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Why are lactose and maltose classified as reducing sugars?

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Tall, dense stalks of sugarcane grow on both sides of a narrow path, with green leaves arching overhead and sunlight illuminating the plants. — sugar Lines of sugarcane before harvest.

Lactose and maltose are classified as reducing sugars because they possess a free or potentially free aldehyde group that is available to participate in chemical reactions, such as oxidation.12

In the case of lactose, the two constituent monosaccharides—β-d-galactose and glucose—are linked in a manner that leaves the aldehyde group at the anomeric carbon of the glucose unit free to react. Similarly, maltose is composed of two glucose units linked such that a free aldehyde group remains available.2

This characteristic distinguishes them from nonreducing sugars like sucrose and trehalose, where the linkage between the two monosaccharide units involves the anomeric groups of both, effectively "locking" them and preventing the availability of a free aldehyde or keto group for reaction.3