Drying is one of the preservation methods of food and one of the oldest methods of food processing. In general, the meaning of drying and dehydration is the same. There are many ways to dry, the simplest is sunshine, which uses solar energy to get out of the food. Drying can be divided into atmospheric drying and vacuum drying. In the case of atmospheric drying, the gas phase is mainly an inert gas, which is called a drying medium, which has the function of taking away the vaporized moisture carrier when dry, and when the vacuum is dried, the gas phase is mainly composed of low-pressure steam, which is sucked by a vacuum pump. Remove.
Drying is a method of dehumidifying by thermal energy. There are three drying methods according to the way of heat energy transfer:
1 Hot air drying (convection type) This method directly uses high temperature hot air as a heat source to transfer heat to the material by convection heat transfer. Hot air is the heat carrier and the wet body. Generally, hot air drying is carried out under normal pressure. In the case of vacuum drying, since the gas phase is at a low pressure state, its heat capacity is small, and it is impossible to directly use air as a heat source, and other heat sources must be used.
2 Contact drying (conduction mode) This method is a method of transferring heat to the material in contact with the wall surface by heat conduction from the wall surface. The heat source can be water vapor, hot water, gas, hot air, and the like. Contact drying can be carried out under normal pressure or under vacuum. When operating under normal pressure, although there is heat exchange between the material and the gas, the gas is not a heat source, and the gas always acts as a moisture carrier, that is, the gas flow is accelerated and eliminated. The role of vaporization of water. Vacuum contact drying is a drying method widely used in the food industry.
3 Radiation drying This method uses infrared, far infrared, microwave, radio waves and other energy sources to transfer heat to materials for drying. As with contact drying, radiant drying can also be used at atmospheric or vacuum drying. Radiation drying is also an important drying method in the food industry. With the development of industry and agriculture, microwave technology has become more and more widely used in food drying. This is because the microwave drying method has a series of advantages compared with other drying methods: (1) fast drying speed and short drying time. Since the microwave can penetrate deep into the material rather than relying on the heat conduction of the material itself, the entire heating and drying process can be completed in only 1/10-1/100 of the time of the general method. (2) The product quality is high, because the heating time is short, the food color, fragrance, taste and nutrient loss can be kept small. (3) Reflecting sensitivity, easy to control with conventional heating method, whether using electricity, steam or hot air, etc., materials need to be warmed up for a certain period of time to reach a certain temperature. When the machine fails or stops heating, the temperature of the material needs to be lowered. Heating for a long time, and using microwave heating, by adjusting the power of the microwave, the heating of the material can be instantaneously changed, thereby facilitating automatic control. (4) Heating and uniform microwave heating is carried out simultaneously in all parts of the material, avoiding the hardening or uneven heating of the surface of the material caused by the temperature gradient formed by the external method from the outside. (5) The heating process has the ability to automatically balance. When the frequency and electric field strength are constant, the microwave absorption power of the material during the drying process mainly depends on the dielectric loss of the material. The dielectric loss of different substances is different. For example, the dry matter of water is large, so it absorbs more energy, the water evaporates quickly, and the microwave does not concentrate on the dry matter. It avoids the overheating of the material and has the ability of automatic balance to ensure The original characteristics of the substance. (6) High thermal efficiency and low equipment footprint. The microwave heating equipment itself does not consume heat, and most of the thermal energy acts on the material. The thermal efficiency is high and has little effect on the ambient temperature. The volume of the microwave heating equipment is much smaller than that of the conventional method.
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