® Ready to Eat-Cereals " Texturized Vegetable Protein and Miller, 1973). Pressure at the die can be obtained by decreasing the size of the die orifice, i.e. the smaller the die orifice, the more pressure placed upon the die plate. Conversely, the feed zone has a large depth screw channel or large flow area while the metering zone has a small depth screw channel or small flow area. The ratio in screw channel depth in the feed zone to that of the metering zone is known as the compression ratio (Harper, 1989). The larger the difference in channel depth between the feed zone and the metering zone, the more pressure exerted at the die. Problems could occur if the die has a large resistance to flow, and result in pressure build-up in the feed zone. This leads to plastification of the ingredients in that zone and instable working of the extruder. However, if the die has a low resistance to pressure, it leads to too much pressure build up in the metering zone and no pressure in the feed zone. This results in surging of the extruder (Janssen, 1989).

Heat input to the product during extrusion cooking comes from both internal and external forces. Steam can be injected into the barrel of the preconditioner, heat can be transferred from a steam jacket encasing the barrel, or frictional heat can develop on the barrel wall and in the die area during the rotation of the screw and flow of the product (Clark, 1978; Dziezak, 1989).

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