Why is starch a non-reducing Sugar?

Starch is considered a non-reducing sugar because its glucose units are connected by acetal (glycosidic) linkages that leave no free hemiacetal—and therefore no free aldehyde/ketone—able to reduce reagents such as Benedict’s or Fehling’s solutions.

Reducing sugars must present a free carbonyl in equilibrium with a cyclic hemiacetal; this open-chain aldehyde/ketone can reduce Cu²⁺ to Cu⁺ in classical tests.

In starch, most anomeric carbons are “locked” as acetals in α(1→4) and α(1→6) glycosidic bonds, preventing ring opening to a free carbonyl, so reduction does not occur.

Structure of starch

• Amylose: Linear polymer of α-D-glucose linked by α(1→4) bonds; anomeric carbons form acetal linkages, eliminating the free hemiacetal needed for reducing behavior.
• Amylopectin: Branched polymer with α(1→4) backbones and α(1→6) branch points; these linkages also consume anomeric centers, again preventing a free carbonyl.

Reducing end nuance

Polysaccharides may have at most a single terminal “reducing end,” but in bulk starch this single end is negligible compared with the vast number of acetal-linked residues, so starch is operationally classified as non-reducing in standard tests.

Experimental consequence

Because there is no free aldehyde/ketone to reduce Cu²⁺, starch gives a negative (blue) Benedict/Fehling test, unlike reducing sugars such as glucose or maltose that yield brick-red Cu₂O.