Freeze-drying is a food preservation technique based on the drying of certain compounds by sublimation of the water they contain.
A freeze-dryer is the device used for freeze-drying. It consists of the following components: dry chamber or freeze-drying chamber: this is where the substance to be freeze-dried is placed; condenser with cooling circuit: this communicates with the dry chamber and is where the steam produced during sublimation is condensed; and, finally, a vacuum system.
This technique does not alter the product’s physico-chemical structure, and its low moisture content means that it can be preserved without a cold chain, giving it high microbiological stability.
Freeze-drying extracts 95% of the water contained in a foodstuff, which translates into a major advantage in terms of transport costs, since it enables larger quantities of goods to be loaded without the need for a cold chain. At the end of the freeze-drying process, the food is transformed into a rigid structure that retains its shape and volume, but with reduced weight, while maintaining its nutritional and organoleptic characteristics. Once rehydrated, they regain their original texture, aroma and flavor.
How a food freeze-dryer works
To introduce a material into the freeze-dryer, it must first be frozen, to preserve its molecular structure. To do this, a freezer capable of reaching sub-zero temperatures is used. The frozen material is then introduced into the freeze-dryer, where a vacuum pressure is created by a vacuum pump. The vacuum sublimates 95% of the water present in the sample. The product is thus dried without the need for temperatures above ambient.
The heat contained in the freeze-dryer provides the energy needed to transform the water into a gas, speeding up the process. The remaining gas is condensed at low temperature, returning it to a solid state.
Freeze-drying process
There are four main stages in freeze-drying foods:
- Préparation.
- Congélation.
- Séchage primaire.
- Séchage secondaire.
Advantages of freeze-drying over other techniques :
- This results in products that regenerate very quickly.
- The shape and characteristics of the final product are essentially the same as those of the original.
- This process is ideal for drying thermolabile substances.
- Oxidizable constituents are protected.
- The final moisture content is very low.
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