THE PROTEIDS OF MILK. 135 



salts appear necessary, and that coagulation can be delayed or prevented 

 by decalcifying the fluid. This is most readily done by adding a small 

 quantity of a soluble oxalate. 1 Peptone has, as with blood, a retarding 

 effect on coagulation. 2 



Green 3 has suggested that there is a definite relationship between 

 the ferment and the calcium salt, resembling that which exists between 

 pepsin and hydrochloric acid. Hammarsten 4 and, later, Einger 5 showed 

 what this relationship is. The formation of casein from caseinogen is, in 

 fact, a double process : the first action is that of the ferment which 

 converts the caseinogen into what we may call " soluble casein " ; the 

 second action is that of the calcium salt which precipitates the casein 

 as curd, which is probably caseate of lime. This may be shown by 

 taking a solution of caseinogen and adding rennet ; if the mixture is 

 warmed to 40 C., no visible change occurs ; but nevertheless soluble 

 casein, and not caseinogen, is now present. If the mixture is now boiled 

 to destroy the ferment, cooled, and a drop of 2 per cent, calcium chloride 

 added, the formation of a curd takes place. 6 



Casein and caseinogen differ in several of their properties. The 

 curd of caseinogen precipitated by acetic acid is not nearly so coherent 

 as the curd of casein produced by rennet. The precipitability of 

 caseinogen by acid is not prevented by the addition of an oxalate, 

 and there is 13 per cent, more calcium phosphate used up in rennet 

 coagulation than in acid precipitation. 7 



The action of rennin upon caseinogen is not a simple conversion of 

 that proteid into one of a more insoluble kind ; but just as the fibrin 

 ferment splits the molecule of fibrinogen into an insoluble proteid, fibrin, 

 and a soluble globulin which passes into the serum, so rennin splits 

 the caseinogen molecule into two parts : one part is the curd or casein ; 

 the other is a soluble proteid which passes into the whey, and is termed 

 " whey proteid " by Hammarsten. This is the equivalent of the lacto- 

 protein of other investigators. Some of these state it is like a proteose 

 or peptone. It is certainly not coagulated by heat ; it is precipitable by 

 saturating with magnesium sulphate ; rennet has no further action on 

 it. It does not, however, give the pink biuret reaction. 8 It contains 

 C., 50-3 ; and N., 13'2 per cent. 9 



Caseinogen. This proteid may be precipitated from milk by the 

 addition of acids like acetic, or by saturation with salts like sodium 

 chloride and magnesium sulphate, or by half-saturation with ammonium 

 sulphate. In all cases the fat of the milk is entangled with the 

 precipitate. Caseinogen may be most readily prepared free from fat 

 by first half-saturating the milk with ammonium sulphate ; the precipi- 

 tate is collected, well washed with half-saturated solution of the same 



1 Arthur and Pages, Arch, de physiol. norm, et path., Paris, Ser. 5, tome ii. ; Compt. 

 rend. Soc. de biol., Paris, tome xliii. The addition of oxalates does not absolutely decalcify 

 blood or milk ; the calcium in close combination with the proteid remains unprecipitated. 

 See Schafer (Proc. Physiol. Soc., 1895, p. xviii) ; Hammarsten (Ztschr. f. j)hysiol. Chem., 

 Strassburg, 1896, Bd. xxii. S. 333), and also the article in this book on Blood. 



2 Edmunds, loc. cit. 3 Journ. Physiol., Cambridge and London, vol. viii. p. 371. 



4 ' Zur Kenntniss des Kaseins," Nova Acta Reg. Soc. Sclent., Upsala, 1877. Festschrift. 



5 Journ. Physiol., Cambridge and London, vol. xi. p. 464. 



6 Here the analogy of casein and fibrin breaks down. In blood coagulation the cal- 

 cium salts assist in the genesis of the fibrin ferment rather than in the formation of 

 fibrin from fibrinogen (Hammarsten, loc. cit.} 



7 D. F. Harris, Journ. Anat. and Physiol., London, 1894, vol. xxix. p. 188. 



8 Halliburton, Journ. Physiol., Cambridge and London, vol. xi. p. 462. 



9 Koster, Jahresb. ii. d. Fortschr. d. Thier-Chem., Bd. xi. S. 14. 



