Atmospheric freeze drying
Although sublimation in normal freeze drying occurs under a vacuum, it is perfectly possible for sublimation occur in air at atmospheric pressure, as long as the partial pressure of water vapour below the psat. Indeed, this is by far the most common form of freeze drying on planet earth - it comes naturally, in a mass of snow and ice, which cover most of the earth's surface. Experiments in ambient freeze-drying of food products are dated Meryman (1959), who freeze dried small samples of the liver and recirculation of air through the Packed desiccant bed. Production processes, use of atmospheric freeze drying is currently in operation (Dtech A/S, Norway), in which the freeze dried food particles in fluidized bed of cold air with relative humidity of 30%. Gas raises the humidity in the fluidized bed, but then recirculated air through the condenser to remove the water before reheated. Capacitor and integrated heater with heat pump, and this allows to save the electric power, to be made which is not possible with conventional freeze dryer design.
Other workers continued to Meryman idea to use absorbent material, but it was included in the immediate vicinity of drying the material in a boiling layer.
This means that there is no capacitor costs and molecules of water vapor not have to travel far, as soon as they left the food, which increases the speed of drying. Process, in fact, internally controlled mass transfer 1 cm-size of solid particles. The disadvantage is that the desiccant should be separated completely from food and then that the risk of contamination of food dehydrator remains...
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Food refrigeration and freezing