We know the Starch is a polysaccharide consisting of two components, .... amylose and amylopectin.
Amylose component of the starch is actually a linear or u can say non-breanched polymer of glucose.
The glucose units are joined to each other ..with alpha-1-4-glycosidic linkage.
So because of this kind of structure.... The Amylose is found in coiled form and each single coil posess six glucose residues.
Principle
the significant feature of the helical coils of amylose is that... These helical coils absorve iodine....
And this reaction produces a blue colored complex which we can measured colorimetrically.