The purpose of pre-freezing in a freeze dryer and how long it takes to pre-freeze

The freeze-drying process of the freeze-drying machine mainly includes three stages: pre-drying, sublimation drying, and analytical drying. The pre-freezing stage is the process of freezing products containing a large amount of moisture into a solid state. The pre-freezing time is generally about 2 to 4 hours, mainly depending on the product thickness, characteristics, and pre-freezing temperature.
The purpose of pre-freezing in a freeze dryer is threefold:
① Form crystals that are conducive to sublimation. It is known that most frozen products are solutions, which may have more than one ingredient. Some also contain biologically active substances such as proteins and organisms, which may form a glassy state during freezing. After a while, an upper dry layer and a lower frozen layer will form. When the products in the frozen layer continue to dry, the sublimated airflow must pass through the drying layer. Since the glassy state is almost continuous, there is no channel for the sublimated airflow, and the continued drying of the product will be affected. , drying failed.
② Ensure that all products have the same fine structure. When freeze-drying, all products should usually be frozen to a certain low temperature. The entry temperature is usually 5°C and last for a period of time. Ensure that all products reach the same temperature before cooling down and pre-freezing together. , so that the freezing process of all products is exactly the same to prevent uneven product structure and different colors. In order to prevent the product from bubbling when sublimating under vacuum, the temperature during pre-freezing must be lower than the eutectic point temperature of the product. Generally, it should be at least 5°C lower than the eutectic point temperature of the product.
③ Fix the product. If the product fails to freeze into a solid, vacuum it directly. Under the action of vacuum refrigeration, when the vacuum reaches a certain level, the air dissolved in the water will "boil" and bubble, and the product will escape from the container. When the pressure is lower than 6.11mbar, the product freezes into ice, but has no fixed shape. The glass container may even break and the freeze-drying fails.