A food extruder consists of a flighted Archimedes screw which rotates in a tightly fitting cylindrical barrel. Raw ingredients are preground and blended before being placed in the feed end of the extrusion screw. In many cases, these ingredients are partially heated and moisturized in a preconditioning chamber which can be of the atmospheric or pressure cooker type. The action of the flights on the screw push the food product forward and in so doing, work and mix the constituents into a viscous dough-like mass. Heat is added to the food dough as it passes through the screw by one or more of three mechanisms: (1) viscous dissipation of mechanical energy being added to the shaft of the screw, (2) heat transfer from steam or electrical heaters surrounding the barrel, and (3) direct injection of steam which is mixed with the dough in the screw.