102 REPORTS ON THE STATE OF SCIENCE.—1918. 
Chick and Martin showed that both acid and alkaline solutions of 
casein show an increase in viscosity as the amount of acid or alkali 
present is increased, indicating the greater adsorption of water by the 
casein-salt particles than by casein alone. (Cf. Z Chem. Ind. Koll. 
11, 102-105 (1912).) 
Casein adsorbs acids, the amount adsorbed being almost directly 
proportional to the acid concentration. (Cf. TANGL, Chem. Zeit. 
(1908) 1, 1288.) 
The part played by casein in the colloid reactions of milk is very 
pronounced, and is discussed under “The Coagulation of Milk.” 
The Coagulation of Milk. 
The coagulation or “curdling” of milk can be brought about by 
addition of acids, or by means of rennet, an enzyme obtained from 
the stomach of the calf. The casein in the milk is not acted upon by 
rennet in alkaline solutions, but only in acid or neutral solutions, 
and the greater the degree of acidity of the milk, the more rapidly 
does the rennet act. Dilution of the milk with water inhibits the 
action. 
The natural curdling of milk is due to the acidity produced by 
bacterial action, which sets free lactic acid. Various investigators 
have proved that it is the actual hydrogen ion concentration that 
matters, and not the nature of the acid. The time required for 
clotting is proportional to the hydrogen ion concentration. (Cf. 
Michaelis and Mendlessohn, Biochem. Zeit. 58, 315 (1915).) 
These investigators found that the optimum concentration of 
hydrogen ions required for the precipitation of casein by acids in 
pure solutions or in milk = 25 x 10°. The minimum concen- 
tration required was found by Allemann to be 13 x 10°. (Cf. 
Biochem, Zeit. 45, 346-358 (1912).) 
Orla-Jensen showed that equivalent quantities of different acids 
are necessary to curdle equal quantities of the same milk ata given 
temperature, but the acids act at a different rate. (Cf. Oversigt. K. 
Danike. Vidensk. Selsk. Forh., pp. 287-309 (1914).) 
The whole question of the coagulation of milk by acids, and 
especially by rennet, is intimately connected with the question of 
the calcium content of milk, and any complete discussion must 
commence from this point. 
In milk about 76 per cent. of the total calcium is combined with 
the casein ; the total calcium content amounts to about 018 per cent. 
That calcium which is not united to the casein (about 24 per cent.) is 
present as phosphate and citrate. (Cf. Trunz, Zeit. f. physiol. Chem., 
40, 263 (1903-4), Rona and Michaelis. Biochem. Zeitsch.,21, 114 (1909).) 
When milk is heated, the calcium content falls. Thus Sdéldner 
showed that a milk containing 18 mgms. of Ca. per 100cc. when cold, 
lost 14 mgms. (7.¢., 17°5 per cent.) on boiling ; in two other instances 
the loss amounted to 18 per cent. and 24 per cent. respectively. A 
similar decrease took place in the content of P20;. Séldner inferred 
that the Ca. was united with phosphorus as mono- and di-calcium 
phosphate; this on boiling was converted into the insoluble tri-phos- 
phate, and so precipitated. (Cf. Sdldner, Landw. Versuchs. (1888), p. 
351, Boekhout and de Vries, Landw. Versuchs. (1901), p. 221.) 
