A piston assembly for a food extruder having a food reservoir with a chamber and an inner surface, a piston rod and a piston dish. The piston rod slidably extends into the chamber of the food reservoir. The piston dish is attached loosely to the piston rod and has a lower segment, an upper segment, an annular groove and a sealing ring. The lower segment has an outer surface with an inclined bottom edge. The upper segment is hollow and is formed with the lower segment. The annular groove is formed in the outer surface of the lower segment adjacent to the upper segment. The sealing ring is mounted in the annular groove and slidably presses against the inner surface of the food reservoir to form an airtight seal.