There are three stages in the process: Freezing, Sublimation drying (Primary drying), and desorption drying (Secondary drying).
The freezing phase is the most critical in the entire freeze-drying process. It is the base of a success freeze drying. It is important to freeze foods completely to avoid water boiling during vacuum drying. In addition, rapid freeze food can avoid large ice crystals form, which may deteriorate the final product quality.
During the primary drying phase, the pressure is lowered through the application of a high vacuum, and heat is applied to provide the energy needed for the ice to sublime. This initial drying phase removes about 95% of the water present in the food. This step can take several hours to days.
Secondary drying phase is to remove unfrozen water molecules remaining after primary drying. During this phase, the temperature is raised higher than in the primary drying phase in order to vaporize the water molecules. Pressure is frequently lowered during this phase, but not in all cases.