MICRO-ORGANISMS AND ENZYMS 305 



the soluble calcium salts have been removed by pre- 

 cipitation with ammonium oxalate or by dialysis is 

 not coagulated by rennet-enzym until a soluble cal- 

 cium salt is added. We may, therefore, summarize 

 as follows what appears to be fairly well established 

 in explanation of the coagulating action of rennet: 



(1) That milk-casein is the only substance in milk 

 involved in the rennet coagulation, excepting phos- 

 phates of calcium and other soluble salts of calcium. 



(2) That in rennet coagulation, no change of reaction 

 or acidity occurs ; the milk becomes neither acid nor 

 alkaline through rennet action. (3) That the two 

 active agents in the rennet coagulation of milk are 

 rennet-enzym and soluble calcium salts. 



Relation of casein and paracasein. In the fore- 

 going discussion of the process of rennet coagula- 

 tion, there is nothing to indicate just what happens 

 to milk-casein in being changed into paracasein, 

 or, in other words, just how paracasein really dif- 

 fers from milk-casein. It must be confessed that 

 we do not know at all clearly, although there are 

 many suggestions. We know only this with cer- 

 tainty, that milk-casein does not readily coagulate 

 in the presence of dilute calcium salts at ordinary 

 temperatures, but paracasein does. Otherwise the 

 general properties of casein and paracasein are 

 very similar. Some hold that the difference is 

 purely physical, the paracasein consisting of larger 

 particles than the casein. While the ultramicro- 

 scopic study (p. 143) of rennet coagulation enabled 

 the observers to see the minute particles of casein 

 come together and form larger aggregations under the 

 action of rennet, this does not show whether this 



