Freeze drying is the removal of water by sublimation from the frozen state (ice). In this process, the food is first frozen and then subjected to a high vacuum, whereby the water ice evaporates without melting. The water vapor released is condensed on the surface of a condenser at very low temperature. The heat needed for sublimation is supplied by radiation or conduction. Freeze drying occurs in two stages: sublimation of the ice, and desorption of the moisture adsorbed in the dry matrix. A model for lyophilization kinetics in a greatly simplified system is developed. Applications of freeze drying in the food industry are reviewed, and batch and continuous freeze dryers are described. Microwave freeze-drying is discussed.
Freeze concentration is the removal of water from a frozen food liquid as ice crystals. Since freeze concentration occurs without heating or boiling, thermal damage and loss of volatile aromas are largely avoided. The main disadvantage of freeze concentration is the loss of solute, which may be considerable at high concentration ratio.