Starch reacts with iodine because iodine molecules enter the spiral circle of starch and form starch iodine complex. As for color, it is related to the length of starch sugar chain. When the chain length is less than 6 glucose groups, it will not develop color; When the average length of the chain length is 20 glucose groups, it will be red; When there are more than 60 glucose groups, it is blue.
Extended data
The color reaction between starch and iodine requires certain conditions, and the final color results are different under different conditions. There are three types of starch: soluble starch solution, potato pulp and glutinous rice thick pulp. It can be found that the soluble starch solution appears blue when it meets iodine. Potato pulp turns purple when it meets iodine; Thick glutinous rice paste will turn purple when it meets iodine.
Starch freshness. By observing the newly prepared starch solution, the starch solution prepared for one week and the starch solution prepared for one month, and observing their color reactions after meeting with iodine, we can find that the starch solution prepared for one week and one week is blue, while the starch solution prepared for one month is purple-blue. It can be seen that the color reaction between starch and iodine will be affected by the freshness of starch.
When starch molecules are heated, the spiral circle will expand due to expansion, and iodine molecules cannot form stable inclusion compounds in the spiral circle through hydrogen bonds, resulting in the gradual fading of the original color. Therefore, the color reaction of starch should be observed at 30℃ or below.
Baidu Encyclopedia-Principles of Iodine Coloring for Starch
? Baidu encyclopedia-starch
Baidu encyclopedia-iodine