The infusion unit comprises a receptacle (25, 27) for receiving at least one ingredient in bulk form for the preparation of a beverage, as well as a brewing piston (33) arranged for insertion in the receptacle and forming a brewing chamber therewith. The brewing piston (33) is provided with a sealing arrangement (35) for sliding and sealing engagement with an inner wall of the receptacle when the brewing piston is in the receptacle. A hydraulic actuator (51) controls the movement of the brewing piston (33) so as to adjust the volume of the brewing chamber by moving the brewing piston in a brewing position inside the receptacle. A hot water delivery circuit (39) is further provided, for delivering hot water in the interior of the brewing chamber and performing a brewing cycle.