lo6 THE CARBOHYDRATES 



General Properties of Dextrins. 



From what has been said above, it will be seen that the 

 term dextrin comprises a number of substances some of which 

 are not at all well defined. The following may, however, be 

 regarded as approximately representing the characteristics of 

 all substances included in this group :■ — 



1. They are amorphous substances which are readily soluble 

 in water to form gummy solutions, which are used as a sub- 

 stitute for natural gum ; they are precipitated from aqueous 

 solutions by the addition of alcohol. 



2. Unlike starch inulin and glycogen, dextrin does not 

 give a precipitate with basic lead acetate. 



3. As their name implies, they are strongly dextro-rotatory, 

 in which respect they resemble starch. 



4. They give either a red colour or no colour at all with 

 iodine. 



5. They are not fermentable by yeast alone, but are fer- 

 mented by a mixture of yeast and diastase acting together, 

 which is no doubt due to their slow hydrolysis in the first 

 place by the diastase and the subsequent fermentation of the 

 maltose so produced. 



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 Diill * 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 



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



