Dehydration effectively preserves apples and bananas, and once dry, the fruit retains its flavor, texture, color and nutritional qualities. When properly dried and stored in cool temperatures, fruit remains edible for six to 12 months. According to the North Dakota State University Extension, apples and bananas are excellent fruits for drying.
The best apples for dehydrating are fully ripe and ready to eat. Bananas should be evenly yellow or have only small, brown flecks. Use high-quality fruit, because drying won't improve the quality of bad fruit. Discard fruit with bruises, soft spots, decay or mold.
Wash and peel the apples, and then remove the cores. Cut the apples into slices about 1/8 inch thick. Peel the bananas, and then cut them into pieces 1/4 to 3/8 inch thick. The pieces can be cut either lengthwise or crosswise.
Pretreatment is crucial -- without it, dehydrated fruit can develop harmful bacteria. Pretreatment also preserves the natural texture and color of the fruit. You can easily and safely pretreat the fruit with an acidic solution, and this can be done several ways. To use powdered ascorbic acid, mix 2 1/2 tablespoons of the powder into 1 quart of cool water. You can also use equal parts lemon juice and water, or citric acid mixed at a rate of 1 teaspoon for every 1 quart of water. Soak the fruit in the pretreatment solution for 10 minutes, then remove it with a slotted spoon.
Electric dehydrators are easy to use and produce high-quality dried food. Drying time may require only a few hours or as long as a day, depending on the type of dehydrator, as well as room temperature and humidity. Arrange the apple and banana slices on the dehydrator trays in a single layer, not touching. Set the dehydrator at 140 degrees Fahrenheit. Turn the fruit every two hours.
The quality won't be as good as fruit dried in an electric dehydrator, but you can dry fruit in your oven if its lowest setting can maintain a temperature below 200 F. Higher temperatures are too hot for drying. To dry fruit in an oven, spread a clean cloth on each oven rack. Use thin, porous fabric such as cheesecloth or muslin. Arrange the fruit, not touching, on the fabric-covered racks, and then return the racks to the oven. Open the oven door 2 to 4 inches, and place a portable fan where it can blow toward the open door. Turn the fruit every two to three hours. Oven dehydration isn't a safe method for households with pets or small children.
Drying fruit in the sun is appropriate only when the outdoor temperature reaches at least 85 F by noon and air humidity is lower than 60 percent. Outdoor fruit dehydration is impossible in cool or humid areas. To dry fruit outdoors, place the fruit on clean screens or racks. Cover the fruit with thin, porous fabric such as cheesecloth or muslin. Turn the fruit once every day until dehydration is complete. Cover the fruit at night or bring it indoors.
Apples and bananas are adequately dehydrated when the fruit is pliable but not sticky. Let the fruit cool for 15 minutes, and then cut it in half. If you see moisture, return the fruit to the dehydrator. It's best to err on the side of caution, because fruit that is slightly too dry is safer than food that isn't dry enough. Store the dehydrated apples and bananas loosely in a glass jar for four to seven days, shaking the jar daily. If condensation appears on the inside of the jar, the fruit requires additional dehydration time. Once it's dried, continue to store the fruit in glass jars. Store the jars in a cool, well-ventilated area.