How To Achieve Efficient Pre-freezing?

2025-09-28 09:48:03
How To Achieve Efficient Pre-freezing?
Freezing wet materials is a preparatory step for vacuum freeze-drying (also known as freeze-drying), and is therefore also called pre-freezing. Pre-freezing is the first stage of freeze-drying and a crucial step in determining the success of the freeze-drying process. Before drying, the material must be pre-frozen to a temperature below its eutectic point to ensure that the free water within the material is completely solidified, forming a stable solid skeleton that facilitates the sublimation of ice crystals during the subsequent drying process. Process parameters such as pre-freezing time, pre-freezing temperature, and pre-freezing rate are key factors affecting the energy consumption and quality of the freeze-dried product.

1. Prefreezing Time

If the prefreezing time is too long, energy consumption will increase. If the prefreezing time is too short, the material will not be frozen solid. During vacuum drying, any remaining liquid within the material will evaporate rapidly, causing the liquid to condense and the material to shrink. Dissolved gases in the liquid will also rapidly escape under vacuum, causing the material to bubble and lose its original shape. Therefore, accurately determining the prefreezing time for the material is crucial for reducing freeze-drying energy consumption and improving freeze-dried product quality.

2. Prefreezing Temperature

If the prefreezing temperature is too high, the material may not be frozen solid, causing expansion and bubbling during vacuum sublimation drying, leading to freeze-drying failure. Conversely, if the temperature is too low, energy consumption will increase. Relevant research indicates that the prefreezing temperature of the material should be 10–20°C below its eutectic point. Otherwise, significant energy losses will result.

3. Prefreezing Rate

The prefreezing rate is another important factor affecting freeze-drying energy consumption. Slow freezing forms large, fewer ice crystals. However, these large ice crystals leave behind large pores after sublimation, increasing the sublimation drying rate. However, these large ice crystals reduce the surface area within the material, thus reducing the desorption drying rate. Rapid freezing causes the ice crystals to complete their crystallization process before they have time to grow, resulting in smaller, more numerous ice crystals. These small ice crystals form fine pores after sublimation, increasing resistance to water vapor migration and reducing the sublimation drying rate. Because smaller ice crystals increase the surface area within the material, this facilitates the desorption of bound water during the desorption drying process.

4. Conclusion

It can be seen that selecting appropriate values for pre-freezing time, pre-freezing temperature, and pre-freezing rate is crucial for reducing freeze-drying energy consumption and improving product quality.