Dear Marjorie,

starch molecules have a helix-shaped secondary structure. It consists of two different types of polysaccharides, linear amylose and branched amylopectin. Inside the amylose which forms a helix shape, iodine can lodge forming a charge-transfer-complex of the type In– (I3–, I5–, or I7–) causing the deep blue color. The compound is unstable and decomposes with time, or when heated, and the dark blue color disappears.

Best wishes,

Johannes

Mr Haedrich,

Thank you for the answer, very clear. For the amylose structure, I understood that the structure change and is not able to fix the iodine when it is heated. But do you know if after cooling, the molecule find its initial helix structure again ? 

There are microstructural changes in the amylose depending on the temperature. Changes are little in a range up to 80°C, but may be considerable above. Conformational changes may be reversible depending on the damage that has occurred to the structure by heating.

Well in my experiment they have been heated at 105°C for 15 min (for the worst case) so it may explain the fact that iodine can't fix amylose again.

Thank you very much for the explanation and the time dedicated to it :)

During heat treatment, the crystalline lattice of the starch granules melts (ca. 60-65°C) and amylose molecules are released into the solution. beyond, at ca. 90-95°C, a large increase in viscosity is observed when gelatinization occurs which is irreversible. On cooling, the starch chains (mainly amylose) in the gelatinized paste tend to associate, leading to the formation of a more ordered structure which is termed retrogradation. This results in a viscosity increase due to the formation of a firm gel, depending on the source of starch. So I believe that any temperatures beyond 60 °C (not 80°C as I stated before) will already lead to irreversible structural changes not allowing the previous starch-iodine complex to re-form.