REACTIONS OF PROTEINS 15 



milky appearance in the water draining from the 

 mass. 



The gluten obtained is greyish yellow in colour and 

 elastic, but should not be sticky. The product, how- 

 ever, varies largely, depending upon the quality of the 

 wheat from which the flour was made (see p. 161). 



9. Qualitative Reactions of the Proteins. 



Although some reactions are given by all proteins, other tests 

 are responded to by only a certain number. The special colour 

 tests mentioned below will thus be found to succeed only in some 

 cases. Again, some proteins (legumin, vegetable albumins, 

 peptones) are soluble in water, others (globulins) are soluble in 

 saline solutions, while certain insoluble members (gluten) dissolve 

 only in strong acids or alkalies. 



The following tests may be carried out with gluten, pea meal 

 (containing legumin), ^%g albumin, gelatine, and casein, only the 

 first two of which, however, can be regarded as plant products. 



General Reactions : — 



a. Heat a little of the substance with dry soda-lime 

 in a hard-glass tube. The protein-nitrogen is converted 

 into ammonia, which can be recognised by smell, turning 

 red litmus blue, etc. 



b. Xantho-proteic Reaction, — The substance is boiled 

 for some time with concentrated nitric acid. The yellow 

 solution obtained becomes orange on addition of excess 

 of concentrated NaOH solution. This reaction depends 

 upon the formation of nitrophenolic compounds by the 

 oxidising action of the HNO3. 



c. Biuret Reaction. — To the solution of the substance, 

 if soluble, is added a small quantity of CuSO^ solution 

 and excess of NaOH solution. A violet colour is pro- 

 duced which deepens on heating. 



In the case of insoluble proteins, e.g. gluten, the sub- 

 stance is dissolved in concentrated NaOH solution with 

 application of heat, and the CuSO^ solution then added. 



