Catalyzes the liberation of maltose from 1,4-alpha-D glucans. 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. Monomer. Endosperm. There are at least three alleles; SD2H; SD1 and SD2L. The sequence of SD2H is shown here. The three alleles show different thermostabilities and different affinities for soluble starch. Belongs to the glycosyl hydrolase 14 family.