302 SCIENCE AND PRACTICE OF CHEESE-MAKING 



boric acid introduced into cheese through the rennet- 

 extract is too small to identify by delicate chemical 

 tests. Commercial rennet-extracts vary in strength 

 and new lots always need testing before being used 



(P- 430). 



Strength of rennet-enzym in coagulating milk- 

 casein. How powerful the action of rennet-enzym 

 is in coagulating milk-casein can be seen in cheese- 

 making, where we use only about one part of rennet- 

 extract for 4,000 or 5,000 parts of milk, and it must 

 be kept in mind that rennet-extract is only a dilute 

 form of the rennet-enzym. It has been estimated that 

 one part of pure rennet-enzym can coagulate three 

 million parts of milk. Apparently, rennet-extract 

 does not exhaust itself by its own action, a general 

 characteristic of enzyms, but can be repeatedly used; 

 at least this is theoretically true. For example, if we 

 could recover from whey and curd the rennet used 

 in coagulating milk, it would coagulate an equal quan- 

 tity again. As stated already, one of the most char- 

 acteristic properties of an enzym is that it can produce 

 very powerful effects without itself being affected in 

 any way. 



Explanation of the coagulating action of rennet- 

 enzym. A large amount of effort has been devoted 

 to the study of the coagulating effect of rennet-enzym 

 in order to ascertain just what the rennet does to the 

 milk-casein to make it coagulate. Many different ex- 

 planations have been offered, but in the present state 

 of our knowledge it is impossible to give an explana- 

 tion of the process that can be regarded as satisfac- 

 tory and conclusive. The most we can do here to 

 advantage is to present the details of the process, so 



