Starch is polymer, when you see you will find that starch doesn't contain an aldehyde group.
It lacks a free aldehyde or ketone group that would allow the starch structure to be opened up.
Iodine testing can be used to identify starch.
There is no free carbonyl group in starch.
Even though it contains numerous reducing glucose molecules, only the terminal glucose possesses hemiacetal when connected with a glycosidic bond, and this is insufficient to provide a favorable result for reducing sugars.
