Starch when mixed with Iodine turns a deep shade of blue because starch contains amylose. Amylose is a soluble component of starch as well as other carbohydrates.
Amylose molecules are made up of single strands of glucose molecules shaped like springs. When iodine is added to a starch, it adheres to the beta amylose molecules because of their solubility. The starch pushes the iodine into a line in the middle of the amylose coils and creates a transfer of charge between the iodine and starch. This causes a change in the arrangement of electrons and energy level spacings. The new spacings absorb visible light differently and create the deep blue color.
Iodine is often used to test foods for starch. Adding iodine to potatoes, dried beans, rice, bread and pasta creates the deep blue reaction to indicate the presence of starch. To perform the test, use a dropper to add one or two drops of iodine to each type of food. Use other foods that do not contain starch, such as apple slices, carrots and sugar, for comparison. Test both a ripe and unripe banana, and notice that the unripe banana contains starch while the ripe banana does not. This demonstrates how the amount of starch decreases as a fruit ripens.