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Quality for Keeps: Freezing Vegetables
Barbara J. Willenberg
Associate State Food and Nutrition Specialist
Department of Food Science and Human Nutrition
Frozen foods can add variety to your meals year-round. As with any method
of food preservation, following specific guidelines will assure you of high
quality, safe food. For additional information, refer to other Human Environmental
Sciences guides in the Quality for Keeps freezer series.
Blanching
Blanching, or scalding vegetables in boiling water or steam for a short period
of time, is a must for almost all vegetables to be frozen except onions and
green peppers. It slows or stops the action of enzymes. Up until harvest time,
enzymes cause vegetables to grow and mature. If vegetables are not blanched,
or not blanched long enough, the enzymes continue to be active during frozen
storage causing off-colors, off-flavors and toughening.
In addition, blanching cleanses the surface of dirt and spoilage organisms,
brightens the color and helps retard loss of vitamins. It also wilts or softens
vegetables and makes them easier to pack.
Blanching time is crucial and varies with the vegetable and size of the pieces
to be frozen. Under-blanching speeds up the activity of enzymes and is worse
than no blanching. Over-blanching causes loss of flavor, color, vitamins and
minerals. Follow recommended blanching times for specific vegetables.
Water blanching For home freezing, the best way to blanch vegetables is in boiling
water. Use a blancher with a basket and cover, or fit a wire basket into a
large kettle with a lid.
Use one gallon of water per pound of prepared vegetable. Using these proportions,
the water should continue to boil when vegetables are lowered into water.
Put vegetables in blanching basket and lower into vigorously boiling water.
Place a lid on the blancher. Start counting blanching time as soon as the
water returns to a boil. Keep heat high so that water continues to boil throughout
the blanching process. See recommended blanching times for specific vegetables.
Steam blanching
Heating in steam is the recommended method for grated summer squash and sprouts.
For broccoli, pumpkin, sweet potatoes and winter squash, both steaming and
boiling are satisfactory methods. Steam blanching takes about 1-1/2 times
longer than water blanching.
To steam, use a pan with a tight-fitting lid and a basket that holds the
food at least three inches above the bottom of the pan. Use 1 to 2 inches
of water in the pan, bring to a boil and leave on high throughout the blanching
process.
Place vegetables in the basket in a single layer so that steam reaches
all parts quickly. Cover the pan and start counting time as soon as the lid
is on.
Microwave blanching Research has shown that microwave blanching is not always an effective
method, as some enzymes may not be inactivated. This could result in low-quality
frozen vegetables with off-colors, off-flavors and poor texture. If blanching
is done in a microwave oven, follow individual manufacturer’s instructions.
Microwave blanching does not save time or energy.
Cooling
As soon as blanching is complete, cool vegetables quickly and thoroughly
to stop the cooking process. To cool, plunge the basket of vegetables immediately
into a large quantity of cold water. Change water frequently or use cold running
water or iced water. If ice is used, about one pound of ice for each pound of
vegetable is needed. Cool vegetables for the same amount of time as they are
blanched. Drain vegetables thoroughly after cooling. Extra moisture can cause
a loss of quality when vegetables are frozen.
Thawing and using
Follow the guidelines below to keep frozen vegetables safe and preserve their
color, flavor, texture and nutritive value:
Don’t thaw frozen vegetables before cooking (with a few exceptions).
Corn-on-the-cob should be thawed so the kernels will not be warmed while the
cob interior is still cold. In addition, greens, broccoli and asparagus will
cook more uniformly if thawed slightly and broken apart before cooking. Thaw
in the refrigerator or under cold running water. Never allow to thaw at room
temperature.
When cooking frozen vegetables, bring a small amount of water to boil and
add the frozen vegetables. Bring water to a boil again, cover the pan and lower
the heat. Cook just until vegetables are fork tender, usually about half the
cooking time for the same fresh vegetable. See Table 2 for a timetable for
cooking frozen vegetables.
Prepare only enough frozen vegetables for one meal. Any leftovers could
be used in salad. Do not refreeze cooked, frozen vegetables.
Other methods of cooking frozen vegetables include steaming, stir frying,
pressure cooking or microwaving. Frozen vegetables can be added without thawing
to soups or stews. Add them near the end of cooking to prevent texture loss.
Many frozen vegetables can be baked in a covered, greased casserole in the oven.
Partially thaw and separate pieces first. Although baking time for frozen vegetables
varies, the approximate time for baking most partially thawed vegetables is
45 minutes at 350 degrees Fahrenheit. This is not a time- or energy-efficient method,
however, unless other foods are being baked in the oven at the same time.
Freezing instructions for specific
vegetables
Asparagus
Select young tender spears. Wash thoroughly and sort into sizes. Trim stalks
and remove scales with a sharp knife. Cut into even lengths to fit containers.
Water blanch small spears 2 minutes, medium spears 3 minutes, and large
spears 4 minutes. Cool promptly, drain, package, seal and freeze.
Beans: green, snap or wax Select young tender pods when the seed is first formed. Wash in cold
water and cut into 1-inch or 2-inch pieces or slice lengthwise.
Water blanch 3 minutes. Cool promptly, drain, package, seal and freeze.
Beans: lima, butter or pinto Harvest while the seed is in the green stage. Wash, shell and sort
according to size.
Water blanch small beans 2 minutes, medium beans 3 minutes, and large beans
4 minutes. Cool promptly, drain, package, seal and freeze.
Beets Select deep, uniformly red, tender, young beets. Wash and sort according
to size. Trim tops, leaving 1/2-inch of stem and tap root, to prevent bleeding
of color during cooking
Cook in boiling water until tender -- for small beets 25 to 30 minutes;
for medium beets 45 to 50 minutes. Cool promptly in cold water. Peel, remove
stem and tap root and cut into slices, julienne strips or cubes. Package,
seal and freeze.
Broccoli Select firm, young, tender stalks with compact heads. Remove leaves
and woody portions. Separate heads into convenient-size sections and immerse
in brine (4 teaspoons salt to 1 gallon water) for 30 minutes to remove insects.
Drain and split lengthwise so flowerets are no more than 1-1/2 inches across.
Water blanch 3 minutes in boiling water or steam blanch 5 minutes. Cool
promptly, drain, package, seal and freeze.
Brussels sprouts Select green, firm and compact heads. Trim, removing coarse outer
leaves. Wash thoroughly and immerse in brine (4 teaspoons salt to 1 gallon
of water for 30 minutes) to remove insects. Sort into small, medium and large
sizes.
Water blanch small heads 3 minutes, medium heads 4 minutes, and large heads
5 minutes. Cool promptly, drain, package, seal and freeze.
Cabbage (including Chinese cabbage) Frozen cabbage or Chinese cabbage is suitable for use only as a cooked
vegetable. Select freshly picked, solid heads. Trim coarse outer leaves from
head. Cut into medium to coarse shreds or thin wedges, or separate head into
leaves.
Water blanch 1-1/2 minutes. Cool promptly, drain, package, seal and freeze
Carrots Select young, tender, coreless carrots. Remove tops, wash and peel.
Leave small carrots whole. Cut others into thin slices, 1/4-inch cubes or lengthwise
strips.
Water blanch small whole carrots 5 minutes, diced or sliced 2 minutes,
and lengthwise strips 2 minutes. Cool promptly, drain, package, seal and freeze
Cauliflower Choose compact, snow-white heads. Trim off leaves, cut head into
pieces about 1 inch across and immerse in brine (4 teaspoons salt to 1 gallon
of water) for 30 minutes to remove insects. Drain.
Water blanch for 3 minutes in water containing 4 teaspoons salt per gallon
of water. To prevent darkening, add 2 to 3 tablespoons of lemon juice per
gallon of blanching water. Cool promptly, drain, package, seal and freeze.
Celery Celery loses its crispness when frozen. The frozen product is suitable
only for cooked dishes. Select crisp, tender stalks, free from coarse strings.
Wash thoroughly, trim and cut stalks into 1-inch lengths.
Water blanch for 3 minutes. Cool promptly, drain, package, seal and freeze.
Corn Select only tender, freshly-gathered corn in the milk stage. Husk
and trim the ears, remove silks and wash.
Corn-on-the-cob Water blanch small ears (1-1/4 inches or less in diameter) 7 minutes,
medium ears (1-1/4 to 1-1/2 inches in diameter) 9 minutes, and large ears
(more than 1-1/2 inches in diameter) 11 minutes.
Cool promptly and completely to prevent a “cobby” taste. Drain,
package, seal and freeze.
Whole kernel corn Water blanch 4 minutes on the cob. Cool promptly, drain and cut
from cob. Cut kernels from cob about two-thirds the depth of the kernels.
Package, seal and freeze.
Cream style corn Water blanch 4 minutes on the cob. Cool promptly and drain. Cut
off kernel tips and scrape cobs with the back of a knife approximately half
the depth of the kernel to remove the juice and the heart of the kernel.
Package, seal and freeze.
Another way to prepare cream style corn for freezing is to cut and scrape
the corn from the cob without blanching. Place the cut corn in a double boiler
and heat with constant stirring for about 10 minutes or until it thickens;
cool by placing the pan in ice water. Package, seal and freeze.
Dill Do not wash dill. Break dill heads off stems. Place heads in rigid
containers, seal and freeze.
Eggplant Harvest before seeds become mature and when color is uniformly dark.
Wash and peel. If eggplant is to be fried, cut in 1/3-inch slices. For casseroles
or mixed vegetables, dice or cut in strips. Work quickly, preparing only enough
eggplant for one blanching.
Water blanch diced pieces or strips 2 minutes in 1 gallon of boiling water
containing 4-1/2 teaspoons citric acid or 1/2 cup lemon juice. One-third-inch
slices should be blanched for 4 minutes. Cool, drain, package, seal and freeze.
Note Slices to be fried should be packed between sheets of freezer wrap
for easy removal.
Greens (Includes beet, chard, collard, kale, mustard, spinach and
turnip greens.) Select young, tender green leaves. Wash thoroughly and cut off woody
stems. Cut leaves of chard into pieces.
Water blanch collards 3 minutes and all other greens 2 minutes. Blanch
tender young leaves 1-1/2 minutes. Cool, drain, package, seal and freeze.
Note Do not steam blanch greens.
Fresh herbs Wash, drain and pat dry with paper towels. Wrap a few sprigs of leaves
in freezer film wrap and place in a freezer bag. Seal and freeze.
Chop and use in cooked dishes. Herbs prepared in this way are usually not
suitable for garnish, as they become limp when thawed.
Kohlrabi Select young, tender, mild-flavored kohlrabi, small to medium in
size. Cut off tops and roots. Wash and peel. Leave whole or dice into 1/2-inch
cubes.
Water blanch whole kohlrabi 3 minutes and cubes 1 minute. Cool promptly,
drain, package, seal and freeze.
Okra Select young tender pods and separate into small pods (4 inches or
under) and large pods. Wash. Remove stems at the end of the seed pods, being
careful not to cut into the seed pod.
Water blanch small pods 3 minutes and large pods 4 minutes. Cool promptly
and drain. Leave whole or slice crosswise. Package, seal and freeze.
For frying. After blanching, coat okra with cornmeal or flour. Tray freeze
in a single layer on shallow trays. Place in freezer just long enough to freeze
firm. Package, seal and return to freezer immediately.
Onions Bulb onions store well in a cool, dry place, so freezing is usually
not necessary. It is not necessary to blanch mature onions before freezing.
Just dice or slice, package, seal and freeze.
Onion rings Wash, peel and slice onions. Separate into rings. Water blanch
for 10 to 15 seconds. Cool promptly, drain and coat with flour. Dip in milk.
Coat with a mixture of equal parts of cornmeal and pancake mix. Arrange in
a single layer on a tray. Freeze. Pack into containers using freezer wrap
to separate the layers. Seal and return to freezer immediately. Fry frozen
rings in 375 degree F oil until golden brown.
Peas: edible podded Select bright green, tender pods. Wash, remove stems and blossom
ends and any strings. Leave whole.
Snow peas Also called sugar or Chinese peas. Should be harvested when peas
are just barely visible in the pods. To freeze, water blanch in boiling water
for 2 minutes.
Sugar snap peas Should be picked when the pods are round and fully mature, 2 to
3 inches long. To freeze, water blanch in boiling water for 2 to 3 minutes.
Cool, drain, package, seal and freeze.
Peas: black-eyed or field Select pods when seeds are tender and well-filled. Wash, shell and
discard overmature and immature seeds and those injured by insects. Wash again.
Water blanch 2 minutes. Cool promptly, drain, package, seal and freeze.
Peas: green Harvest when pods are filled with young, tender peas that have not
become starchy. Wash and shell.
Water blanch 1-1/2 minutes. Cool, drain, package, seal and freeze.
Peppers: bell or sweet Sweet or bell peppers can be frozen without blanching. Blanched peppers
are limp and easier to pack; however, they can only be used in cooked dishes.
Select crisp, tender, green or bright red pods. Wash, cut out stems, cut in
half and remove seeds and white membrane. Cut in halves, slices, 1/2-inch strips,
rings or dice depending on intended use.
Blanched Water blanch halves 3 minutes, strips or rings 2 minutes. Cool
promptly, drain, package, seal and freeze.
Unblanched Package raw, seal and freeze.
Peppers: hot To prevent burning hands when handling hot peppers, wear rubber gloves.
Do not touch eyes. Wash peppers and peel by:
Placing in a 400 degree F to 450 degree F oven for 6 to 8 minutes. Remove
from heat and allow to stand in a wet towel to steam for 15 minutes. Remove
skin, stem and seeds.
Or blister the skin of the peppers thoroughly on a hot range or with a flame,
turning frequently to prevent scorching. Steam peppers as directed above. Slash
skin and insert knife at tapered end, pulling the skin off toward stem. Remove
stem and seeds.
Flatten whole peppers to remove air. Pack into containers. For ease in separating
when thawing, place freezer wrap between peppers. Seal and freeze.
Potatoes Because fresh potatoes are available year-round, most people do not
find it practical to freeze potatoes at home. However, if potatoes are to be
home frozen, it’s best to freeze cooked, mashed potato patties, baked
stuffed potatoes or french fries.
Baked stuffed potatoes or mashed potato patties
Prepare according to your favorite recipe, cool quickly in the refrigerator,
then wrap in moisture/vapor proof packaging. Seal and freeze. The recommended
maximum storage time is one month at 0 degrees Fahrenheit. To serve, unwrap and reheat
in a 350 degree F oven.
French fried potatoes Pare and cut potatoes lengthwise into strips about 1/4-inch thick.
Rinse quickly in cold water and dry well on paper towels.
Oven method Arrange potato strips in shallow baking pan, brush with melted
butter or margarine, and bake at 450 degrees Fahrenheit just until they begin to brown,
turning occasionally. Cool quickly in the refrigerator.
Oil method Blanch potato strips in vegetable oil heated to 370 degrees Fahrenheit until
tender but not brown. Drain and cool quickly in refrigerator.
Place prepared strips in moisture/vapor proof containers or bags. Seal
and freeze. The maximum recommended storage time is two months at 0 degrees Fahrenheit.
To serve Return frozen potatoes to baking sheet and bake at 450 degrees Fahrenheit until golden brown, turning occasionally. Or deep-fat fry frozen potatoes
at 390 degrees Fahrenheit until golden and crisp.
Pumpkin Select full-colored mature pumpkins with fine texture. Wash, cut
into cooking-size sections and remove seeds.
Cook until soft in boiling water, in steam, in a pressure cooker, in an
oven or microwave oven. To cool, place pan containing pumpkin in cold water
and stir occasionally. Remove pulp from rind and mash. Package, seal and freeze.
Note Small pumpkins can be pierced and baked whole on a tray in an oven
or microwave oven until soft. After cooling, peel, remove strings and seeds
and mash. Package, seal and freeze.
Squash: summer (Cocozelle, Crookneck, Straightneck, White scallop,
Zucchini) Choose young squash with tender skin. Wash and cut in 1/2-inch slices.
Water blanch 3 minutes. Cool promptly, drain, package, seal and freeze.
Grated zucchini (for baking) Choose young tender zucchini. Wash and grate without peeling. Steam
blanch in small quantities for 1 to 2 minutes until translucent. Drain well
and pack in containers in amounts needed for recipes. Cool by placing the
containers in cold water. Seal and freeze. If watery when thawed, drain the
liquid before using the zucchini.
Squash: winter (Acorn, Banana, Buttercup, Butternut, Golden Delicious,
Hubbard) Select firm, mature squash with a hard rind.
Prepare same as for pumpkin.
Sweet potatoes Choose medium to large sweet potatoes that have been cured for at
least one week. Sort according to size and wash.
Cook until almost tender in water, steam in a pressure cooker, oven or
microwave oven. Let stand at room temperature until cool. Peel sweet potatoes,
cut in halves, slice or mash.
If desired, to prevent darkening, dip whole sweet potatoes or slices for
5 seconds in a solution of 1 tablespoon citric acid or 1/2 cup lemon juice to
1 quart water. To keep mashed sweet potatoes from darkening, mix 2 tablespoons
orange or lemon juice with each quart of mashed sweet potatoes. Pack into containers,
seal and freeze.
Baked Wash, trim and heat potatoes in oven at 350 degrees Fahrenheit without peeling
until slightly soft. Cool, remove peel and wrap individually in aluminum foil.
Place in freezer bags and freeze. Complete the baking in an oven at 350 degrees Fahrenheit immediately before serving, leaving the potatoes wrapped in foil.
Tomatoes Select firm, ripe tomatoes with deep red color. Frozen tomatoes will
have a mushy texture when thawed and are suitable only for cooking, i.e. in
soups, stews, spaghetti sauces, etc. In addition, tomatoes that are frozen
raw become watery and develop an off-flavor after a short period in the freezer.
Tomatoes that are too ripe for safe canning, but still sound and free from
decay, can safely be frozen.
Raw Wash and dip in boiling water for 30 seconds to loosen skins. Core
and peel. Freeze whole or in pieces. Pack into containers, leaving 1-inch
headspace. Seal and freeze.
Juice Wash, sort and trim firm, vine-ripened tomatoes. Cut in quarters
or eighths. Simmer 5 to 10 minutes. Press through a sieve. If desired, season
with 1 teaspoon salt to each quart of juice. Pour into containers, leaving
1-1/2-inches headspace. Seal and freeze.
Stewed Remove stem ends, peel and quarter ripe tomatoes. Cover and cook
until tender (10 to 20 minutes). Place pan containing tomatoes in cold water
to cool. Pack into containers, leaving 1-inch headspace. Seal and freeze.
Tomato products such as sauce, puree, catsup and chili sauce can be frozen.
Prepare as usual, cool rapidly, pack into rigid containers, leaving headspace,
and freeze.
Green tomatoes Select firm, sound green tomatoes. Wash, core and slice 1/4-inch
thick. No blanching is necessary.
For frying. Pack slices into containers with freezer wrap between the
slices. Seal and freeze.
Turnips or parsnips Select small to medium, firm turnips or parsnips that are tender
and have a mild flavor. Wash, peel and cut into 1/2-inch cubes.
Water blanch for 2 minutes. Cool, package, seal and freeze.
Mashed Cut into chunks and cook until tender. Drain, mash, cool and pack
into containers. Seal and freeze.
Table 1 Approximate yield of frozen vegetables from fresh
Asparagus 1 crate (12, 2-pounds bunches) yields 15 to 22 pints
1 to 1-1/2 pounds yields 1 pint
Beans, lima (in pods) 1 bushel (32 pounds) yields 12 to 16 pints
2 to 2-1/2 pounds yields 1 pint
Beans, snap, green and wax 1 bushel (30 pounds) yields 30 to 45 pints
2/3 to 1 pound yields 1 pint
Beets (without tops) 1 bushel (52 pounds) yields 35 to 42 pints
1-1/4 to 1-1/2 pounds yields 1 pint
Squash, summer 1 bushel (40 pounds) yields 32 to 40 pints
1 to 1-1/4 pounds yields 1 pint
Squash, winter
3 pounds yields 2 pints
Sweet potatoes
23 pounds yields 1 pint
Tomatoes
1 bushel (45 pounds1) yields 26 to 34 pints
Tomatoes, for juice
1 bushel (45 pounds1) yields 20 to 28 pints
1As defined by the Missouri Department of Agriculture.
Table 2 Timetable for cooking frozen vegetables1
Time to cook after water returns to a boil.
Asparagus 5 to 10 minutes
Beans, lima Large type
6 to 10 minutes
Baby type
15 to 20 minutes
Beans, snap, green or wax 1-inch pieces
12 to 18 minutes
Julienne strips
5 to 10 minutes
Beans, soybeans, green
10 to 20 minutes
Beet greens 6 to 12 minutes
Broccoli 5 to 8 minutes
Brussels sprouts 4 to 9 minutes
Carrots 5 to 10 minutes
Cauliflower 5 to 8 minutes
Chard 8 to 10 minutes
Corn Whole-kernel
3 to 5 minutes
On-the-cob (thawed)
3 to 4 minutes
Kale 8 to 12 minutes
Kohlrabi 8 to 10 minutes
Mustard greens
8 to 15 minutes
Peas, green 5 to 10 minutes
Spinach 4 to 6 minutes
Squash, summer 10 to 12 minutes
Turnip greens
15 to 20 minutes
Turnips
8 to 12 minutes
1Use 1/2 cup lightly salted water for each pint (2 cups) of vegetables
with these exceptions: lima beans, 1 cup; corn-on-the-cob, water to cover.
Tips for successful freezing
Select varieties suitable for freezing. Check a seed catalog
or ask the grower.
Work under sanitary conditions.
Select young, tender vegetables of good quality that are
fresh from the garden. Freezing does not improve quality. Sort for size, ripeness
and color.
If the vegetables cannot be frozen immediately, refrigerate
them.
To prevent loss of quality and nutrients, work in small quantities,
enough for only a few containers at a time.
Wash and drain all vegetables before removing skins or shells.
Wash small lots at a time through several changes of cold water. Lift the produce
out of the water so the dirt washed off will not get back on the food. Do not
let the vegetables soak.
Remember that yields vary depending on the condition of the
produce as well as the preparation and packing methods used. See Table 1 for
the approximate yield of frozen vegetables from fresh.
Prepare each vegetable as directed in this guide and refer
to MU publication GH 1501, Freezing Basics, for more detailed information on
the freezing process and recommended storage times.
Types of packs
Dry pack
Dry packing is recommended for all vegetables because it results in a good
quality product and preparation for freezing and serving is easier. After
vegetables are blanched, cooled and drained, package quickly in rigid freezer
containers or freezer bags. Remove as much air as possible from bags. Leave
1/2-inch headspace for rigid containers. Seal tightly, label and freeze.
Tray pack
A variation of dry packing is tray packing. After vegetables are blanched,
cooled and drained, spread in a single layer on shallow trays and freeze.
Leave in the freezer just long enough to freeze firm. Longer exposure to dry
freezer air will result in moisture loss and quality changes. When frozen,
promptly package leaving no headspace, seal tightly, label and return to the
freezer. The advantage of tray packing is that vegetable pieces remain loose
and can be poured from the container and the package reclosed.
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