DAIRYING 5 



215. The term specific gravity means that if a certain quan- 

 tity or measure of water at a given temperature weighs 100 or 

 1000 pounds, the same quantity of milk serum, or skim milk, 

 weighs 104 or 1040 pounds, and the same quantity of butter fat 

 weighs 90 or 900 pounds. This difference in weight between the 

 milk serum and the fat is the cause of cream separation. If the 

 butter fat were free to move in the serum there would be a 

 complete separation of the fat by allowing milk to stand, but this 

 is not the case. The curd or casein of the milk and other sub- 

 stances present act as a retarder on the fat from coming to the 

 surface. A certain amount of the fat of milk rises when the 

 milk is left standing quietly and forms a layer of cream on the 

 surface, while the skim milk underneath retains some fat, depend- 

 ing on certain conditions. The amount of fat left in the skim 

 milk is the standard commonly used for measuring the efficiency 

 of different methods of cream separation. 



216. The condition of the serum varies in different lots of 

 milk. The casein is not always in the same condition, and this 

 influences the thickness or viscosity of the serum. The greater 

 the viscosity, the more resistance there is to the movement of 

 the butter fat, and the less the viscosity, or the thinner the serum, 

 the easier the fat rises. 



This viscosity of the serum is not only influenced by the con- 

 dition of the curd or casein, but by the amount of milk sugar, 

 etc., in solution in the serum. Milk rich in fat contains usually 

 more solids-not-fat than thinner milk, and the more solids in 

 the serum the greater its viscosity and its resistance to the 

 separation of the fat. If the serum of milk was always the same, 

 rich milk would skim easier than thin milk. 



217. The size of the fat globules in each lot of milk has 

 considerable influence on cream separation. It has been found 

 by looking at a drop of milk under a microscope that the fat is 

 distributed through it in extremely small drops or globules, and 

 that these globules vary in size. As a rule, they are larger in the 

 milk of a fresh cow than when the same cow is a stripper. It is 

 evident that the larger the fat globules, the quicker they will 

 separate from the serum, and this, together with the increased 



