Provided are devices and methods for discharging pretreated biomass from a pressurized reactor. Pretreated, hot biomass is added under high pressure at the top of a discharge vessel, which is partially filled with water or aqueous solution. The vessel comprises water jets situated on its sides through which water or aqueous solution is injected in order to establish a strong turbulence in the lower part of the vessel. This turbulence, preferably a vortex turbulence or “hydrocyclone,” blends biomass and water and disintegrates any clumped material prior to its ejection at high speed as a slurry through an orifice or valve at the bottom of the vessel. A stable temperature stratification is established within the discharge vessel, whereby hot water remains at the top of the vessel, reducing heat and steam loss from the pretreatment reactor.