Starch indicator is usually amylose, and starch is colored with iodine. Amylose is blue when exposed to iodine, amylopectin is purple when exposed to iodine, and dextrin is blue-purple, purple and orange when exposed to iodine.

Amylose, also known as sugar starch, is a straight chain of glucose connected by glycosidic bonds and curled into a spiral secondary structure. Amylose includes corn, potato and glutinous rice.