In recent years, with the rapid development of the food industry, fried food has been widely involved in children's food, fast food, meat-filled food, quick-frozen food, microwave food and pasta products and many other fields. Food frying technology continues to improve, the scale of production continues to expand, and frying equipment is also developing in the direction of factory, serialization and intelligence.
Among them, the typical continuous fryer has developed from a simple type in the 1980s to a modern intelligent type: the frying width is 250, 400, 600 and 800mm, etc.; the frying length is 2.5, 4, 6 and 8m, etc.; the heating methods used are electric, gas or coal-fired.
Intermittent fryers have also developed in the direction of fuel saving, energy saving and environmental protection, and there have been fryers with integrated oil-water structure. This paper focuses on the current situation of food frying technology and the principle, structure and characteristics of typical frying equipment, aiming to provide a reference for the innovation and development of food frying technology and the improved design of frying equipment.
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There are many kinds of food frying technology, according to the air pressure when frying can be divided into atmospheric pressure frying and vacuum frying; according to the way of frying, it can be divided into continuous frying and intermittent frying.
1. Atmospheric pressure frying
Atmospheric frying refers to the air pressure inside the fryer when it is working, which is the most commonly used frying technique. Its advantages are convenient processing, good brittleness and good flavor of the processed products, etc., and the disadvantage is that the loss of nutrition and natural color during the frying process is very large. Room temperature frying is more suitable for the frying of grain foods, such as fried pastries and bread.
2. Vacuum frying
The absolute pressure of vacuum frying is lower than atmospheric pressure, and it is processed in the case of relative lack of oxygen. The food processed with this technology can maximize the preservation of the flavor and nutrient composition of the raw materials, effectively preventing the oxidation and deterioration of edible oils and fats. The features of this technology are as follows:
1) Can reduce the evaporation temperature of moisture in the material, compared with atmospheric pressure frying, heat energy consumption is relatively small, frying temperature is greatly reduced, can reduce the loss of vitamins and other heat-sensitive components in food, is conducive to maintaining the nutritional components of food, avoid food coking.
2) Can cause a hypoxic environment, can effectively kill bacteria and some harmful microorganisms, reduce the oxidation rate of materials and frying oil, provide conditions to prevent material "browning", inhibit material mildew and bacterial infection, is conducive to the extension of product storage period.
3) Under a sufficiently low pressure, the material organization will have a certain expansion effect due to the reduction of external pressure. The vacuum state also shortens the time for impregnation, degassing and dewatering of the material.
4) With the help of the role of pressure difference, the movement of material molecules and gas diffusion in the material are accelerated, thereby improving the speed and uniformity of material handling.
3. Batch frying
When using this technique to fry food, the frying oil is first heated to a specified temperature, and then the material is fed into the frying equipment. After processing, the product is removed and a new material to be fried is added. Since the feeding of the material is intermittent, it is called the intermittent frying technique. The application of this technology is relatively common, and its advantage is that the technical content is relatively low, suitable for small-scale production; the disadvantage is that the product will cause the original heat loss during the feeding process, which takes a long time.
4. Continuous frying
When using this technology to fry food, the feeding of the material is continuous, the material is fed into the fryer and then moves with the mesh belt in the frying oil, and then the processed product is exported from the outlet. Because the products processed with this technology have a consistent frying temperature and time, the products have a constant appearance, flavor, tissue and shelf life, and have a good oil filtration effect, which can reduce the content of fried odors and free fatty acids.
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Fryer is the key equipment of food frying processing, according to the air pressure when frying, can be divided into atmospheric pressure fryer and vacuum fryer. Atmospheric pressure fryers are generally open and widely used.
According to the way of frying, it can be divided into continuous fryer and intermittent fryer. According to the liquids required to be added when processing food, it can be divided into traditional fryers and water filter fryers.
In addition, there are special fryers specially designed for a certain or a certain type of material. This article only introduces a few typical atmospheric fryers.
1. Traditional continuous fryer
The traditional continuous fryer is the main model of frying process application in the convenience food production line, and is also an indispensable frying equipment for mechanized large and medium-sized food processing plants, which is divided into Yz250 type, Yz400 type and Yz600 type according to its processing capacity, and the width of the mesh belt is 250, 400, 600mm, etc. The larger the mesh belt width, the higher the productivity.
1) Structure
The traditional continuous fryer is composed of a double mesh with stepless variable speed conveyor system, an external circulation filtration system composed of the main tank tank and the auxiliary tank, a PID oil temperature automatic controller, an XSM rpm linear speed instrument and a rope lifting device, and its structure is shown in Figure 1.
2) Features
1) According to the needs of the frying material, the traditional continuous fryer can use salad oil, peanut oil, rapeseed oil and palm oil for deep-fried meatloaf, rice cake, potato cake, various mixed cakes, french fries, potato chips, fish balls and meatballs.
2) The traditional continuous fryer adopts the device of extracorporeal filtration circulation oil circuit, so that the oil is heated by the heater in the flow of uniform circulation in the main oil tank, so that the temperature of the oil rises evenly and stably. In addition, when the circulating oil passes through the double-layer (coarse) filter of the auxiliary tank, the residue of large and small particles in the oil is removed, ensuring the cleanliness of the oil, so that the fried material remains clean at any time.
3) The structure of the traditional continuous fryer is refined and reasonable, and the function is advanced. The use of double mesh belt stepless speed adjustment, not only to ensure the different frying time of the frying, but also to ensure that the fried material in the oil layer under the 2-3cm of the stable heating and transportation; the use of gantry and lifting mechanism, the cover can be easily lifted and lowered, easy to fry the food processing.
2. Water filter fryer
Water filter fryers, also known as oil-water mixed fryers, include small intermittent water filter fryers and large water filter continuous fryers, which have different scopes of application.
1) How it works
The principle is shown in Figure 2. The water filter fryer is designed by using the principle of different specific gravity of oil and water, incompatibility and automatic stratification. In the tank, there is a high temperature oil layer (heating oil layer), a buffer oil layer (filter oil layer) and a water layer (sedimentation layer). The slag chips falling from the fried material fall into the water layer through the buffer oil layer before the high-temperature oil layer is carbonized, and are regularly discharged from the slag discharge port. The debris floating on the surface of the hot oil is filtered out after entering the filter tank driven by the hot oil circulation pump. In addition, due to the buffer oil layer, the amount of one-time refueling is twice that of the same wide fryer, but the actual fuel consumption of the material is only 70% of the fuel consumption of the same width fryer.
Because the water molecules in the water below continue to enter the high-temperature oil layer during the frying process, supplementing the moisture in the frying layer, it not only ensures the frying quality of the material, but also greatly extends the service life of the oil, which is 2 to 3 times that of the ordinary frying method.
2) Structure
The structure of the water filter continuous fryer is basically the same as that of the standard continuous fryer, but the water layer is added to the oil tank, the structure is more complicated, and it is mainly used in the production line of large food processing enterprises to achieve mass production. The following focuses on the structure, working principle and characteristics of a small intermittent water filter fryer.
(1) The structure of the water filter batch fryer
The water filter batch fryer is mainly composed of a shell, an oil tank and a heating pipe, as shown in Figure 3. Drain and drain ports are opened in the lower part of the side wall of the oil tank.
(2) The working principle of the water filter batch fryer
Before use, pour water into the oil tank, where the water level does not exceed the center of the oil scale, and then pour in the oil. The specific gravity of oil and water is different and the incompatibility of each other makes the oil and water layered, and the upper layer of oil is heated by using a heating pipe, while the heat pipe blocks the downward transfer of heat, which plays a role in reducing the disturbance of the lower oil layer when the upper layer of fried food and the frying oil is turned. The control panel is equipped with a smart meter that automatically controls the oil temperature and keeps it in the right range.
(3) Characteristics of water filter batch fryer
1) The use of central heating, oil and water mixing structure mode, can improve the heating speed, energy saving and fuel saving, can be fried at the same time a variety of food.
2) In the process of frying, due to the specific gravity relationship between vegetable oil and animal oil, the animal oil leached from the fried meat food naturally sinks into the lower layer of vegetable oil, so it is ensured that the working frying oil in the upper layer is always kept pure. It can fry a variety of foods at the same time, without stringing each other' taste, effectively reducing the oxidation degree of the fried oil and inhibiting the increase in acid prices.
3) The food residue during the frying process sinks into the water, does not carbonize, and effectively controls the production of carcinogens. Residues can be removed in time to achieve self-cleaning, which is one of the reasons why the oil remains pure. The concentration of fumes is 1.96 mg/m³, which is much lower than the concentration of 6161 mg/m³ in ordinary fryers.
4) Due to the automatic control of oil temperature and water temperature, it effectively reduces the degree of oxidation of fried oil (that is, the oil is not easy to paste), completely changes the structure of traditional frying equipment, fundamentally overcomes the drawbacks of traditional fryers, and the fried food is not only good in color and flavor, but also has a clean and beautiful appearance, improves product quality, and is widely used in hotels, hotels, fast food restaurants, guest houses, troops, colleges and enterprise canteens.
3. The working principle and structure of the oil-pouring fryer
The oil-pouring fryer is suitable for low-temperature fried food, such as fried chili oil. The oil temperature required for such materials is low, and the oil temperature is too high to fry the paste, so it is not possible to use a mesh belt type continuous fryer or a water filter fryer. At present, most of the frying of materials such as peppers is artificially fried, which has problems such as low productivity, easy to fry paste, and workers are often choked by spicy oil smoke. Based on the characteristics of materials such as chili peppers, specially designed oil-pouring fryers can be used. At present, such fryers are rare in China.
The structure of the oil-pouring fryer consists of an oil connection plate, a sieve, a spreader, a heating pipe, a fuel tank, a smoke exhaust cover and a frame, as shown in Figure 4.
2) How it works
When the oil fryer works, the fried food to be fried (such as cut chili diced peppers) is first spread evenly on the sieve piece by the spreader, and at the same time, the fried oil heated by the heating pipe flows into the oil pipe from the tank under the action of the oil pump, and is sprayed onto the sieve piece through the oil pipe. Because the sieve blade is driven by the reducer, the fried food is fried for frying processing. The frying oil sprayed into the sieve disc flows into the oil receiving tray through the small hole in the sieve, and then filters through the return pipe and filter funnel to flow back into the tank, and so on.
3) Features
(1) The contact time between the fried oil and the food to be fried is short, so as to ensure that the food to be fried will not be fried.
(2) When frying peppers and other materials, the oil smoke is particularly spicy and pungent, which is harmful to the respiratory system of the processing personnel. Therefore, there are smoke extraction devices above the sieve blade and above the fuel tank, thus ensuring that the operator's body is not endangered.
(3) The oil temperature of the fried material will be reduced, but it can continue to be recycled after returning to the tank for heating, saving fuel and making full use of heat energy.
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Fried food is one of the traditional foods in China, which is deeply loved by people because of its crisp and delicious, fragrant aroma and ability to increase appetite. However, the hazards of food processed by traditional frying methods and the energy and energy consumption of fryers have also attracted widespread attention. Therefore, the development and improvement of frying technology and frying machines should be based on solving the above two problems.
1. It is necessary to advocate "green frying" to reduce the production of toxic substances such as acrylamide in fried foods. In the frying method, vacuum frying technology is used as much as possible; in the frying equipment, the water filter fryer and vacuum fryer are the mainstays. In the future, we should further strengthen the research and development of vacuum frying technology and oil-water hybrid frying technology, as well as the improvement of related frying machines and the research and design of utility models.
2. At present, most of the existing fryers are mainly in the form of electric heating, which consume a lot of electricity (especially large continuous fryers), and put forward new challenges in today's increasingly tight electricity consumption;
In addition, most of the oil after processing is naturally cooled, and it needs to be reheated the next time it is processed, which consumes a lot of heat energy. In the future, we should strengthen the improvement of the design and development of oil fryers that use energy other than electric energy, such as gas-fired fryers and coal-fired fryers. In the effective use of energy, the fryer design as far as possible to consider the installation of thermal insulation device, so that the processing of the completed hot oil can be insulated, reduce the loss of energy.
Source: Food R&D and Production Finishing
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