1. Type and size of materials Due to the wide variety of materials and conditions, the microwave vacuum drying process is not fixed. In fact, during the microwave vacuum drying process, the inside of the material gradually forms a loose porous shape, and the thermal conductivity inside thereof begins to weaken, that is, the material gradually becomes a poor thermal conductor. As the drying process progresses, the internal temperature will be higher than the outside. The larger the material volume, the larger the internal and external temperature gradients. The internal heat conduction cannot balance the temperature difference generated by the microwave, and the temperature gradient reaches an unacceptable level. Therefore, the material should generally be pretreated to a smaller granule or sheet to enhance the drying effect. Powdered products are unique in microwave drying. When they are stacked together, they should not be regarded as many small particles, but as a whole, and special attention should be paid to the temperature difference between the inside and the outside of the layer. Generally, when the material appears in a large form, the microwave power is reduced when the material is near the deceleration drying period, thereby effectively reducing the temperature difference between the inside and the outside, and the disadvantage is that the drying time is prolonged.

2. Vacuum degree The lower the pressure, the lower the boiling point temperature of water, and the faster the water diffusion rate in the material. The degree of vacuum in the microwave vacuum cavity is mainly limited by the breakdown electric field strength, because in the vacuum state, the gas molecules are easily ionized by the electric field, and the breakdown field strength of the air and water vapor decreases with the pressure; the lower the electromagnetic wave frequency, The gas breakdown field strength is smaller. The gas breakdown phenomenon is prone to occur in the microwave feed coupling port and in the field where the field strength is concentrated. The occurrence of breakdown discharge not only consumes microwave energy, but also damages components and generates large microwave reflections, shortening the life of the magnetron. If the breakdown discharge occurs on the surface of the food, the food will be scorched, and the field strength of 20 kV/m can generally penetrate the food (different dielectric constant). Therefore, it is very important to choose the correct degree of vacuum. The higher the vacuum, the better. The excessive vacuum will not only increase the energy consumption, but also increase the possibility of breakdown discharge.

3. Microwave power Microwave has the property of selectively heating substances. Water is a substance with very strong molecular polarity, which is more susceptible to heat by microwaves. Therefore, the higher the water content, the easier it is to absorb microwaves and the faster the heat generation. When the moisture content is reduced, the ability to absorb microwaves is also reduced. Generally, in the early stage of drying, the moisture content of the material is high, and the microwave power has a higher influence on the drying effect. Continuous microwave heating can be used. At this time, most of the microwave energy can be absorbed by water, and the water rapidly migrates and evaporates; at constant speed and slow drying. During the period, as the water is reduced, the required microwave energy is also small, and pulse gap type microwave heating can be employed.

Taking pineapple pulp as raw material, the effect of microwave power on the drying effect of pineapple powder during microwave vacuum drying was studied. Experiments show that the water content of pineapple pulp decreases with the increase of microwave power. The microwave emission power has a significant effect on the appearance quality of the material. When the microwave power is greater than 2 kW, the material is locally overheated and sparking occurs, and internal scorching occurs, the color is deteriorated, and the gloss is lowered. Therefore, under the premise of ensuring that the material reaches the required water content, reducing the microwave power can improve the appearance quality of the dry product and save energy.

4. Microwave time The choice of microwave vacuum drying time is very important and is also affected by many factors. The effect of microwave time on the drying effect of vacuum microwave drying pineapple powder was studied by using pineapple pulp as raw material, and the following results were obtained. Under the vacuum of 98.2~99.2kPa, the moisture content of the wet base in the initial stage of drying changes little, because the water molecules inside the material have not fully absorbed a large amount of microwave energy, and the heat source is insufficient; Continued, the polar molecular vibration inside the material is intensified, more energy is converted into heat, and the movement of water molecules is promoted, and the moisture content of the material changes greatly. In the late stage of microwave vacuum drying, the inside of the material gradually forms a loose porous shape, and the thermal conductivity inside the material begins to weaken, and the moisture content tends to be stable. In addition, the drying time is also affected by the requirements for the moisture content of the finished product. If the finished product is generally dried, the moisture content can be controlled at 3% to 5%. If the requirement is as low as 1% or less, the drying time should be extended accordingly.

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