396 



CHEMISTRY OF THE DIGESTIVE PROCESSES. 



specific rotatory powers and reduction coefficients ; but they scarcely give 

 adequate proofs that they are describing pure substances. 1 They found that 

 even after twelve months a portion of the dextrin remained unconverted into 

 maltose, and the substance so remaining was unfermentable by yeast. This 

 substance is y-achroodextrin, and is formed together with some maltose by the 

 splitting up of /3-achrob'dextrin, which in its turn is formed by a similar 

 decomposition of a-achroodextrin. 



The following is a summary of their results : 



The digestion of starch by diastatic enzymes consists of a breaking 

 up, through several more or less well-defined stages, by a process of 

 gradual hydrolysis, of a very complex molecule into a much simpler one, 

 and might be represented schematically by the following general 

 equation, which cannot be made more definite, because we are un- 

 acquainted with the molecular weights of starch and dextrin, only 

 knowing that they are very large 



Starch Maltose 



(C 6 H. 10 5 ) n + (H 2 0) m = (C I2 H 22 U ,H 2 0) + 



Dextrin 



That is, starch and water, in presence of a suitable ferment, yield 

 maltose and different dextrins, but we are ignorant of the value of 

 n, in, and p. 



Attempts to carry too far the analogy between the action of 

 dilute mineral acids and that of the diastatic ferments on starch, 

 led, as already stated, to an error, which persisted for several years, 

 as to the products of the latter action. Nevertheless, a close analogy 

 does exist between the two processes ; both are essentially hydration 

 processes; and in both the same stages may be observed. They 

 only differ in two respects, first, that the dilute acid at boiling tem- 

 perature acts much more rapidly; secondly, that it proceeds a 

 stage further, and very rapidly converts the maltose formed into grape- 

 sugar. 



These successive changes may be best observed by boiling with very 

 dilute acid ('2 per cent, or less). Soluble starch is first formed, giving, on 

 neutralisation, a blue with iodine ; next, is an intermediate stage, in which a 

 violet is obtained followed by a stage giving a red colour (erythrodextrin) ; 

 and finally a stage is reached at which a coloration is no longer obtained 



1 According to Brown and Morris (see Trans. Chem. Soc., London, 1885, p. 527 ; 1889, 

 p. 462), the chemical and physical properties of these different achroodextrins might be 

 given by a variable mixture of one achroodextrin possessing no reducing power with mal- 

 tose. They admit the existence besides achroodextrin, of maltodextrin (Herzfeld). a body 

 intermediate between achroodextrin and maltose, but more nearly allied to the latter. 



