Iodine Test for Starch and Glycogen

The use of Lugol's iodine reagent (IKI) is useful to distinguish starch and glycogen from other polysaccharides. Lugol's iodine yields a blue-black color in the presence of starch. Glycogen reacts with Lugol's reagent to give a brown-blue color. Other polysaccharides and monosaccharides yield no color change; the test solution remains the characteristic brown-yellow of the reagent. It is thought that starch and glycogen form helical coils. Iodine atoms can then fit into the helices to form a starch-iodine or glycogen-iodine complex. Starch in the form of amylose and amylopectin has less branches than glycogen. This means that the helices of starch are longer than glycogen, therefore binding more iodine atoms. The result is that the color produced by a starch-iodine complex is more intense than that obtained with a glycogen-iodine complex.

Method

Add 2-3 drops of Lugol's iodine solution to 5 ml of solution to be tested. Starch gives a blue-black color. A positive test for glycogen is a brown-blue color. A negative test is the brown-yellow color of the test reagent.