Endo-1,4-mannanase, a crucial enzyme for depolymerization of seed galactomannans and wood galactoglucomannans. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 5.0. Optimum temperature is 45 degrees Celsius. Belongs to the glycosyl hydrolase 26 family.