CH. VII.] POTATOES AND FLOUR. 173 



206. Pound the residue from the above in a mortar with a 

 2 per cent, solution of sodium carbonate and filter. Acidify the 

 filtrate with acetic acid. A precipitate of casein is formed, which 

 is soluble in excess of acid. To the filtrate from this apply the usual 

 protein colour reactions : they are usually all obtained owing to 

 the presence of proteoses, peptones and various animo-acids. 



D. Potatoes. 



207. Scrape the clean surface of half a potato with a pen- 

 knife, keeping the scrapings as fine as possible. Place the scrapings 

 in a beaker of water, stir well, and strain through fine muslin into- 

 another beaker. Allow this to stand for a few minutes and then 

 note the white deposit of starch. Pour off the supernatant fluid 

 and reserve it for the next exercise. Fill the beaker containing 

 the starch with water, stir well, and again allow the starch to settle. 

 By repeating this process of lixiviation the starch can be obtained 

 quite pure. Examine a little microscopically and note the charac- 

 teristic form of the grains (see Ex. 132) . Heat a little with \vater, 

 cool, and add iodine. A deep blue colour is obtained. 



208. Filter the fluid A, and test portions of the filtrate for 

 proteins by the usual colour tests. Only small quantities of protein, 

 are found to be present, the most marked reaction being Millon's. 



E. Flour. 



White flour from the endosperm of wheat grains 

 contains 70 to 75 per cent, of starch, about 8 per cent, of 

 protein and about i per cent, of fat. The proteins are 

 gliadin (soluble in 70 to 80 per cent, alcohol), and glutelin 

 (soluble in alkali). When treated with water these two 

 proteins form a sticky mass called gluten, the viscidity 

 being due to the gliadin. Thus grains poor in gliadin, as 

 rice and oats, do not form dough when mixed with water. 



Flour only contains glucose if germination has taken 

 place before milling. 



Whole flour is obtained from the whole of the grain, 

 except the outer husk and outer part of the bran. It is 



