I. Overview Food is highly susceptible to contamination during production, storage, transportation and sale. Conventional techniques such as high temperature, drying, blanching, pasteurization, freezing, and preservatives can be used to achieve insecticidal sterilization and preservation of food. But it often affects the original flavor and nutrients of the food. Microwave insecticidal sterilization is to make microorganisms in foods, and at the same time, it is combined with microwave thermal effect and non-thermal effect, which causes the protein and physiological active substances in the body to mutate, which leads to the growth and development of microbial growth and death, and achieves food sterilization. The purpose of preservation. 1. Thermal effect of microwave energy: Under the action of a certain intensity microwave field, the insects and bacteria in the food will be heated by the microwave energy due to the molecular polarization phenomenon, thereby denaturation of the protein and loss of biological activity. The thermal effect of microwave mainly plays a role in rapid temperature sterilization; 2. Non-thermal effect of microwave energy: The high-frequency electric field also changes the membrane potential and polar molecular structure; the protein and physiologically active substances in the microorganisms are mutated, and the vitality or death is lost. It has played a special role in conventional physical sterilization in sterilization and is one of the causes of bacterial death. 3. Microwave sterilization and preservation are the result of the combined effects of microwave thermal and non-thermal effects. Therefore, the microwave sterilization temperature is lower than the conventional method. Generally, the sterilization temperature of the conventional method is 120 ° C - 130 ° C, the time is about 1 hour, and the microwave sterilization temperature is only 70 ° C - 105 ° C, and the time is about 90-180 seconds. Third, the microwave device: The microwave heat sterilization device comprises the following parts: 1. The portion for generating microwaves is mainly composed of a microwave generator and a microwave conduit; Nowadays, large-scale production mainly uses conveyor belt tunnel furnaces. Fourth, the advantages of microwave sterilization: 1, short time, fast Conventional thermal sterilization transfers heat from the surface of the food to the interior by means of heat conduction, convection or radiation. It takes a long time to reach the sterilization temperature. Microwave sterilization is a direct interaction between microwave energy and microorganisms such as food and bacteria. The thermal effect and non-thermal effect work together to achieve rapid heating and sterilization, and the treatment time is greatly shortened. The sterilization effect of various materials is generally 3-5 minutes. 2, low temperature sterilization to maintain nutrients and traditional flavor Microwave sterilization is sterilized by special thermal and non-thermal effects. Compared with conventional thermal sterilization, the desired sterilization effect can be obtained at a relatively low temperature and in a short time. Practice has shown that the general sterilization temperature can achieve the effect at 75-80 ° C. In addition, the microwave treatment food can retain more nutrients and color, fragrance, taste, shape and other flavors, and has a puffing effect. For example, conventionally heat treated vegetables retain 46-50% of vitamin C, while microwave treatment is 60-90%. Conventional heated pig liver vitamin A is maintained at 58%, while microwave heating is 84%. 3. Saving energy Conventional thermal sterilization often has heat loss in the environment and equipment, while microwaves directly treat the food, so there is no additional heat loss. In addition, its conversion efficiency of electric energy to microwave energy is 70-80%, compared with 30-50% of electricity. 4, the surface and the interior are carried out simultaneously 5, easy to control Microwave food sterilization treatment, the equipment can be used immediately, without the thermal inertia of conventional thermal sterilization, the operation is flexible and convenient, the microwave power can be continuously adjusted from zero to the rated power, and the transmission speed is continuously adjusted from zero for easy control. 6, simple equipment, advanced technology Compared with conventional sterilization, microwave sterilization equipment does not require boilers, complex piping systems, coal yards and transportation vehicles, as long as it has basic conditions of water and electricity. 7, improve working conditions, save floor space The working environment temperature of the equipment is low and the noise is small, which greatly improves the working conditions. The entire microwave equipment operator only needs 2-3 people. It is widely used in the heating, drying and sterilization of beef jerky, pork chop, fish fillet, sauce, meat, duck and chicken. After the microwave products are sterilized by microwave, the freshness, tenderness and flavor of the meat products are kept the same. The health indicators can be completely lower than the national food hygiene standards. The shelf storage time can reach 1-2 months. The microwave sterilization and preservation technology of meat products The successful application has been extended from the original shelf life of 3 days to 1-2 months, and the technical achievements have been raised to a new stage. Fifth, there are problems with microwave heating: (1) Uneven microwave heating 1. Reasons for uneven microwave heating: (B) the impact of microwave on the human body From the perspective of the principle of microwave action, the human body also absorbs microwaves, so microwave radiation will also cause certain harm to the human body. Usually when the human body is exposed to radiation, the skin always feels hot first, so it can be avoided in time. However, when exposed to microwave radiation, due to its penetrability, the tissue in the body will also heat at the same time, and the nerves in the human body are relatively small, so often the organs with low heat resistance have been damaged when the heat is not felt. If the blood vessels, eyes and testicles are vulnerable to microwaves, the common diseases of radar workers are cataracts and male infertility. The use of microwaves requires attention to safety, but it does not require as much tension as radioactivity. (3) Microwave sterilization bag breaking In the microwave sterilization operation process, in addition to the attention that metal containers and aluminum-plated or aluminum composite bags cannot be used, there is also a problem that the sealed bags are broken in the sterilization process, which is not easy to solve. It is also not good to seal after sterilization. At present, microwave sterilization can be carried out before packaging or after packaging. When the packaged food is subjected to microwave heat sterilization, the pressure in the bag will be too high to swell the package bag. Therefore, the whole microwave heat sterilization process should be carried out under pressure, or the package should be placed in a pressurized glass container for treatment (four ) The problem of discoloration. In the microwave sterilization of mustard and other products, the problem of discoloration of mustard products was also found. Sixth, microwave heating technology application prospects Microwave heating technology is widely used, and microwave heating technology is only a small part of its application in food processing. 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 the existing technology. For example, a combination refrigerator of a microwave oven and a refrigerator; a microwave kettle and a microwave curling iron, a microwave drying and a washing machine combined with a microwave drying, a washing machine; a microwave water heater, a low temperature moisture measuring instrument, and the like. The development of microwave heating technology is closely related to people's understanding and mastery, as well as market demand. Among them, perfecting microwave heating equipment and processing technology is especially important.
Second, the principle of microwave sterilization:
2. The furnace body or the cavity portion is made of a material that can reflect microwaves, and can generate microwave resonance;
3. There is also a microwave stirring or dispersing device in the furnace;
4. Seal the door to prevent microwave leakage;
5. The operational control section includes a safety interlock.
Conventional thermal sterilization begins at the surface of the material and is then transferred to the interior by heat transfer. There is an internal and external temperature difference. In order to maintain the flavor of the food and shorten the processing time, the inside of the food does not reach a sufficient temperature to affect the sterilization effect. Since the microwave has a penetrating effect, when the food is processed as a whole, both the surface and the inside are simultaneously acted upon, so the sterilization is uniform and thorough.
(1) Microwave heating is selective, even in the same microwave field, different food materials have differences in temperature rise;
(2) Microwave has good penetrability, which is affected by reflection, penetration, refraction, absorption, etc. in actual heating, even though there may be a large difference in thermal energy generated by various parts of the same food material;
(3) The sharp angle concentration of the electric field, some also called the edge effect. The microwave acts as a kind of electric wave, and the electric field has sharp angle concentration, which is the main reason for the uneven heating of microwave food. The electric field will concentrate in the corners, and these parts will produce more heat and heat up. In order to overcome the difficulties of the water chest effect and the hot spots, many improvements have been made on the container. For example, try to use a rounded container of the right size and a ring-shaped container. Plasticized foods with sharp corners. In order to overcome the limitations of microwave heating, the combination of microwave and far infrared heating methods has become a new trend in microwave oven development.