Principle
The principle of starch detection is to use the property of starch that turns blue when exposed to iodine.
Because starch has the property of turning blue when exposed to iodine, it is determined by the structural characteristics of starch itself.
Starch is a polymeric carbohydrate, which is formed by the polymerization of glucose molecules. The basic building block is α-D-glucopyranose, with the molecular formula (C6H10O5 )n. There are two types of starch: straight-chain starch and branched-chain starch. The former is an unbranched helical structure; the latter is formed by 24-30 glucose residues linked by α-1,4-glycosidic bonds first and last, and α-1,6-glycosidic bonds at the branched chains.
Starch is a polymeric carbohydrate, a polysaccharide composed of a single type of sugar unit. The basic unit of starch is α-D-glucopyranose, and the covalent polymer formed by glucose stripped of water molecules and linked together by glycosidic bonds is the starch molecule.
The molecular formula of free glucose is C6H12O6, while the dehydrated glucose unit is C6H10O5. Therefore, the starch molecule can be written as (C6H10O5)n, with n being an indefinite number. The number of structural monomers (dehydrated glucose units) that make up a starch molecule is called the degree of polymerization and is expressed as DP.
Starch is composed of α-1,4 glycosidic bonds. Subsequently, starch has been divided into straight-chain molecules, which are composed of D-hexa-ring glucose via α-1,4-glycosidic bonds, and branched molecules, which branch at α-1,6-glycosidic bonds, with the rest being α-1,4-glycosidic bonds.