(C) Iodine Test for Starch Iodine test was used to test for the presence of starch in the given food material. When iodine solution was mixed with starch it turned from brown to dark blue in color. The reaction could take place even with little amount of starch as iodine test was a sensitive one. Iodine solution penetrated easily into the starch when the cell walls have been destroyed, hence chopping and crunching or biscuit-cream cracker, potato and groundnut were needed. IN the experiment conducted, biscuit-cream cracker, potato and groundnut showed positive results against the test, indicating starch was present in these food samples. However different color intensity was shown.
Starch was a polysaccharide consisting of glucose units joined together by glycosidic bonds. Starch existed in two types of molecules which were amylose and amylopectin. Generally starch contained more amylopectin (65% to 85%) compare to amylose. The reaction between amylose and iodine was said to account the color change of the solution as the present of amylose was higher in most of the starch product. Amylose molecules consisted of single, mostly unbranched chains of glucose molecules and had helical structure which was a coil or spring. Iodine on its own was insoluble in water, therefore potassium triiodide solution was used as a reagent, as it dissociated, the iodide ion reacted reversibly with the iodine to yield the triiodine ion. A further reaction between a triidide ion and an iodine molecule yielded the pentaiodide ion which eventually will get stuck or slipped in the coil of the beta amylose molecules. The iodine was then forced into a linear arrangement in the middle groove of the amylose coil, causing the transfer of charge in between starch and iodine to occur. Moreover, as the electron arrangement changed, the spacing between energy level interrupted too. Visible light at