THE INDIVIDUAL GROUPS OF PROTEINS 457 



PHOSPHOPROTE1NS. 



The phosphoproteins constitute the greater part of the protein 

 present in the food-stuffs of young mammals and of embryo birds. They 

 are present in milk, in the eggs of birds and the eggs of frogs and 

 fish. Their characteristic is that they contain phosphorus to the 

 extent of nearly I per cent In this respect they resemble the nucleo- 

 proteins and were formerly termed nucleoalbumins, but they differ 

 from nucleoproteins in that the phosphorus is probably in combination 

 with one of the amino acids ; in the nucleoproteins the phosphorus is 

 present in the nucleic acid with which the protein is combined. 



MILK. 



Milk contains three proteins, caseinogen (or casein), lactoglobulin, 

 lactalbumin, the carbohydrate lactose, butter fat, together with small 

 amounts of lecithin and a yellow pigment, salts, chiefly calcium 

 phosphate, and also in small quantities citric acid, creatine, allantoine. 

 The milk ot different animals has not the same percentage composi- 

 tion : 



Cow's milk and goat's milk are very similar. Human milk contains 

 less protein and more lactose. Mare's and ass' milk most nearly 

 approach human milk in composition. The other milks contain nearly 

 5 times as much fat and differ considerably. 



Cow's Milk. 



Appearance. 



Milk is a white or pale yellow fluid which is opaque except in thin 

 layers. The peculiar non-transparent appearance is due partly to an 

 emulsion of finely divided fat particles and partly to the opalescence 

 of the calcium salt of caseinogen. 



Theyktf particles of varying size are visible under the microscope 

 and some of them exhibit Brownian movement. 



The specific gravity of milk varies from 1028-1035. It is usually 

 taken with a lactometer, a specially graduated and delicate hydrometer. 

 It rises gradually for some time after it has been drawn and the specific 

 gravity should not be taken till after 5 hours. In mixing milks the 

 formation of air bubbles must be carefully avoided as they are held 

 rather tenaciously and cause errors in the determination of the specific 

 gravity owing to the removal of the fat, the lightest constituent, which 

 floats to the surface. Skimmed milk has a higher specific gravity, 

 from 1033-1037. 



