Prepared foods, also known as ready-to-prepare foods, refer to agricultural, livestock, and aquatic products as raw materials, which are properly conditioned and packaged, and stored in freezing (-18°C), refrigerated (below 7°C) or controlled atmosphere, etc. Treat products that can be stored and sold at room temperature and can be eaten directly or after simple processing or heat treatment before consumption.
The essence of prepared food is a kind of convenience food, which has the characteristics of convenience, speed, delicious flowers, nutrition and safety. Because of its high added value and demand for localized taste, it is recognized as one of the products with development potential in the domestic food industry. With the innovation of process technology, the improvement of the cold chain circulation system, the update of packaging materials and the development of high-efficiency thawing equipment, the new type of prepared food has an infinitely broad market prospect. The fried conditioned food has the advantages of unique flavor, simple conditioning and edible, can be stored at room temperature, etc., and can be accepted by the majority of consumers. In today’s fast-paced life, although people have mixed opinions on it, there is no doubt that the representative bulk fried seasoning foods such as fried instant noodles, fried French fries and fruit and vegetable chips have become indispensable to people. food.
Frying technology can be divided into three categories: atmospheric frying, reduced pressure frying and high pressure frying.
The pressure in the oil kettle for normal pressure frying is the same as the ambient atmospheric pressure, and it is usually open. Therefore, atmospheric frying is more suitable for frying grain food. Such as the dehydration of fried cakes, fried bread, fried instant noodles, etc.
Low-pressure frying, also known as vacuum frying, is a process in which food is fried, dehydrated and dried in edible oil to fully evaporate the moisture in the raw materials. The frying temperature in the frying kettle is generally below 100 °C, and the vacuum degree is 92~98.7 kPa. The method can keep the product in good color, shape and crispness, and the deterioration degree of the product and oil will be correspondingly reduced due to the low oxygen environment, and the loss of nutrients will be less.
High-pressure frying is a frying method in which the pressure in the frying pot is higher than normal pressure. High-pressure frying can solve the problem of affecting food quality due to long-time frying. This method has high temperature, less volatilization loss of moisture and oil, and the product is crispy on the outside and tender on the inside. It is most suitable for frying meat products, such as fried chicken legs, fried Steak etc.
Compared with traditional heating, microwave heating is significantly different, mainly due to the difference in heating form and heating speed, resulting in different distribution of food heat energy and moisture, thus affecting the inherent quality of food. Traditional heating needs to heat the environment and the heat medium, and it takes a longer time to reach the required heating temperature, while microwave heating can directly absorb microwaves and heat the food immediately. Microwave heating of food is through the alternating periodic changes of the electric and magnetic fields of microwaves, so that the dipole molecules (such as water, protein, etc.) in the food continue to rotate back and forth with the change of the magnetic field, and friction, collision, vibration, extrusion, etc. The action of the kiln generates heat energy, so that the food is heated, the heating speed is fast, the time-consuming is short, and the obtained product is of high quality. Combining microwave and vacuum frying can not only dehydrate quickly in a short period of time, but also maintain the quality of products under negative pressure conditions, so as to realize microwave efficient frying of conditioned food.
01 Atmospheric pressure fried food
1. Atmospheric pressure frying
Atmospheric frying as a food processing method can be traced back to 1600 BC and ancient Egypt. The time when frying was actually used as a food processing method is not clearly recorded, but the word “frying” comes from the Latin and Greek word for “roasting”, which suggests that frying may have developed from the roasting technique. Because of its early origin, atmospheric frying has relatively mature research on this frying technology and equipment at home and abroad, and is currently popular at home and abroad.
2. Atmospheric pressure frying equipment
The atmospheric frying equipment mainly includes electric heating frying equipment, gas frying equipment, continuous deep fryer, oil-water mixing frying equipment, and microwave atmospheric frying equipment.
When the electric heating frying equipment is in operation, the material is placed in the material mesh basket and fried in oil. After frying, take out the basket together. This frying method has the following disadvantages:
(1) During the frying process, the oil is in a high temperature state, the oil is rapidly oxidized and deteriorated, the viscosity increases, and it becomes dark brown after repeated use for several times, which is inedible.
(2) Residues are accumulated on the bottom of the pot, and as the frying time prolongs, the oil becomes more turbid, and the residues attached to the surface of the product will accelerate the deterioration of the food and seriously affect the health of consumers.
(3) The oil used for frying food repeatedly for a long time at high temperature will generate various forms of oleopolymers with different toxicity, which can lead to nerve paralysis, gastric tumor and even death.
Gas-fired frying equipment is a kind of frying equipment that uses gas as energy. It is equipped with automatic ignition and artificial ignition, flame control, frying time, temperature control, air pressure adjustment, etc. It is safe, convenient and hygienic to use. It is an ideal oil. frying equipment. The continuous deep fryer equipment is characterized by no frying cage, but it can make all materials immersed in oil for continuous frying. The heating of the oil is carried out outside the pot, and it has a hydraulic device that can lift the entire conveyor frame and its accessory parts from the oil tank, which is convenient for maintenance. The oil-water hybrid fryer has completely changed the structure of the traditional atmospheric frying equipment. It scientifically adopts the oil-water mixing technology and uses a new type of burner. Sealed heating effectively controls the lower oil temperature during the heating process, slows down the oxidation of frying, prolongs the service life of frying oil, and improves the quality of the product. Greatly enriched and revolutionized frying technology.
3. Research progress of atmospheric frying
The frying process is carried out under normal pressure, and the oil temperature is above 160 ℃. In the case of high temperature, a special aroma will be produced. Some scholars have studied the oil intake and texture changes in fried potato chips. Potato chips and untreated samples that were blanched in hot water at 85 °C for 3.5 min were tested at oil temperatures of 120, 150, and 180 °C. deep fried. The results show that the frying temperature and pretreatment method can significantly affect the final fat content of fried potato chips, and the higher the temperature, the lower the fat intake of potato chips. But frying temperature and pretreatment method had no significant effect on the final texture of fried potato chips.
In recent years, the use of microwave heating instead of traditional frying methods has become increasingly popular. Some scholars have studied the effect of pre-drying time on microwave low-oil potato chips. In order for the potato chips to have the flavor of fried food, while avoiding the low fat content of atmospheric frying, a layer of oil is applied to the surface of the potato chips with a brush before baking, so that the potato chips can be “fried” during the microwave treatment process. “surroundings. The purpose of the microwave treatment is to dry and ripen the potato chips. After the potato chips are oiled, they are baked in a microwave oven until they are ripened. The results show that the use of microwave to produce potato chips can reduce the oil content to less than 20%. Microwave frying can be considered as a suitable alternative to traditional frying.
4. Problems existing in atmospheric frying
Atmospheric frying will bring many problems due to the high temperature, such as the destruction of the nutrients of the food at high temperature, and the color, aroma and taste will be affected. In addition, high temperature makes the oil fumes, pollutes the environment, and increases fuel consumption. At the same time, repeated frying at high temperature will produce thermal oxidation reaction and generate peroxides of unsaturated fatty acids, which can directly hinder the body’s absorption of oil and protein. Reduce the nutritional value of food.
The ingredients in the oil undergo many chemical reactions, resulting in rancid frying oil, thickening of the oil, and bad taste. It may even produce some substances that are harmful to the human body or even carcinogenic, affecting the health of consumers, such as acrylamide. Atmospheric pressure and high temperature fried foods have a high oil content, which makes some special groups (such as obese, cardiovascular patients, etc.) want to eat but dare not eat them, so that the processing range and sales of atmospheric fried foods are greatly restricted. Therefore, reducing the oil content of normal-pressure fried products is also an urgent problem to be solved.
02 Vacuum fried prepared food
Vacuum frying is a new type of food processing technology developed in the late 1960s and early 1970s. Adaptability. The technology has been greatly developed in the United States, Europe and Japan in the 1970s and 1980s. Many vacuum-fried foods have been studied internationally, including fruits: apple chips, banana chips, mango chips, etc.; vegetables : Carrots, potatoes, green beans, sweet potatoes, etc.; Meat and aquatic products: fish fillets, beef, shrimp, etc. In recent years, my country has successively developed vacuum-fried products such as edamame, red dates, taro, broad beans, sea asparagus and other fruits and vegetables, and the research on vacuum-fried technology has been continuously deepened.
1. The principle of vacuum frying technology
The essence of vacuum frying is to fry, dehydrate and dry food in edible oil under negative pressure conditions. The boiling point of water at normal pressure is 100°C. The vacuum system is in a negative pressure state relative to atmospheric pressure, and as the pressure decreases significantly, the boiling point of water will also decrease accordingly. Under 10-100 mm Hg vacuum. The boiling point of water is 10-55°C. In the gasification and separation of water-containing food, vacuum is closely related to low temperature. Operating under vacuum can effectively avoid a series of problems caused by high temperature treatment, such as polymerization deterioration of frying oil, browning reaction of food itself, loss of nutrients, etc.
2. Characteristics of vacuum frying process
The products made by vacuum frying have remarkable puffing effect. This is because in the negative pressure state, with oil as the heat transfer medium, the moisture (free moisture and part of the bound water) inside the food will quickly evaporate and spray out, so that the tissue forms a loose and porous structure.
(2) Color retention and fragrance retention
Using vacuum frying, the raw materials are processed in a low temperature, low pressure grease environment. Since most of the flavors in food are water-soluble, after the dehydration process, these flavor components are further concentrated, so the original flavor of the food can be well preserved. In addition, due to the low oxygen environment, the food is prone to browning and fading, maintaining its original color. But for pigments that contain oil soluble. Such as chlorophyll, carotenoids, etc., the pigments are easily dissolved during the frying process, so pretreatment is required to ensure the stability of the pigments.
(3) The product has low oil content
Due to the negative pressure. When frying, the gas quickly diffuses into the pores of the food, preventing the oil from penetrating into the pores of the product. After frying, the product is still kept under vacuum for degreasing to prevent the pressure from negative. When the pressure returns to normal pressure, the pressure in the pores of the product rapidly increases to the normal pressure level, and the grease on the surface of the product penetrates into the pores, so the grease content of the product is low.
3.Research progress of vacuum frying pretreatment
Due to consumers’ health concerns, fat content has become an important control indicator during the processing of fried products. Therefore, many experts and scholars are devoted to the research of reducing the fat content of fried products, and it is often necessary to pre-process the samples: such as blanching, dipping, film coating, and pre-drying.
4.Problems existing in vacuum frying technology
my country is rich in fruit and vegetable resources, and it has great advantages to expand the production of fruit and vegetable chips, which are not only convenient to carry, but also make full and rational use of resources. The vacuum frying technology is becoming more and more mature, there are more and more types of processing, and the development of products is changing with each passing day, but there are still some problems:
(1) The product has high fat content
Fruit and vegetable crisp products on the market are high in fat. Excessive fat not only increases the production cost, but also shortens the storage period of the product, and more importantly, brings certain harm to the health of consumers, such as coronary heart disease and heart disease. Therefore, reducing the fat content of fried foods should be the goal that food researchers and food business circles strive to achieve.
The currently feasible simple solution is to increase the solid content and the coating technology, but there are still some problems with the coating technology. Because edible colloids can hinder the penetration of oil at the same time. It also hinders the evaporation of water. Therefore, the final moisture content of the product will be high. We can develop special frying qualities with high solids content as needed. It can not only reduce the fat content and production cost of the product, but also improve the storage stability.
(2) Lack of process research for some products
Although many fruits and vegetables have been applied to vacuum frying technology, and relatively perfect process flow and process parameters have been established. However, some new varieties of fruit and vegetable raw materials cannot use ordinary vacuum frying technology due to their unique physical and chemical composition. process and parameters. Therefore, in order to promote the comprehensive development and application of vacuum frying technology and the development of new products, it is also very important to study the vacuum frying technology of special fruits and vegetables.
(3) There is a lack of technological research to maintain the quality of fried products during storage stability and shelf life
Vacuum fried products have a fluffy and crispy texture because they absorb moisture easily and because they contain a lot of fat. It is prone to rancidity and oil oxidation. The most important quality of vacuum fried products is crispness. To maintain this state, the moisture content should be well controlled below 3%, so how to control the moisture content and oxygen content on the packaging of fried products to avoid Moisture absorption and oil oxidation are the focus of research.
(4) Save energy consumption
At present, the heating source of vacuum frying technology used in China is generally steam energy or electric energy, which consumes a lot of energy and has a high production cost. How to make vacuum frying efficient, it is urgent to develop new technologies, such as using microwave sources as heating sources, combining microwave heating and vacuum frying technology to improve production efficiency and improve product quality at the same time.
There are many kinds of frying techniques and equipment for preparing food, and each equipment has its own characteristics and scope of application. However, every equipment and technology has more or less defects, so people are constantly using various new technologies and new methods to produce high-quality, low-fat products. Using microwave vacuum technology and frying technology as well as some pretreatment methods, the products can be dehydrated quickly in a short period of time, reducing the company’s production costs, improving product quality, and achieving high efficiency and energy saving.