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Freeze-drying has several advantages over other food preservation methods. Freeze drying is actually a three step process. It takes the three steps to freeze-dry the food, and a fourth step to reconstitute it just before it is eaten.

Lets look at the advantages of freeze drying over other food preparation methods. This will explain why freeze dried food is particularly well-suited for long term food storage programs.

Foods that are frozen retain their fresh flavor and nutritional content, but they also require constant, low temperature storage conditions, and most importantly electrical power. Canned and dehydrated foods are shelf-stable, but the high temperatures used in manufacturing can alter the flavor, texture and nutritional value of the food. Freeze drying comprises the best of these other processing methods. Freshness, aroma, and color are similar to frozen food, while also providing the shelf stability of dehydrated and canned food.

The freeze drying process varies with different foods in terms of time, temperature, pressure, and other intermediate steps. To begin with, bacterial counts are taken of the foods. Since thick meats do not freeze dry well, they will be cut into smaller pieces. This may apply to other foods as well.

Meats and seafoods must be cooked before freeze drying. Most fruits and vegetables are simply washed first. However, vegetables such as peas and corn must be first be blanched.

The food is then placed on trays, the trays are placed in carts, and the carts are wheeled into cold rooms where the temperature can be as low as -40 F. This causes the food to be frozen very quickly.

The carts are then moved from the cold room into the vacuum drying chamber where a process known as sublimation takes place. Sublimation is where a solid material is forced to change its current state into a gaseous state without first ever becoming a liquid. This begins with a vaccum pump removing the air from the vacuum chamber. Next, the temperature is raised to 100 F.

Since the pressure in the chamber is below the point where water can simultaneously exist as a solid, liquid, and gaseous state (called the triple point), the heat causes the ice crystals to change into water vapor. The vapor is then drawn off from the chamber, leaving only the food.

At this point, the dried food will have small voids or pockets, similar to a sponge. These voids are where the ice crystals where present. At this stage, the food will absorb water much more easily when it is prepared for eating.

The dried food will next be checked for moisture content and purity. Certain foods are then broken into small pieces or even ground into a powder. Meals are prepared by blending the different ingredients together, and the food is then packed in airtight containers such as pouches and cans. The packaging is very important as it prevents the food from reabsorbing moisture from the air.

In the end, freeze-dried foods have several advantages over other types of foods used in long term food storage. Freeze dried foods retain all the fresh taste and nutritional content of frozen foods. Also, unlike dehydrated foods which shrink and shrivel under the high temperature drying process, freeze dried foods maintain their original shape and texture. They also weigh less than fresh food and take much less storage space. As long as they are kept in their original packaging, these foods stay fresh longer and can be stored at room temperatures.