NITROGENOUS COMPOUNDS 75 



Casein, the chief nitrogenous constituent of milk, is a 

 phospho-protein. It is obtained as a curdy precipitate when 

 acids are added to milk from which the fat has been pre- 

 viously separated. To purify it, the precipitate must be dis- 

 solved in alkali and reprecipitated several times, and then 

 extracted with alcohol and ether. Though itself an acid, it 

 can, like other proteins, form salts with acids. These are 

 soluble in water, and the casein precipitated by acids is there- 

 fore soluble in excess. 



The potassium, sodium, and ammonium salts of casein are 

 also soluble. The clear solutions pass through clay filters, and 

 are not precipitated by charcoal or other porous substances. 

 In these salts it has an equivalent weight of 1135, and the 

 acid is at least tetrabasic, probably hexa-basic. The com- 

 mercial product known as " Eucasein " is the ammonium 

 salt. " Nutrose " and " plasmon " are sodium and potassium 

 caseinates. 



The caseinates of calcium and other earths form opalescent 

 solutions, from which the protein is precipitated by charcoal 

 and other finely divided insoluble substances. These solu- 

 tions cannot, therefore, pass through clay filters. 



In milk, the casein is present in the form of a calcium salt 

 (calcium dicaseinate), in intimate association with calcium 

 phosphate probably as a true double salt. When casein is 

 precipitated, the calcium phosphate is also thrown down. The 

 term " casein " is often loosely applied to this complex pro- 

 duct which occurs in milk, and also to the coagulum produced 

 by the action of rennet. It should, however, be reserved 

 strictly for the free acid, i.e. the phospho-protein, pure and 

 simple. The term " caseinogen " is now generally used to 

 distinguish the original substance, i.e. calcium caseinate, which 

 occurs in milk. 



It is difficult to say with certainty whether this caseinogen 

 is in true solution or, as Fleischmann has it, merely suspended 

 in a bulky colloidal condition. As a calcium salt it is not 

 coagulated by heat even on boiling. The so-called " haptogen 

 membrane " the scum which forms on the surface when milk 

 is warmed is produced by coagulation of the albumin and 



