The invention concerns a mixing device (1) having: —a whipper housing (2), a rotor (3) and a back wall (4), the whipper housing and the back wall forming a whipper chamber in which is lodged the rotor, —the whipper housing having a beverage inlet (11) and a beverage outlet (7), —a drive shaft (41) for driving the rotor, said drive shaft being supported by the back wall, —the rotor presenting a radial back surface (33) facing the whipper housing back wall (4) and a radial front surface (32) opposed to the radial back surface —the whipper housing having a front wall (21), said front wall facing at least a part of the radial front surface (32) of the rotor, and wherein the whipper housing front wall (21) presents bumps (5) elevating from the surface (211) of the front wall, the upper surface (51) of each bump being flat and the edges (52) of said upper surface being sharp.