Extrusion Processes

Extrusion processing of foods has been practiced extensively for many years. Both single-screw and twin-screw extruders are used for commercial production of a wide variety of food products, ranging from snack half-products, textured vegetable protein, animal feed, expanded ready-to-eat cereals, and flat breads. Before embarking on a discussion of the performances of the various extruder types, it is essential that the reader have some understanding of the rheological properties of food extrudates. For a non-Newtonian extrudates, problems in integrating the nonlinear differential equation again arise. Control of product temperature is an important issue in the extruder design. In addition to heat conducted through extruder surfaces, other thermal inputs are employed in food processing to augment (or minimize) heat generation by viscous dissipation of mechanical energy. Since the extrudate undergoes chemical and physical changes during its transit through the extruder, there is a great deal of interest in the residence-time distributions in different extruder types.