Beta-amylase activity. Can use p-nitrophenyl maltopentaoside (PNPG5) as substrate only in reduced form. Can play a minor role in the starch degradation and maltose metabolism in chloroplasts during the night. More active on phosphorylated glucan. Interacts directly 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. Thioredoxins f1, m1, and y1 mediate the reversible reductive activation of oxidized BAM1. Optimum pH is 6-8. Expressed in leaves, roots, flowers, pollen, and seeds. Normal growth rate and starch breakdown in leaves during the night. Belongs to the glycosyl hydrolase 14 family.