What are the typical reducing sugars found in fruit juice?

• A. Glucose and Fructose
• B. Sucrose and Lactose
• C. Maltose and Galactose
• D. Ribose and Deoxyribose

Answer: (A)

Reducing sugars are those that can act as a reducing agent because they have a free carbonyl group (an aldehyde or a carbonyl that can form one). In fruit juice, the main sugars are glucose and fructose. Glucose is an aldose with a reactive aldehyde form in its open-chain form, so it readily reduces oxidizing agents. Fructose is a ketose, but it can tautomerize to an aldose under certain conditions, giving it reducing capability as well. Sucrose, on the other hand, is non-reducing because the glycosidic bond links both anomeric carbons of glucose and fructose, preventing the open-chain form from forming and thus blocking reduction. Other sugars like lactose and maltose can be reducing but are not the typical major sugars found in fruit juice, and ribose or deoxyribose are primarily associated with nucleic acids rather than common fruit juice sugars.