Principles Of Extruded Food Processing

As the food material with some moisture content passes through the extruder, it encounters various forces such as the push of the screw, the obstruction of the inner wall of the sleeve, the reverse screw, the forming mold, and the heat from the outer wall of the sleeve. This leads to the generation of a significant amount of frictional and conduction heat. Additionally, the heating effect of the frictional heat causes the material to be in a high-pressure state of 3~8MPa and at a high temperature of above 200°C inside the barrel. At this pressure, the water in the material does not boil and evaporate due to the extrusion temperature being higher than the saturated vapor pressure of the water. This leads to the material being in a molten state.

Once the material is extruded from the die, there is a sudden drop in pressure to normal pressure, which causes the water in the material to flash and evaporate instantly. This leads to the temperature dropping to around 80°C, resulting in the material becoming an expanded food with a porous structure and a certain shape. To prevent overheating, a cooling device is added, which ensures that the temperature is below 100°C before the material exits the die. Therefore, the extrusion process involves a combination of various physical and chemical processes that lead to the creation of unique expanded food products.