Neal Foley, age 13, of Providence, R.I., for his question:
HOW IS FOOD DEHYDRATION ACCOMPLISHED?
A popular method used in food preservation is drying, or dehydration. To accomplish this procedure, the moisture must be removed from the food.
Microorganisms cannot grow on dry food. Drying also reduces the size and weight of foods, making them easier to transport and store.
Dried foods play an important part in feeding troops in wartime. Soldiers can easily carry small packages of dried foods such as milk, eggs and powered coffee.
Food processors blanch vegetables and some fruits before drying to prevent changes caused by enzymes. In blanching, the foods are exposed to steam or boiling water.
Processors often treat apples, apricots, pears and peaches with sulfur dioxide gas to prevent enzyme and other chemical changes, especially browning.
Food can be dried in the sun, in kilns, in special machines called dehydrators and in spray chambers.
Sun drying is the oldest method of drying food. Processors spread the food on trays and expose it to the sun. After several days, enough moisture has evaporated to make the food safe for storage. Processors sun dry many fruits, some vegetables and some fish.
Kiln drying uses heat from a furnace or stove to evaporate moisture from food. The furnace or stove stands on the lower floor of a building called a kiln. The food is put on the slanted floors of the upper stories and heat rises through the openings between the slats.
Kiln drying may take several days. During this period, processors turn and move the food several times to make sure it becomes thoroughly dry.
Dehydrators take less time than the other means of drying to process the same amount of food. Some dehydrators use a partial vacuum to make water evaporate at a low temperature. Due to the lower temperature, fewer chemical changes caused by heat take place.
Processors dehydrate eggs, milk, fruit and vegetable juices and other liquid or partly liquid foods by spray drying. In this method, liquid food is sprayed through nozzles into specially designed drying chambers. Food particles collect at the bottom of the chambers as powder.
Hot blast drying is used chiefly for vegetables. Dehydrators force hot air over food put on trays in special chambers. As the heated air flows over the food, it absorbs and carries off moisture.
Freeze drying removes water from food while the food is still frozen. The frozen food is cooled to about minus 20 degrees Fahrenheit. Then it is placed on trays in a vacuum chamber and heat is carefully applied.
In this method, the frozen water in the food is evaporated without melting. The food does not undergo high temperatures until most of the moisture has been removed.