In the iodine method, which components yield blue color?

• A. Plant starch, amylose, and amylopectin
• B. Glycogen and amyloid
• C. All carbohydrates
• D. Starch only

Answer: (A)

The iodine test yields a blue color because iodine forms a complex with the helical structures found in starch. Plant starch consists of two components, amylose (mostly linear) and amylopectin (branched). Both contribute to the characteristic blue color when iodine is applied, with amylose driving the strong blue-black result and amylopectin contributing to the overall blue depending on the amount of helical content. Glycogen, though a glucose polymer, is highly branched and does not form the same stable helical cavities, so it does not produce the characteristic blue with iodine. Other carbohydrates don’t form this starch-iodine complex, so they don’t show the blue color.