THE PR O TEWS OF MILK. 1 3 5 
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. Eammarsten 4 and, later, Ringer 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 X., 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. ; C'ompt. 
rend. Soc. dc 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. physiol. Chem., 
Strassburg, 1896, Bd. xxii. S. 333), and also the article in this book on Blood. 
2 Edmunds, loc. eit. 3 Joum. Physiol., Cambridge and London, vol. viii. p. 371. 
4 ; ' Zur Kenntniss des Kaseins," Nova Acta Reg. Soc. Scient., Upsala, 1877. Festschrift. 
5 Joum. 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. tit. ) 
7 D. F. Harris, Joum. Anat. and Physiol., London, 1894, vol. xxiw p. 188. 
8 Halliburton, Joum. Physiol., Cambridge and London, vol. xi. p. 462. 
9 Koster, Jahresb. ii. d. Fortschr. d. Thier-C'hem., Bd. xi. S. 14. 
