56 CARBOHYDRATES AND THEIR [CH. 



speaking, be filtered, but generally, when hot, it passes to some extent 

 through ordinary filter-paper. Starch is insoluble in alcohol and is pre- 

 cipitated by it. 



The most characteristic reaction of starch is the blue colour it gives 

 with iodine solution. This blue colour disappears on heating, but re- 

 appears again on cooling. Starch is precipitated from "solution" by 

 half saturation with ammonium sulphate : it does not reduce Fehling's 

 solution. 



By boiling with dilute acids, starch is first converted into " soluble 

 starch " which still gives a blue colour with iodine. On further boiling, 

 various dextrins (see dextrins) are obtained which give either purple, 

 red or no colour with iodine. The final product, after prolonged boiling 

 with acids, is glucose. Hydrolysis with diastase yields dextrin and 

 maltose (see diastase, p. 73). 



Expt. 54. Preparation of starch from Wheat. Starch may be prepared from 

 a cereal by the following method. 



Take 25 gms. of flour and make it up into a dough with a little water. Allow it 

 to stand for half an hour. Then tie a piece of muslin over the top of a beaker which 

 is filled with water. Place the dough on the top of the muslin and rub it gently with 

 a glass rod. The starch will be separated from the gluten, and will be washed 

 through the muslin and on standing will sink to the bottom of the beaker. Allow 

 this to stand till the starch has settled, then decant off the bulk of the liquid. Filter 

 off the starch, and wash well with water, then with alcohol and finally with ether. 

 Dry in the steam-oven. 



For the detection of starch in green leaves, see Expt. 77. 



Expt. 55. Tests for starch. Take a small quantity of the starch prepared in the 

 previous experiment and shake up with a little cold water in a test-tube. Filter, and 

 test the filtrate with a drop of iodine (in potassium iodide) solution. No blue colour 

 is obtained. Pour a drop of the iodine solution on the residue in the filter. It turns 

 deep blue. 



Weigh out 2 gms. of the starch prepared in the last experiment, and mix it into 

 a thin cream with a little water. Boil rather more than 100 c.c. of water in an evapo- 

 rating dish, and then gradually add to it the starch paste, keeping the water boiling 

 all the time. An opalescent " solution " is obtained. With a few c.c. of the solution 

 in each case make the following tests: 



(a) Add 1-2 drops of iodine solution. A blue colour is obtained. Heat the solu- 

 tion : the blue colour disappears, but reappears on cooling. 



(6) Add an equal volume of alcohol : the starch is precipitated. 



(c) Add an equal volume of saturated ammonium sulphate solution : the starch 

 is precipitated, i.e. by half saturation with this salt. 



(d) Add basic lead acetate solution : the starch is precipitated. 



Expt. 56. Hydrolysis of starch. To 50 c.c. of the starch solution prepared in the 

 last experiment add 1 c.c. of strong sulphuric acid. Boil for 10-20 minutes in a 



