Food scientists test for starch and fat in foods by adding chemical reagents that react in predictable ways to these nutrients. Lugol's iodine is used to identify the presence of starch, and Sudan III is used to identify the presence of fat.
Lugol's iodine is used when testing foods for their starch content. The solution is yellow-brown, but when it reacts chemically with starch, a blue-black substance called iodide starch is produced, according to Scribd. To perform the test, the tester needs two test tubes, a test tube rack, an eyedropper, distilled water, Lugol's iodine, starch powder and a piece of bread. The tester puts a few drops of distilled water and starch powder into one of the test tubes and a few drops of distilled water and a small piece of bread into the other. Then, the tester places several drops of the Lugol's iodine into each test tube. The test tube that turns the darkest color is the one that contains the highest starch content.
Testing for fat works on the same principle. The tester may use either a liquid containing fat, such as milk, or a solid that has been finely ground. For a solid, the tester adds distilled water until the test tube is roughly half full. Then he adds a few drops of Sudan III to each of the test tubes and shakes them. A red-stained oil layer will form on top of the liquid indicating the presence of fat. The larger the oil layer, the higher the fat content of the sample.