Commercial Food Drying

Since sun drying takes a long time and exposes foods to the weather and to the action of insects, most foods are now dried by various commercial processes, although raisins are still sun dried. Among the commercial types of drying, the three most important are conventional (heat), vacuum (pulls the water out), and freeze-drying (ice crystals vaporize).

Conventional Drying

Conventional drying uses heat to evaporate the water. In one method, the food is spread on a slatted floor or on shelves within kilns or drying rooms. A blower then passes hot air from a heater over and through the food. In tunnel drying, food is placed on trays or “carts”, which are moved through a tunnel of carefully controlled hot air. Liquids can be dried by either spray drying or drum drying. In the former, a fine spray of the liquid is dried very quickly in mid-air. Spray drying is used to produce such foods as nonfat dried milk and some types of instant coffee. Drum drying occurs when liquid is poured over the very hot surface of a drum dryer, an apparatus resembling a large barrel. The dried food can then be peeled off like tissue paper, ground into flakes, and packaged. Some mashed-potato flakes and quick-cooking hot cereals are dried in this way.

Vacuum Drying

Vacuum drying dehydrates foods to very low moisture levels (1 to 3 percent) through the use of a vacuum. Milk, tomato paste, orange juice, and coffee are often concentrated by vacuum drying. The food is placed in a chamber, and the surrounding pressure is reduced below atmospheric pressure, which lowers water’s boiling point. The water is more easily boiled off at this lower boiling temperature. This drying method preserves flavor and color while increasing shelf life considerably, but it has two drawbacks: it is expensive, and it requires that the dehydrated food be stored in airtight containers to prevent rehydration by drawing moisture from the air.

Freeze-Drying

Freeze-drying consists of first freezing the food and then placing it in a vacuum, where the ice sublimates to a vapor. This process of sublimination is the most effective method for drying foods, because it does not subject the food to high heat, which alters a food’s flavor, color, and structure. The heat used is low enough to prevent melting of the ice, while warm enough to assist in evaporation of the water. As a result, freeze-dried products yield the highest quality, store indefinitely, and can be reconstituted easily. The process, however, is more costly than conventional drying.