CHEMICAL AND PHYSICAL BEHAVIOR OF CASEIN 

 . SOLUTIONS. 



By JACQUES LOEB. 



{From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, January 6, 1921.) 



1. L. L. Van Slyke and J. C. Baker described in 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 simultaneous high-speed mechanical 

 stirring. . . . After standing 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 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 approximately 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. 



1 Van Slyke, L. L., and Baker, J. C, /. Biol. Chem., 1918, xxxv, 127. 

 2Loeb, J., /. Gen. Physiol., 1918-19, i, 237. 



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