Yingtai:Basic Process of Freeze-Drying Technology

Yingtai:Basic Process of Freeze-Drying Technology

Freeze-drying, also known as lyophilization, generally consists of three stages: pre-freezing, sublimation drying, and desorption drying.

Stage 1: Pre-Freezing

Pre-freezing is the process of solidifying the free water in the material, which helps to maintain the same shape in the dried product as it had before drying. This prevents irreversible changes such as bubbling, shrinkage, concentration, and solute migration during vacuum drying, and minimizes changes in solubility and the characteristics of biological materials due to temperature decreases. Pre-freezing plays a crucial role in the quality of the freeze-dried product and must meet two basic requirements: appropriate cooling rate and final temperature.

The cooling rate depends on factors such as the temperature of the cooling source, the quantity and volume of the samples, the surface area of the container, and the heat transfer between the sample container and the cold environment. If the cooling rate is too fast, intracellular ice formation can occur; if it is too slow, it may cause damage or death to living materials. Rapid freezing results in smaller ice crystals, which leads to more uniform appearance and consistent color in the product, reduces protein denaturation, and lowers the death rate of biological materials. The final temperature of pre-freezing should be lower than the eutectic point or glass transition temperature of the solution to ensure complete freezing of the suspended biological material. Otherwise, some liquid may "boil" and create bubbles during vacuum extraction, causing uneven product surfaces. For less critical products, the final temperature of pre-freezing should be just a few degrees below the eutectic point, generally 10°C to 20°C lower.

Stage 2: Sublimation Drying

Sublimation drying, also known as primary drying, involves placing the frozen product in a sealed vacuum chamber and applying heat to allow ice crystals to sublimate into water vapor, thereby dehydrating the product. The drying process progresses from the surface inward, and the voids left by the sublimated ice become the passageways for the subsequent sublimation water vapor. When all ice crystals are removed, the primary drying phase is complete, removing approximately 90% of the total water content. This process involves both heat transfer and mass transfer, with heat supplied by heating plates and sublimated water vapor exiting through the gaps in the dried layers. Throughout this stage, the material must remain frozen. Excessive temperature could cause ice crystals to melt, so the temperature of the frozen layer should be kept below its eutectic point. Additionally, heating should not be too rapid to avoid exceeding the collapse temperature, which is the temperature at which some dried products lose rigidity and become sticky, increasing in density, and darkening in color. Collapse affects the passage of water vapor through the freeze-dried layer, leading to partial melting of the product.

Stage 3: Desorption Drying

Desorption drying, also known as secondary drying, follows the completion of primary drying. Residual water, which was not frozen, remains adsorbed on the capillary walls and polar groups within the dried material. After secondary drying, the residual moisture content of the product generally ranges between 0.5% and 4%. The duration of desorption drying depends on factors such as the type and shape of the product, the required residual moisture content, and the performance of the freeze-drying machine.