CONSTITUTION OF CASEIN. 55 



2Ca 3 (P0 4 ) 2 is precipitated, the molecule (casein) being modified 

 by the heating, a portion having no acidic function being removed. 

 On treating with rennet a similar change takes place, a con- 

 siderable portion of the casein molecule being split off, also having 

 no acidic function, and a salt of the composition 



Ca 3 H 4 (Cas)2Ca 3 (P0 4 ) 2 



being precipitated, and the sodium passing to the whey and 

 neutralising a portion of the acidity. 



On the other hand, Milroy states that there is no change in 

 the concentration of hydrogen ions on curdling milk with rennet, 

 but it must be remembered that precipitation of calcium phos- 

 phates may remove hydrogen ions from the solution. 



There is still some uncertainty as to the change that takes 

 place when milk is curdled by rennet ; Hammarsten's view is 

 that the casein is split up into two compounds, and it is quite 

 certain that the curd only contains 86 per cent, of the casein 

 nitrogen, while the remaining 14 per cent, remains in the whey 

 in the form of a whey protein which contains a lower percentage 

 of nitrogen than casein, but, nevertheless, Harden and MacCulluni 

 and Bosworth are of opinion that casein is not split up by rennet, 

 and no nitrogen or phosphorus is separated in the whey ; the 

 apparent discrepancy is probably partly due to the fact that 

 other proteins have been included in the casein, when the action 

 of rennet on milk has been considered, while the purification 

 of casein for experiments on casein solutions has been a process 

 sufficiently drastic to alter the casein. 



Van Slyke and Bosworth consider that the casein separated 

 by porous pots consists of calcium casein plus calcium hydrogen 

 phosphate, and is neutral to phenol-phthalein, and do not con- 

 sider there is any combination, as if the casein be taken up with 

 water the calcium phosphate can be separated by centrifuging ; 

 their conclusion does not quite follow, however, from the facts. 



It is noticed that the sulphur is lower than the phosphorus, 

 although its atomic weight is slightly higher. By treating casein 

 with alkalies a portion of the sulphur is removed as sulphide. 

 It is possible that in the purification of the casein by solution 

 in dilute alkali and precipitation by acids that a small amount 

 of decomposition sets in. 



The following are the amounts of various acids (calculated as 

 c.c. of normal solution per litre of milk) required to precipitate 

 the casein on boiling : 



Hydrochloric and sulphuric, . . 8-6 c.c. 



Acetic and lactic, . . . . 9 to 10 c.c. 



Citric, 13-5 c.c. 



Oxalic, 28-5 c.c. 



Phosphoric, 34 to 35 c.c. 



