Microwave drying equipment is a common method in modern drying. It uses hot air as drying medium, uses heat sources (coal, petroleum, natural gas, etc.) to provide heat, and blows hot air into oven or drying chamber through blower to speed up air flow and transfer heat to materials.
Water on the surface of materials is heated and vaporized and diffused into the surrounding air. In gas, when the surface moisture content of the material is lower than its internal moisture content and forms a moisture gradient, the internal moisture diffuses to the surface until the moisture content in the material drops to a certain extent; at the same time, the surface temperature of the material is higher than the material center after heating and forms a temperature gradient, which promotes the heat transfer from the surface to the center. In the drying process, mass transfer and heat transfer occur simultaneously and in opposite directions.
The hot-air drying of coriander was studied. It was found that the drying time of the blanched coriander was shortened obviously because of the long high-speed drying stage during the whole drying process; the higher the drying temperature, the greater the wind speed, the smaller the thickness of the packing layer, the higher the drying rate and the shorter the drying time; the higher the temperature, the wind speed and the paving time were. The rehydration of coriander dry products increased first and then decreased.
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