Starch-iodine coloration as an index of differential degradation by the amylases

Abstract

Although the intimate nature of the coloured iodine compounds formed by starch and starch dextrins remains obscure, this colour reaction has played a prominent part in the study of amylases. Parallel observations on the changes in iodine coloration and the increase in reducing power during the enzymic degradation of starch led first to the recognition of the fact that amylases from different sources induce distinctive types of breakdown. Under the action of the so-called “saccharogenic” (or β ) amylase from ungerminated barley and other cereals about 60 % of the starch substance is transformed into maltose, but the iodine colour changes only to violet or mauve. In contrast, most amylases are of the so-called “dextrinogenic” type, having the capacity to destroy completely the iodine colouring property. The colour changes typically from blue, through shades of violet, rose brown, and orange, until at the “achroic point” the colour of added iodine is not altered. Enzymes from different sources are distinguishable, however, in that the increases in reducing power which accompany the destruction of the iodine colouring property vary considerably.