616 MILK. 



the white color of the milk is. due partly to the casein and calcium phos- 

 phate. SOLDNER has prepared two calcium compounds of casein with 

 1.55 and 2.36 per cent CaO, and these compounds are designated di- and 

 tricalcium casein by COURANT. 



Besides the rather earlier investigations on the salts of casein by SOLD- 

 NER, COURANT, ROHMANN, LAQUETJR, RAUDNITZ l and others we have 

 the recent observations and theoretical discussion of ROBERTSON 2 on the 

 composition, nature and dissociation of the caseinates. We can here 

 only refer to this and the earlier investigations. 



Casein solutions do not coagulate on boiling, but solutions of casein- 

 lime are covered, like milk, with a pellicle. They are precipitated by 

 very little acid, but the presen.ce of neutral salts retards the precipitation. 

 A casein solution containing salt or ordinary milk requires, therefore, 

 more acid for precipitation than a salt-free solution of casein of the same 

 concentration. The precipitated casein dissolves very easily again in 

 a small excess of hydrochloric acid, but less easily in an excess of acetic 

 acid. The combination between casein and acid, like other protein 

 and acid compounds, is precipitated by neutral salts. These acid solu- 

 tions are precipitated by mineral acids in excess. 3 Casein is precipitated 

 from neutral solutions or from milk by common salt, containing calcium 

 or magnesium sulphate, in substance, without changing its properties. 4 

 Metallic salts, such as alum, zinc sulphate, and copper sulphate, com- 

 pletely precipitate the casein from neutral solutions. 



On drying at 100 C., casein, according to LAQUEUR and SACKUR, 

 decomposes and splits into two bodies. One of these, called caseid, is 

 insoluble in dilute alkalies, while the other, the isocasein, is soluble therein. 

 The isocasein is a stronger acid and has other precipitation limits and 

 a rather lower equivalent weight than the casein. 



The property which is the most characteristic of casein is that it coagu- 

 lates with rennet in the presence of a sufficiently large amount of lime- 

 salts. In solutions free from lime-salts the casein does not coagulate 

 with rennet, but it is changed so that the solution (even if the enzyme 

 is destroyed by heating) yields a coagulated mass, having the properties 

 of a curd, if lime-salts are added. The rennet enzyme, rennin, has there- 

 fore an action on casein even in the absence of lime-salts. These last 



1 Soldner, Die Salze d. Milch, etc., and Maly's Jahresber., 25; Courant 1. c.; 

 Rohmann, Berl. klin. Wochenschr., 1895; Laqueur, 1. c., and Hofmeister's Beitrage, 

 7; Raudnitz, Ergebn. d. Physiol., 2, Abt. 1. 



2 Journ. of physik. Chem., 11 and 12; Journ. of biol. Chem., 5. 



3 In regard to the acid combinations of casein and the ability to take up acid, 

 see Laxa, Milchwirkch. Centralbl., 1905; Long, Journ. Amer. Chem. Soc., 29; L. 

 and D. van Slyke, Amer. Chem. Journ., 38; Robertson, Journ. of biol. Chem., 4. 



4 See the works of Hammarsten and Schmidt-Nielsen, Hammarsten's Festschrift, 

 1906. 



