Extrusion principle

Materials containing a certain amount of water are pushed by the screw in the extruder barrel, blocked by the mold and cutting device in the barrel, and are also heated by external friction heat and materials and materials. Screw and barrel. The material is placed in a high temperature environment of 3 ~ 8MPa and 120 ~ 200℃, and higher temperatures can be reached as needed.

Because the pressure exceeds the saturated steam pressure at the extrusion temperature, the material will boil and evaporate water in the extrusion cylinder.

At such high temperatures, shear forces and pressures, the material is in a molten state. When the material is forced out of the die, the pressure suddenly drops to normal pressure, at this time, the water flashes, there is a similar “explosion” situation, and the product expands.

Water evaporates from the material, taking away a lot of heat, making the material drop from the high temperature in the extrusion process to the lower temperature of 80 ° C. Due to the decrease in temperature, the material solidifies from the state at the time of extrusion and retains the puffed shape.

The ingredients of puffed foods are mainly starch. Therefore, the puffing state is mainly completed by starch. In the state of high temperature and pressure, starch particles are gelatinized first, and then the molecules are combined and crosslinked under high temperature and high shear to form a network structure. After extrusion, cooling and curing, the structure becomes the skeleton of the expanded food structure, and the ingredients of other raw materials are filled in it.

Therefore, the content of starch in the raw material directly affects the expansion degree of the product. In the absence of starch in the raw material, there is basically no puffing effect.

Extrusion process flow

The production process of extruded food is crushing → mixing (wetting) conditioning → conveying → feeding → extrusion → shaping, cutting → baking → oiling, seasoning → packaging.

1) Crushing

In order to make the raw materials mixed evenly in the extrusion and cooking process, so that the starch is fully gelated, which is conducive to expansion, each material (corn should be husked and germ first) is crushed to 30-40 mesh size, and the twin screw extruder is crushed to 60 mesh. Above.

2) Mix raw materials

The different raw materials and auxiliary materials are mixed evenly in a certain proportion in the powder mixer, and the amount of water added is determined according to the difference in climate, ambient temperature and humidity. The total moisture of the mixed raw materials is controlled at 13% ~ 20%.

3) Extrusion

Extrusion is the key to the whole process, which directly affects the texture and taste of the product. There are many variables that affect extrusion, such as the moisture content of the material, temperature, pressure, screw speed, the type of raw material and its proportion in the extrusion process.

The raw material with low amylose content has high α degree and better swelling effect. The different protein and fat content in the raw material will also have an impact on the puffing quality. The material with high protein content has low expansion degree during extrusion. When the fat content exceeds 10%, it will affect the expansion rate of the product, and a certain amount of fat can improve the product. Texture and taste. Different types and models of extruders have different optimum extrusion process parameters.

4) Shaping and cutting

After extruding from the die hole, the extruded material is cut by the rotary tool near the die hole or pulled to the forming machine.

5) Bake

The extruded semi-finished product has a high moisture content, which needs to be sent to the tunnel oven through the belt conveyor for further baking, so that the moisture content is less than 5%, in order to extend the shelf life, and produce a special aroma after baking to improve the quality.

6) Seasoning

It’s in the seasoning machine. The vegetable oil and cream mixed in a certain proportion are heated to about 80 ° C, and the oil is evenly sprayed on the surface of the material that rolls with the rotation of the seasoning machine through the atomizing nozzle. The purpose of spraying is to improve the taste; The second is to make the material easy to stick to the seasoning, and then spray the seasoning, and evenly sprinkle the powder seasoning on the surface of the continuous rolling material through the duster equipped with a propeller, and then the finished product is obtained. To prevent moisture and ensure crispness, condiments should be packed immediately.

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