 From ancient Egyptians and Greeks to colonists and early settlers of the New World, dehydration or drying food is one of the oldest methods of preservation. The Great Depression of the 1930s contributed to an increase in home drying in the United States. Many homemakers dried foods the same way early settlers had. During World War II, science and technology were applied to dehydrating food to feed the troops in the field. Ground food had the advantages of being lightweight, compact, and easy to transport with a reasonably long shelf life. Research done during and after this period led to increased commercial drying of a wide variety of foods. Hi, I'm Julie Cascio with the University of Alaska Fairbanks Cooperative Extension Service. Research on food preservation is an ongoing process. The United States Department of Agriculture and the Cooperative Extension Service continuously apply new research findings to their recommendations for food preservation techniques. The guidelines in this module may be revised as additional knowledge is gained that may increase the margin of safety or improve the quality of home-preserved foods. Consult your local Cooperative Extension Office annually for updated information. Drying preserves food by removing enough moisture so that bacteria, yeasts, and molds cannot grow and spoil the food. Enzymes are chemicals in all fruits and vegetables which cause them to ripen. Enzymes may cause browning, loss of vitamins, and loss of flavor. Drying slows down but does not stop the action of enzymes. Pretreating with heat, ascorbic acid, or sulfur dioxide can stop enzyme action and oxidation. Brown foods such as fruit leathers, dried fruits, or dried vegetable chips can be eaten dry. Other foods may be rehydrated or reconstituted, which means to return water to dried foods. The optimum temperature for drying food is 140 degrees Fahrenheit. If higher temperatures are used, the food will cook instead of dry. Cooking at these higher temperatures is called case hardening. The food cooks on the outside, trapping the moisture. Since the moisture cannot escape, the food will eventually mold from the inside. The drying process should never be hurried by raising the drying temperature. Foods are dried by using the right combination of warmth, air currents, and low humidity. In drying, a warm temperature allows the moisture to evaporate. Air currents speed up drying by moving the surrounding moist air away from the food. Low humidity allows moisture to move from the food to the air. If the surrounding air is humid, then drying time will be increased. There are several methods for dehydrating food. Sun drying and solar drying occur out of doors. These methods are suitable for high acid foods. Room drying, oven drying, and portable electric dehydrators are indoor methods. Room drying differs from sun drying since it takes place indoors in a well-ventilated attic, room, car, camper, or screened-in porch. Herbs, hot peppers, nuts in the shell, and partially dried sun-dried fruits are the most common air-dried items. Herbs and peppers can be strung on a string and hung in the air or tied in bundles and suspended from overhead racks until dry. Enclosing them in paper bags with openings for air circulation protects them from dust, loose insulation, and other pollutants. Temperature is key to drying foods in the oven. Turn your oven to 140 degrees Fahrenheit and open the door about 2 to 6 inches to increase air circulation. Place a fan outside the open oven door to also improve the circulation. Of course, this is not safe if small children are around. Because the oven door is left open, the temperature inside the oven will vary. An oven thermometer placed near the food gives an accurate reading of the temperature inside. Adjust the temperature dial of the oven to achieve the needed 140 degrees Fahrenheit inside. Use trays narrow enough to clear the sides of the oven. Cake racks placed on top of cookie sheets make a good option for many foods. The trays should be narrow enough to clear the sides of the oven and about 3 to 4 inches shorter to allow for air circulation. The oven racks holding the trays should be 2 to 3 inches apart. Before using this method, test your oven to determine if it can hold an inside temperature of 140 degrees Fahrenheit or lower. If the temperature remains above 140 degrees Fahrenheit, case hardening is an issue. Choose another method for drying foods. Be aware that food takes twice as long to dry in an oven as in a dehydrator. The oven is not as efficient as a dehydrator and it uses more energy. Oven dried foods usually are darker, more brittle, and less flavorful than food dried by an electric dehydrator. Another method of drying is to use food dehydrators. A food dehydrator has an electric element for heat and a fan and vents for air circulation. It's designed to dry foods fast at 140 degrees Fahrenheit. The major disadvantage of a dehydrator is its limited capacity. Features of a portable food dehydrator may include double wall construction of metal or high grade plastic, wood is not recommended because it's a fire hazard and it's difficult to clean. Teating elements, a countertop design, a dial for regulating the thermostat that can be set from 85 to 160 degrees Fahrenheit, a fan or blower, 4 to 10 open mesh trays made of sturdy lightweight plastic for easy washing, the UL seal of approval, and a timer. Often the completed drying time may occur during the night and a timer could turn the dehydrator off and prevent scorching. It's a nice feature but often expensive. Air may flow through the dehydrator horizontally or vertically. In a unit with horizontal airflow, the heating element and fan are located on the side of the unit. Advantages of horizontal airflow are several different foods can be dried at one time. All trays receive equal heat penetration and juices or liquids do not drip down into the heating element. This dehydrator is metal with a wood grain coloring on it. In a unit with a vertical airflow, the heating element and fan are located at the base or in the lid. If different foods are dried at the same time, the flavors can mix. For units with the heating element located at the base, such as this one, liquids can drip inside. Whichever method you choose to dehydrate food, drying does not improve the quality of foods so always start with the best produce available. Drying time depends on the type of food, the size of the pieces, the moisture content of the food and the drying method. It may take four hours to three days to dry. Most fruits and fruit leathers will feel leathery when dry. This apple fruit leather has a nice leathery feel to it. The moisture content will be about 20 percent, which is fine for these high acid foods. To test for dryness, cut several cooled pieces in half. I have these apple slices here and you can cut them in half and look at them and if you notice beads of moisture on them, you should know that they're not quite dry yet. There should be no visible moisture and you should not be able to squeeze any moisture from the fruits. Some fruits may remain pliable, but they should not be sticky or tacky. If a piece is folded in half, it should not stick to itself. This one does, it's not ready to go. Berries should be dried until they rattle when shaken. Have some blueberries here and you can really hear how they're rattling. Vegetables when dry are described as tough, these carrots, you can really feel that they're tough, brittle, crisp or cracking hard. Some would shatter if hit with a hammer. Vegetables should have 10 percent moisture content when dried as they are a low acid food susceptible to bacteria contamination. Near the end of drying, check vegetables frequently and remove those that appear dried. A more precise method would be to weigh the product before and after drying to determine if 90 percent of the moisture has evaporated. After drying, cool 30 to 60 minutes before packaging. Drying food warm can lead to sweating and moisture buildup, however excessive delays in packaging could allow the moisture to re-enter the food. So remember, if you've dried food outdoors, it must be pasteurized before it is packaged. Pasteurize dehydrated food to kill the insects and their eggs by freezing below 0 degrees Fahrenheit at least 48 hours or heating in an oven at 160 degrees Fahrenheit for 30 minutes. When dried fruit is taken from the dehydrator or the oven, the remaining moisture may not be distributed equally among the pieces because of their size or their location in the dehydrator. Conditioning is a process used to equalize the moisture and reduce the risk of mold growth. To condition the fruit, take the dried fruit that has cooled and pack it loosely in glass or plastic jars. Seal the containers and let them stand for 7 to 10 days. The excess moisture in some pieces will be absorbed by the dryer pieces. Shake the jars daily to separate the pieces and check the moisture condensation. If condensation develops like it has in this jar, then return the fruit to the dehydrator for more drying. After conditioning, package and store the dried fruit. Proper storage prevents insects and rodents from eating the food. It also keeps moisture out and saves nutrients. Use glass jars or vapor-proof freezer cartons with tight-fitting lids. Heavy-duty plastic bags with pressed-together seals are acceptable but are not insect or rodent-proof. Screw lids or covers on glass jars to prevent insect contamination, but it is not necessary to heat process the jars. Metal containers may be used if dried food is first placed in plastic bags. Dried fruits and vegetables will keep up to one month at room temperature in a cool, dry, dark area. For storage up to one year, place tightly-wrapped dried food in the freezer. Low temperatures extend the shelf life. Dried fruits can be eaten as nutritious snacks or can be soaked in water or juice for 1 to 2 hours and used in favorite recipes. Sliced vegetables can be eaten dried as tasty vegetable chips for dips, though most vegetables are refreshed with water before use. To add water back to vegetables, soak in water for 1 to 2 hours. Use two cups of water for each cup of dried food. I have one cup of water here for about a half a cup of mixed vegetables. Using boiling water speeds up the soaking period. Dried vegetables can be added directly to soups or stews without pre-soaking, drawing on the liquid in the soup or stew for rehydration during cooking. Make sure there is enough liquid in the soup for that rehydration. We have discussed different methods of dehydrating food, foods that are commonly dehydrated, considerations for drying high acid foods like fruits and low acid foods like vegetables, and how to determine if the foods you have dehydrated is dry enough. Now it's your turn to dry food for your eating pleasure.