Because the aglycone is a hemiacetal, lactose undergoes mutarotation. For the same reason lactose is a reducing sugar. The free aldehyde formed by ring opening can react with Benedict's solution. Thus, a solution of lactose contains both the α and β anomer at the “reducing end” of the disaccharide.

Why are lactose and maltose reducing sugars?

Maltose (glucose + glucose) and lactose (galactose + glucose) have a free aldehyde group and thus are reducing sugars.

Why lactose is non reducing sugar?

Lactose is composed by the combination of a glucose and a galactose molecule. Lactose undergoes mutarotation and it is hence a reducing sugar. Note: Monosaccharides are considered as reducing sugars.