IO MODERN METHODS OF TESTING MILK 



ond change takes place, the free casein taking up the 

 acid without definite combination and forming a sub- 

 stance which is familiar as the curd of sour milk. Sim- 

 ilar changes occur when milk is treated with other 

 acids, such as hydrochloric, acetic, sulphuric, etc. Free 

 casein is insoluble in water and also in very dilute acids 

 at ordinary temperatures. The action of acids on cal- 

 cium casein and on free casein is hastened by increase 

 of temperature. Casein dissolves easily in an excess 

 of acid, forming soluble casein salts. 



(3) Action of alkalis on milk-casein. Dilute solu- 

 tions of alkalis (caustic soda, ammonia, etc.) act upon 

 casein and its salts with acids, forming compounds 

 that dissolve easily in water. These alkali compounds 

 of casein are not affected by rennet. Some of these 

 compounds are found in commerce as food and me- 

 dicinal preparations under such names as Plasmon, 

 Nutrose, Santogene, Eucasein, Galactogene, etc. 



(4) Action of heat on milk-casein. Heat alone un- 

 der ordinary conditions, even at the boiling point of 

 water, does not coagulate the casein in milk. Casein 

 may be coagulated by heating under pressure at a tem- 

 perature of about 270 F. The browning of milk 

 heated under pressure is more or less due to changes 

 in the casein. The formation of a peculiar skin on 

 the surface of milk heated above 140 F. is largely 

 due to the calcium casein of the milk and not to albu- 

 min as was formerly supposed. The skin itself con- 

 tains practically all of the constituents of the milk and 

 may be regarded as a kind of evaporated milk. 



(5) Action of rennet on milk-casein. One of the 

 most characteristic properties of the calcium casein 



