168 THE CHEMISTRY OF SOME FOODS. [CH. VII. 



iSS. To 20 cc. of milk in a 100 cc. measuring cylinder add 

 65 cc. of distilled water and 15 cc. of i per cent, acetic acid. Mix 

 thoroughly and allow to stand for about 5 minutes. Mix again and 

 filter through a pleated paper. The filtrate may have to be passed 

 through the paper again, but can usually be obtained perfectly clear. 

 Label the filtrate A. 



189. Treat some of the precipitate obtained in the previous 

 exercise with a little water and about i cc. of 2 per cent, sodium 

 carbonate. Shake vigorously in a test-tube. A milky suspension 

 of fat in an alkaline solution of casein is obtained. 



190. To a portion of this suspension cautiously add acetic 

 acid. The casein is reprecipitated, when the solution is definitely 

 acid to litmus. 



191. Transfer some of the precipitate obtained in Ex. 188 or 

 in Ex. 190 to a test-tube. Add about 2 cc. of " glyoxylic reagent " 

 (Ex. 23), and then 2 cc. of pure sulphuric acid. Mix by gentle 

 agitation. As the casein dissolves in the hot acid a fine purple 

 glyoxylic reaction is developed, due to the presence of tryptophane 

 in casein. 



192. Heat another portion of the precipitate with Millon's 

 reagent (Ex. 22). The precipitate turns brick red, owing to the 

 presence of tyrosine in casein. 



193. Squeeze the remainder of the precipitate obtained in 

 Ex. 188 between filter paper to express as much fluid as possible. 

 Reserve a portion for Ex. 194. 



Place a piece about the size of a pea in a dry test-tube and add 

 10 drops of pure sulphuric acid. Heat over a small flame until the 

 mass charrs. Then cautiously add one drop of pure nitric acid, 

 taking care that an explosive reaction does not endanger yourself or 

 your neighbours. Heat again over the flame until white sulphuric 

 fumes appear in the tube. (Unless the solution is heated until it 

 fumes in the tube the temperature will not rise sufficiently for the 

 complete and rapid oxidation of the organic material.) If the 

 solution again charrs add another drop of nitric acid with the same 



