Extrusion technology and its application

1. The principle of extrusion puffing

The material containing a certain amount of moisture is pushed by the screw in the barrel of the extruder and blocked by the mold and the shut-off device in the barrel, and is also heated by the friction heat from the outside and the material and the screw and barrel. , so that the material is in a high temperature environment of 3 ~ 8MPa and 120 ~ 200 ℃, and can reach a higher temperature according to needs.

Since the pressure exceeds the saturated vapor pressure at the extrusion temperature, the material will boil and evaporate of moisture in the extrusion cylinder.

Under such high temperature, shear force and pressure, the material is in a molten state. When the material is forcibly squeezed out of the die mouth, the pressure suddenly drops to normal pressure, and the water flashes at this time, resulting in a situation similar to "explosion", and the product expands accordingly.

The moisture evaporates from the material and takes away a lot of heat, so that the material instantly drops from the high temperature during the extrusion process to a relatively low temperature of 80°C. Due to the decrease in temperature, the material is solidified and formed from the state when extruded, and maintains the puffed shape.

The ingredients of extruded puffed food are mainly starch. Therefore, the formation of the puffed state is mainly completed by starch. In the state of high temperature and high pressure, the starch granules first gelatinize, and then the molecules are combined and cross-linked under the action of high temperature and high shear to form a network structure. After extrusion, cooling and solidification, the structure becomes the skeleton of the puffed food structure, and the ingredients from other raw materials are filled in it.

Therefore, the content of starch in the raw material directly affects the puffing degree of the product. In the absence of starch in the raw material, the puffing effect is basically not produced.

2. Features of extrusion technology

1) Improve practical quality, easy to store

The use of puffing technology can make corn and sorghum, which originally 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, the practical quality is improved, and the food has a unique flavor of light body, crispness and rich aroma.

2) Improve product quality and reduce harmful substances

The extrusion process is typically a high temperature, high pressure, short time process, which results in limited material handling and thus retains almost all the nutrients in the raw material 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 variety of products

There are a wide variety of raw materials that can be processed. Not only can the grains, potatoes, beans and other grains be deeply processed, the coarse grains can be made finely, and exquisite snacks can be produced; but also fruits and vegetables, spices and some animal proteins can be processed. When producing puffed snacks, products with different shapes can be produced by using the same extruder only by changing the raw materials and the die.

4) High production efficiency and low production cost

The production capacity of foreign large-scale twin-screw extruders reaches dozens of tons per hour, and the operation is simple. Compared with the traditional cooking method, using modern extrusion technology to process products has significantly reduced time consumption, energy consumption, labor force, and plant occupation.

5) Less waste and no waste

After the raw material is puffed, its utilization rate is more than 98℅. Generally speaking, except that a small amount of raw materials are needed as primers when starting up and shutting down, almost no waste is discharged during the entire production process, and there is no waste of raw materials.

3. Extrusion equipment

The extruder was developed from a simple molding machine, initially only used in the plastics industry, and later in the food and other industries. The extruders currently used are mainly screw extruders, which can be divided into two types: single-screw and twin-screw according to the structure.

The single-screw extruder propels the material by a screw that gradually increases from thin to thick and the pitch from wide to narrow. At present, the single-screw extruder produced in my country basically uses a screw with a shallow thread height and a rotating speed of 300-400 r/min. It can generate high grinding and high shearing force, and the grain raw material stays in the barrel for only a short time. In 10-20s, the temperature of the material before ejection is as high as 130-140℃.

The twin-screw extruder has a pair of counter-rotating or co-rotating screws that are continuously meshed with each other, and the material is forcibly conveyed by the principle of positive displacement.

In the process of material conveying, pressure backflow is rarely formed, which can make the local pressure rise sharply, speed up the puffing process, and obtain high output.

4. Process flow of extrusion puffing

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

1) Crush

In order to mix the raw materials evenly and fully gelatinize the starch during extrusion and cooking, which is conducive to puffing, each material (corn should be removed from the husk and germ first) is crushed to a particle size of 30-40 mesh, and the twin-screw extruder is crushed to a size of 60 mesh. above.

2) Raw material mixing

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

3) Extrusion puffing

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, the temperature, pressure, screw speed, the type of raw materials and their proportions during the extrusion process, etc.

The raw material with low amylose content has high α degree after puffing, and the puffing effect is better. Different protein and fat contents in the material also have an impact on the puffing quality. Materials with high protein content have low puffing degree during extrusion; when the fat content exceeds 10%, it will affect the puffing rate of the product, and a certain amount of fat can improve the product. Texture and flavor. Different types and models of extruders have different optimal process parameters for extrusion.

4) Shaping and cutting

After the puffed material is extruded from the die hole, it is cut and formed by a rotary cutter close to the die hole or pulled to a shaping machine.

5) Bake

The extruded semi-finished product has high moisture content and needs to be sent to the tunnel oven for further baking through the belt conveyor, so that the moisture content is lower than 5%, so as to prolong the shelf life, and at the same time produce a special aroma after baking to improve the quality.

6) Seasoning

in a rotisserie. The vegetable oil and cream mixed in a certain proportion are heated to about 80 ℃, and the oil is evenly sprayed on the surface of the material that is tumbling with the rotation of the seasoning machine through the mist nozzle. The purpose of spraying is to improve the taste; the second is to make the material easily stick to the seasoning.

Then the seasoning is sprayed, and the powdered seasoning is evenly sprinkled on the surface of the continuously rolling material through a powder sprayer equipped with a screw propeller, and the finished product is obtained. To prevent moisture and ensure crispness, seasoned products should be packaged immediately.

5. Multi-field application of extrusion technology

1) Application in snack food processing

Puffed food is the first successful product to apply extrusion technology to food processing. Using rice, corn and other grains and potatoes as the main raw materials, after extrusion and cooking, it is puffed into a loose and porous product. After baking, dehydration or frying, a layer of delicious seasoning is sprayed on the surface. , puffed shrimp, etc. belong to this category.

The other type is the puffed sandwich snack food, which is made by co-extrusion puffing, that is, the grain material is extruded to form a hollow tube, and various ingredients such as egg yolk powder, powdered sugar, milk powder, seasonings, and spices are pressed together. After a certain proportion is added, it is fully stirred and mixed to form a sandwich material with good fluidity. Through a sandwich pump and a co-extrusion die, the filling is injected into the middle of the tube while the puff is extruded. The puffed sandwich small food has a crisp taste and a variety of flavors with the change of the sandwich filling. By changing the formula of the sandwich material, various nutritionally enhanced foods and functional foods can be processed.

2) Application in grease leaching

For oil crops with high oil content, after shelling (skin), the oil is first processed by an extrusion extruder to extrude part of the oil in advance, and form particles with a certain structure before leaching, which is an ideal new technology. . At present, there are extrusion extruder manufacturers in the United States, Brazil, India, Switzerland and other countries. Among them, the extrusion extruder with pre-pressed and used for processing high oil content produced by ANDERSON Intrntionl Corp of the United States is particularly striking.

In addition, the invention of the extruder and its application in the solvent leaching process of oil flakes has greatly increased the efficiency of the solvent leaching process. The flakes in the extruder are squeezed by a series of intermittent spirals and rubbed by the fixing bolts protruding from the barrel, so that more oil cells are broken.

While the flakes are mashed, steam is injected into the barrel containing the flakes, and the flakes are cooked in the barrel for 15-20s. Cooking changes the protein properties of the flakes, making them softer.

At the same time, when the steam enters the barrel and mixes with the flakes, the pressure inside the barrel changes the steam from the gas phase to the liquid phase, releasing a large amount of heat, which produces an effective cooking action in the extruder, and the liquid penetrates the flakes , to fully penetrate the blank.

In addition, after the oil blanks are completely cooked, they are released from the outlet of the extruder. Due to the sudden drop in pressure, the moisture in the blanks will immediately change from liquid phase to gas phase.

When the liquid turns into a gas, it absorbs the surrounding heat, which cools the protein and creates a rigid, porous protein structure that expands.

3) Application of extrusion technology in the preparation of resistant starch

The application of extrusion puffing technology in the pretreatment of resistant starch preparation is because in the preparation process of resistant starch, extrusion puffing plays the role of pre-gelatinization and improves the gelatinization degree of starch.

It has been reported that the gelatinization degree of starch can reach more than 90% after extrusion and puffing, while the gelatinization rate of traditional technology is only 80-90%.

Only when starch is completely gelatinized, can amylase and puralanase fully act on it to generate amylose molecules of a certain length. By adjusting the action conditions of the enzyme, the yield of resistant starch can be improved.

4) Application in the production of textured vegetable protein 

The production of textured vegetable protein uses plant-based protein as raw material, such as soybean protein isolate, wheat protein, etc., after extrusion and shearing, the tertiary structure of protein molecules It is destroyed to form a relatively linear protein molecular chain. Under a certain temperature and moisture, due to the high shear force and the directional flow of the screw, when it is extruded through the die outlet, the protein molecules become fibrous. structure.

After the vegetable protein is textured, it improves the taste and elasticity, expands the scope of use, and improves the nutritional value. Compared with animal protein, it has low price, no cholesterol, long shelf life, easy coloring, and easy to add flavor and flavor. It can be prepared into a variety of different foods.

It can be added to meat raw materials, used as meat filler, or replaced with meat, fish and poultry to make imitation meat food. The United States has added such meat supplements to hamburger steaks, meatloaf, and sandwiches, with up to 30% of meat substitutes in hamburger steaks.

5) Application of extrusion technology in the development of health care mixed powder

Black rice, barley, and buckwheat flour are used as raw materials. These materials have high nutritional value and health care functions. However, due to their hard texture, they are difficult to gelatinize and digest and absorb during normal cooking. Therefore, they must undergo special processing. , extrusion technology is the ideal method.

In addition, due to the uneven distribution of various nutrients in each raw material, people's needs for various nutrients are often not met.

6. Development prospects of extrusion technology

One purpose of food processing is to make full use of existing food resources and develop new sources of raw materials to develop a variety of popular foods.

The emergence of puffing technology can be said to have opened up a new way for the application of raw materials such as grains, starches, etc., which we call coarse grains, in the food industry.

Moreover, puffed food generally needs to be seasoned, so the development of the puffed food processing industry will definitely drive the development of the seasoning industry, and at the same time, it will also drive the development of packaging technologies such as films.

In addition, with the development of the food industry, the emergence of new technologies and processes, and the improvement of people's living standards, extrusion technology and its equipment are also developing in a better direction, producing more popular low-oil, health-care and natural food.

Such as low temperature, ultra-low temperature puffing technology, chemical puffing technology, ultrasonic puffing technology may be applied in practice in the near future; and microwave puffing technology, baking puffing technology as new puffing technology has attracted people's attention and gradually applied in production.

According to reports, with starch as the main raw material, with 9% corn protein isolate, 1.5% palm oil, under the condition of 50% water content, 500W, 2450MHz microwave radiation for two minutes can obtain good raw material puffed food.

Vacuum frying and puffing technology is of great significance for improving the quality of food and reducing the degree of deterioration of oil. If frying is carried out at a vacuum of 20mmHg and a temperature of 100°C, water vapor will be generated at 60°C; if frying at 80-120°C, the moisture in the raw materials can be fully evaporated, and the volume of water will be reduced when the water evaporates. Significant expansion.

In addition to the obvious puffing effect of the products prepared by vacuum frying, the frying time is also relatively shortened.

In short, with the improvement of people's living standards, the production of puffed food by extrusion technology and the application of extrusion technology in other fields have very broad prospects in my country.

Extrusion puffing technology, microwave puffing technology, baking puffing technology, etc. will be the development direction of puffing technology; vacuum frying puffing technology is an effective improvement method to maintain the vitality of frying puffing technology.

Carrying out research on puffing theory and technology, developing new sources of raw materials, developing new extrusion puffing equipment, organic combination of extrusion puffing technology and other food processing technologies will be the focus and hot spot of puffing technology development.