State Nutrition Specialist
Dehydration is one of the oldest methods of food preservation, though methods for drying food have become sophisticated over time. For many people, dehydrating food at home is a convenient way to preserve foods.
Molds, yeast and bacteria need water to grow. When foods are sufficiently dehydrated, microorganisms cannot grow and foods will not spoil. Dried fruits and fruit leathers may be used as snack foods; dried vegetables may be added to soups, stews or casseroles. Campers and hikers value dried foods for their light weight, keeping qualities and ease of preparation.
The nutritive value of food is affected by the dehydration process. Vitamins A and C are destroyed by heat and air. Using a sulfite treatment prevents the loss of some vitamins but causes the destruction of thiamin. Blanching vegetables before drying (to destroy enzymes) results in some loss of vitamin C and B-complex vitamins as well as the loss of some minerals, because these are all water soluble. Yet blanching reduces the loss of thiamin and vitamins A and C during dehydration and storage.
Dried foods have more calories on a weight-for-weight basis because of their nutrient concentration. For example, 100 grams of fresh apricots has 51 calories, whereas 100 grams of dried apricots has 260 calories. In general, dried foods are not a major part of the American diet, so nutrient loss is not a concern. Nutritive value, as well as flavor and appearance, is best protected by low temperature and low humidity during storage.
Foods can be dehydrated by various means: the sun, a conventional oven, an electric dehydrator or, for herbs only, a microwave oven. Dehydration, like other preservation methods, requires energy. Unless sun drying is possible, the energy cost of dehydrating foods at home is higher than for canning and, in some cases, more expensive than freezing.
Solar drying is a modification of sun drying in which the sun's rays are collected inside a specially designed unit with adequate ventilation for removal of moist air. The temperature in the unit is usually 20 to 30 degrees F higher than in open sunlight, which results in a shorter drying time. While solar drying has many advantages over sun drying, lack of control over the weather is the main problem with both methods.
Oven drying is the most practical way to experiment with dehydration. It requires little initial investment, protects foods from insects and dust and does not depend on the weather. Continual use of an oven for drying is not recommended because ovens are less energy-efficient than dehydrators, and energy costs tend to be high. It is difficult to maintain a low drying temperature in an oven, and foods are more susceptible to scorching at the end of the drying period. Oven-dried foods are usually darker, more brittle and less flavorful than foods dried by a dehydrator.
An electric deyhdrator produces a better-quality dried product than any other method of drying. Electric dehydrators are self-contained units with a heat source, a ventilation system, and trays to place the food on. They are used to dry foods indoors. Therefore, as with oven drying, they don't depend on the weather. Such dryers can be purchased or made at home and vary in sophistication and efficiency. Although an electric dehydrator requires a fairly high initial investment, it maintains low temperatures and uses less energy than an oven.
It is not recommended that microwave ovens be used for drying foods, because the food will partially cook before it dries, imparting an overcooked flavor. Microwave ovens can be used to dry some herbs quickly—but watch them carefully to prevent them from catching on fire. Check the owner's manual for drying recommendations.
Drying times in conventional ovens or dehydrators vary considerably depending on the amount of food dried, its moisture content and room temperature and humidity—and in the case of oven drying, the use of fans. Some foods require several hours, and others may take more than a day. Interrupting drying time, or prolonging it by using lower temperatures, may result in spoilage.
Air temperature and circulation must be controlled during the drying process. If the temperature is too low or the humidity too high (resulting in poor circulation of moist air), the food will dry more slowly than it should and microbial growth can occur. Watch temperatures closely at the beginning and end of the drying period. If the temperature is too high at first, a hard shell may develop on the outside, trapping moisture on the inside. This condition is known as case hardening.
Temperatures that are too high at the end of the drying period may cause food to scorch. Temperatures between 120 and 140 degrees F are recommended for drying fruits and vegetables. Temperatures up to 150 degrees F may be used at the beginning, but should be lowered as food begins to dry. For at least the last hour of the drying period, the temperature should not exceed 130 degrees F.
When using the oven drying method, it is important to know if your oven can maintain the appropriate temperatures. Use an oven thermometer to test the temperature of the oven at its lowest setting. Many ovens cannot maintain a temperature below 200 degrees F. If your oven cannot, it is too hot for successfully drying food, so alternative drying equipment must be used.
Trays upon which the food is placed must be at least 1-1/2 inches narrower than the inside of the oven to allow for air circulation. Allow at least 2-1/2 inches between trays and 3 inches of free space at the top of the oven for good air circulation.
Arranging food layers
About 4 to 6 pounds of food can be dried in an oven at one time. Place food on drying trays, or on wire cooling racks covered with cheesecloth or nylon netting, to allow easy removal of dried food. Pieces of food should be in a single layer. Do not place food directly on oven racks. Cookie sheets are acceptable only for fruit leathers, which do not require good air circulation.
Setting the temperature
Place an accurate, easy-to-read thermometer on the top rack toward the back. Preheat oven to 150 degrees F. For gas ovens, if temperature cannot be maintained below 200 degrees F, it may be possible to use only the pilot light. For electric ovens, use only the bottom element, disconnecting the broiler element if necessary. Arrange trays in the oven to allow for adequate air circulation. Prop oven door open at least 4 inches.
Place a fan outside the oven door to aid air circulation. Move it from side to side occasionally. The room also should be well ventilated. Oven drying, particularly if a fan is used, should be done with caution if small children are around.
Maintaining the temperature
Maintain the temperature at 140 degrees F. Watch the temperature even more carefully toward the end of the drying process. To prevent scorching, lower temperature to 120 degrees F if possible. Examine the food often and turn trays frequently, removing foods as they dry. Refer to MU Extenson publication GH1563, How to Dehydrate Foods, for directions about specific food products.
When using a dehydrator, load food on trays in single layers so that pieces do not overlap. This arrangement allows air to circulate through the trays. A constant temperature of 140 degrees F is necessary for dehydrator drying. Large pieces, such as apricot halves, should be turned halfway through the drying time. Pieces near the sides of the tray should be moved to the center. Stir small pieces with your fingers (make sure they're clean) every one to two hours, separating bits that stick together. It may be necessary to rotate the trays within the dryer at least once during the drying period. Vegetables usually take six to 16 hours to dry. Fruits can require up to 48 hours.
Never dry sulfured fruits in an oven or dehydrator, because the sulfur dioxide fumes can be irritating. Use sulfite dips or steam, water or syrup blanching in place of the sulfur treatment.
Different foods requiring similar drying times and temperatures can be dried together. Vegetables with strong odors or flavors (garlic, onion and pepper) should be dried separately. Don't dry strong-smelling vegetables outside in an electric dehydrator, because dehydrators are not screened and insects may invade the food.
Choosing a dehydrator
Because an electric dehydrator can be an expensive investment, choose a specific brand or model carefully. Refer to the sidebar for features to look for and evaluate before making your investment.
Judging when food is dry requires experience. It is better to overdry than to underdry. When in doubt, continue drying for an additional 15–30 minutes. Check for doneness. Allow the product to cool before testing.
Vegetables are sufficiently dried when they are leathery or brittle. Leathery vegetables will be pliable and spring back if folded. Edges will be sharp. Corn and peas will shatter when hit with a hammer.
Fruits are adequately dried when moisture cannot be squeezed from them, and if they are tough and pliable when cut. Fruit leathers may be slightly sticky to the touch but should separate easily from the plastic wrap.
Meats should be extremely dry unless they are to be refrigerated or frozen for long-term storage. Meat is sufficiently dried when it is dark-colored and fibrous and forms sharp points when broken.
Herbs are dried when brittle. Their leaves shatter when rubbed together.
Even when a food tests dry, it may not be uniformly dry. Also, there is a chance of contamination of dried foods, especially if racks have been exposed to the open air for any time. Therefore, conditioning and pasteurizing should be done before storing.
Conditioning is the process used to equalize, or evenly distribute, moisture left in the food after drying. It is usually done to fruits, herbs and seeds to improve storage, because it decreases the chance of spoilage, especially by molds. To condition a food, follow these steps:
- Cool foods on trays.
- Pour into a large, nonporous container of food-grade material; fill to about two-thirds full.
- Cover container and place in a convenient, warm, dry place. Shake container daily or stir contents at least once a day for 10–14 days.
- Check for condensation on the lid and any signs of spoilage. If condensation occurs, return food to the dryer to finish the product. Recondition after it is dry.
- Cool thoroughly before packaging.
If any sign of mold growth occurs, destroy the product.
Freshly dried fruit can be added to the conditioning batch within the first five days. Conditioning time will need to be lengthened to accommodate the additional food.
Pasteurizing is recommended for foods that have been contaminated before or during storage. It can be used as a second treatment for vegetables held in storage if the vegetables do not have any mold on them. Keep in mind that this treatment can cause quality changes. To pasteurize, use one of the two following methods:
- Freezer—Seal dried food in a heavy, plastic bag after drying (and conditioning, if necessary). Place in a freezer at zero degrees F for a minimum of 48 hours.
- Oven—Place the food in a single layer on a tray or in a shallow pan. Place in an oven, preheated to 160 degrees F, for 30 minutes. Cool and package for storage.
Packaging dried foods
Package dried foods in glass jars, food-grade plastic storage containers, or plastic food-storage bags. Make sure the package has an airtight seal. It is a good idea to package dried foods in small amounts, because after the package is opened, the food can absorb moisture from the air and quality deteriorates.
Storing dried foods safely
Store containers of dried foods in a cool, dark, dry area, such as a basement or cellar. Exposure to humidity, light or air decreases the shelf life of foods. The lower the temperature, the better: Foods stored at temperatures under 60 degrees F will keep about one year. At 80–90 degrees F, the food begins to deteriorate within several months. The shelf life of fruits increases three to four times for every 18 degrees F drop in temperature.