328 PEACTICAL PHYSIOLOGY 



The rennin comes from a precursor pro-rennin in the gastric mucous 

 membrane. 



The stages can therefore be tabulated as follows : 



Pro-rennin + HC1 



Rennin Caseinogen (soluble) 

 Sol. Casein + Ca 



Insol. Casein or Clot. 



(Cf. Blood.) 



In order to study the conditions necessary for the clotting of milk a solution 

 of caseinogen may be prepared by the following method (Ringer's) : 



300 c.c. of milk are mixed with an equal bulk of water, and 10% acetic 

 acid is added till all the caseinogen has been precipitated. The precipitate is 

 filtered off and thoroughly washed with distilled water until the washings are no 

 longer acid in reaction. It is then removed from the filter paper, and ground up 

 in a mortar with solid calcium carbonate. The resulting paste is thrown into 

 500 c.c. of water placed in a tall vessel, and the solution is allowed to stand for 

 several hours. The fat, which was contained in the precipitate, rises to the 

 surface, the calcium carbonate sinks to the bottom, and the intervening fluid 

 contains the caseinogen in combination with calcium as calcium caseinogenate, 

 which is soluble in water (Osborne). 



Three samples of the opalescent solution are removed by means of a pipette, 

 and placed in three test tubes labelled A, B, and <?. To A are added a few drops 

 of rennin ; to B a few drops of a 0*5 % phosphoric acid and some rennin ; 

 to C a few drops of a '2 % solution of calcium chloride and some rennin. 



The three test tubes are placed in the water bath at 40 C., when it will be 

 noticed that coagulation occurs only in B and C, in which, besides the ferment, 

 soluble calcium salts are present. 1 In A, although no visible change has taken 

 place, the caseinogen has been converted into the so-called soluble casein, and all 

 that is necessary for the production of clotting is the presence of calcium in. 

 solution. That this is so can be demonstrated by boiling the solution A so as to 

 destroy the ferment, then cooling and adding a few drops of a 2 % solution of 

 calcium chloride, when a clot will at once form. 



The fluid left after the clotting of the caseinogen is known as whey 

 in this case rennet whey. If the caseinogen be got rid of by acid, it 

 is known as " acid whey " ; if by " salting out," as " salt whey " ; if by 

 alcohol, "alcoholic whey," and so on. These wheys are different in 

 composition; for instance, rennet whey and acid whey contain lact- 

 albumin, salt whey and alcoholic whey do not. 



EXPERIMENT VI. Apply the xanthoproteic reaction to some acid 

 whey: a positive result is obtained. Apply also the other protein 

 colour tests. Acidify some of the whey with acetic acid and boil ; the 



1 The phosphoric acid added to B brings some of the Ca salts suspended in the 

 opalescent fluid into solution. 



