How does iodine react with starch?
Starch consists of two types of molecules, amylose - a soluble starch and amylopectin. When starch is mixed with iodine in water, an intensely blue colored starch/iodine complex is formed. Many of the details of the reaction are still unknown. Apparently, the Iodine gets stuck in the coils of beta amylose molecules.
color in the presence of iodine. The triiodide ion ion slips into the coil of the starch causing an intense blue-black color. Starch Test: Add Iodine-KI reagent to a solution or directly on a potato or other materials such as bread, crackers, or flour. A blue-black color results if starch is present.
- Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Three important polysaccharides, starch, glycogen, and cellulose, are composed of glucose. Starch and glycogen serve as short-term energy stores in plants and animals, respectively. The glucose monomers are linked by α glycosidic bonds.
- In lab, we used Benedict's reagent to test for one particular reducing sugar: glucose. Benedict's reagent starts out aqua-blue. As it is heated in the presence of reducing sugars, it turns yellow to orange. The "hotter" the final color of the reagent, the higher the concentration of reducing sugar.
- Starch Test: Add Iodine-KI reagent to a solution or directly on a potato or other materials such as bread, crackers, or flour. A blue-black color results if starch is present. If starch amylose is not present, then the color will stay orange or yellow.
Lugol's Iodine (IKI) test for the presence of starch: Iodine dissolved in an aqueous solution of potassium iodide - reacts with starch producing a deep blue-black color. This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine.
- Starch, whether it's in the form of amylose or amylopectin, is not a reducing sugar. The first ring cannot open up because there's no hydrogen on the circled oxygen to allow for ring opening. So polysaccharides, such as starch, are not reducing sugars.
- Since cellulose does not have a helical structure like amylose, it does not bind to iodine to form a coloured product. Humans are unable to metabolize cellulose as a source of glucose since our digestive juices lack the enzymes that can hydrolyze the glycosidic linkages.
- In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions interact with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents.
Updated: 2nd October 2019