Acts as endo-acting enzyme with a requirement for at least five sugar moieties for effective catalytic activity. Hydrolyzes carob-galactomannan (locust bean gum) effectively and to a smaller extent guar gum, but not yeast mannan. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 5.4. Optimum temperature is 85 degrees Celsius. Belongs to the glycosyl hydrolase 26 family.