Involved in galactan degradation (PubMed:17056685, PubMed:27501980). Degrades arabinose-free galactan to galactooligosaccharides, producing galactotetraose as the main product along with galactotriose, galactobiose, and galactose (PubMed:27501980). Is also able to degrade galactotetraose, galactotriose and galactobiose, suggesting an additional exo-mode of activity (PubMed:27501980). 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 (Probable). The enzyme specifically hydrolyzes (1->4)-beta-D-galactosidic linkages in type I arabinogalactans. Binds 1 Ca(2+) ion per subunit. Optimum pH is 6.5 with AZCL galactan as substrate. Optimum temperature is 50 degrees Celsius with AZCL galactan as substrate. Repressed by the transcriptional regulator GanR and induced by galactobiose. Also repressed by glucose. Belongs to the glycosyl hydrolase 53 family.