Could be involved in the degradation of glucomannan and catalyzes the endo hydrolysis of beta-1,4-linked mannan, galactomannan and glucomannan. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. The production of the two beta-mannanases seems not to be caused by proteolytic cleavage. Belongs to the glycosyl hydrolase 26 family.