate, no vertical current move- 

 ments exist in the milk. It is difficult, however, to prevent currents 

 arising in creaming operations, due to cooling. The colder portion 

 of the milk, being of greater specific gravity, sinks to the bottom, 

 and the warmer portion, being lighter, rises to the top. In this way 

 the collection of fatty globules on the surface is disturbed and 

 impeded. The descending currents carry away more fat with them 

 from the cream layer than the ascending currents bring back to, 

 the surface. It is only after the entire mass of the milk assumes 

 the same temperature as the surrounding air, and when no further 

 changes owing to temperature are induced, that the fatty globules 

 can follow without disturbance their tendency to collect on the 

 surface. For creaming the following conditions are necessary: 



(1) Milk should be set immediately after milking, since the con- 

 ditions of coagulation of the caseous matter are then most perfect. 



(2) Cream-raising ought to be carried on at the highest possible 

 temperature, in order to avoid, as much as possible, the resistance 

 the fatty globules meet with in coming to the top. 



(3) The milk of large and well-fed cows should preferably be 

 used, since it is very probable that such milk will possess the usual 

 properties of milk, and especially will undergo a proper coagulation 

 of the caseous matter. 



(4) The progress of lactic fermentation, which unfavourably 

 influences the coagulation of the caseous matter, should be retarded 



