MILK. 305 



in reaction, the casein appears to remain in solution, but- is prob- 

 ably only swollen as in milk, and the liquid contains at the same 

 time a large quantity of calcium phosphate without any precipitate 

 or any visible suspended particles. The casein solutions containing 

 lime are opalescent and have on warming the appearance of milk 

 deficient in fat. Therefore it is not impossible that the white 

 color of the milk is due partly to the casein and calcium phosphate. 



Casein solutions do not coagulate on boiling, but are covered, as 

 milk, with a skin. It is precipitated by very little acid, but the 

 presence of neutral salts retards the precipitation. A casein solu- 

 tion containing salt, or ordinary milk, requires, therefore, more acid 

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

 centration. The precipitated casein dissolves very easily again in a 

 small excess of the acid, but less easily in an excess of acetic acid. 

 The acid solutions are precipitated by mineral acids in excess. 

 Casein is precipitated from neutral solutions or from milk by 

 common salt or magnesium sulphate in substance without changing 

 its properties. Metallic salts, such as copper sulphate, completely 

 precipitate the casein from neutral solutions. 



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

 coagulates with rennet in the presence of a sufficiently great amount 

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

 coagulate. According to SOXHLET and SOLDNER, the soluble lime- 

 salts are only of essential importance in coagulation, while the cal- 

 cium phosphate is without importance. The chemical processes 

 which take place in the rennet coagulation have not been thorough- 

 ly investigated ; still several observations seem to show that casein 

 splits partly into a difficultly soluble body paracasein or cheese, 

 whose composition closely resembles that of casein and which forms 

 the chief product, and partly into an easily-soluble substance, 

 similar to albumose, whey-albumin, which is deficient in carbon and 

 nitrogen (50. 3# C and 13.2$ N, KOSTNEB) and which is produced 

 in very small quantities. Paracasein is not further changed by the 

 rennet enzyme, and it has not the same property of holding calcium 

 phosphate in solution as casein has. If rennet be added to a casein 

 solution free from lime, the solution does not coagulate, but the 

 casein is changed so that the solution, after the enzyme has been 

 destroyed by rapid heating, acts like a paracasein solution, on the 



