Freeze drying is a method of preserving biological material ranging from flowers to whole animals. The most common use of this technique (called lyophilization) is to preserve food in a way that protects the taste and pleasing appearance. Freeze-dried foods are sent on space missions, and are recommended for disaster survival kits. While it is more convenient to purchase freeze-dried products from a supplier, it is possible, with practice, to make them at home.
How it Works
Lyophilization involves a process called sublimation. This is when water goes from a solid (frozen) state to a gaseous state without first becoming a liquid. If food or other biological material is frozen and not sealed in protective packaging, the moisture vaporizes and leaves it over time. It can then be reconstituted in water with little loss of quality.
If you have a refrigerator with a freezer, and a wire rack such as those used to cool baked goods, you have the equipment you need to freeze dry food. Apples are a good choice to experiment with since they are inexpensive and lend themselves well to this method of preserving. Wash, peel and core an apple. Slice it thinly and lay the slices on the rack, leaving a little space for air to circulate between. Leave the slices undisturbed for about a week. Take one slice out and let it thaw. If you have let it dry long enough the fruit will still look fresh and edible after it thaws. If moisture is still present, the apple will turn black. Experiment with different fruits, vegetables and meats to determine drying times for your freezer.
Speeding up the Process
For people who are serious about preserving food by this method, freeze dryers are available for purchase. These machines create a vacuum to draw moisture out of frozen food, greatly reducing the drying time over the basic method. Some machines freeze and vacuum; for others food pieces must already be frozen solid before placing in the machine for best results. Prices range from $500 up to several thousands of dollars for freezer/vacuum machines.