ABSTRACT
Freeze-drying, also known as lyophilization, commonly refers to freezing of water in a substance followed by subsequent sublimation of ice crystals in the frozen material. Freeze-drying has become a well-established unit operation in the pharmaceutical and food industries. This chapter provides a description of the general physicochemical and engineering principles of freeze-drying, provides underpinning information on the freezing of water, and introduces typical foods available for industrial and consumer uses as ingredients and ready-to-use or ready-to-eat foods. Although atmospheric freeze-drying is possible, most existing freeze-dryers operate using the vacuum freeze-drying principle, whereby ice temperature can be controlled by pressure. The drying chamber pressure is controlled by the vacuum pump equipment, while the refrigeration unit is used to maintain the condensing surface at a lower temperature than that of ice sublimation. Raoult’s law shows that small solutes, which may have a significant mole fraction in a food, may significantly depress the freezing temperature.
