Freeze drying is based on the physical process of sublimation: in a vacuum, water changes from its solid, frozen state directly into vapour – the ice evaporates. The product to be dried is first deep frozen at normal pressure, usually by cryogenic liquefied gases. It is then placed under vacuum, so the frozen moisture within the product sublimates and is removed as vapour. This allows water to be extracted from foodstuffs without heating or lengthy drying processes which would cause nutrients, aromatic substances and vitamins to be lost.
These essential ingredients are largely unaffected by freeze drying. The extraction of water with this method causes only slight alteration to the internal structure of the food. The dried cells form a sponge-like structure, with cavities that absorb moisture rapidly when the product is placed in water. In ideal cases, rehydrated freeze dried food is almost indistinguishable from fresh food.
Typical examples of freeze dried foods include coffee granules and other instant drinks, spices, and fruit for muesli mixtures. Sometimes freeze dried foods are high fliers – they are included in the food packs of astronauts.
COBRA dry screw vacuum pumps are particularly suited to the evacuation of drying chambers in foodstuffs freeze drying systems. They achieve the required vacuum levels without operating fluids and guarantee high pumping speeds throughout their working range, from atmospheric to ultimate pressure.