CHAPTER XL 



THE ESTIMATION OF PROTEINS. 



PROTEINS may be either estimated collectively as total proteins, 

 or separate determinations of casein and albumin can be per- 

 formed. 



Total Proteins from Total Nitrogen. The determina- 

 tion of total proteins is generally performed either by making 

 an estimation of total nitrogen in milk, and multiplying this 



100 



by 6-39 ( = tg - K , as both casein and albumin contain this amount 

 ID-DO 



of nitrogen), or by precipitating the proteins as copper salts by 

 Ritthausen's method. 



Ejeldahl's Method. To determine total nitrogen the method 

 of Kjeldahl is the most convenient. Five grammes of milk are 

 weighed into a round bottomed hard glass flask of about 150 c.c. 

 capacity and 20 c.c. of pure sulphuric acid added. This is 

 placed over a small flame and heated till thoroughly charred, the 

 water being evaporated during this heating ; the flame is now 

 removed, and about 10 grammes of potassium bisulphate or 

 sodium sulphate added, and a small drop of mercury. A funnel or 

 pear-shaped bulb with an elongated projection is placed in the 

 neck of the flask, and heat applied, the flame being gradually 

 increased as frothing ceases. In about half an hour the liquid 

 becomes colourless ; it is allowed to cool, diluted with water, 

 and transferred to the distillation flask. This may conveniently 

 be of copper or brass, and, for ease of manufacture, can be in the 

 form of a bottle ; the digestion flask is washed out with water, 

 care being taken that any white crystals which form on cooling, 

 and become yellow on dilution, are transferred to the distillation 

 flask. A cork carrying a dropping funnel with stopcock, and a 

 wide tube with one or more bulbs blown on it, is fitted ; one end 

 of the tube is connected with a condenser, the other end of 



N 

 which is made to dip just below the surface of 50 c.c. of ^ 



sulphuric acid. 



One hundred c.c. (more or less) of a 30 per cent, solution of 



171 



