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.
Let’s 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.
Frozen foods retain fresh flavor and nutritional value, but require uniform, low temperature storage conditions, and that takes energy. Dehydrated and canned foods are shelf-stable, but high-temperature processing can degrade flavor, texture and nutritional content. Freeze-drying combines the best of these processing methods. It preserves freshness, color and aroma similar to frozen food, while providing the shelf-stable convenience of canned and dehydrated food.
The freeze drying process of different foods varies with temperature, time, pressure, and other intermediate steps. Foods are first tested for bacterial counts. Next, the food may be cut into smaller pieces, depending on its original size. For example, thick meats do not freeze dry very well, so they must be cut into smaller chunks before freeze drying.
Meat and seafood are then cooked before freeze drying. Fruits and vegetables are washed first. Some vegetables such as peas and corn have to 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.
Next, the carts are moved from the cold room into the vacuum drying chamber. The drying stage involves a process known as sublimation. Sublimation is where a solid material is forced to change state into a gaseous state without ever becoming a liquid. The process is started by removing the air with a vacuum pump. The temperature is then raised to about 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 is filled with tiny voids, similar to a sponge. These voids are where the ice crystals were once present. This makes the food easier to absorb water when it is prepared for eating.
The dried food is then tested for moisture content and purity. Some foods may be ground into small pieces or even into a powder. Different ingredients are then blended together and the resulting food is packaged in airtight containers to prevent them from absorbing moisture from the air.
Freeze-dried foods have several advantages over other types of food for long term storage. They retain virtually all their fresh food taste and nutritional content. They maintain their original shape and texture, unlike dehydrated foods which shrink and shrivel due to high temperature processing. They weigh less than fresh food and take much less storage space. Finally, they stay fresh and can be stored at room temperature in their original packaging.