Low beta-amylase activity. Interacts poorly with starch or other alpha-1,4-glucan. Hydrolysis of (1->4)-alpha-D-glucosidic linkages in polysaccharides so as to remove successive maltose units from the non-reducing ends of the chains. Redox regulation; active in reducing conditions, inactive in oxidizing conditions. Optimum pH is 6. Slightly by cold stress. Normal growth rate and starch breakdown in leaves during the night. Belongs to the glycosyl hydrolase 14 family.