Freeze drying, lyophilisation
Lyophilisation, commonly referred to as freeze-drying, is the process of removing water
from a product by sublimation and desorption. The aim of the process is to preserve sensitive material that cannot be dried by evaporation at elevated temperature
due to the degradation of specific components at high temperature which would result in a loss of taste or other quality aspects.
Field of application
The technique is used for drying coffee
, soup vegetables, flowers, instant meals, fish, meat
Description of techniques, methods and equipment
This process is performed in lyophilisation equipment, which consists of:
- a drying chamber with temperature controlled shelves (this can be a batch chamber, where the trays remain fixed on the heating plates through the drying operation, or a semicontinuous type, in which the trays move through a vacuum lock into a drying tunnel)
- a condenser, to trap water removed from the product in the drying chamber
- a cooling system, to supply refrigerant to the shelves and the condenser
- a vacuum system, to reduce the pressure in the chamber
- a condenser, to facilitate the drying process.
If the incoming product is a liquid (e.g. coffee extract) the liquid is frozen in two steps (each with different freezing
temperatures and times) and ground. The solid material is then fed (manually or mechanically) onto the trays in a drying chamber. The temperature of the drying chamber is well below 0°C. The exact temperature depends on the product being dried. A vacuum is applied in the chamber. The ice evaporates under these conditions. This evaporation causes a further decrease in the temperature of the product, which is compensated for by adding heat (by means of heating plates) to the product through the temperature-controlled trays. The evaporated water is re-frozen on the surface of a condenser, which has a temperature well below the sublimation temperature under the existing conditions in the chamber. From time to time the condenser is de-iced by heating the condenser surface. The water is liquified and drained. The vacuum is maintained by a vacuum pump. The outlet of the vacuum pump
is emitted into the air
. To prevent solids entering and damaging the vacuum pump a filter is usually applied in front of the pump.