v] HYDROLYZING ENZYMES 57 



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 by hydrolysis of starch, (a) By diastase from 

 leaves of the Pea (Pisum sativum). Weigh out lOgms. 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. 75) and filter. The filtrate will contain 

 diastase (see also Expts. 78-80). Then add the diastase extract to the starch solu- 

 tion 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 of the liquid is withdrawn from time to time and tested with iodine, it will be found 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 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 96-98 / alcohol and filter. A sticky mass of dextrin is left 

 which should be extracted with a little hot alcohol and then reserved for the next 

 experiment. To show the presence of maltose, the alcoholic extract is evaporated to 

 dryness on a water-bath, the residue taken up in a little water and the osazone test 

 made (see p. 49) with the solution. Crystals of maltosazone will separate out. 



(b) By diastase from germinating Barley (Hordeum vulgare). Grind well 25 gms. 

 of barley grains in a coffee-mill. Put the flour into a flask and extract with 96-98% 

 alcohol by heating on a water-bath. This will largely free the grain from sugars. 

 Make a starch " solution" of the residue by boiling with 500 c.c. of water and filtering 

 through fine muslin. 



