Food Safety Feature Articles
Janet Hackert, Regional Nutrition and Health Education Specialist, Harrison County, University of Missouri Extension
With food prices on the rise, many people are becoming more interested in growing and preserving their own food. More and more are canning and freezing, while others are discovering, or rediscovering, the science of drying foods. The first dehydrator was introduced in France in 1795. Ever since, people have been storing home-dried food in a cool, dry, dark place for long periods and keeping it safe to eat.
Dehydrating removes the water from food, which is how properly dried and stored foods last for so long. Molds, yeast and bacteria that make food spoil quickly need water to live and grow. With lower moisture content, these microorganisms cannot survive.
As with most processing methods, drying foods affects the nutritional value of the food. The heat and air used in dehydration destroys both vitamins A and C. Blanching vegetables before drying stops the enzyme action that causes the produce to decay. Blanching also reduces the amount of water soluble nutrients like vitamin C, B vitamins and some minerals. However, blanching helps maintain levels of vitamin A, vitamin C and thiamin during the drying process and storage. With most of their water removed, the nutrients in dried foods are more concentrated, and are higher in calories and fiber per weight compared to their fresh counterparts.
Foods can be dried in the sun or in a solar drier, but using an oven or electric dehydrator is more reliable than depending on the weather. Oven drying is a great way to try food dehydration because it involves little, if any, added equipment. It is not, however, a very efficient way to dry foods. Those wanting to do more preservation may want to invest in an electric dehydrator designed specifically for this task.
An electric dehydrator uses warm air and good air circulation to remove moisture from food. A drying temperature of 140 degrees F is recommended. Dehydrators can dry fruit, vegetables, meat and herbs. Drying times vary from a few hours to a full day. Times depend on the moisture content, amount of food, room temperature and humidity.
Selecting a dehydrator
Perhaps the most convenient and efficient way to dry food is with a commercial dehydrator. The National Center for Home Food Preservation suggests looking for the following features in a dehydrator:
- Double wall construction of metal or high grade plastic
- Enclosed heating elements for safety
- Countertop design for ease of use
- An enclosed thermostat from 85 to 160 degrees F
- Fan or blower to move moist air away from drying food
- Four to 10 open mesh trays made of sturdy, lightweight plastic for easy washing
- UL seal of approval
- One-year guarantee
- Dial for regulating temperature
- Timer so dehydrator will shut off when done, even if unattended, to prevent scorching
Some produce works better than others. Here are some of those that are considered excellent choices for drying:
- Citrus peel
Others good for drying:
- Green beans
And some are just not recommended for drying:
- Berries with seeds
- Citrus fruits
- Winter squash
As with any preservation method, use high quality food when drying. Food does not increase in quality with preservation and storage, so starting with the best produce results in the best quality dried food.
For fruits that have some bruises or are overripe, fruit leather would be the solution. The fruit is processed into a sauce before drying, so the appearance of the individual pieces of fruit is not as important and blemishes can be removed from the mix.
Each fruit or vegetable has its own recommendations for preparation, but generally it is best to start by making sure the fruits or vegetables are clean. Remove core, seeds, stem or other parts that would not typically be eaten. Whether a particular fruit or vegetable is peeled or not depends on the item and on personal preference for the final product.
Many fruits and vegetables can be dried either whole, chunked or sliced, again depending on shape preference. Keep in mind that the bigger the size of the piece, the longer the drying time needed.
The next step in drying fruits and vegetables is a pretreatment. For vegetables, this involves blanching to destroy the enzymes in these plant products. This reduces loss of color and flavor during drying and storage.
Fruits have several options for pretreatment, including sulfuring or using a sulfite dip, soaking in ascorbic acid, using a honey dip or syrup, or steam blanching. Sulfuring requires special equipment that allows sulfur fumes to penetrate the fruit. Using a sulfite dip is a little easier, but with both methods one ends up with a product with sulfur residues that can have an adverse effect on asthma sufferers. When a sweetened dip or pretreatment is used, a sweeter product results, but sugar content and calories also increase.
Dipping fruit in a liquid containing ascorbic acid is perhaps the most common pretreatment. Mix 1 teaspoon of powdered ascorbic acid in 2 cups of water. Crystalline ascorbic acid can be purchased where canning supplies are available. Vitamin C is ascorbic acid, so these vitamin tablets can be crushed and used — six tablets of 500 mg is equivalent to the 3000 mg of ascorbic acid found in a teaspoon. Soak fruit for drying in the mixture for three to five minutes. Remove, drain and lay on dehydrator’s drying racks to dry.
Ascorbic acid is also found in lower concentrations in other products. Commercial mixtures sold for keeping fruit fresh for canning or freezing contain ascorbic acid and sugar. These can also be used, adding 1½ tablespoons to 1 quart of water. Then soak, drain and dry as with the pure ascorbic acid.
Fruit juices, like orange, pineapple, lemon, grape and cranberry — ones that are high in vitamin C — can also be used as a pretreatment. These add sugar to the final product along with the flavor of the juice itself. Again, soak the fruit in the fruit juice for three to five minutes, drain and lay out on trays to dry.
With all these ascorbic acid pretreatments, use the dipping liquid twice then add more ascorbic acid, ascorbic acid mixture, or high vitamin C content fruit juice, whichever method is being used.
Place fruit or vegetable pieces in a single layer on drying trays. Do not let pieces touch or overlap. Spreading the pieces out slightly allows for good air circulation around each one, making the most of the heat circulating in the dehydrator. Here are some examples of drying times for commonly dried fruits and vegetables:
- Apple slices or rings: 6 to 12 hours
- Apricot halves: 24 to 36 hours
- Pear halves: 24 to 36 hours (slices take less time)
- Peppers, 1-inch squares: 8 to 12 hours
- Tomato slices or sections: 10 to 18 hours
Examine a sampling of items to determine dryness. For vegetables, the final product should be at about 10 percent moisture content — this means they will be brittle or crisp. Fruits are generally not reconstituted like most vegetables are, so they are not dried to the same extent. Fruits will be at about 20 percent moisture content when dried — this means that a piece that is cooled will not show any wetness when cut in half. At this point, it should not be sticky.
Let fruit cool for 30 to 60 minutes after drying and before packaging to prevent moisture from building up inside the package. Use containers that are both air-tight and moisture-tight.
Conditioning fruit helps even out the moisture content within a package,
preventing mold growth within that package. Because fruit has a higher
moisture content, and because it is not likely to be even from tray
to tray and from one part of the tray to another, condition fruit before
To do this, loosely pack dried and cooled fruit in plastic or glass jars. Seal them and let stand for seven to 10 days. Shake the jars every day — this helps keep the pieces separate. It also helps distribute the moisture within the jar and among the fruit pieces. If condensation shows up in the jar, the moisture level is too high — return fruit to dehydrator and dry fruit again, then condition it again.
From the time the fruit or vegetable pieces are dry until the time they are finally stored, be careful not to let too much moisture back in. This can come from condensation in the package, water on a work surface or moisture from the air. A less humid day is preferable when packaging dried fruits and vegetables.
There are several good resources for more information on drying foods
at home. University of Missouri Extension has the following guides:
Drying Foods GH1562
How to Dry Foods at Home GH1563
How to Use Dried Foods GH1564
These can be picked up from a local MU Extension office or found online at http://extension.missouri.edu/publications/. For more information on drying produce, visit your local MU Extension office, or contact Janet Hackert at 660-425-6434 or HackertJ@missouri.edu.
University of Georgia Cooperative Extension Service has published a comprehensive book on drying foods, as well as other food preservation methods, called So Easy to Preserve. The same information is available online at http://www.uga.edu/nchfp/.
Last update: Tuesday, August 26, 2014