Extrusion of Foods
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Food extrusion refers to a process in which a food material is forced to flow, under one or more varieties of conditions of mixing, heating and shear through a die which is designed to form and/or puff-dry the ingredients. The early 1950s saw the first application of extrusion for producing dry expanded pet foods in the United States. Pet foods have grown into the largest single commercial application of extrusion cooking. The most basic soy-based raw material for extrusion are pressed flakes which are defatted products containing 50 wt% protein with a variable protein digestibility index of 55–90%. Vitamins and minerals are added during extrusion for fortification of products. Natural and synthetic colorants can be added during extrusion to produce products of different colors. The feeding system can feed materials directly into the extruder or into a preconditioner. Preconditioning is beneficial for extrusion processes involving higher moisture content.