CASEIN 97 



neutralize the alkali, which it was necessary to add to the solution 

 in order to just redissolve the casein was then determined. The 

 point of complete re-solution of the casein was determined by 

 noting the point at which the refractive index of the filtered mix- 

 ture attained a constant (maximum) value. It was found that at 

 "saturation" in solutions containing 1.25 per cent of casein, 1 

 gram of casein = approximately 32 X 10~^ equivalent gram mole- 

 cules of HCl. The acid-equivalent in more dilute solutions ap- 

 pears to be somewhat higher, although I have not measured it 

 directly.* This is attributable to the fact that, as the observa- 

 tions of Van Slyke and Van Slyke show, the hydrochloride of 

 casein tends in some measure to decompose into casein and hydro- 

 chloric acid, and, since free casein is insoluble, an excess of acid 

 over that actually combined with the casein is necessary in order 

 to diminish this (hydrolytic) dissociation sufficiently to keep the 

 casein in solution. The same phenomenon, as we shall see, is 

 encountered in acid solutions of serum globulin. 



Schlossmann (50) and Arny and Pratt (2) have described com- 

 pounds of casein with alums which, especially that with ferric 

 alum, are but sparingly soluble. Upon this fact Arny and Pratt 

 have based a method of estimating casein volumetrically. 



Paracasein, the product of the action of rennet upon casein, 

 resembles casein very closely, save in the greater insolubility of 

 the calcium salt in the presence of an excess of calcimn ions. 

 Calcium caseinate is precipitated by calcium chloride, a smaller 

 amount of calcium chloride being required to bring about complete 

 precipitation, the smaller the proportion of base in the caseinate 

 (i.e., the more acid the solution). This precipitation is still more 

 readily accomplished at higher temperatures (24) (59). Calcium 

 paracaseinate differs from calcium caseinate in that it is precipi- 

 tated by a smaller excess of calcium ions, and, by a given excess, 

 at a lower temperature (24). Paracasein differs, however, still 

 more markedly from casein in the proportion of base which it 

 binds at "saturation" of the base with protein, i.e., when the 

 proportion of inorganic base is sufficient and only just sufficient to 



* The accuracy of this estimate of the combining-capacity of casein for 

 acids at "saturation" of the acid by protein cannot be rehed upon, since the 

 solution necessarily contained a certain quantity of the chloride of the alkah 

 employed to dissolve the casein, and the compounds of casein with acids are 

 very readily precipitated by salts. 



