Exoglycosidase that cleaves the single beta-linked mannose residue from the non-reducing end of beta-mannosidic oligosaccharides of various complexity and length. Prefers mannobiose over mannotriose. Is also severely restricted by galactosyl substitutions at the +1 subsite (By similarity). Has no activity against polymeric mannan. Hydrolysis of terminal, non-reducing beta-D-mannose residues in beta-D-mannosides. kcat is 15 sec(-1) with p-nitrophenyl-beta-mannopyranoside as substrate. Optimum pH is 5.3. Active from pH 4.2 to 6.5. Optimum temperature is 40 degrees Celsius. Glycan metabolism; N-glycan degradation. Homodimer. Belongs to the glycosyl hydrolase 2 family. Beta-mannosidase B subfamily.