Freeze drying aka lyophilization is a process whereby a product is dried under low temperature and vacuum. It is mainly used to dry thermo-labile materials so that its components (proteins, flavors and colors) can be preserved along with its original shape and size.
This method started off being widely used in the pharmaceutical industry to allow serum to be rendered chemically stable and viable without need for refrigeration. The process was applied to penicillin, and became recognized as an important scientific technique for the preservation of biologicals.
Controlled freeze drying keeps the product temperature low enough during the process to avoid changes in the dried product appearance and characteristics. It is an excellent method for preserving a wide variety of heat-sensitive materials such as proteins, microbes, pharmaceuticals, tissues & plasma.
When it comes to food, it is worth noting that not all food is suitable for freeze-drying. Generally, larger-sized foods need to be cut into smaller pieces prior to freeze-drying.
The common freeze-dried food items are coffee, fruit, juice, vegetables, herbs, food flavorings, fish, seafood, eggs, and dairy. As a result, freeze-dried foods can be used for backpacking, camping, military rations, survival food storage, and space exploration. The dried product can then be stored for long periods without the risk of changing composition (i.e. enzymatic, genetic) or being infected by microorganisms.
Freeze drying is also an expensive process due to the high energy consumption. This is why for some foods, it is simply not cost-effective.
There are three stages in the process: freezing, sublimation drying, and desorption drying, with the freezing phase being the most critical. The food has to be frozen rapidly to avoid the formation of large ice crystals, which can deteriorate the final product quality.
To freeze-dry, a few basic elements are required, namely refrigeration system, vacuum system, control system, product chamber or manifold, and condenser.