Basic Principles of Operation

Food pieces are first frozen and then dehydrated under vacuum from the frozen state

This very low temperature drying protects all food quality attributes such as appearance, texture, colour, flavour and vitamins

The principle behind freeze drying is that under certain conditions of low vapour pressure (vacuum), water can evaporate from ice without the ice melting. The process is called sublimation

Within the vacuum chamber, heat is applied to the frozen food to speed sublimation and if the vacuum is maintained sufficiently high and the heat is controlled to approximately -5ºC, then moisture vapour will sublime at a near maximum rate

Sublimation takes place from the surface of the ice and so as it continues the ice front recedes towards the centre of the food sample, the food dries from the surface inwards. Since the frozen food remains rigid during sublimation, escaping water molecules leave voids behind them, resulting in the porous sponge-like dried structure. Thus freeze dried foods reconstitute rapidly but must also be protected from ready absorption of atmospheric moisture and oxygen by adequate packaging