STUDIES RELATING TO THE CHEMISTRY OF 



MILK AND CASEIN.* 



SUMMARY. 



I. The acidity of fresh milk is due to the presence of acid phos- 

 phates. Titration of phosphoric acid with alkali, in the presence 

 of calcium salts, results in hydrolysis of dicalcium phosphate formed 

 during the titration, whereby free calcium hydroxide and phos- 

 phoric acid are first formed and then calcium hydroxide unites 

 with more dicalcium phosphate to form insoluble tricalcium phos- 

 phate. As a result of these reactions more alkali is required to 

 make a solution, containing calcium and phosphoric acid, neutral 

 to phenolphthalein than is required in the absence of calcium. 

 The calcium must be removed previous to titration by treatment 

 of ioo c.c. of milk with 2 c.c. of saturated solution of neutral potas- 

 sium oxalate. 



II. The amount of phosphorus in casein has been commonly given 

 as about 0.85 per ct. By treating a solution of casein in dilute 

 NH4OH with ammonium oxalate and an excess of NH 4 OH and 

 letting stand 12 hours the phosphorus content is reduced to about 

 0.70 per ct. This lower percentage can not be explained as 

 being due to hydrolysis of casein and splitting off of phosphorus. 

 While some of the casein is hydrolyzed, this portion does not enter 

 into the final preparation and does not affect its composition, because 

 the hydrolyzed portion is not precipitated by acetic acid while the 

 unhydrolyzed part is. The higher figure ordinarily given is due 

 to the presence of inorganic phosphorus (dicalcium phosphate) 

 carried from the milk into the precipitated casein and not entirely 

 removed under the usual conditions of preparation. The lower 

 figure corresponds very closely to two atoms of phosphorus (0.698 

 per ct.) in the casein molecule. Analyses of various preparations 

 of casein containing varying amounts of ash show a general cor- 

 respondence between the ash and phosphorus content. 



III. The similarity between the composition of casein and para- 

 casein, and the fact that casein has been shown to have a molecular 

 weight of 8888 + and a valency of 8, while paracasein has been 

 shown to have a molecular weight of 4444 + and a valency of 4, 1 

 seems to be evidence enough for concluding that the transformation 

 of casein into paracasein is a process of hydrolytic splitting, one 



1 Van Slyke and Bosworth. N. Y. Agrl. Expt. Sta. Tech. Bull. No. 26, and Journ. 

 Biol. Chem., 14:227. 



* Reprint of Technical Bulletin No. 37, December. 



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