PROTEINS 237 



coagulum or curd, named by some authorities casein, but 

 often spoken of as paracasein : it may also be termed 

 rennet-curd. The exact nature of the action is not 

 thoroughly understood, but the coagulation apparently 

 takes place in three distinct stages. The first action of 

 the rennet is to change the caseinogen into a soluble 

 body, which we may name " soluble casein " ; almost coin- 

 cident with this, is the production of soluble calcium 

 compounds ; the latter then throw down the casein as 

 a dense coagulum which differs from the curd produced 

 by acids in containing practically all the calcium 

 phosphate of the milk. In it is entangled much of the 

 fat originally diffused through the milk. In the absence 

 of soluble calcium compounds, the " soluble casein " 

 remains in solution or suspension, giving no visible 

 evidence of its existence. 



Soluble calcium salts do not precipitate a curd from 

 fresh unrenneted milk, nor will rennet coagulate milk, 

 from which the calcium salts have been removed by the 

 action of ammonium oxalate until a soluble calcium salt 

 is added. 



As already indicated, caseinogen behaves towards the 

 alkalis soda, and potash as an acid body, uniting with 

 them to form salts which dissolve readily in water ; these 

 salts, although they are decomposed by acids with the 

 precipitates of caseinogen, are not coagulated by rennet. 



The following summary of the names in use for the 

 phosphoprotein of milk and its derivatives may be 

 useful : 



(1) In milk calcium caseinogenate (often loosely termed 

 casein) is present. 



(2) Acids precipitate free caseinogen (sometimes called 

 casein}. 



