Freeze-drying is a cold process that consists in freezing and dehydrating by creating a vacuum, without heat, unlike other dehydrating processes. Nutrients & flavours are retained for a long time, without the need for preservatives or additives. Considered discovered by the Peruvian Incas who were letting potatoes freeze at night and removing their water content by stomping them with their bare feet, the technology was really developed in the twentieth century as a way to preserve medicine drugs, in particular from WWII for blood plasma and penicillin, and food for astronauts. No more bare foot involved!
Instant coffee, rehydratable food for trekking, emergency rations, tiny packs with plain fruits, vaccines, pharmaceutical drugs, chances are high you have tried freeze-dried products already.
What happens now when food is freeze-dried is that it is first frozen, and then a vacuum is created by lowering the pressure (a lot, think space vacuum!), which transforms the water from a solid state in the frozen food directly to a vapor state without going through a liquid one. Imagine an ice cube vaporized without seeing water. This is called sublimation. A cool name to describe the mere change in a physical state, as this is not a chemical reaction!
Compared to other dehydration techniques, this one does not involve heat, which can have a destructive impact on some nutrients and compounds, even relatively low temperature dehydration techniques used in raw food preparation. For example, vitamin C is very sensitive to processes, and we have confirmed ourselves through lab tests that while it totally disappeared from cooked and dehydrated samples, vitamin C was perfectly preserved with freeze-drying.