Disclosed herein are methods, systems, and compositions for the uniform pretreatment of biomass within seconds with low inhibitor formation. The pretreatment process is used to convert biomass to a fuel, sugars, or other useful chemicals by subjecting the feedstock to a rapid retention time under pressure and temperature and/or chemical reactant. The system includes at least one high pressure, steam impermiable plug and a continuously-operating valve discharge apparatus to discharge pretreated feedstock while maintaining uniform pressure on the pretreatment system.