Microwave heating usesmicrowaveThe process of heating an object.
Microwave has obvious characteristics such as wave length (1m~1mm), high frequency (300MHZ~300GHZ) and quantum properties.Microwave technology is widely used in radar, navigation, multi-channel communication, remote sensing and television.Since the 1960s, people have gradually applied microwave heating technology to physical processes such as paper, wood and resin extrusion.In recent years, the introduction of microwave heating technology into the production process of microwave molybdenum industry can not only effectively improve the conversion rate and selectivity of reaction, but also reflect many advantages such as energy conservation and environmental protection. As one of the means to achieve green technology, it has attracted widespread attention.
microwaveIt is a form of energy (not heat), but can be converted into heat in the medium.The reaction of materials to microwave can be divided into four situations: (1) penetrating microwave;(2) Reflected microwave;(3) Microwave absorption;(4) Partially absorbs microwave.
Generally, in the field of energy processing, most of the materials processed are dielectric materials, which usually absorb microwave energy in varying degrees. The coupling between dielectric materials and microwave electromagnetic fields will form various power dissipation to achieve the purpose of energy conversion.There are many ways of energy conversion, such as ion conduction, dipole rotation, interface polarization, hysteresis, piezoelectricity, electrostriction, nuclear magnetic resonance, ferromagnetic resonance, etc. Among them, ion conduction and dipole rotation are the main principles of microwave heating.Microwave heating is a kind of heating method that depends on the object absorbing microwave energy to convert it into heat energy, so that the whole body can rise at the same time, which is completely different from other conventional heating methods.The traditional heating method is based on the principle of heat conduction, convection and radiation to transfer heat from the outside to the material heat. The heat is always transferred from the surface to the inside to heat the material. There is inevitably a temperature gradient in the material, so the heated material is uneven, resulting in local overheating of the material. Microwave heating technology is different from the traditional heating method,It increases the temperature of the heated material by generating "internal friction heat" through the high-frequency reciprocating movement of dipole molecules inside the heated body. Without any heat conduction process, it can make the internal and external parts of the material heated at the same time, and the heating speed is fast and uniform. Only a fraction or a fraction of the energy consumption of the traditional heating method can achieve the heating purpose.From the theoretical analysis, the amount of heat generated by substances in the microwave field has a great relationship with the types of substances and their dielectric properties, that is, microwave has the characteristics of selective heating of substances.[1]
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as everyone knows,Microwave OvenMetal materials cannot be placed inside, because microwave irradiation on the metal surface will completely reflect, that is, it will not work on the metal, which is very similar to light waves, and light irradiation on the mirror will also be completely reflected.However, if microwave acts on non-metallic dielectric, it will be absorbed and penetrated by the dielectric properties, resulting in high-frequency electric and magnetic fields.
The characteristics of microwave heating are mainly reflected in:
1. Dielectric heating effect.The positive ions in the dielectric and the negative electrons nearby exist in pairs. These electrons are closely combined and do not interact with each other. The overall electric field strength of the dielectric is zero to the outside world.If the dielectric is strongly addedelectric field, then positive and negativeElectronicsWill be rearranged immediately.If the electric field is alternating and high-frequency, the frequent rotation of the electronic pairs between molecules will vibrate and generate heat due to friction.
2. Penetration depth of microwave.The microwave intensity will gradually weaken when microwave enters into the dielectric because the dielectric loss absorbs the microwave energy.The microwave energy will decay according to a certain law.
3. Selection of microwave heating mode.The absorption and heating of microwave are related to the loss coefficient.If the food with large loss coefficient and the food with small loss coefficient are heated together, the heating effect is a noticeable problem.
Generally speaking, materials with small loss coefficient (such as glass, plastic, ceramics, etc.) are used as heating containers, and food is added to them. In this way, microwave can only heat food, but the container will not heat.This is what we know in our daily life. Microwave ovens can be used to heat food loaded with ceramics, glass, plastic, etc.When heating wet materials, the parts with more moisture will be heated and dried quickly. Because of its large loss coefficient, other parts will be heated gradually.This will speed up drying.
characteristic
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Microwave heating has the characteristics of fast heating speed, small heat loss, easy operation, etc. It can not only shorten the process time, improve productivity, reduce costs, but also improve product quality.Compared with traditional heating methods, microwave heating has the following characteristics:
Heating is even and fast
The general heating method is to heat the surrounding environment by radiation of heat or convection of hot air to make the surface of the object heated first, and then conduct the interior of the object through heat conduction.This method has low efficiency and long heating time.
The biggest feature of microwave heating is that the microwave is generated inside the object to be heated. The heat source comes from the inside of the object, and the heating is uniform, which will not cause the phenomenon of "immature in the external coke", which is conducive to improving product quality. At the same time, because "simultaneous heating inside and outside" greatly shortens the heating time, the heating efficiency is high, which is conducive to improving product output.The inertia of microwave heating is very small, which can realize the rapid control of temperature rise and fall, and is conducive to the automatic control of continuous production.
Selective heating
The heat produced by microwave heating is closely related to the loss of the heated material.The dielectric constant of various mediums ranges from 0.0001 to 0.5, so the ability of various objects to absorb microwave varies greatly.Generally speaking, medium with large dielectric constant is easy to be heated by microwave, while medium with small dielectric constant is difficult to be heated by microwave.This is the characteristic of microwave heating objects selectively.
Timely control and sensitive response
Conventional heating methods, such as steam heating, electric heating, infrared heating, etc., need a certain time to reach a certain temperature. In case of failure or stop of heating, the temperature will drop for a long time.Microwave heating can quickly adjust the microwave power to the required value in a few seconds and heat it to the appropriate temperature, which is convenient for automatic and continuous production.
Strong field high temperature
The microwave power absorbed in the unit volume of the medium is proportional to the square of the electric field strength, so that the processed object can rise to the required processing temperature in a very short time under a very high electric field strength.Strong field high temperature can also produce sterilization without affecting the product quality.[2]
Strong microwave heating penetration
Far infrared heatingThe frequency of is higher than that of microwave heating, and the heating efficiency should be better. But in fact, it is not. There is also a concept of penetrability.Although far-infrared heating has many advantages and is widely used, far-infrared heating is far inferior to microwave in terms of its penetrating ability to objects.What is penetration?Penetrability is the ability of electromagnetic waves to penetrate into the interior of the medium. When electromagnetic waves enter the medium from the surface and propagate inside it, because the energy is constantly absorbed and can be converted into heat energy, the heat it carries will decay exponentially with the distance deep into the surface of the medium.The penetration depth and wavelength of electromagnetic waves are of the same order of magnitude. Except for larger objects, they can be heated together inside and outside the meter.The wavelength of far-infrared heating is very long, and the penetration ability is poor when heating. Under the far-infrared radiation, only a thin layer of objects is heated, and the heat is mainly transmitted to the interior, which not only takes a long time to heat, but also easily causes uneven heating.According to comparison, the penetration ability of microwave heating is much stronger than that of far-infrared heating.
Clean, sanitary and pollution-free
Generally, industrial heating equipment is relatively large, covers a lot of land, and the ambient temperature is relatively high. The working conditions of operators are poor and the intensity is high.The microwave heating occupies a small area, avoiding high ambient temperature, and the working conditions of workers have been greatly improved.
Effect of microwave on sterilization and preservation of meat food
Small package of meat, chicken leg, chicken wing, beef jerky, dried fish fillet, roast chicken, salted duck, small package of stewed products, meatballs, fish balls, etc.
Expansion and drying
Shrimp slices, sesame seeds, almonds, apple slices, fish slices
Baking and peeling of dried fruits
Melon seeds, peanuts, pine nuts, walnut kernels, cashews, chestnuts, etc
Microwave vacuum drying
High grade Chinese medicinal materials, powder, vegetables and seafood.
Liquor aging and liquid sterilization
Milk, yellow rice wine sterilization, liquor aging, traditional Chinese medicine, high-grade drinks.
Pepper powder, five spice powder, star anise, chicken essence powder and various seasoning powder
Deodorization of soybean
Soybean, soybean powder
Microwave cooking of food
(1) Microwave cooking food
(2) Convenient food in microwave oven
(3) Microwave food container
Microwave drying of food
Microwave vacuum dryer
(1) Advantages
The utilization rate of the plant is high.The production capacity is 3-4 times that of the traditional dryer.
··Fast drying speed and short time.
··Good product quality
··Good sanitary conditions
··Energy saving.Microwave drying can save energy by 20% - 25%.
(2) Disadvantages
·Large investment and power consumption[3]
Microwave thawing of food
(1) Advantages
·Short time
·Small floor area
·Low water loss rate
·The surface is not easy to oxidize
·Not easy to change color
·Less clean water consumption
(2) Application
For meat and its products, aquatic products, fruits and their products, household microwave oven is very convenient and fast for thawing frozen food.
Microwave sterilization and preservation of food
(1) Mechanism
a、Thermal effect: microwave acts on food, and microwave energy is absorbed on the surface and inside of food, resulting in temperature rise.Microbial bacteriaprotein structureChange and losebiological activity。The bacteria will die or will not be able to reproduce due to serious interference.
b、Non thermal effect: it changes the biological arrangement, combination state and movement law of a large number of electrons, ions and other charged particles produced by microorganisms in their biochemical process.In addition, microwave can also lead to relaxation, breakage and recombination of hydrogen bonds in cell DNA and RNA molecular structure, induce gene mutation, and thus interrupt the normal reproductive capacity of cells.
(2) Application
a、It retains more active substances than conventional sterilization methods, and is very suitable for drying and sterilization of ginseng, mushroom, pollen, gastrodia elata and other traditional Chinese medicines and Chinese patent medicines.
b、Disinfection of liquid substances, such as microwave milk sterilizer, not only meets the requirements of colony count, but also improves the stability of milk.
c、The enzyme killing and fresh-keeping of the product can overcome the loss of a large amount of water-soluble nutrients by boiling.For example, the green removal in the tea manufacturing process.
Microwave Baking and Baking of Food
(1) Advantages
·The nutritional value of the product is higher than that of the traditional baking method
·Because it is heated inside and outside at the same time, the time can be reduced to several minutes
·The moisture inside the material vaporizes rapidly and migrates outward, forming numerous tiny channels, which makes the product structure fluffy
·Small equipment area
(2) Disadvantages
The surface temperature is too low during baking to generate sufficient Maillard reaction.[4]
Microwave Puffing
(1) Principle
Utilizing the internal heating characteristics of microwave, the internal part of the material is heated rapidly to produce a large amount of steam, and a large amount of internal steam rushes out to form numerous micro channels, which makes the material tissue expand and loosen.
(2) Advantages
Compared with frying and drying expansion, it does not destroy the original composition of the material, and is easy to deteriorate and has low heat.
matters needing attention
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Almost every family has a microwave oven, because it is convenient to use it to heat food.However, some foods are not suitable for microwave heating, such as boiled eggs, breast milk, green leafy vegetables and processed meat, because they may produce toxins due to microwave heating, leading to certain health diseases and affecting your life.
one
Eggs, with or without shells, will accumulate a lot of moisture when cooked in microwave water.As a result, the microwave oven is like a pressure cooker, and eggs will explode.It should be noted that even the peeled eggs cannot be put into the microwave oven, because there is also a layer of yolk film.
two
Chicken: It will be a big problem to heat chicken in the microwave oven. Because the microwave oven cannot heat chicken evenly, some areas will be heated first.The protein density of chicken is also higher than other red meat, so if the protein decomposition is inconsistent when reheating, it will cause gastrointestinal discomfort.Therefore, in order to ensure uniform heating, the chicken should be frequently turned over.
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Some of the ingredients, such as ham sausage and cod roe, which are packaged with shell or film, should not be directly put into the microwave oven for heating.Because steam will be generated in the process of food heating, and there is no place for the steam of packaged food materials to disperse, it is very easy to cause the rupture of the film and food, or even the rupture of the container.In the process of heating this type of food in the microwave oven, attention should be paid to removing the shell, or cutting a knife or punching several holes in the film, so as to allow steam to escape and avoid danger.
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No matter how much you want to use the remaining green leafy vegetables as leftovers, you should not microwave them.Because highly concentrated nitrates will be reheated and transformed into toxins, turning into carcinogenic nitrosamines.
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In order to save trouble, many friends usually like to put the food packaged in plastic bags or paper bags directly into the microwave oven for a long time.In fact, this is not safe. First of all, whether food is safe or not, plastic is easy to decompose into harmful gases at high temperatures, and even ignite to burn the microwave oven.
six
Fresh asparagus is rich in a variety of antioxidant nutrients, including vitamin C.A new study published in the Norwegian Journal of Scandinavian Agriculture found that heating in the microwave oven would lead to the loss of vitamins in asparagus.Because vitamin C and B vitamins are water-soluble vitamins, their stability is worse, and they are easy to be damaged when exposed to water, heat and light.
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In order to avoid releasing substances harmful to human body during heating, please try not to put the disposable lunch box into the microwave oven for heating.Only polypropylene (PP) is relatively heat-resistant and can withstand a high temperature of about 200 ℃, so it can be heated by microwave and repeated heating will not cause any major problems.The heat resistance of the rest of the lunch boxes is too poor. It's better not to heat them directly.
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1. Do not refreeze the meat thawed in the microwave oven: after the meat is thawed in the microwave oven, the outer layer of low temperature has actually been heated. At this temperature, bacteria can reproduce. Although refreezing can stop its reproduction, it cannot kill live bacteria.If the meat that has been thawed in the microwave oven is put into the refrigerator for freezing, it must be heated to fully cooked.
2. Avoid fried food: high temperature oil will splash and cause fire.In case of fire in the furnace due to carelessness, do not open the door, but turn off the power first, and then open the door to cool down after the fire is extinguished.
3. Avoid overtime heating: put food into microwave oven for thawing or heating. If you forget to take it out, if it takes more than 2 hours, you should discard it to avoid causingfood poisoning。
4. Avoid using ordinary plastic containers: use special microwave ovens to contain food and put it into the microwave oven for heating. First, hot food will deform the plastic container; second, ordinary plastic will release toxic substances, contaminate food and endanger human health.
5. Do not use metal utensils: because the iron, aluminum, stainless steel, enamel and other utensils put into the oven will generate electric sparks and reflect microwave when heated by the microwave oven, which will damage the oven body and heat uncooked food.
6. Do not use closed containers: wide mouth containers should be used when heating liquids, because the heat generated by food heating in closed containers is not easy to dissipate, which makes the pressure in containers too high, which may cause explosion accidents.Even when frying the food with shell, the shell should be punctured with a needle or chopsticks in advance to avoid bursting, splashing and polluting the furnace wall, or splashing and injuring people after heating.
7. Avoid placing the micro oven in the bedroom: at the same time, be careful not to cover the radiator grille on the microwave oven with objects.
8. Do not work in front of the microwave oven for a long time: after starting the micro oven, people should stay away from the microwave oven or at least 1 meter away from the microwave oven.
