58 CARBOHYDRATES [ch. 



and soluble sugar, which is translocated and used as a basis for meta- 

 bolism. During the night in leaves there is also a similar hydrolysis of 

 the starch which has been temporarily stored from the excess of sugar 

 synthesized during the day. 



Starch has a very large molecule and thus a high molecular weight. 

 It is insoluble in cold water. When heated with a little water it gives 

 starch paste, but on boiling with water it gives an opalescent " solution " 

 which really contains starch in the colloidal state as an emulsoid. In 

 this condition it does not diffuse through dialyzing membranes and does 

 not depress the freezing point of water. The " solution " cannot, strictly 

 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. 75). 



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 (or use commercial potato starch) 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 



