CASEIN AND ALWJMJN. 175 



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 42 to 43 C., 

 add 1*5 c.c. of 10 per cent, acetic acid solution, stirring well ; 

 after the expiration of five minutes, filter, and proceed as above 

 described. 



The author finds that it is an advantage 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 6-39. 

 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 directed for total 

 nitrogen . 



Maissen and Musso have 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 " 

 obtained by precipitation thus is raised. In practice, the two 

 errors have a tendency to compensate one another. 



Albumin is estimated by boiling the filtrate from the casein ; 

 it is preferable to use the filtrate from the solution containing 

 salt solution, and this should be brought to a degree of acidity 

 such that 100 c.c. require 2-2 c.c. N alkali for neutralisation ; 

 the filtrate is raised just to boiling over a small flame, and digested 

 on the water-bath for fifteen minutes ; the albumin separates 

 in a pulverulent form. It is collected on a tared filter or, pre- 

 ferably, in a Gooch crucible, and dried at 100 C. ; the nitrogen 

 may be estimated by the Kjeldahl method, and multiplied 

 by 6-39. The albumin is precipitated practically in a state 

 of purity, and no correction for ash need be made, but, owing 

 to the precipitation not being complete, the results are slightly 

 low ; if the casein has not been completely precipitated, a portion 

 may be found with the albumin. 



A small quantity of so-called " lacto-protein " remains in solu- 



