Question

Why is starch a non-reducing sugar?

Answer 1

Hint : The main difference between a reducing sugar and starch is one hydrogen attached to the oxygen. Reducing sugars can reduce others and then oxidise themselves, but starch cannot reduce other substances and thus it is a non-reducing sugar.

Complete Step By Step Answer:
Redox reactions are those in which the oxidation number of a molecule, atom or ion changes. The molecule, atom or ion that loses the electron is called a reducing agent. The reducing agent gives the electron to the accepting molecule, atom or ion on its own.
The molecule, atom or ion that accepts the electron is called an oxidising agent. Oxidising agents accept electrons from others readily.
Some sugars can act as reducing agents, which means they give electrons to other molecules, atoms or ions. But, starch cannot act as a reducing agent.
There is an aldehyde group in some sugars, the sugars become oxidised and the aldehyde group gets converted into carboxylic acid group. The hydrogen attached to the aldehyde group in a sugar is removed from it and thus only the carboxylic acid group remains. Thus this is how sugars can work as reducing agents.
While in the case of starch, it does not possess any free aldehyde group or ketone group which can open up the starch structure. As it lacks a free ketone or aldehyde group, it cannot give out a free electron and thus it cannot work as a reducing agent.

Note :
Starch is a highly branched and highly organised polymeric carbohydrate. Its chemical formula is ${(C_6{H}_{10}{O}_5)}_n$ . Most of the plant matter contains starch in it, thus most of the human diet consists of starch. It can be found in wheat, rice, potatoes, etc.
We can find out if any sugar is a reducing sugar or not. If the sugar contains a hemiacetal group then it is reducing sugar and if the sugar does not contain a hemiacetal group, then it is not reducing sugar. Acetal group is the one where a carbon is attached to two OR groups and hemiacetal group is the one in which the primary carbon is attached to one OR group and one OH group. This makes the structure easily breakable and open-able.