94 DEXTRIN. 



fermenting off the sugar with yeast (O'Sullivan) or by dialysis, since 

 dextrin is noil-diffusible. If however the mixture be warmed with a 

 slight excess of mercuric cyanide and caustic soda, the whole of the 

 sugar is destroyed in reducing the mercuric salt, leaving in solution a 

 non-reducing dextrin 1 . As thus prepared it appears to possess a 

 constant dextrorotatory power (a) D 194*8 [(a)j 216], and as pre- 

 cipitated by alcohol is a white amorphous powder very soluble in water. 



Maltodextrin^. This substance is described as appearing during the earlier 

 stages of a limited hydrolysis of starch-paste with diastase, and it may perhaps 

 similarly occur when saliva or pancreatic juice is employed. It differs from the 

 dextrins previously described as follows. It is more soluble in alcohol and distinctly 

 diffusible ; it reduces Fehling's fluid, has a lower specific rotatory power 



and is completely convertible into maltose by the further action of diastase. It will 

 therefore not be found among the products of a prolonged hydrolytic degradation 

 of starch. 



When starch-paste is hydrolysed outside the body with diastase or 

 with animal enzymes some dextrin is always obtained together with 

 the sugars which make their characteristic appearance during the 

 process. Considerable difference of opinion has been expressed as to 

 the possibility of a complete conversion of these dextrins into sugar by 

 the renewed action of the enzyme upon them after their isolation, but 

 the balance of opinion appears to be that the conversion is in many 

 cases either impossible or takes place with slowness and difficulty. 

 If this is so then the course of an artificial and normal digestion of 

 starch is, as regards the final products, very different in the two cases, 

 for there is no evidence that in the body any carbohydrate is absorbed 

 as dextrin from the alimentary canal. The conditions however under 

 which the two digestions are carried on are markedly different, and 

 more particularly with respect to the very complete and continuous 

 removal of digestive products in the natural process as compared with 

 their "accumulation in an ordinary artificial digestion. Now there is 

 no doubt that the products of an enzymic hydrolysis are inhibitory to 

 the further action of the enzyme 3 , and this is probably the cause of 

 the observed difference. In accordance with this, if a starch digestion 

 be carried on in an efficient dialyser, the starch may be practically 

 entirely converted into sugar, the small residue of dextrin being due 

 rather to inefficiency of the apparatus than to the chemical resistance 



1 It should be carefully borne in mind that probably many forms of dextrin exist, 

 especially among the earlier products of hydrolysis, none of which give any colour- 

 ation with iodine. 



2 Brown and Morris, loc. cit. p. 561. 



3 See also Lindet, Compt. Rend. T. cvin. (1889), p. 453 with special reference to 

 maltose. 



