lO MODERxX METHODS OF TESTING MILK 



ond change takes place, the free casein combining di- 

 rectly with the acid, forming casein lactate, which is 

 familiar as the curd of sour milk. Similar chemical 

 changes occur when milk is treated with other acids, 

 such as hydrochloric, acetic, etc. Both free casein and 

 its familiar salts formed with acids are insoluble in 

 water. The action of acids on calcium casein and on 

 free casein is hastened by increase of temperature. 

 Both casein and casein compounds with acids dissolve 

 in an excess of acid, probably 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-cfiscin. — 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 



