Yingtai: How To Achieve Efficient Pre-Freezing

Yingtai: How To Achieve Efficient Pre-Freezing

Freezing wet materials is a preparatory operation for freeze-drying, also known as pre-freezing. Pre-freezing is the first stage of freeze-drying and is crucial for determining whether the material will successfully complete the freeze-drying process. The material must be frozen to below its eutectic point to ensure that all free water inside the material is completely solidified, forming a stable solid matrix that facilitates the sublimation of ice crystals during subsequent drying. Among the factors influencing the energy consumption and quality of freeze-dried products, pre-freezing time, temperature, and rate are critical parameters.

Pre-Freezing Time: If the pre-freezing time is too long, it leads to increased energy consumption. Conversely, if the pre-freezing time is too short, the material may not be adequately frozen. During vacuum drying, any residual liquid inside the material will quickly evaporate, causing the liquid to concentrate, which shrinks the material's volume. Additionally, gases dissolved in the liquid will rapidly escape in the vacuum, causing bubbling and disrupting the material’s original shape. Therefore, accurately determining the pre-freezing time is crucial for reducing energy consumption and improving the quality of freeze-dried products.

Pre-Freezing Temperature: If the pre-freezing temperature is too high, the material may not fully solidify, leading to expansion and bubbling during vacuum sublimation, which can result in freeze-drying failure. On the other hand, if the temperature is too low, it will increase energy consumption. Studies have shown that the pre-freezing temperature should be 10–20°C below the eutectic point of the material; otherwise, it will cause significant energy loss.

Pre-Freezing Rate: The rate of pre-freezing is another important factor affecting energy consumption in freeze-drying. Slow freezing results in large ice crystals with fewer numbers, which, when sublimated, leave large pores. This increases the sublimation rate but reduces the surface area available for desorption during the drying phase. Rapid freezing causes ice crystals to form quickly with smaller sizes and greater numbers. Small ice crystals, when sublimated, create fine pores, increasing the resistance to water vapor migration and thus reducing the sublimation drying rate. However, the larger surface area due to small ice crystals benefits the desorption of bound water in the drying phase.

Thus, the optimal values for pre-freezing time, temperature, and rate are essential for reducing freeze-drying energy consumption and enhancing product quality.