The Freeze Dry Process
Freeze-drying or lyophilization and can be thought of in terms of holding food in suspended animation. The more water and oxygen you can remove from a food, the longer the food is preserved for.
Lyophilization is the most successful process at removing these substances and so Freeze-Dried food has the longest shelf-life of all preserved foods.
Happily it also happens to be the process that retains more of the food's original nutritional value, taste and texture. To find out why freeze-drying is a better method of preservation than canning or dehydration read this post.
Lyophilization for food is a three stage process: freezing, drying and packaging.
Although larger crystals are easier to freeze-dry, Clarence Birdseye identified that large ice crystals in the formerly living cells of plants and animals burst the cell walls during freezing and thawing, denaturing the texture of the food, and in some cases affecting the nutritional content.
Birdseye solved this issue by exploiting the phenomenon of flash freezing he'd encountered when ice fishing with the Inuit, finding that rapid freezing at extremely low temperatures (between -50 ? to -80 ?) actually prevented the formation of ice crystals.
The physical principal that is exploited in lyophilization is sublimation. Sublimation is the transition of a substance from its solid phase to a gaseous phase without passing through the liquid phase, (think dry-ice).
To ensure that sublimation (desirable) occurs during the drying process rather than melting (undesirable), the water needs to be cooled below its triple point in this initial freezing process. The triple point is the temperature and pressure at which the three phases of a substance can coexist in thermodynamic equilibrium.
So, to ensure the textural and nutritional quality of the food remains, this initial freezing is actually the most important stage in the lyophilization process.
This stage in the process can last several days. Pressure is lowered in the chamber in which the food has been placed through the application of a partial vacuum. Temperature is then increased to the point where the water will sublime, the ice evaporating from the cells dehydrating the food.
Temperature must be carefully controlled as too much heat can alter the cell structure, which again could impair texture, flavour and nutritional value. Too high a temperature can also risks evaporation that is too fast for the pumps to removed from the chamber resulting in condensation.
In this initial drying about 95% of the water is removed.
Some products may undergo a further drying process that aims to remove any unfrozen water molecues. The temperature and sometimes the pressure will be raised (depending on the product) and this second drying can create a product with up to 99% of its water removed.
When freeze-drying is complete the food is vacuum packed, sometimes with an oxygen absorbing sachet added.
Now the food is vacuum packed, it can be stored at room temperature for years without spoiling - perfect for hiking and camping, self-supported expeditions and races, as well as for use as emergency rations.