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The working principle of the freeze dryer is actually to use the physical properties of water sublimation, that is, ice directly changes from solid to gas (water vapor) under low pressure and specific temperature without going through the liquid stage. This process is called freeze drying. Freeze drying can remove moisture from food without destroying the shape and nutritional content of the food.
What is the freeze dryer process?
Freezing stage:
1.Purpose: To completely freeze the water in the material to be dried into ice.
2.Process: The material is placed in the drying chamber of the freeze dryer. The freeze dryer cools the material through the refrigeration system, reducing the temperature of the material to far below its eutectic point. The eutectic point refers to the temperature at which the material components are completely frozen into a solid, usually needing to reach -30°C to -50°C or even lower.
3.Note: The freezing speed affects the size and structure of the ice crystals, which in turn affects the subsequent drying rate and the structure of the final product. Fast freezing forms small ice crystals, and slow freezing forms large ice crystals.
Sublimation drying stage:
1.Purpose: Remove most of the water in the material by sublimation under vacuum.
2.Process:
2.1Evacuation: The vacuum pump of the freeze dryer starts working, reducing the pressure in the drying chamber to 0.1 mbar to 0.6 mbar. This low pressure environment is necessary for sublimation to occur.
2.2Provide sublimation heat: The heating system of the freeze dryer starts to provide gentle heat to the material. This heat is not used to melt the ice, but to provide the heat required for the ice molecules to jump directly from the solid state to the gas state.
2.3Sublimation occurs: Under the combined effect of low pressure and suitable temperature, ice crystals in the material sublimate directly into water vapor. It must be noted that the material temperature must be kept below the eutectic point.
2.4 Condensation: A large amount of water vapor produced by sublimation is captured by the condenser of the freeze dryer. The temperature of the condenser is much lower than the temperature of the drying chamber. The surface of the condenser encountered by the water vapor will quickly condense into frost and adhere to the condenser.
3.Note:
This stage takes the longest time and is the main part of the entire freeze-drying process. The vacuum degree and shelf temperature of the drying chamber need to be precisely controlled, and the material temperature must always be below the eutectic point for efficient sublimation. The vacuum pump needs to ensure continuous operation to maintain a low-pressure environment.
Secondary drying stage:
1.Purpose:
Remove the remaining bound water in the material, the water adsorbed on the molecular structure of the material and not frozen, so that the material reaches the final moisture content.
2.Process:
Increase temperature: In the later stage of primary drying, when most of the ice crystals have sublimated and the material structure has basically been finalized, the shelf temperature can be gradually increased.
3.Desorption:
The increased temperature provides energy to enable the bound water molecules to overcome the adsorption force with the material molecules and desorb from the material. The desorbed water vapor is also condensed by the condenser.
3.Notes:
The temperature at this stage can be higher than the eutectic point of the material, but lower than the collapse temperature or maximum allowable temperature of the material (to prevent the drying structure from collapsing or the material from denaturing). The goal is to thoroughly dry the material and reach the target moisture content.
Post-freeze drying treatment：
When the preset endpoint is reached, turn off the heating system. This operation is usually determined by measuring the material temperature, condenser temperature, vacuum or residual moisture.
Introduce dry inert gas to slowly eliminate the vacuum in the drying chamber.
Under the protection of inert gas or in a low humidity environment, take out the dried product and immediately seal and package it.
What are the advantages of freeze drying?
Maximum retention of morphology, structure and activity: The low-temperature drying process avoids the damage of heat-sensitive substances by high temperature, and the product can maintain its original porous structure and has excellent rehydration.
Extended shelf life: The extremely low water content greatly inhibits microbial growth and chemical reactions.
Lightweight, easy to transport and store.
No need for refrigeration: The finished product can be stored stably for a long time at room temperature
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