Freeze drying is a process that finds application in both the food industry as well as some process industries, such as pharmaceuticals. The processes include first freezing the product, then removing the water by sublimation in which the water in a solid state converts directly to vapor without passing through a liquid state. To carry out the sublimation process, heat is carefully added to the product being maintained under a vacuum in chambers such as one shown in Figure 1.5. To achieve the vacuum a combination of refrigeration and mechanical pumping is employed. The mechanical pump mostly removes air and other noncondensible gases, while the refrigeration system chills an evaporator to perhaps -50°C (-58°F). Water from the product deposits on the evaporator coils in the form of frost. Refrigeration thus serves the initial freezing process as well as the evaporator coils of the vacuum chamber.
The pharmaceutical industry uses freeze drying for certain products where the removal of water at higher temperatures would damage the product. The percentage of food that is preserved by freeze drying is very low, but the process finds a role for certain applications. Once a food product is freeze dried, it needs no further refrigeration and at the time of preparation of eating the addition of water restores the product to a taste remarkably close to the original quality.