Yingtai Freeze Dryer :How to Achieve Efficient Pre-freezing

Yingtai Freeze Dryer :How to Achieve Efficient Pre-freezing

Freezing wet materials is a preparatory step for vacuum freeze-drying (referred to as freeze-drying) and is thus also known as pre-freezing. Pre-freezing is the first stage of freeze-drying and a critical phase that determines whether the material can successfully complete the freeze-drying process. Materials must undergo pre-freezing before drying, where they are frozen to a temperature below their eutectic point to ensure the complete solidification of free water within the material, forming a stable solid framework conducive to the sublimation of ice crystals during subsequent drying. Among the process parameters, pre-freezing time, pre-freezing temperature, and pre-freezing rate are important factors affecting the energy consumption and quality of the freeze-dried product.

01 Pre-freezing Time  

If the pre-freezing time is too long, energy consumption increases. If the pre-freezing time is too short, the material may not be fully frozen. When drying is conducted under vacuum, any remaining liquid inside the material will rapidly evaporate, causing liquid concentration, shrinkage of the material volume, and dissolved gases to bubble out rapidly under vacuum, leading to foaming and destruction of the material's original shape. Therefore, accurately determining the pre-freezing time of the material is crucial for reducing freeze-drying energy consumption and improving product quality.

02 Pre-freezing Temperature  

If the pre-freezing temperature is too high, the material may not freeze completely. During sublimation drying under vacuum, the material may expand and foam, resulting in freeze-drying failure. Conversely, if the temperature is too low, energy consumption will increase. Relevant studies indicate that the pre-freezing temperature of the material should be 10–20°C below its eutectic point. Exceeding this range will cause significant energy loss.

03 Pre-freezing Rate  

The pre-freezing rate is another important factor affecting freeze-drying energy consumption. Slow freezing results in large but fewer ice crystals. However, large ice crystals leave large channels after sublimation, thereby increasing the rate of sublimation drying. On the other hand, large ice crystals reduce the internal surface area of the material, which slows down the desorption drying rate. Rapid freezing causes ice crystals to form before they have time to grow, resulting in small but numerous ice crystals. Small ice crystals form fine channels after sublimation, increasing the resistance to water vapor migration and reducing the sublimation drying rate. However, small ice crystals increase the internal surface area of the material, which is beneficial for the desorption of bound water during the desorption drying stage.

Clearly, appropriate values for pre-freezing time, pre-freezing temperature, and pre-freezing rate are crucial for reducing freeze-drying energy consumption and improving product quality.