Freeze drying

Preserving foods by fast freezing, then exposing them to high vacuum that removes ice by sublimation. Careful control of process conditions gives good rehydration afterwards, with substantial retention of nutrients and of colour, flavour and texture characteristics. The premium drying method for instant coffee, herbs, fruits and emergency rations. The process runs in three stages: freezing, primary drying (sublimation of free ice under vacuum) and secondary drying (desorption of bound water to 1–4% moisture). Because water leaves as vapour without passing through liquid, cell structure survives — giving the characteristic porous texture — though the energy cost makes it the most expensive common dehydration method.