THE INDIVIDUAL GROUPS OF PROTEINS 459 



Separation of Caseinogen and other Proteins. 



10-20 c.c. of milk diluted with 3 volumes of water are gradually 

 acidified with dilute acetic acid avoiding excess ; a flocculent precipitate 

 of caseinogen and fat is formed. The filtrate on nearly neutralising 

 with soda and boiling coagulates (cf. p. 446). It contains the lac- 

 tose and phosphates (below). 



Presence of Lactose and Phosphates. 



Milk turns yellow, then brown, on heating with dilute caustic 

 soda, due to the action of alkali on lactose. 



The filtrate from the separation of caseinogen and the coagulable 

 proteins will show the presence of a reducing carbohydrate; its 

 nature can be determined by preparing the osazone with phenyl- 

 hydrazine and acetic acid. 



The same filtrate will give the reactions for phosphate with nitric 

 acid and ammonium molybdate, or magnesia mixture and ammonia. 



Effect of Salts. 



Saturation of milk with sodium chloride percipitates the calcium salt 

 of caseinogen together with the fat. Saturation of milk with mag- 

 nesium sulphate, or half-saturation of milk with ammonium sulphate, 

 precipitates the calcium salt of caseinogen and the globulin, together 

 with the fat. The solution of the precipitate in water, filtered through 

 a wet paper to remove the fat, is opalescent 



Saturation of milk with ammonium sulphate precipitates the cal- 

 cium salt of caseinogen, the globulin and albumin, together with the fat. 



Human Milk. 



In appearance human milk resembles cow's milk, but it has a differ- 

 ent composition as shown in the table on p. 457. The amount of phos- 

 phates in human milk is also less than in cow's milk. 



Not only is there a difference in composition, but also the caseino- 

 gen of human milk appears to be different. It is more difficult to pre- 

 cipitate this caseinogen with acid. 



The preparation of caseinogen from human milk is best effected by 

 freezing the milk for 2-3 hours after diluting with 5 volumes of water 

 and adding 60-80 c.c. of -iN acetic acid and subsequently shaking 

 and warming to 40 for a few minutes. 



Human milk does not always clot with rennet (p. 461). This de- 

 pends on the smaller quantity of calcium salts in human milk. The 

 clot is also usually not so firm as the clot of cow's milk and the calcium 

 caseate separates in flakes. 



Humanised Milk. 



Cow's milk can be altered in composition so as to approach or be 

 the same as that of human milk. Dilution of cow's milk reduces the 

 amount of protein ; lactose and fat are then added to bring up the con- 

 tent of these constituents. Humanised milk is most easily and best 

 prepared by adding an equal volume of whey to cow's milk and then 

 lactose and fat. If in preparing the whey the milk be continually 

 stirred, the caseinogen is separated and the fat remains in suspension 

 in the liquid. 



