304 SCIENCE AND PRACTICE OF CHEESE-MAKING 



when the rennet-enzym itself had been removed 

 from the field of action. 



(2) Second stage of rennet action; change in 

 calcium salts of milk. — In the second stage of rennet 

 action, it is believed that the rennet-enzym acts upon 

 the insoluble calcium salts of the milk, converting 

 them into a form sufficiently soluble to enable them to 

 coagulate the paracasein. This action appears to take 

 place more slowly than does the conversion of casein 

 into paracasein. This accounts for the period of time 

 that elapses between addition of rennet and coagula- 

 tion ; this time can be shortened by addition of soluble 

 calcium salts. 



(3) Third stage of rennet action; precipitation of 

 uncoagiilated paracasein. — During this period, in- 

 creased viscosity (thickening) and visible coagulation 

 take place. This change, it is generally agreed, is 

 caused by the action, either physical or chemical, of 

 soluble calcium salts upon the uncoagulated paracasein 

 formed during the first stage of the process. After 

 the second stage is completed or nearly so, coagulation 

 commences and proceeds rapidly. The paracasein 

 coagulum (curd) formed in milk always contains in- 

 soluble calcium phosphate, which is probably held in 

 a purely mechanical way, although some believe that 

 it is in combination with paracasein. 



What is the evidence leading us to believe that a 

 soluble calcium salt is necessary for the coagulation of 

 milk-casein ? Two lines of experimental evidence have 

 been furnished, (ist) If we prepare a pure solution 

 of neutral calcium casein or sodium casein, contain- 

 ing no soluble calcium salts, rennet-extract will not 

 coagulate such a solution, but, after the addition of 

 some soluble calcium salt, as calcium chlorid, coagula- 

 tion takes place promptly. (2nd) Milk from which 



