Chemistry: Starch Testing

Student (age 10) tests an apple slice, which stays brown, and a mini marshmallow, which turns black.

Student (age 10) tested several food items. Most of the results turned out as he expected, but he was surprised that peanut butter tested positive for the presence of starch.

Starches or carbohydrates can be detected easily with an iodine solution. Most drugstores carry "tincture of iodine" which contains iodine and alcohol, which can be used for this experiment.  If the iodine turns black or blue, there is starch present. If it remains reddish brown, there is no a significant amount of starch. Obviously once the food has had iodine on it, it must be discarded and cannot be eaten. Bread, pasta, potatoes and fruit such as bananas and apples all contain starches, and therefore are good things to test, but make sure you also have things that do not have starch so they can compare them. Have them look around your kitchen for things to test.
For upper-level students, you can tell them that iodine specifically detects polysaccharides and will not pick up mono or discaccharides.