V] CARBOHYDRATES 59 



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

 boiling all the time. Ah 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 

 round-bottomed flask. Test a portion of the solution with iodine from time to time; 

 a purple, red or brown colour is formed due to the dextrin produced in hydrolysis. 

 To the remainder of the solution after neutralization, using litmus as indicator, add 

 some Fehling's solution and boil. Reduction takes place owing to the glucose formed 

 in hydrolysis. 



Dextrins. 



These compounds occur in the plant as transitory substances, since 

 they are formed as intermediate products of the hydrolysis of starch by 

 diastase. They are also formed on heating starch or by boiling it with 

 mineral acids (see previous experiment). The hydrolysis of starch to 

 dextrins is fairly rapid, but the conversion of dextrins into maltose is a 

 much slower process. 



Both starch and dextrins have the same empirical formula. Various 

 forms of the latter have been identified, such as amylodextrin which gives 

 a blue colour with iodine, erythrodextrin which gives a brownish-red 

 colour with iodine, and achroodextrin which gives no colour with iodine. 

 The dextrins are readily soluble in water ; they are precipitated by 

 alcohol but not by basic lead acetate. On hydrolysis with acids, they 

 are converted into glucose. 



Expt. 57. Preparation of dextrin hy hydrolysis of starch, (a) By diastase from 

 leaves of the Pea (Pisum sativum). Weigh out 10 gms. of commercial potato starch 

 and make it into a solution in 250 c.c. of boiling distilled water as in Expt. 55 and 

 cool. Then weigh out 10-15 gms. of fresh leaflets of the Pea {Pisum sativum) and 

 pound them well in a mortar. Add to the pounded mass 100 c.c. of distilled water 

 and a few drops of chloroform (see maltase, p. 77) and filter. The filtrate will contain 

 diastase (see also Expt. 78). Then add the diastase extract to the starch solution in 

 a flask, plug with cotton-wool and put in an incubator for 48 hrs. If a little of the 

 liquid is withdrawn from time to time and tested with iodine, it will be found that 

 the blue colour due to the starch gradually disappears and is replaced by the brownish- 

 red colour due to dextrin. After 48 hrs. there will be no trace of blue colour ; then 

 filter the liquid and concentrate the filtrate on a water-bath to a syrup. Treat the 

 residue with about 30 c.c. of 96-98 % alcohol and filter. A sticky mass of dextrin i 



