A mixer having i) a vessel defining a mixing chamber surrounded by boundary walls for receiving a mixture containing lumps therein, ii) a plurality of mixing blades, and iii) a driving mechanism supporting the mixing blades in the mixing chamber for mixing motion relative to the vessel so as to mix the mixture within the mixing chamber of the vessel, further includes at least one lump conditioning blade. The lump conditioning blade is supported by the driving mechanism such that the lump conditioning blade is movable in a working direction along a respective one of the boundary walls with a working edge of the blade being maintained at a prescribed space from the boundary wall which is effective to reduce a lump size of the lumps in the mixture as the mixture passes between the working edge of the lump conditioning blade and the boundary wall.