PROTEINS 



147 



2. The second assumption is that all proteins contain 16 per cent of 

 nitrogen, in which case the ratio of the protein to nitrogen is 100:16 

 or 6.25. Since the nitrogen content varies from 15.5 to 18.7 per cent, 

 the use of the factor 6.25 may give too high or too low results, depending 

 upon the protein under consideration. In the case of the cereals, the 

 nitrogen content is higher than 16 per cent, and the factor 5.7 more 

 nearly represents the ratio between the protein and the nitrogen. With 

 milk the factor 6.37 is used. In general, however, the factor 6.25 is used 

 and is the one that is employed in the calculation of general tables of 

 analysis for the composition of food materials. 



It should be observed that the term "crude protein" has nothing to 

 do with the purity of the protein, but merely j-epresents results obtained 

 by laboratory procedure. 



Purpose of reagents used in determining total nitrogen 



In the Kjeldahl determination of total nitrogen, carbon and hydrogen 

 must be oxidized and the nitrogen converted into ammonium sulfate. 

 Sulfuric acid is the oxidizing reagent that is used. The material first 

 becomes black and charred, but after prolonged heating it clears up and 

 becomes free from carbon. The action of the sulfuric acid may be repre- 

 sented by the following equation: 



H0SO4 -» H.O + SOo + O 



The sodium or potassium sulfate added raises the boiling point of 

 the solution and thus aids in the oxidation of the material. A small 

 amount of copper selenite is also added and acts as a catalyst to promote 

 oxidation. The oxidation of protein may be represented by the oxida- 

 tion of a fragment of a protein, for example, glycine. 



CH2(NH2)-COOH + 30-> 2CO2 + HgO + NH3 



2NH3 + H0SO4 -^ (NH4)2S04 



Through the addition of concentrated sodium hydroxide, ammonia is 

 liberated and collected by distilling it into a known amount of standard 

 acid. From the amount of acid neutralized by the ammonia, the nitrogen 

 and, hence, the crude protein (N X 6.25) are obtained by a simple 

 calculation. 



REVIEW QUESTIONS ON PROTEINS 



1. Name two of the chief proteins in (1) milk, (2) eggs, (3) blood, (4) wheat; 

 one protein in (5) com and (6) peas. Which of these proteins probably contain (a) 

 phosphorus? (b) sulfur? 



