NOVELTY - Biomass containing lignocellulose is processed such that cellulose and hemicellulose are made accessible for further processing, by repeated compressions in a reciprocating piston press, where loose biomass is continuously fed into a piston chamber in front of a piston which moves the loose biomass into a tubular reaction chamber in which the biomass is compressed for producing a mechanically-induced water vapor explosion and autohydrolysis under simultaneous displacement of compressed biomass through the reaction chamber.    USE - Processing of biomass e.g. straw, containing lignocellulose such that cellulose and hemicellulose are made accessible for further processing, typically decomposition, preferably enzymatic decomposition, for use in production of biofuels e.g. ethanol, butanol, hydrogen, methanol and biogas.    ADVANTAGE - The method is economical, and enables efficient mechanical tearing of cellulose fibers, which makes lignocellulose components of cellulose and hemicellulose accessible to subsequent enzymatic decomposition to their respective monomers. The method can be carried out with less consumption of water, chemicals and energy.    DETAILED DESCRIPTION - Biomass e.g. straw, containing lignocellulose is processed such that cellulose and hemicellulose are made accessible for further processing, typically decomposition, preferably enzymatic decomposition, by repeated compressions in a reciprocating piston press, where loose biomass is continuously fed into a piston chamber in front of a piston which moves the loose biomass into a tubular reaction chamber in which the biomass is compressed for producing a mechanically-induced water vapor explosion and autohydrolysis under simultaneous displacement of compressed biomass through the reaction chamber.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of mechanical steam explosion for simultaneous pretreatment and feeding of straw into a biogas reactor, alternatively direct or indirect feeding into thermal, chemical, thermochemical or other bioreactor.