Freeze drying is a process that removes water from food and other materials. The process starts by first freezing the food, then placing it under a vacuum. The vacuum environment allows the ice to sublime, or transform from solid to vapor. Freeze drying is also known as Lyophilization or Cryodesiccation.
Freeze drying has four stages:
- Pretreatment – Food can be prepared before freeze drying it. This is done to change the drying process, or the final dried product. Pretreatment can be as simple as slicing the food to certain thicknesses. Or by adding citric acids to maintain the appearance of the final dried foods. Some recipes need to be modified for freeze drying. Ingredient flavors might change from either the drying, or the reconstituting processes.
- Freezing – Food is cooled below and to left of its triple point to make sure sublimation will happen during the drying stages.
- Drying
- Primary drying – This stage removes the free ice by applying a small amount of heat. The heat provides the energy needed to sublime the free ice without thawing the food. Primary drying removes about 95% of the moisture.
- Desorption [Secondary drying] – Bonded water molecules are released during this phase. It happens by increasing the heat. This stage actually starts before the sublimation/primary drying is complete.
This final stage brings the moisture content to between 1% and 4%.
Sources:
https://en.wikipedia.org/wiki/Freeze-drying#cite_note-multiple-7
http://www.fda.gov/ICECI/Inspections/InspectionGuides/ucm074909.htm
http://www.spscientific.com/freeze-drying-lyophilization-basics/