126 THE CARBOHYDRATES 



6. They do not reduce Fehling's solution when pure. 



7. They are converted into glucose on hydrolysis with 

 mineral acids. 



As has already been mentioned, starch when suddenly 

 heated to about 200 is converted into a substance commercially 

 known as dextrin. The use of starch for stiffening linen 

 depends on some such similar change produced in the starch 

 by the heat of the iron. 



Although a great many different dextrins have from time to 

 time been described, comparatively few of them are sufficiently 

 well defined to warrant any description here. The three follow- 

 ing, in addition to maltose and isomaltose, were isolated by 

 Lintner and Dull * from the products of the action of malt 

 extract on starch by a long process of fractional precipitation 

 with alcohol. 



Amylo-dextrin. This substance, which is regarded by these 

 authors as the chief constituent of soluble starch, is a white 

 powder which is slightly soluble in cold water, but readily in 

 hot. It is strongly dextro-rotatory (a D = + 1 96), does not re- 

 duce Fehling's solution, and gives a blue colour with iodine. 



Erythro-dextrin. This is a solid which dissolves readily in 

 water, has a rotatory power of a D = + 196, and with iodine 

 produces a red-brown colour. 



The existence of erythro-dextrin as a chemical entity is, 

 however, disputed by Ost, who says that it is a mixture of 

 achroo-dextrin with starch ; an artificial mixture of achroo- 

 dextrin with a half per cent of starch also produces a red 

 colour with iodine. 



Achroo-dextrin. This substance is optically active, has the 

 value a D = + 192, gives no colour with iodine, and has a 

 sweetish taste. 



COMMERCIAL DEXTRIN. 



Commercial dextrin is prepared by heating starch to about 

 230-260; it is a yellowish-brown powder, while that prepared 

 by acid hydrolysis of starch is an almost colourless solid with 

 a choncoidal fracture, or else a white powder resembling 

 starch. It is composed chiefly of achroo-dextrin mixed with 



* Lintner and Dull : " Ber. deut. chem. Gesells.," 1893, 26, 2533. 



