Prefreezing is to freeze the water in the material into ice to create conditions for subsequent sublimation drying. Through prefreezing, the water in the material is fixed in a solid ice crystal state to avoid direct vaporization of water in a vacuum environment, which will cause the material structure to be destroyed. The prefreezing process can also make the solutes in the material evenly distributed in the ice crystal framework, which is conducive to maintaining the original shape and structure of the material during the sublimation process. Appropriate prefreezing temperature and rate are crucial to the quality of freeze-dried products. If the prefreezing temperature is too high or the rate is too slow, it may cause the ice crystals to be too large, destroy the cell structure of the material, and affect the rehydration and activity of the product; if the prefreezing temperature is too low or the rate is too fast, it may increase energy consumption and be unfavorable for the sublimation process.
