Method of preserving food; see food technology. The product to be dried is frozen and then put in a vacuum chamber that forces out the ice as water vapor, a process known as sublimation.
Many of the substances that give products such as coffee their typical flavor 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.