Involved in galactan degradation (PubMed:15312766). Degrades arabinose-free galactan to galactooligosaccharides, producing galactotetraose as the main product along with galactotriose, galactobiose, and galactose (PubMed:15312766). May hydrolyze the beta-1,4-galactan linkages of the galactan portion of arabinogalactan type I, a pectic plant polysaccharide from which most of the arabinose has been removed (By similarity). The enzyme specifically hydrolyzes (1->4)-beta-D-galactosidic linkages in type I arabinogalactans. Binds 1 Ca(2+) ion per subunit. Belongs to the glycosyl hydrolase 53 family.