The principle of extrusion technology
The material containing a certain amount of moisture is pushed by the screw in the sleeve of the extruder and blocked by the discharge abrasive and the intercepting device in the sleeve. In addition, it is also affected by friction heat from the outside and the material, the screw, and the inside of the sleeve. The heating effect keeps the material at a high temperature of 3-8MPa and 120-200°C, and can reach a higher temperature as required.
Since the pressure exceeds the saturated vapor pressure at the extrusion temperature, the material will boil and evaporate moisture in the extrusion barrel. Under such high temperature, shear force and pressure, the material assumes a molten state. When the material is forced out of the die mouth, the pressure suddenly drops to normal pressure, and at this time, the moisture will flash, resulting in a situation similar to "explosion", and the product will expand accordingly.
Moisture evaporates from the material, taking away a lot of heat, so that the material instantly drops from the high temperature in the extrusion process to a relatively low temperature of 80°C. Due to the decrease of temperature, the material is solidified from the state when extruded, and the shape after puffing is maintained.
The main ingredient of extruded puffed food is starch. Therefore, the formation of the puffed state is mainly completed by starch. Under high temperature and high pressure, the starch granules first gelatinize, and then under the action of high temperature and high shear, the molecules combine and crosslink each other to form a network structure. After the structure is extruded, cooled and solidified, it becomes the skeleton of the puffed food structure, and the ingredients in other raw materials are filled in it. Therefore, the content of starch in the raw material directly affects the degree of puffing of the product. In the absence of starch in the raw material, the puffing effect will basically not occur.
Features of extrusion technology
1) Improve practical quality, easy to store
The puffing technology can make the rough and hard tissue structures of corn and sorghum that contain more cellulose and vitamins become bulky and soft, and the Maillard reaction generated during the puffing process increases the color, aroma and taste of the food. Thereby improving the practical quality, so that the food has a unique flavor of light body, crispness and strong fragrance.
2) Improve product quality and reduce harmful substances
The extrusion process is a typical high-temperature, high-pressure, short-time process, which limits the material handling, thus retaining almost all the nutrients in the raw materials in the final product. At the same time, the high-temperature short-time process can still reduce undesirable product factors like harmful enzymes and microorganisms.
3) Wide applicability of raw materials and many types of products
There are many kinds of raw materials that can be processed. It can not only carry out deep processing of grains, potatoes, beans and other grains, make coarse grains finely processed, and produce exquisite snack foods; but also process fruits and vegetables, spices and some animal proteins. When producing puffed food, the same extruder can be used to produce products with different shapes only by changing the raw materials and die heads.
4) High production efficiency and low production cost
The large twin-screw extruder has a production capacity of tens of tons per hour and is easy to operate. Compared with the traditional cooking method, processing products with modern extrusion technology has significantly reduced time consumption, energy consumption, labor force, and occupied workshops.
5) Less waste and no waste
When starch is used to brew wine and make sugar, after the raw materials are expanded, the utilization rate reaches more than 98℅, the sugar yield increases by 12℅, and the wine yield increases by 20℅. In addition, generally speaking, except that a small amount of raw materials are needed as primers during startup and shutdown, there is almost no waste discharge during the entire production process, and there is no waste of raw materials.
Extrusion equipment
The single-screw extruder propels the material by a screw that gradually becomes thicker and the pitch gradually narrows. At present, the single-screw extruder produced in my country basically adopts a screw with a shallow thread height, and the speed is 300-400r/min. It can produce high grinding and high shearing force, and the residence time of the grain raw material in the barrel is only 10-20s, the temperature of the material is as high as 130-140°C before being released from the mold.
The twin-screw extruder has a pair of counter-rotating or co-rotating screws that are continuously engaged with each other, and the material is forced to be conveyed by the principle of positive displacement. Pressure reflux is rarely formed during the material conveying process, which can sharply increase the local pressure, speed up the puffing process, and obtain high output.
