Catalyzes the reversible phosphorolysis of nigerose. Also shows a weak activity on kojibiose. nigerose + phosphate = beta-D-glucose 1-phosphate + D-glucose Does not require divalent metal ions. kcat is 67 sec(-1) for nigerose. kcat is 110 sec(-1) for D-glucose. Optimum pH is 7.0. Optimum temperature is 40 degrees Celsius. Homodimer. Belongs to the glycosyl hydrolase 65 family.