Freeze-drying is another method of preserving food. During this process, water is removed from food while the food is still frozen by a process known as sublimation. The frozen food is cooled to about -20° F (-29° C). Then it is placed on trays in a refrigerated vacuum chamber, and heat is carefully applied. As a result, any water in the food is changed directly from ice to water vapor without first changing into water. Freeze-dried products include: soups, tea, and instant coffee.
Source: Excerpted from FDA/CFSAN Food Safety A to Z Reference Guide, September 2001