What is a positive test for starch?


Quick Answer

The test that is usually used to identify the presence of starch in a sample is the iodine test. One of the two components of starch, specifically amylose, reacts to iodine, generating a deep blue color. Although the presence of simple carbohydrates is also detected using Benedict's solution, starch and other complex carbohydrates do not react positively to this reagent, unless they are decomposed.

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Full Answer

To perform the iodine test, one must add the iodine reagent to the material to be tested, whether it's a solution or a solid piece such as a slice of potato or bread. If a blue-black color appears, it means starch is present. A yellow or orange coloration indicates that starch amylose is not present. If the material that is tested contains only cellulose or disaccharides, a positive result does not appear.

The iodine reagent used for the starch test is usually prepared by dissolving iodine in water, after potassium iodide has been added to improve iodine's solubility. The resulting compound is a triiodide ion complex that has the ability to produce the intense blue coloration.

Some cereals, such as rice, wheat and corn and some vegetables store significant amounts of starch. Starch is made out of two fractions called amylose and amylopectine, but the latter does not participate in the reaction with iodine.

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