Starch is a carbohydrate found in many different dietary categories. You may test for the presence of starch using an iodine solution. When starch is present, the iodine becomes brown and then blue-black or purple. The presence of starch can also be detected by heating a sample in a container with a narrow opening (called a crucible) made of platinum or glass. As the starch begins to turn brown, it begins to smell like burnt sugar. Starch also gives off gas when heated - this is called "exhalation" and you should avoid heating samples that contain starch because this will cause the starch to go up in smoke. Iodine solutions are used by food scientists to detect the presence of starch because it changes color when it comes into contact with starch. This test is useful for screening large quantities of material because any substance that will react with iodine and give off starch will show up as a positive result. This method cannot distinguish between different types of starch; for this purpose, there are several other tests available. One common alternative is to use an amyloglucosidase enzyme solution which will break down starch into sugars that can be measured using glucose oxidase. Another option is to use an infrared microscope to look for differences in the way waves are reflected from areas containing starch vs. those that don't.