Freeze drying is a technology that uses the principle of sublimation to dry items. It is a process in which the material to be dried is quickly frozen at a low temperature, and then in an appropriate vacuum environment, the frozen water molecules are directly sublimated into water vapor to escape. The drying rate of freeze dryer is the amount of vaporized water that passes through the unit drying area and reaches the gas space in the unit time.
Moisture in food can be divided into bound moisture and unbound moisture. At a certain temperature, the vapor pressure of unbound moisture is constant, which is equal to the vapor pressure of pure water at the same temperature, but the vapor pressure of bound moisture varies with the different binding forces. In this way, when a sufficiently moist food is dried, the first thing that comes out must be unbound water that has no binding force (generally). At this time, the vapor pressure on the food surface of the lyophilizer is almost equal to the vapor pressure of pure water, and the vapor pressure must remain unchanged before this part of the water is completely evaporated, and the drying rate will remain unchanged under certain circumstances. That is to say, when the external drying conditions are constant, the drying rate of the food will not change. The so-called "external drying conditions" refer to the temperature, humidity, flow rate of the air for hot air drying, and for vacuum drying, it refers to the vacuum degree, heat source temperature, contact between the heat transfer surface and the material, etc.
Therefore, in a typical drying process, there are a constant speed drying stage and a reduced speed drying stage. In fact, there is no absolutely clear boundary between the so-called bound water and unbound water, so also no clear boundary between the two stages of the freeze drying machine. In addition, the drying mechanism in the reduced speed drying stage is much more complicated than that in the constant-speed stage, so the shapes of the drying curves in this stage are also various.
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