ACTION OF THE ENZYMES. Ill 



solvents of various kinds, the stomach of the calf, which con- 

 tains the enzyme in enormous quantities. Among these 

 sodium chloride and hydrochloric acid solutions may be men- 

 tioned. Glycerine is also used extensively. When purer 

 preparations are required, precipitation with- alcohol, nitra- 

 tion, and re-solution in water are necessary. 



Caseinase behaves like other ferments in that it is readily 

 destroyed at a rather low temperature. A few minutes' 

 exposure in a neutral solution at 70 C., or in an acid solu- 

 tion at 63 C., suffices to do away with all milk-curdling 

 properties. In an alkaline medium the ferment is affected 

 rapidly even at a very low (room) temperature. Though 

 active in a faintly acid or alkaline medium, 'caseinase acts 

 best in a neutral medium and at a temperature of about 

 40. Below and above this point the rapidity with which 

 milk is made to coagulate falls off rapidly. 



The mechanism of the coagulation of milk has been studied 

 by a number of authors. Briefly summarized it may be 

 stated as follows: Milk contains a protein substance to 

 which the name caseinogen has been given (casein of HAM- 

 MARSTEN), the change of which into casein (paracasein of 

 HAMMARSTEN) is the essential change in the curdling of 

 milk. Casein is formed from caseinogen under a number 

 of circumstances. This occurs when acid is added to the 

 milk, or when any acid-producing change occurs in the 

 milk such as "souring " under the influence of bacteria, 

 or when an electric current is passed through the milk. 

 But milk is caused to coagulate most rapidly in ordinary 

 alimentation through the addition of caseinase. In the gastro- 

 intestinal tract two agencies are active in bringing about the 

 coagulation of ingested milk: first, the hydrochloric acid of 

 the gastric juice, and secondly, the presence of caseinase in 

 the gastric and pancreatic secretions. Another source of 

 caseinase in the alimentary tract is found at times in cer- 

 tain bacteria which may be present. 



When caseinase is added to milk the latter soon sets into 



