Starch can be divided into amylose (sugar starch) and amylopectin (colloid starch). The former is a spiral structure without branches; The latter consists of 24~30 glucose residues connected end to end by α- 1, 4- glycosidic bond, and the branch is α- 1, 6- glycosidic bond.

Starch includes amylose and amylopectin. Amylose contains hundreds of glucose units while amylopectin contains thousands of glucose units. Among natural starch, linear starch accounts for 20% ~ 26%, which is soluble, and the rest is amylopectin. When detected by iodine solution, amylose solution is blue, and amylopectin turns red-brown after contacting with iodine. (The reason is that amylose with long spiral segments can form a complex with long-chain poly i3- to produce blue. Amylose-iodine complex contains 19% iodine. The combination of amylopectin and iodine produces red-purplish red, because the branches of amylopectin are too short to form long-chain poly I 3- )

Starch contains more than two components with different properties. Soluble starch that can be dissolved in hot water is called amylose. It can only swell in hot water, and the insoluble one is called amylopectin.