From the biochemical point of view, nucleic acid (RNA) and deoxyribonucleic acid (DNA) that bacteria normally grow and reproduce are curled macromolecules closely connected by several hydrogen bonds. Microwave leads to the relaxation, breakage and recombination of hydrogen bonds, thus inducing genetic gene or chromosome distortion or even breakage. Microwave sterilization uses electromagnetic field effect and biological effect to kill microorganisms. Practice has proved that microwave device has obvious advantages in sterilization temperature, sterilization time, product quality maintenance, product shelf life and energy saving.
principle
Embedding bovine nose: microwave sterilization
Question: What are the characteristics of microwave sterilization?
Inlaid ox characters:
Microwave is an electromagnetic wave with a frequency of 300 MHz to 300 GHz.
Microwave thermal effect is a process in which microwave directly interacts with matter and converts UHF electromagnetic waves into heat energy. Microwave sterilization is the result of microwave thermal effect and biological effect. The potential distribution of microwave on the cross section of bacterial cell membrane affects the concentration of electrons and ions around the cell membrane, thus changing the permeability of the cell membrane. As a result, bacteria are malnourished, unable to metabolize normally, and their growth and development are hindered and they die.
From the biochemical point of view, nucleic acid (RNA) and deoxyribonucleic acid (DNA) that bacteria normally grow and reproduce are curled macromolecules closely connected by several hydrogen bonds. Microwave leads to the relaxation, breakage and recombination of hydrogen bonds, thus inducing genetic gene or chromosome distortion or even breakage. Microwave sterilization uses electromagnetic field effect and biological effect to kill microorganisms. Practice has proved that microwave device has obvious advantages in sterilization temperature, sterilization time, product quality maintenance, product shelf life and energy saving.
principle
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1. Thermal effect of microwave energy: Under the action of a certain intensity of microwave field, insects and fungi in food will absorb microwave energy due to molecular polarization and heat up, thus making them protein denatured and lose their biological activity. The thermal effect of microwave mainly plays the role of rapid heating and sterilization;
2. Non-thermal effect of microwave energy: High-frequency electric field also changes its membrane potential and polar molecular structure, causing protein and physiologically active substances in microorganisms to mutate, lose vitality or die. It plays a special sterilization role that conventional physical sterilization does not have, and it is also one of the causes of bacterial death.
3. Microwave sterilization and preservation is the result of the combined effect of microwave thermal effect and non-thermal effect. Therefore, the temperature of microwave sterilization is lower than that of conventional methods. Generally, the sterilization temperature of conventional methods is about 1 20℃-1hour at 30℃, while the microwave sterilization temperature is only about 70℃- 105℃ for 90- 180 seconds.
microwave device
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The microwave heating sterilization device comprises the following parts:
1. The microwave generating part is mainly composed of a microwave generator and a microwave conduit;
2. The furnace body or cavity part is made of materials that can reflect microwaves and generate microwave resonance;
3. There is a microwave stirring or dispersing device in the furnace;
4. Seal the door to prevent microwave leakage;
5. The operation control part includes safety interlock device.
At present, the conveyor tunnel furnace is mainly used in mass production.
superiority
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1, short time and fast speed.
Conventional thermal sterilization is to transfer heat from the surface of food to the inside through heat conduction, convection or radiation. It often takes a long time to reach the sterilization temperature. Microwave sterilization refers to the direct interaction between microwave energy and microorganisms such as food and its bacteria, and the combined effect of thermal effect and non-thermal effect realizes rapid heating sterilization, which greatly shortens the treatment time, and the sterilization effect of various materials is generally 3-5 minutes.
2. Low-temperature sterilization retains nutrients and traditional flavor.
Microwave sterilization uses special heat and non-thermal effect to sterilize. Compared with conventional thermal sterilization, it can obtain the required disinfection and sterilization effect at a lower temperature and in a shorter time. Practice shows that the general sterilization temperature can reach the effect at 75-80℃. In addition, the food treated by microwave can retain more nutrients and flavor such as color, aroma, taste and shape, and has the function of puffing. For example, the vitamin C content of vegetables processed by conventional heating is 46-50%, while that of vegetables processed by microwave is 60-90%, that of pig liver heated by conventional heating is 58%, and that of pig liver heated by microwave is 84%.
Step 3 save energy
Conventional thermal sterilization often has heat loss in environment and equipment, while microwave directly acts on food, so there is no additional heat loss. In addition, the conversion efficiency from electric energy to microwave energy is 70-80%, which generally saves electricity by 30-50%.
4. The surface and the interior are carried out simultaneously.
Conventional thermal sterilization starts from the surface of the material and then diffuses to the inside through heat conduction, and there is a temperature difference between inside and outside. In order to keep the flavor of food and shorten the processing time, the internal temperature of food is often not enough, which affects the sterilization effect. Because of the penetration of microwave, the surface and interior of food are affected at the same time, so disinfection and sterilization are uniform and thorough.
5, easy to control
Microwave food sterilization treatment, the equipment can be used immediately, without the thermal inertia of conventional thermal sterilization, and the operation is flexible and convenient. The microwave power can be continuously adjusted from zero to rated power, and the transmission speed can be continuously adjusted from zero, which is convenient for control.
6. Simple equipment and advanced technology.
Compared with conventional disinfection and sterilization, microwave sterilization equipment does not need boilers, complicated piping systems, coal yards and transport vehicles, as long as it has basic conditions such as water and electricity.
7. Improve working conditions and save floor space.
The working environment of the equipment has low temperature and low noise, which greatly improves the working conditions. The whole microwave equipment only needs 2-3 operators. It is widely used for heating, drying and sterilizing beef jerky, pork breast, fish fillet, sauce meat, duck meat, chicken and other products. After microwave sterilization, the freshness, tenderness and flavor of meat products remain unchanged, and the hygiene index can be completely lower than the national food hygiene standard. Shelf storage time can reach 1-2 months. The successful application of microwave sterilization and preservation technology of meat products has extended the preservation period from 3 days to 1-2 months, and raised the technical achievements to a new stage.
Application object
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Microwave equipment can disinfect different packaged and unpackaged items, and can be used for:
● Foods: bread, moon cakes, noodles, tofu, dried tofu, etc.
● Vegetables: pickles, bamboo shoots and mushrooms.
● Fruits: litchi, longan, etc.
Dairy products, condiments, flavors and fragrances, instant noodle soups, hot pot seasonings and various liquids can all be sterilized.
exist problem
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(a) Uneven microwave heating
1. Reasons for uneven microwave heating:
(1) Microwave heating is selective. Even in the same microwave field, different ingredients have different temperature rises.
(2) Microwave has good penetrability and is affected by reflection, penetration, refraction and absorption in actual heating, even though the heat energy generated by different parts of the same food may be very different;
(3) The sharp concentration of electric field, some of which are also called edge effect. As a kind of electric wave, microwave has sharp electric field concentration, which is the main reason for the uneven heating of food by microwave. The electric field will be concentrated in the corners, and these parts will generate more heat and heat up quickly. In order to overcome the adverse effects of water chestnut effect and hot spots, people have made many improvements to containers, such as using rounded containers and circular containers with appropriate sizes as far as possible to shape food with sharp corners. In order to overcome the limitations of microwave heating, it has become a new trend to combine microwave and far infrared heating methods.
(B) the impact of microwave on the human body
According to the principle of microwave, the human body will also absorb microwave, so microwave radiation will also cause some harm to the human body. Usually, when the human body is irradiated, the skin always feels hot first, so it can be avoided in time. However, when exposed to microwave radiation, because of its penetrability, the tissues in the body will also heat up at the same time, and there are fewer nerves in the human body, so those organs with low heat resistance are often damaged before they feel the heat. For example, blood vessels, eyes and testicles are vulnerable to microwave damage, and common diseases of radar workers include cataract and male infertility. Pay attention to safety when using microwave, but don't be too nervous as when treating radioactivity.
(3) microwave sterilization and bag breaking
In the process of microwave sterilization, in addition to paying attention not to use metal containers and aluminum-plated or aluminum-coated bags, there is also the problem that the sealed bags are broken during sterilization, which is difficult to solve and it is difficult to adopt the method of sealing after sterilization. At present, microwave sterilization can be carried out before packaging or after packaging. When the packaged food is sterilized by microwave heating, because the pressure in the bag is too high, the packaging bag will burst, so the whole process of microwave heating sterilization should be carried out under pressure, or the packaging should be treated in a glass container with pressure.
(4) discoloration problem
In the microwave sterilization of mustard tuber and other products, the discoloration of mustard tuber products was also found.
Application prospect
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Microwave heating technology is widely used, and its application in food processing is only a small part of it. It can be said that in the process of understanding the characteristics of microwave heating, people are developing more and more new devices and instruments in combination with existing technologies. Such as a combined refrigerator of a microwave oven and a refrigerator; Microwave kettle and microwave curling iron, microwave drying washing machine combined with microwave heating washing machine; Microwave water heater, low temperature hygrometer, etc. The development of microwave heating technology is closely related to people's understanding and mastery and market demand, among which improving microwave heating equipment and processing technology is particularly critical.