The freeze drying process plays with the physical properties of the water. It is commonly known that water boils at 100ºC at the sea level, at 120ºC in a pressure cooker and 80ºC at the top of a mountain.
The evaporation temperature depends on the pressure. If the pressure is reduced enough, in what is commonly called vacuum, the water can boil at room temperature.
If we combine the effects of vacuum with low temperatures, we are in the water sublimation region, that is to say, the ice turns into water vapour without becoming liquid. Sublimation is the secret of freeze drying, this is what allows drying and dehydrating food without breaking its molecular bonds.