CORRECTNESS OF THE CHEMICAL VIEWPOINT 53 



1918 a method for preparing "pure casein" from skimmed milk, 

 which consisted 



"in the gradual addition of acid and its immediate distribution through 

 the mass of milk without causing coagulation of casein at the point 

 where the acid first comes into contact with a portion of the milk. 

 This result can be accomplished by introducing the acid below the 

 surface of the milk with high-speed mechanical stirring. After stand- 

 ing under gentle stirring for 3 hours with acidity just below the point of 

 casein coagulation, addition of acid is continued slowly, accompanied 

 as before by rapid stirring in order to obtain the particles of casein 

 coagulum in the finest possible state of division." 



The coagulated casein is then centrifuged and after repeated 

 washings is found free from Ca and inorganic P. As Van Slyke 

 and Baker point out, the pH of this casein coagulum is about 

 4.5 to 4.6, i.e., it is slightly below the isoelectric point. The 

 essential feature of Van Slyke and Baker's method therefore 

 consists in slowly bringing the milk or casein solution approxi- 

 mately to the pH of the isoelectric point of casein. The writer 

 has shown that gelatin gives off all ionogenic impurities at the 

 isoelectric point and Van Slyke and Baker's experiments show 

 that the same method works also with casein. The casein 

 prepared after Van Slyke and Baker's method is also free from 

 albumin since this latter protein is soluble at pH 4.5 or 4.7, 

 and is, hence, removed from the insoluble isoelectric casein by 

 washing. 



In our experiments 1 we used casein prepared after Van Slyke 

 and Baker's method from skimmed milk and in addition from a 

 "pure casein" of the market. Both preparations gave practi- 

 cally the same result. In order to remove traces of fat from the 

 casein the latter was washed in acetone. 



It is not possible to prepare 1 per cent casein solutions, except 

 with a few acids, on account of the low solubility of the casein 

 salts with acids. It is, however, possible to compare casein 

 chloride and casein phosphate in 1 per cent solutions. One gram 

 of isoelectric casein, prepared after Van Slyke and Baker, was put 

 into 100 c.c. of watery solution containing 1, 2, 3, etc., c.c. of 

 0.1 N HC1 or 0.1 N H 3 PO 4 . The pH of the casein solution was 

 , J., J. Gen. PhysioL, vol. 3, 547, 1920-21. 



