Freeze drying, which scientists might call lyophilization, uses both a vacuum and a freezing process to remove water from perishable foods and medicines. The result is a product that can be stored at room temperature for years without spoilage, or packed in limited storage spaces and reconstituted with water later. Makers of instant coffee often use a freeze drying process, as do dietitians creating meals for campers, soldiers and astronauts. This process is also used in the pharmaceutical industry to preserve the integrity of air or moisture-sensitive medicinal compounds.
Drying foods with heat for preservation is an ancient concept, but it has certain drawbacks. The water inside the food is in liquid form, but the heat of the sun or other source slowly converts it to a gas. As the liquid passes out of the food, the cell walls are often damaged and the essential flavor and texture of the food is lost. Adding water to foods dehydrated with heat does not always restore flavor or texture. This is why freeze drying has an advantage over heat dehydration in the preservation process.
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 process 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, 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.