76 THE CHEMISTRY OF CATTLE FEEDING 



globulin. It contains fat and other substances mechanically 

 associated. It is probable, however, that the caseinogen does 

 undergo change when milk is boiled. Solutions of pure cal- 

 cium caseinate become turbid on warming to about 35 C., 

 owing to a partial hydrolytic separation of the base. The tur- 

 bidity disappears again on cooling, probably owing to reforma- 

 tion of the original salt. These changes may be roughly 

 indicated by an equation as follows : 



Ca-caseinate + 2H 2 O ^ Ca(OH) 2 -f casein 



By the action of the rennet ferment, casein is converted 

 into a soluble form called para-casein. This substance reacts 

 with calcium salts, forming calcium para-caseinate, which is 

 insoluble, and is therefore precipitated as a curd. If all the 

 calcium salts be first removed from solution by precipitation 

 with ammonium oxalate, the first stage of the change, i.e. the 

 conversion of casein into soluble para-casein, takes place, but 

 the second does not, and the curd is not formed. 



Vitellin, the chief protein in the yolk of birds' eggs, and 

 ichthulin, obtained from the ova of fish, both belong to the 

 phospho-protein group. 



Histones and Protamines. These exhibit a strongly basic 

 character, owing to the preponderance of basic radicles in the 

 molecule. They are precipitated from aqueous solutions by 

 alkalis, and even by ammonia, but are soluble in excess. 



The histones are coagulated by heat only in the presence 

 of salts. They are precipitated by the alkaloid reagents from 

 neutral solutions, whereas other proteins react only in the 

 presence of acids. The percentage of sulphur in histones is 

 smaller, and that of the nitrogen larger, than in the typical 

 compounds. Histones can unite with serum-albumin, oval- 

 bumin, and with casein, and the compounds come down as 

 precipitates when these substances are added to neutral solu- 

 tions. Globin, the essential protein radicle of haemoglobin, is 

 a histone. 



Protamines are even more strongly basic than histones. 

 They are precipitated by alkaloid reagents, even from alkaline 

 solutions, and are not coagulated by heat. Protamines contain 



