CHAPTER XV 

 THE STABILITY OF PROTEIN SOLUTIONS (Continued) 



B. THE STABILITY OF SOLUTIONS OF CASEIN IN WATER 1 



Since isoelectric casein is practically insoluble in water it is 

 easy to study the mechanism of solution of granules of casein in 

 aqueous solutions of acid and alkali. When this is done it is 

 found that this mechanism is entirely different in the two media. 

 In an alkaline solution, e.g., NaOH, casein granules dissolve very 

 much as do particles of sodium oleate, the solution of which is 

 accompanied by phenomena of spreading. According to Quincke 

 such phenomena of spreading are due to a sudden lowering of 

 surface tension between the surface layer of soap and water, 

 whereby projecting small particles of the surface are torn off so 

 that the surface of the granules soon becomes smooth. This 

 happens in the case of casein granules in alkali. There is no 

 swelling noticeable in the particle. 



The forces which drive the Na caseinate into solution are not 

 the forces of the Donnan equilibrium. If this were the case the 

 rate of solution of the granules should reach a maximum at a 

 pH of between 10.0 and 12.0 and should then diminish. As a 

 matter of fact the rapidity of solution increases indefinitely with 

 the pH of the NaOH. In M/2 NaOH the solution of the granule 

 occurs almost instantaneously. This agrees with the fact that 

 solutions of Na caseinate in water require very high concentra- 

 tions of NaCl or LiCl or NH 4 C1 for precipitation. 



A Na caseinate solution of pH 7.0 was prepared containing 

 2 gm. of originally isoelectric casein in 100 c.c. solution. Five 

 cubic centimeters of this solution were added to 5 c.c. of solutions 

 of different salts also of pH 7.0. No precipitation was observed 

 when the concentration of NaCl in the caseinate solution was 

 M or that of LiCl was 3>^ M, or that of NH 4 C1 was 2 M. 



1 LOEB, J., and LOEB, R. F., J. Gen. Physiol., vol. 4, p. 187, 1921-22. 



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