40 ACTION OF SALIVA UPON STARCH. [BOOK II. 



It is precipitated from its solutions by tannic acid and by alcohol. 

 The property of being precipitated by tannic acid permits of the 

 separation of soluble starch from the dextrins and maltose which are 

 formed at further stages of the reaction. 



By adjusting the proportion of diastatic ferment to starch-paste 

 and testing sufficiently soon, it will be discovered that at a certain 

 stage, liquefaction has been the only result, neither dextrins nor 

 sugar being yet present. 



Production of Erythrodextrins and Sugar. 



When the ferment has acted longer upon starch solution, iodine 

 produces a violet or red colour. At the same time the solution is 

 found to contain dextrins and a sugar. By adding tannic acid to the 

 solution if violet, the yet undecomposed soluble starch is precipitated 

 and then a liquid of a more or less deep red colour is obtained. This 

 liquid contains a body, or bodies, isomeric with starch, precipitable 

 from the solution by the addition of alcohol, and to the product of 

 precipitation the name of erythrodextrin has been given. It has been 

 surmised that more than one erythrodextrin exists. That a definite 

 body erythrodextrin actually exists appears, however, very doubtful, 

 the reactions which were held to prove its existence being explainable 

 on the hypothesis that it is a mixture of varying quantities of un- 

 altered starch with achroodextrin and maltose. 



Erythrodextrin, or the mixed product so named, is soluble in 

 water, precipitable from its solution by alcohol, but not by tannic acid. 



The solution is coloured red by iodine. 



When erythrodextrin is subjected to the further action of a 

 diastatic ferment it is decomposed into two isomeric dextrins of 

 different reactions, termed achroodextrin and maltodextrin, and into 

 maltose. 



Production of Achroodextrin , Maltodextrin and Maltose. 



When a starch solution is subjected at a suitable temperature to 

 the prolonged action of a diastatic ferment, the clear solution is found 

 not to be coloured red by iodine, but to assume a yellow tinge, 

 which becomes gradually fainter until no colouration is produced by 

 the reagent. This is the so-called ' achromic point ' of Dr Roberts. 

 By causing a large quantity of diastatic enzyme to act upon a suffi- 

 ciently dilute starch mucilage, the whole of the stages of the trans- 

 formation, culminating in the achromic point, can be brought about 

 almost instantaneously. 



Influence of Dr Roberts's experiments have led him to the con- 

 temperature elusion that the diastatic activity of saliva increases 

 upon diastatic w ith rise of temperature up to about 30 C., and that it 



continues steady from this temperature to about 45 C. 



and then declines, being finally extinguished between 



