Starch is a polysaccharide and a nutrient stored in plants. It mainly exists in seeds and tubers. It is a colorless and odorless white powder with a density of1.499 ~1.513. It is hygroscopic. It consists of amylose (starch granules) and amylopectin (starch cortex). Their proportion in starch varies with plant species.

Amylose is a chain compound formed by combining glucose with α- 1, 4- glycoside bond, which can be hydrolyzed into maltose by amylase. The content of starch is about 10~30%. Soluble in hot water, no paste. Blue in iodine.

Glucose molecules in amylopectin are linked by α- 1, 4- glycosidic bond, and some are linked by α- 1, 6- glycosidic bond. So there are branches, about 20 glucose units have one branch, and only the peripheral branches can be hydrolyzed into maltose by amylase. Insoluble in cold water, it swells into paste in hot water. It will turn purple or reddish purple when it meets iodine.

Starch can be gradually decomposed by amylase or acid, and the process is as follows:

Starch → red dextrin → colorless dextrin → maltose → glucose.