Fish Products and Byproducts Technology

Unit 6 - Extrusion

6.5. Extrusion processing steps
Ingredients are propelled along the barrel of the extruder by one or two screws. The mechanical screw action and the friction it creates blends, shears and cooks the material (Held at 120-175ºC for about 30 seconds). At the end of the barrel, the mixture is forced through a die at high pressure. Because of the difference between pressure inside and outside of the barrel, the material expands, and according the die shape and the cutter blade action, it creates a pellet of varying size, shape and density. Schematic representation of extrusion cooking is given in figure 2.
There are three zones in single screw or twin screw extruders.

• Input zone
• Kneading zone
• Cooking zone

Input zone - where th raw materials are introduced
Kneading zone - Where the ingredients are subjected to extreme mixing, pressure and stream heat
Cooking zone - uniformly blending the dry preconditioned ingredients, injected into the extruder on a cooking-extrusion system.