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. Dried 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. Pre-treating with heat, ascorbic acid, or sulfur dioxide can stop
enzyme action and oxidation. Some 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 -- the 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 the 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),
enclosed heating 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. The 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 the 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 4 hours to
3 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. I 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 bacterial
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. Packaging 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 in their eggs by
freezing below zero 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 drier pieces. Shake the jars daily to separate the pieces,
and check the moisture condensation. If condensation develops -- like it has in
this jar -- then we return the fruit to the dehydrator for more drying.
After conditioning, package and store the dried fruit.
If condensation develops in the jar, return the fruit to the dehydrator for
more drying. After conditioning, package and store the 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 press-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 1 month at
room temperature in a cool, dry, dark area. For storage up to 1 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 2 cups of water for each cup of dried food. I have 1 cup of
water here for about 1/2 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. Just make sure there is enough liquid in the soup
for that rehydration. We have discussed different methods of dehydrating
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 food you have dehydrated is dry enough. Now it's your turn to dry food
for your eating pleasure.