freeze-drying

Method of preserving food (see food technology) or other perishable materials. The product to be dried is frozen and then put in a vacuum chamber that forces out the ice as water vapour, a process known as sublimation. The most widely-known freeze-dried product is instant coffee.

Many of the substances that give products such as coffee their typical flavour are volatile, and would be lost in a normal drying process because they would evaporate along with the water. In the freeze-drying process these volatile compounds do not pass into the ice that is to be sublimed, and are therefore largely retained.