306 SCIENCE AND PRACTICE OF CHEESE-MAKING 



physical change was accompanied by any chemical 

 change in the milk-casein. 



Dissolving or digesting action of rennet-enzym. — 



Rennet-extract has the power of dissolving paracasein, 

 this peptic action being slow but continuing for a long 

 time in cheese. Whether one enzym does both the 

 coagulating and the digesting, or whether there are 

 two specific enzyms (rennin and pepsin), each per- 

 forming its special kind of work, is not fully settled, 

 but, as already stated, the results of most recent in- 

 vestigations point to two distinct enzyms. 



Conditions of action of rennet-enzym. — The con- 

 ditions under which rennet-enzym coagulates milk- 

 casein have been extensively studied and we will now 

 consider some of the more important ones. The 

 rapidity and completeness of coagulation of milk- 

 casein by rennet-enzym are dependent upon the fol- 

 lowing conditions : 



(i) The presence of soluble calcium salts appears 

 to be necessary for the coagulation of milk-casein by 

 rennet-enzym. This has been discussed already. 



(2) Effect of acids. — Milk must be neutral or acid 

 in reaction in order to be coagulated by rennet-enzym. 

 Free acids or acid salts favor the action. All acids, 

 whether organic or inorganic, show very marked effect 

 upon the coagulation, though they differ from one 

 another in respect to the extent of influence which 

 they exert on rennet action. The more acid there is 

 in the milk, up to a certain limit, the more quickly does 

 coagulation by rennet-enzym take place. Milk sour 

 enough to curdle is not coagulated by rennet; sim- 

 ilarly, sour buttermilk is not coagulated. The follow- 

 ing table shows the results of some work done at the 



