Freeze drying first involves cooling the food or chemical compound, often far below the freezing point of water. At this point, all of the water contained inside the food should be frozen into solid crystals. The basic structure of the fruit, vegetable or meat has not changed, but the water content is in the solid state. Half of the vacuum freeze dryer has been accomplished at this point through temperature reduction.
The drying process involves the use of a vacuum chamber. The frozen foods or chemicals are placed into the vacuum chamber, and the surrounding air is pumped out. If this process were performed at room temperature, the food would most likely be destroyed as the liquid water forced its way into the vacuum chamber. However, the frozen water crystals actually change from solid ice to a gas, bypassing the liquid state altogether.
This process is called sublimation. It's the same effect that causes solid dry ice to virtually disappear when struck with a hammer. During the freeze drying process in food drying equipment, the solid water converts to a steamy gas in the vacuum chamber, leaving all of the solid food materials dehydrated.
Once the water has been removed through freeze drying, the dried foods or chemicals are often stored in vacuum-sealed packs to keep air and moisture from reaching them. These packs can be stored at room temperature, since bacteria and other harmful organisms cannot survive without air anyway. Freeze drying also leaves behind tiny pores where the frozen water crystals used to be. This is why instant coffee grounds mix so quickly and thoroughly with hot water. Freeze drying is also a popular process for creating 'space age' ice cream desserts and backpack-friendly meals for hikers and campers.