Hydrolyzes galactomannan found in plant cell wall, by cleaving alpha-1,6-D-galactose side-chains from the mannan backbone. Appears to act in synergy with mannanase (ManA) to elicit hydrolysis of galactomannan. Has greater activity against galactomannans with decreased degree of polymerisation values. To a lesser extent, is also able to degrade other galactosides containing alpha-1,6-linked D-galactose, such as melibiose and stachyose. Hydrolysis of terminal, non-reducing alpha-D-galactose residues in alpha-D-galactosides, including galactose oligosaccharides, galactomannans and galactolipids. Optimum pH is 8.2. Optimum temperature is 50 degrees Celsius. Displays half maximal activity in 25 minutes and 7 minutes when pre-incubated at 50 degrees Celsius and 55 degrees Celsius, respectively. Belongs to the glycosyl hydrolase 27 family.