It was previously thought that starch adsorbs iodine, and the wavelengths of the visible light absorbed by the iodine move to a short wavelength, and the brown iodine becomes blue. In the same way, amylopectin and dextrin can also adsorb iodine, but the degree of adsorption is different, so the color is different. The strong basis for this explanation is that the iodine starch solution disappears in the blue when it is heated. This is thought to be the cause of the desorption of the molecular kinetic energy after heating.

In recent years, advanced analytical techniques, such as X ray and infrared spectroscopy, have been applied to study the blue substance produced by iodine and starch. It is proved that the reason why the color and the adsorption of iodine and starch are mainly due to the inclusion compound.

What is the inclusion? Amylose is a long spiral body of α - glucose molecules condensed into a spiral, and each of the glucose units still has hydroxyl groups exposed to the helix. The iodine molecules interact with these hydroxyl groups so that the iodine molecules are embedded in the axis of the amyloid. The effect of iodine and starch is called inclusion, and the product is called inclusion.

In the inclusion compound of starch and iodine, each iodine molecule cooperates with 6 glucose units. The starch chain is spiral in diameter 0.13pm, and iodine molecule is in the axis of helix.

The color of the inclusion complex of starch and iodine, which is related to the degree of polymerization or relative molecular mass of starch. In a certain degree of polymerization or molecular weight range, with the increase of polymerization degree or relative molecular mass, the inclusion of color ratio from colorless, orange, pink, purple to blue. For example, when the degree of polymerization of amylose is 200 to 980 or the range of relative molecular mass is 32000 to 160000, the color of the inclusion complex is blue. A lot of branched amylopectin, the average degree of polymerization of the amylopectin on the branched chain is 20~28, so that the inclusion complex is purple. The dextrin has a lower degree of polymerization, showing a brown, red, and light red. The next table is the degree of polymerization of starch and the color of the formation of iodine inclusion.

• Print
• Close This Page
• Recommendation：
• 
• Browse：