Quality for Keeps: Drying Foods
Karla Vollmar Hughes and Barbara J. Willenberg
Department of Food Science and Human Nutrition
Of all food preservation methods, that of drying foods has received the most
widespread and enthusiastic publicity in recent years. Actually, drying is one
of the oldest methods of food preservation. Techniques have been passed from
one generation to another based on what worked and what didn't. Methods used
for drying food have become sophisticated over time. Initially, salting and
drying in the sun, an open room or on stove tops were the accepted methods.
It wasn't until 1795 that the first dehydrator was introduced, in France, for
the purpose of drying fruits and vegetables. Today, the variety of dried foods
in the marketplace has created a multimillion dollar industry. For many people,
drying food at home is a convenient way to preserve foods.
Benefits
Molds, yeast and bacteria need water to grow. When foods are sufficiently dehydrated
(dried), 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.
Nutritional value
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, B-complex
vitamins and some minerals because these are all water soluble. On the other
hand, blanching does reduce loss of vitamins A, C and thiamin during dehydration
and storage.
There are more calories in dried foods on a weight-for-weight basis because
of the concentration of nutrients. For example, 100 grams of fresh apricots
have 51 calories, while 100 grams of dried apricots have 260 calories. In general,
dried foods are not a major part of the American diet and nutrient loss is,
therefore, not a concern. Nutritive value, as well as flavor and appearance,
is best protected by low temperature and low humidity during storage.
Drying methods
Foods can be dehydrated by various means: the sun, a conventional oven, an electric
dehydrator or a microwave oven (for herbs only). Drying, 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
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. Missouri weather is not suitable for
sun or solar drying because there are few consecutive days of high temperatures
and low humidity. It is likely that the food will sour or mold before drying
is completed.
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. Also, it is difficult to maintain a low drying
temperature in the oven, and foods are more susceptible to scorching at the
end of the drying period. Oven-dried foods usually are darker, more brittle
and less flavorful than foods dried by a dehydrator.
Foods can be dried on trays in an electric dehydrator,
a self-contained unit with a heat source and ventilation system. Electric dehydrators
are used to dry foods indoors. Such dryers can be purchased or made at home
and vary in sophistication and efficiency. Although the initial investment is
fairly high for an electric dehydrator, it maintains low temperatures and uses
less energy than an oven. The quality of the product is better than with any
other method of drying. As with oven drying, there is no dependence on weather
conditions.
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 the use of fans, for oven drying). Some foods require several
hours and others may take more than a day. Prolonging drying time (by using
lower temperatures) or interrupting drying time may result in spoilage.
It is important to control air temperature and circulation 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
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 degrees Fahrenheit to 140 degrees Fahrenheit are recommended for drying fruits and vegetables. Temperatures
up to 150 degrees Fahrenheit 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 Fahrenheit.
Procedures for drying
When using the oven-drying method, it
is important to control the temperature. Use an oven thermometer to test the temperature of the oven
at its lowest setting. Many ovens cannot maintain a temperature below 200 degrees Fahrenheit. This is too hot for successfully drying food, and 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.
About 4 to 6 pounds of food can be dried in an oven at one time. Place food
on drying trays, or 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.
Place an accurate, easy-to-read thermometer on the top rack toward the back.
Preheat oven to 150 degrees Fahrenheit. For gas ovens, if temperature cannot be maintained
below 200 degrees Fahrenheit, 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 should be well ventilated, also. Oven drying,
particularly if a fan is used, should be done with caution if small children
are around.
Maintain the temperature at 140 degrees Fahrenheit. Watch the temperature even more
carefully toward the end of the drying process. To prevent scorching, lower
temperature to 120 degrees Fahrenheit if possible. Examine the food often and turn trays
frequently, removing foods as they dry.
When using a dehydrator, load food on trays in single layers so that pieces
do not overlap. This allows air to circulate through the trays. A constant temperature
of 140 degrees Fahrenheit 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 1 to 2 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.
Note
Never dry sulfured fruits in an oven or dehydrator,
because the sulfur dioxide fumes can be irritating.
Use sulfite dips, steam or water 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.
Because an electric dehydrator can be an expensive investment, choose a specific
brand or model carefully. Refer to the information in Table 1, for features
to look for and evaluate before making your investment.
When is it dry?
Judging when food is dry requires experience. It is better to overdry than to
underdry. When in doubt, continue drying for an additional 15 to 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 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 in color,
fibrous, and forms sharp points when broken.
Herbs are dried when brittle. Their leaves shatter when rubbed together.
After drying
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 period of time. Therefore, conditioning and pasteurizing should
be done before storing.
Conditioning is the process used to
equalize (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
2/3 full.
- Cover container and place in a convenient, warm, dry place. Shake container
daily or stir contents at least once a day for 10 to 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.
Note
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 0 degrees Fahrenheit for a minimum of 48 hours and
up to two weeks.
- Oven
Preheat oven to 175 degrees Fahrenheit. Spread food loosely on a 2-inch-deep
tray, and place in the oven (15 minutes for fruit and 10 minutes for vegetables).
Cool and package for storage.
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.
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 Fahrenheit will keep approximately one year, at 80 degrees Fahrenheit to 90 degrees Fahrenheit the food begins to deteriorate within several months. For every 18 degrees Fahrenheit drop in temperature, the shelf life of fruits increases three to four times.
GH1562, reviewed May 1994