General process of vacuum freeze drying
The process of vacuum freeze-drying includes pretreatment, pre freezing, sublimation, desorption and post-treatment.
-
Before vacuum freeze-drying, the materials should be pretreated firstly, such as the selection and washing of fruits and vegetables, the concentration of liquid and so on. After completion, the materials should be evenly packed in suitable containers to ensure that the evaporation surface of the materials should be as large as possible and the thickness should be as thin as possible. The specific parameters need to be determined according to the specific material characteristics and energy consumption of cold drying equipment.
-
Then the prepared materials are put into the metal plate suitable for the plate layer of the freeze-drying box. The metal plate is usually made of aluminum plate or thin stainless steel plate to ensure smooth bottom surface and good heat transfer effect. After all these are completed, we will start to enter the pre freezing stage.
-
Pre freezing. Before the beginning of pre freezing, the freeze-drying box can be cooled, and then the product can be put into the pre freezing, or directly put into the product for pre freezing. In this process, to ensure that all the materials are frozen, the pre freezing temperature is generally 5 ~ 10 ℃ lower than the eutectic point of the materials, generally below - 30 ℃. The pre freezing process can be divided into quick freezing (cooling 10-15 ℃ per minute) and slow freezing (cooling 1 ℃ per minute). After the material has dropped to the lowest pre freezing temperature, it will stay at this temperature for 1-2 hours to freeze the product completely, and then it can be transferred to the sublimation stage. In addition, the screw vacuum pump manufacturer added that the pre freezing and drying modules in vacuum freeze-drying may be integrated or separated (that is, pre freezing and sublimation drying are not separated in one container).
-
When the temperature of the cold trap is lower than - 40 ℃, the vacuum pump is started for vacuumizing operation. The vacuum system needs to pump the freeze-drying box from atmospheric pressure to about 10-30pa or even lower in a certain period of time. When the vacuum degree reaches the requirements of the process, the materials in the box will be heated to provide the heat required for water sublimation and start sublimation. In the process of vacuum system, it is necessary to control the pressure in the freeze-drying box through pressure regulation and air filling method, so as to ensure the sublimation rate and facilitate the heat transfer. However, the sublimation temperature should not exceed the eutectic point of the material and should be lower than the disintegration temperature of the dry material.
-
The specific time of sublimation drying is related to the thickness and shape of the material, the amount of heat supply, and the speed of water vapor removal, generally in 10-30 hours. After the initial sublimation drying, it is necessary to extend the drying time for 0.5-1 h to completely eliminate the residual ice crystals in the products and the uneven drying of the products.
-
Desorption. Sublimation drying removes free water from the material. When the sublimation drying is finished, it will enter the absorption drying stage to remove the combined water in the material and the water adsorbed in the drying layer. In this process, the temperature of the product can rise rapidly to the maximum allowable temperature, and the freeze-drying can be finished after holding at this temperature for a period of time. However, due to the decrease of water vapor quantity, the temperature of cold trap and the pressure in freeze-drying box, it is not conducive to heat transfer. It is necessary to speed up the sublimation and desorption by means of pressure regulation and inflation.
-
At the end of desorption, the material is discharged. Finally, the material is processed by vacuum packaging or vacuum inflatable packaging. The materials treated by vacuum freeze-drying process can be stored for a long time without deterioration without low temperature storage. It is light in weight and convenient for long-distance transportation. So far, the whole process has been completed.
