Endo-1,4-beta-xylanase involved in the hydrolysis of xylan, a major structural heterogeneous polysaccharide found in plant biomass representing the second most abundant polysaccharide in the biosphere, after cellulose. Hydrolyzes xylans from oat spelt and birchwood at similar rates, but it has no detectable activity toward Avicel or carboxymethyl cellulose. Endohydrolysis of (1->4)-beta-D-xylosidic linkages in xylans. Optimum pH is 2.0. Optimum temperature is 50 degrees Celsius. Glycan degradation; xylan degradation. Belongs to the glycosyl hydrolase 11 (cellulase G) family.