Do you know the characteristics of starch?

Starch has the characteristic of turning blue when exposed to iodine, which is determined by the structural characteristics of starch itself. Starch is a white amorphous powder composed of 10% to 30% linear starch and 70% to 90% branched starch. Water soluble amylose curls into a spiral shape through intramolecular hydrogen bonds. If iodine solution is added, the iodine molecules in the iodine solution will be embedded into the gaps of the spiral structure, and with the help of van der Waals force, they will be linked to linear starch to form a complex. This complex can uniformly absorb visible light other than blue light (with a wavelength range of 400-750 nm), thus making the starch solution appear blue.

The color mechanism of starch and iodine is that linear starch turns blue when it encounters iodine, branched starch turns purple red when it encounters iodine, and dextrin turns blue purple, purple, orange, and other colors when it encounters iodine. These color reactions have high sensitivity and can be used as quantitative and qualitative methods for identifying starch, as well as for analyzing iodine content. It is used in the textile industry to measure the completeness of fabric desizing. Why does iodine appear different colors when it comes into contact with starch or dextrin

Previously, it was believed that starch could adsorb iodine, causing the wavelength of visible light absorbed by iodine to shift towards shorter wavelengths, and the brown iodine solution would turn blue. Similarly, branched starch and dextrin can also adsorb iodine, but the degree of adsorption varies, resulting in different colors. The strong basis for this explanation is that the blue color of iodine starch solution disappears when heated. This is believed to be due to the increase in molecular kinetic energy after heating, causing desorption.

In recent years, advanced analytical techniques such as X-ray and infrared spectroscopy have been used to study the blue compounds formed by iodine and starch, proving that the color development of iodine and starch is mainly due to the formation of inclusion complexes, in addition to adsorption reasons. What is a composite material? Amylose starch is composed of α- Glucose molecules condense into a long spiral shape, with hydroxyl groups still exposed outside each glucose unit. Iodine molecules interact with these hydroxyl groups, causing them to embed into the axis of the starch spirochete. The interaction between iodine and starch is called inclusion complex, and the resulting product is called inclusion complex.

In the inclusion complex formed by starch and iodine, each iodine molecule is coordinated with 6 glucose units, and the starch chain is wound into a spiral shape with a diameter of 0.13 pm. The iodine molecule is located at the axis of the spiral.

The color of the inclusion complex formed by starch and iodine is related to the degree of polymerization or relative molecular weight of starch. Within a certain range of polymerization degree or relative molecular weight, as the polymerization degree or relative molecular weight increases, the color of the inclusion complex changes from colorless, orange, light red, purple to blue. For example, when the degree of polymerization of linear starch is 200-980 or the relative molecular weight range is 32000-160000, the color of the inclusion complex is blue. Branched starch with many branches has an average degree of polymerization of 20-28 on the straight chain, resulting in a purple inclusion complex. The degree of polymerization of dextrin is lower, appearing brownish red, red, light red, etc. The following table shows the degree of polymerization of starch and the color of iodine inclusion complexes generated. Table 7-1 Degree of polymerization of starch and color of iodine inclusion complex formation. Degree of polymerization of glucose units 3.8 7.4 12.9 18.3 20.2 29.3 34.7 or higher

The color of the inclusion complex is colorless, light red, brown red, purple, blue purple, and blue. The inclusion complex formed by starch and iodine is most stable at pH 4, so its color reaction is obvious in slightly acidic solutions.

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