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Freeze drying process

The freeze drying process can be described as a soft drying process, in which the water is extracted from the product without breaking its molecular bonds to preserve its taste and texture.

How freeze drying works?

The freeze drying process plays with the physical properties of the water. It is commonly known that water boils at 100ºC at the sea level, at 120ºC in a pressure cooker and 80ºC at the top of a mountain.

The evaporation temperature depends on the pressure. If the pressure is reduced enough, in what is commonly called vacuum, the water can boil at room temperature.

If we combine the effects of vacuum with low temperatures, we are in the water sublimation region, that is to say, the ice turns into water vapour without becoming liquid. Sublimation is the secret of freeze drying, this is what allows drying and dehydrating food without breaking its molecular bonds.

Graph of the freeze-drying process.

Freeze drying process 
step by step: 

Freeze drying process starts with the fresh product at room temperature (20ºC) and atmospheric pressure (1bar/1000mbar).

1 The first step is freezing the product temperature down to -40ºC.

2 Then, reducing the pressure until the vacuum is close to 0.01 mbar.

3 The heating that causes sublimation can then start under this vacuum condition.
The water that is contained inside the product (in ice form ) becomes gas (water vapor).

4 The water vapor is collected in the freeze dryer condenser.

5 The atmospheric pressure is recovered to extract the freeze dried food and then, it is vacuum packed.