iodine does not mechanically change starch. it chemically reacts with it to give a blue/black colour

"Iodine is a common general stain used in thin-layer chromatography. It is also used in the Gram stain as a mordant, after the sample is treated with crystal violet. In particular, iodine forms an intense blue complex with the glucose polymers starch andglycogen. Many applications rely on this property: - Iodometry. The concentration of an oxidant can be determined by adding it to an excess of iodide with a little free iodine, to destroy elemental iodine/triiodide as a result of oxidation by the oxidant. A starch indicator is then used as the indicator close to the end-point, in order to increase the visual contrast (dark blue becomes colorless, instead of the yellow of dilute triiodide becoming colorless). - An Iodine test may be used to test a sample substance for the presence of starch. - The Iodine clock reaction is an extension of the techniques in iodometry. - Iodine solutions are used in counterfeit banknote detection pens; the premise being that counterfeit banknotes made using commercially available paper contain starch. - Starch-iodide paper are used to test for the presence of oxidants such as peroxides. The oxidants convert iodide to iodine, which shows up as blue. A solution of starch and iodide can perform the same function. - During colposcopy, Lugol's iodine is applied to the vagina and cervix. Normal vaginal tissue stains brown due to its high glycogen content (a color-reaction similar to that with starch), while abnormal tissue suspicious for cancer does not stain, and thus appears pale compared to the surrounding tissue. Biopsy of suspicious tissue can then be performed. This is called a Schiller's test." Source and further information: http://en.wikipedia.org/wiki/Iodine Further information: http://en.wikipedia.org/wiki/Staining