MICRO-ORGANISMS AND ENZYMS 303 



far as they appear to be worked out. The rennet 

 coagulation of milk-casein is believed to take place 

 in three quite distinct stages or phases, as follows : 

 (i) Change of casein into paracasein; (2) change 

 of the calcium salts of the milk into soluble form ; 

 and (3) precipitation of uncoagulated paracasein by 

 the soluble calcium salts. 



(i) First stage of rennet action; change of casein 

 into paracasein. — The change of casein into paracasein 

 is wholly dependent on the action of rennet-enzym. 

 There is no change visible to the eye, neither increase 

 of consistency (viscosity) nor any apparent coaguk- 

 tion. In the absence of soluble calcium salts, the 

 paracasein that has been formed remains in this un- 

 coagulated condition. The action in this stage of the 

 process takes place as well in the cold as at higher 

 temperatures. What evidence have we that casein is 

 changed into paracasein before coagulation takes 

 place ? This is shown experimentally as follows : To 

 a solution containing some salt of casein, free from 

 soluble calcium salts, we add rennet-extract. No co- 

 agulation takes place. This solution is heated high 

 enough to destroy the power of the rennet to a.ct and 

 then cooled, after which calcium chlorid or some other 

 soluble calcium salt is added, when coagulation ap- 

 * pears at once. It may be stated here that one of the 

 / most characteristic differences between milk-casein 

 and paracasein is that soluble calcium salts da not 

 coagulate milk-casein at ordinary temperatures, but 

 they do cause coagulation of paracasein. In the fore- 

 going experiment, rennet does something to the casein 

 compound which causes the casein* to do what it could 

 not do before, that is, coagulate at ordinary tem- 

 peratures by addition of soluble calcium salts, even 



