Yingtai: The Basic Process of Freeze-Drying Technology

Yingtai: The Basic Process of Freeze-Drying Technology  

Freeze-drying generally consists of three stages: pre-freezing, primary drying (sublimation drying), and secondary drying (desorption drying).  

1. Pre-Freezing  

Pre-freezing involves solidifying the free water in the material, ensuring that the dried product retains the same form as before drying. This step prevents irreversible changes such as bubbling, shrinkage, concentration, and solute migration during vacuum drying, while also minimizing the reduction in solubility and alterations in the biological characteristics of the material due to temperature changes. Pre-freezing plays a crucial role in determining the quality of the freeze-dried product and must meet two basic requirements: an appropriate cooling rate and a suitable final temperature.  

The cooling rate depends on factors such as the temperature of the cold source, the sample volume, container size and surface area, and the heat transfer between the sample container and the cold environment. If the cooling rate is too fast, intracellular ice formation may occur; if too slow, it may lead to cellular damage or death. Slow cooling results in larger ice crystals, while rapid cooling produces smaller ones.  

Products frozen quickly exhibit a uniform appearance and consistent color. Smaller ice crystals cause less protein denaturation and aggregation, and for living materials, rapid freezing reduces mortality and saves freezing time. The final pre-freezing temperature should be below the eutectic point or glass transition temperature of the solution to ensure complete freezing. Otherwise, during vacuum application, slight liquid "boiling" may occur, producing bubbles and causing an uneven product surface. For less demanding products, the final pre-freezing temperature only needs to be slightly below the eutectic point, typically 10°C to 20°C lower.  

2. Primary Drying (Sublimation Drying)  

Primary drying, also known as sublimation drying, involves placing the frozen product in a sealed vacuum chamber and applying heat to sublimate the ice crystals into water vapor, thereby dehydrating the product. Drying progresses from the outer surface inward, with the voids left by sublimated ice crystals forming escape channels for subsequent water vapor.  

When all ice crystals are removed, the first stage of drying is complete, eliminating approximately 90% of the total moisture. This process involves both heat and mass transfer: the heat absorbed by sublimation is supplied by heating plates, while the sublimated water vapor escapes through gaps in the dried layer. However, during drying, the unfrozen portion of the material must remain frozen. If the temperature is too high, ice crystals may melt, so the frozen layer temperature should always stay below the eutectic point. Additionally, excessive heating can cause the dried layer to exceed the collapse temperature.  

The collapse temperature refers to the point at which certain dried products lose rigidity and undergo a collapse-like phenomenon, losing their porous structure and becoming sticky, denser, and darker in color. Collapse obstructs the escape of water vapor from the freeze-dried layer, potentially leading to partial melting of the product.  

3. Secondary Drying (Desorption Drying)  

Secondary drying, also called desorption drying, follows the completion of the first stage. At this point, some unfrozen water remains adsorbed on the capillary walls and polar groups of the dried material. After secondary drying, the residual moisture content typically ranges between 0.5% and 4%.  

The duration of desorption depends on factors such as the type and shape of the product, the required residual moisture content, and the performance of the freeze-drying equipment.