94 DEXTRIN. 



tained by fermenting off the sugar with yeast (O'Sullivan) or by 

 dialysis, since dextrin is non-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 pre- 

 pared it appears to possess a constant dextrorotatory power (a) D = 

 194-8 [(a)j= 216], and as precipitated 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 dextriiis 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 pro- 

 longed 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 dur- 

 ing 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 evi- 

 dence 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 continu- 

 ous removal of digestive products in the natural process as com- 

 pared 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 dia- 

 lyser, the starch may be practically entirely converted into sugar, 

 the small residue of dextrin being due rather to inefficiency of the 



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

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

 ation with iodine. 



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



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

 maltose. 



