Catalyzes the reversible epimerization of cellobiose to 4-O-beta-D-glucopyranosyl-D-mannose (Glc-Man). Can also use lactose, epilactose, mannobiose and cellotriose. Highly specific for oligosaccharides linked by the beta-1,4-glycosidic linkage. Shows preference for lactose. D-cellobiose = beta-D-glucosyl-(1->4)-D-mannopyranose kcat is 80.8 sec(-1) for cellobiose. kcat is 111 sec(-1) for lactose. Optimum pH is 6.3. Optimum temperature is 80 degrees Celsius. Belongs to the cellobiose 2-epimerase family.