MECHANISM OF METABOLISM 



213 



Apparently not all organisms which liquefy gelatin are able to de- 

 compose egg albumin; we must conclude that the enzyme liquefy- 

 ing gelatin is different from the proteolytic enzyme dissolving egg- 

 white. 



COAGULATING ENZYMES. The blood-clotting enzyme (throm- 

 base) does not occur in microorganisms. Rennet, however, is found 

 in many species. Rennet is extracted from the stomach of calves 

 and pigs and used to set the curd in milk for cheese making. The 

 enzyme acts upon the casein in milk, decomposing it into paracasein 

 and some soluble protein. The time of coagulation depends upon 

 the temperature of the milk and the concentration of the rennet. 

 This coagulation of milk is quite different from the acid curd, where 

 the insoluble casein is precipitated by the acid. If enough acid is 

 added, the milk curdles immediately; if there is not enough acid, 

 there will be no curd, not even after a long time. An acid curd can 

 be brought back to the original state by an addition of alkali, while 

 a rennet curd by no means can be changed back to casein. Rennet- 

 forming bacteria are found in milk and dairy products, in soil and other 

 habitats. They will coagulate milk without causing any appreciable 

 increase of acidity. They all seem to digest the curd after it is formed 

 (see the above table). The relation between proteolytic and rennet 

 enzymes will be discussed in a later chapter. 



Rennet is sometimes called chymosin; the Society of American 

 Bacteriologists uses the German word "lab." 



