The present invention relates to an apparatus, which can be part of a pre-treatment system in a plant for the production of fuels, e. g. bio-ethanol, derived from plant biomass, e. g. first generation crops, such as grain, sugarcane and corn or second generation crops such as lignocellulosic biomass. The invention relates to an apparatus for processing, such as fluffing and mixing, at least two media, such as a solid, e. g. biomass, and a fluid, e. g. steam, so as to rendering the first medium susceptible to efficient receiving of energy and/or mass which is provided by localized release of the second medium. Although the description of the present invention focuses on biomass, it is envisaged that the invention is generally applicable to control the mixing of at least two media by crossing their stream of while dispersing at least one of them.