The iodine test provides an indication of the extent to which the long-chain α-glucans of the starch (amylose and amylopectin) have been degraded to sugars and smaller polysaccharides. As soon as the helix structure of the starch is no longer present, iodine can no longer be captured by it, and the solution stops turning dark purple. A so-called "Blausud" (literally a "blue batch") or wort that tests positive for iodine would subsequently cause issues downstream in the production process, such as sensory and microbiological problems as well as turbidity.
wort, beer
High molecular weight dextrins and starch present in wort or beer are precipitated through the addition of ethanol, centrifuged, dissolved in phosphate buffer, followed by the addition of an iodine solution. Depending on the molecular weight and degree of branching of the erythrodextrins and starch, a red to blue color forms, the intensity of which is measured photometrically.