208 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Properties. 



Commercial dextrin obtained by heating starch is a yellow-brown 

 powder. 



The dextrin obtained by the hydrolysis of starch by malt extract 

 is a white powder resembling starch. It is composed chiefly of achroo- 

 dextrin and is soluble in water giving a sticky solution. The solution 

 has a faint sweet taste and a peculiar smell. 



The dextrin obtained by the action of barley diastase is similar in 

 appearance. 



Reactions. 



(1) Action of Alcohol. 



Dextrin is insoluble in alcohol and is precipitated from its solution 

 in water by excess of alcohol. 



(2) Action of Iodine. 



Commercial dextrin generally gives a reddish-brown coloration 

 on treating with 1-2 drops of iodine solution; this is due to the 

 presence of erythrodextrin. The coloration disappears on heating 

 and reappears on cooling. 



The dextrin produced by malt diastase gives no colour with iodine. 

 That produced by barley diastase gives a blue colour. 



(3) Basic Lead Acetate. 



Dextrin is not precipitated from solution by basic lead acetate. 



(4) Ammonium Sulphate. 



Dextrin is not precipitated by half saturation with ammonium 

 sulphate. 



(5) Fehling's Solution. 



The dextrins reduce Fehling's solution slightly. 



(6) Reduction of Dye-stuffs. 



Solutions of dextrin produce a red colour on warming with picric acid 

 and sodium hydroxide. 



(7) Hydrolysis. 



Dextrin is easily hydrolysed into glucose by boiling its solution 

 with dilute sulphuric acid for a few minutes. The presence of glucose 

 is shown by neutralising the solution with soda and testing with 

 Fehling's solution. 



(8) Action of Alkali. 



On warming a solution of dextrin with alkali it becomes yellow or 

 yellow-brown in colour. 



(9) Fermentation. 



Dextrin is not fermented by yeast. 



(10) Rotation. 



Dextrin has a high dextrorotatory power. 



