Which sugar is non-reducing?

• A. Sucrose
• B. Glucose
• C. Galactose
• D. Maltose

Answer: (A)

Reducing sugars can open up to reveal a free aldehyde or ketone that can reduce other substances; non-reducing sugars lack a free carbonyl because their anomeric carbons are tied up in glycosidic bonds. Sucrose is non-reducing because the glycosidic linkage joins the anomeric carbon of glucose to the anomeric carbon of fructose, so neither end has a free carbonyl to act as a reducing agent. In contrast, glucose and galactose have free aldehyde groups in their open forms, making them reducing sugars, and maltose has a free reducing end as well, enabling it to reduce oxidizing agents.