ESTIMATION- OF CASEIN AND ALBUMIN. 129 



with acetone to remove the fat. Finally, the precipitate is dried 

 at 75° to 80° C, and weighed. 



They have proved experimentally that the protein is com- 

 pletely separated, and that the weight is not increased appre- 

 ciably by the condensation of the formaldehyde. 



Estimation of Casein and Albumin. 



Hoppe-Seyler's Method consists in diluting 10 grammes of 

 milk with about 100 c.c. of water and precipitating the casein 

 by the addition of dilute acetic acid ; carbon dioxide is passed 

 into the solution in order to complete the precipitation. The 

 precipitate is allowed to settle, and the liquid decanted throujjh 

 a tared filter and treated in exactly the same manner as the 

 copper precipitate in Ritthausen's method. The casein, after 

 drying, is ignited, and the weight of the ash, less the weight of 

 the ash of the filter, subtracted from the total weight. 



Ritthausen prefers to precipitate the casein at a temperature 

 of 40^ C, omitting the passage of the carbon dioxide. Van 

 Slyke has compared these two methods, and finds that the results 

 are identical ; he gives the preference to that of Ritthausen, as 

 being carried out with greater facility, and operates as follows : — 

 Dilute 10 grammes of milk with 90 c.c. of water at 4"J to 13° C, 

 add I'd c.c. of 10 per cent, acetic acid solution, stirring well; 

 after the expiration of five minute.'^, filter, and proceed as above 

 described. 



The author finds that it is an atlvautage to dilute 10 grammes of 

 milk with 90 c.c. of a mixture of equal parts of saturated salt 

 solution and water. 



Frenzel and Weyl have proposed the use of sulphuric acid, 

 but Van Slyke has found that either a slight deficiency or excess 

 of acid causes inaccuracy. He, therefore, does not recommend 

 the method. 



Instead of weighing the casein, the nitrogen in the precipitate 

 may be estimated by Kjeldahl's method, and multiplied by 6 38. 

 In this case it is not necessary to dry the casein, but the preci- 

 pitate with the filter may be dropped into a digestion flask, the 

 acid added, and the method performed as dii-ected for total 

 nitrogen. 



JIaissen and ilusso ha\"e proposed the use of rennet for pre- 

 cipitation of the casein, but this is not accurate. 



As casein is not entirely insoluble in water, especially in the 

 presence of acid, the results have a tendency to be low, especi- 

 ally if the nitrogen be estimated. .On the other hand, it is 

 difficult to wash the casein absolutely free from other milk 

 solids {? calcium citrate) : hence the weight of the " casein " 



9 



