<<Deborah's remarks: I offer this document directly from its source with permission without any pretty re-format.>>
no. 9.308
Drying Vegetables
by P. Kendall, P. DiPersio and J. Sofos1
Quick Facts...
Successful drying depends on heat, air dryness and air circulation.
Select vegetables to be dried at peak flavor and quality.
Blanch vegetables before drying to stop enzyme action and enhance destruction of
microorganisms.
Package dried foods in tightly sealed containers and store in a cool, dry place.
Drying is one of the oldest methods of food preservation. Drying preserves foods
by removing enough moisture from food to prevent decay and spoilage. Water content
of properly dried food varies from 5 to 25 percent depending on the food. Successful
drying depends on:
enough heat to draw out moisture, without cooking the food;
dry air to absorb the released moisture; and
adequate air circulation to carry off the moisture.
When drying foods, the key is to remove moisture as quickly as possible at a temperature
that does not seriously affect the flavor, texture and color of the food. If the
temperature is too low in the beginning, microorganisms may survive and even grow
before the food is adequately dried. If the temperature is too high and the humidity
too low, the food may harden on the surface. This makes it more difficult for moisture
to escape and the food does not dry properly.
Although drying is a relatively simple method of food preservation, the procedure
is not exact. A "trial and error" approach often is needed to decide which techniques
work best.
Nutritional Value of Dried Foods
Drying, like all methods of preservation, can result in loss of some nutrients. Nutritional
changes that occur during drying include:
Calorie content: does not change, but is concentrated into a smaller mass as moisture
is removed.
Fiber: no change.
Vitamin A: fairly well retained under controlled heat methods.
Vitamin C: mostly destroyed during blanching and drying of vegetables.
Thiamin, riboflavin, niacin: some loss during blanching but fairly good retention
if the water used to rehydrate also is consumed.
Minerals: some may be lost during rehydration if soaking water is not used. Iron
is not destroyed by drying.
For best retention of nutrients in dried foods, store in a cool, dark, dry place
and use within a year.
Drying Trays
Drying trays can be simple or complex, purchased or built. Good air circulation without
reaction between food and trays is most important. For small amounts of food and
trial runs, cheesecloth or synthetic curtain netting stretched over oven racks, cake
racks, broiler racks or cookie sheets work well. Attach with clothes pins. For large
quantities of food, use shallow wooden or plastic trays with slatted, perforated
or woven bottoms.
If preparing your own trays, do not use galvanized screening for tray bottoms. It
has been treated with zinc and cadmium, which can cause a harmful reaction when in
contact with acid foods. Other metals such as aluminum also are not advisable because
they may discolor and corrode with use. If used, line with cheesecloth or synthetic
curtain netting to keep food from touching the metal. A liner also helps keep foods
from sticking to trays and prevents pieces of food from falling through.
Wash trays in hot, sudsy water with a stiff brush. Rinse in clear water and air dry
thoroughly before and after each use. A light coat of fresh vegetable oil or nonstick
substance helps protect wood slats and makes cleaning easier.
If trays are used in an oven, they should be 1 1/2 inches smaller in length and width
than the oven dimensions to allow for good air circulation. When stacking trays,
place blocks of wood 2 inches or higher between trays.
Selecting Vegetables
Select vegetables at peak flavor and eating quality. This usually is just as they
reach maturity. Sweet corn and green peas, however, should be slightly immature so
they retain their sweet flavor before their sugars change to starch. Refer to Table
1 for fresh-to-dried ratios for a variety of produce.
Table 1: Yield of dried vegetables.
Amount purchased or picked
Amount dried product
Produce
Pounds
Pounds
Pints
Beans, lima
7
1 1/4
2
Beans, snap
6
1/2
2 1/2
Beets
15
1 1/2
3 to 5
Broccoli
12
1 3/8
3 to 5
Carrots
15
1 1/4
2 to 4
Celery
12
3/4
3 1/2 to 4
Corn
18
2 1/2
4 to 4 1/2
Greens
3
1/4
5 1/2
Onions
12
1 1/2
4 1/2
Peas
8
3/4
1
Pumpkin
11
3/4
3 1/2
Squash
10
3/4
5
Tomatoes
14
1/2
2 1/2 to 3
Source: Drying Foods at Home
, Marjorie M. Philips, Extension Service. University of Arkansas, Little Rock, Arkansas
72203.
Picking activates enzymes that cause color, flavor, texture, sugar content and nutrient
changes in vegetables. To control such changes, prepare the produce immediately after
gathering and begin processing at once. Thoroughly wash or clean produce to remove
any dirt or spray. Drain thoroughly. Shake leafy vege-tables well. Sort and discard
any food with decay, bruises or mold. Such defects may affect all pieces being dried.
Follow preparation steps outlined in Table 2.
Pretreating Vegetables to Enhance Quality and Safety
Pretreating vegetables by blanching in boiling water or citric acid solution is recommended
to enhance the quality and safety of the dried vegetables (see Table 2). Blanching
helps slow or stop the enzyme activity that can cause undesirable changes in flavor
and texture during storage. Blanching also relaxes tissues so pieces dry faster,
helps protect the products vitamins and color and reduces the time needed to refresh
vegetables before cooking. In addition, research studies have shown that pretreating
vegetables by blanching in water or citric acid solution enhances the destruction
of potentially harmful bacteria during drying, including
Escherichia coli
O157:H7,
Salmonella species and Listeria monocytogenes.
Blanching
Water blanching is recommended over steam blanching or blanching in a microwave because
water blanching achieves a more even heat penetration than the other two methods.
Plain water or water with added citric acid may be used. Citric acid acts as an anti-darkening
and anti-microbial agent. Prepare the citric acid water by stirring 1/4 teaspoon
(1 gram) of citric acid into one quart (approximately one liter) of water.
Work with small amounts so plain or citric acid water doesn't stop boiling. Watch
closely and precook as follows:
Fill large kettle half full with plain or citric acid water and bring to a boil.
Put no more than one quart of the vegetable pieces in a cheesecloth or other mesh
bag. A 36-inch cloth square gathered at the corners works well. Secure ends.
Drop vegetable bag in boiling water, making sure water covers the vegetables. Shake
bag so hot water reaches all pieces.
Start timing as soon as vegetables are in boiling water. Adjust heat to ensure continuous
boiling.
Heat for length of time shown in Table 2.
Drop bag in very cold water to cool (same time as blanched).
Drain on paper towel or cloth.
Drying Methods
Arrange pretreated vegetables on drying trays in single or thin layers, 1/2 inch
deep or less. Dry in dehydrator or oven as described below.
Dehydrator Drying.
Thermostatically controlled electric dehydrators are recommended for home food drying.
They are relatively inexpensive, convenient for drying large or small batches of
food, and easy to use. The best dehydrators have thermostatically controlled heat
settings and fans that blow warm air over the foods. Some models have a heat source
at the bottom and removable, perforated trays (for air circulation) stacked above
the heat source. Dehydrators should be used indoors in a dry, well-ventilated room.
Food on lower trays near the heat source will often dry more rapidly than food on
higher trays and, therefore, trays should be rotated throughout drying.
Oven Drying
If you do not have access to a food dehydrator, either a gas or electric oven may
be used to dry vegetables. Both require careful watching to prevent scorching. Proper
temperature and ventilation are most important in oven drying. To oven dry, preheat
oven at lowest setting (140 to 150 degrees F), then adjust the thermostat and prop
the oven door open to achieve a consistent oven temperature of 140 degrees F, and
to allow moist air to escape.
Conventional ovens may not maintain consistent temperatures at low settings. To ensure
maintenance of 140 to 150 degrees F, monitor oven temperature using a calibrated
oven thermometer. Place the oven thermometer directly on the oven rack or tray and
check it every two hours throughout drying.
Place trays of prepared food in oven. Stack trays so there is at least 3 inches of
clearance at the top and bottom of the oven and 2 1/2 inches between trays. Shift
trays, top to bottom and front to back, every half hour. Stir food often if it is
1/2-inch deep or more. Single layers need no stirring. Food scorches easily toward
the end of drying time; therefore, turn the heat off when drying is almost complete
and open the door wide for an additional hour or so.
Testing for Dryness
Foods should be dry enough to prevent microbial growth and subsequent spoilage. Dried
vegetables should be hard and brittle.
Remove a small handful of food and cool for a few minutes before testing for dryness.
When warm or hot, foods seem more soft, moist and pliable than they actually are.
See Table 2 for dryness tests for individual foods.
Post-Drying Treatment
Conditioning.
When drying is complete, some pieces will be moister than others due to size and
location during drying. Conditioning distributes residual moisture evenly in dried
food. In doing so, it reduces the chance of spoilage. See fact sheet 9.309,
Drying Fruits
, for details on how to condition dried foods. Because vegetables dry to a nearly
waterless state, conditioning them is not always necessary.
Packaging and Storing
Pack cooled, dried foods in small amounts in dry, scalded glass jars (preferable
dark) or in moisture- and vapor-proof freezer containers, boxes or bags. Metal cans
may be used if food is first placed in a freezer bag.
To protect from insects and reabsorption of moisture, seal lids onto containers.
Wrap the edge where the lid meets the container with a plasticized, pressure-sensitive
tape or clean, 1-inch cloth strip dipped in melted paraffin. Bags may be heat-sealed
or closed with twist ties, string or rubber bands.
Label containers with the name of the product, date, and method of pretreatment and
drying. Store in a cool, dry, dark place. Properly stored, dried vegetables keep
well for six to 12 months. Discard all foods that develop off smells or flavors or
show signs of mold.
Using Dried Vegetables
One cup of dried vegetables reconstitutes to about 2 cups. To rehydrate and cook
leafy or tender vegetables (spinach, kale, cabbage, chard, tomatoes), cover with
hot water and simmer to desired tenderness. Soak root, stem and seed vegetables (carrots,
green beans, peas, corn) before cooking. Cover with cold water and soak 30 to 90
minutes, or cover with boiling water and soak 20 to 60 minutes. After soaking, simmer
until tender.
Dehydrated vegetables have a unique texture and flavor. They are best used as ingredients
for soups, casseroles, sauces, stuffings and stews.
References
DiPersio, P.A., P.A. Kendall, M. Calicioglu and J.N. Sofos. (2003). Inactivation
of Salmonella during drying and storage of apple slices treated with acidic or sodium
metabisulfite solutions. J. Food Prot. 66:2245-2251.
DiPersio, P.A., P. A. Kendall, Y. Yoon and J.N. Sofos. (2004). Inactivation of Salmonella
during drying and storage of Nantes carrot slices treated with steam blanching, water
blanching or immersion in a 3.23% salt solution before drying, or oven heating after
drying. IFT Annual Meeting Poster. Abstract available at:
http://ift.confex.com/ift/2004/techprogram/paper_7122.htmDiPersio, P.A., P. A. Kendall, Y. Yoon and J.N. Sofos. (2004). Inactivation of Salmonella
during drying and storage of Nantes carrot slices treated with steam, water or acid
blanching before dehydration. Poster session: Intl. Assoc. Food Prot. Annual Meeting.
Abstract available at
www.foodprotection.org.
Table 2: Steps for drying vegetables. (See text for details.)
Vegetable
Preparation
Blanching Time* (mins.)
Drying Time (hrs.)
Dryness test
Asparagus
Wash thoroughly. Halve large tips.
4-5
6-10
Leathery to brittle
Beans, green
Wash. Cut in pieces or strips.
4
8-14
Very dry, brittle
Beets
Cook as usual. Cool, peel. Cut into shoestring strips 1/8" thick.
None
10-12
Brittle, dark red
Broccoli
Wash. Trim, cut as for serving. Quarter stalks lengthwise.
4
12-15
Crisp, brittle
Brussels sprouts
Wash. Cut in half lengthwise through stem.
5-6
12-18
Tough to brittle
Cabbage
Wash. Remove outer leaves, quarter and core. Cut into strips 1/8" thick.
4
10-12
Crisp, brittle
Carrots, parsnips
Use only crisp, tender vegetables. Wash. Cut off roots and tops; peel. Cut in slices
or strips 1/8" thick.
4
6-10
Tough to brittle
Cauliflower
Wash. Trim, cut into small pieces.
4-5
12-15
Tough to brittle
Celery
Trim stalks. Wash stalks and leaves thoroughly. Slice stalks.
4
10-16
Very brittle
Chili peppers, green
Wash. To loosen skins, cut slit in skin, then rotate over flame 6-8 minutes or scald
in boiling water. Peel and split pods. Remove seeds and stem. (Wear gloves if necessary.)
None
12-24
Crisp, brittle, medium green
Chili peppers, red
Wash thoroughly. Slice or leave whole if small.
4
12-24
Shrunken, dark red pods, flexible
Corn, cut
Husk, trim. Wash well. Blanch until milk in corn is set. Cut the kernels from the
cob.
4-6
6-10
Crisp, brittle
Eggplant
Wash, trim, cut into 1/4" slices.
4
12-14
Leathery to brittle
Horseradish
Wash, remove small rootlets and stubs. Peel or scrape roots. Grate.
None
6-10
Brittle, powdery
Mushrooms**
Scrub. Discard tough, woody stalks. Slice tender stalks 1/4" thick. Peel large mushrooms,
slice. Leave small mushrooms whole. Dip in solution of 1 tsp. citric acid/quart water
for 10 minutes. Drain.
None
8-12
Dry and leathery
Okra
Wash thoroughly. Cut into 1/2" pieces or split lengthwise.
4
8-10
Tough, brittle
Onions
Wash, remove outer paper skin. Remove tops and root ends, slice 1/8 to 1/4" thick.
4
6-10
Very brittle
Parsley, other herbs
Wash thoroughly. Separate clusters. Discard long or tough stems.
4
4-6
Flaky
Peas
Shell and wash.
4
8-10
Hard, wrinkled, green
Peppers, pimentos
Wash, stem. Remove core and seeds. Cut into 1/4 to 1/2" strips or rings.
4
8-12
Tough to brittle
Potatoes
Wash, peel. Cut into 1/4" shoestring strips or 1/8" thick slices.
7
6-10
Brittle
Spinach, greens
like Kale, Chard, mustard
Trim and wash very thoroughly. Shake or pat dry to remove excess moisture.
4
6-10
Crisp
Squash, summer or banana
Wash, trim, cut into 1/4" slices.
4
10-16
Leathery to brittle
Squash, winter
Wash rind. Cut nto pieces. Remove seeds and cavity pulp. Cut into 1" wide strips.
Peel rind. Cut strips crosswise into pieces about 1/8" thick.
4
10-16
Tough to brittle
Tomatoes
Steam or dip in boiling water to loosen skins. Chill in cold water. Peel. Slice 1/2"
thick or cut in 3/4" sections. Dip in solution of 1 tsp. citric acid/quart water
for 10 minutes.
None
6-24
Crisp
* Blanching times are for 3,000 to 5,000 feet. Times will be slightly shorter for
lower altitudes and slightly longer for higher altitudes or for large quantities
of vegetables.
** WARNING: The toxins of poisonous varieties of mushrooms are not
destroyed by drying or by cooking. Only an expert can differentiate between poisonous
and edible varieties.
1
P. Kendall, Colorado State University Extension foods and nutrition specialist and
professor, food science and human nutrition; P. DiPersio, Colorado State University
Ph.D. candidate, food science and human nutrition; and J. Sofos, Colorado State University
professor, department of animal sciences.10/98. Revised 7/04.
