Freezing is one of the easiest, most convenient, and least time-consuming
methods of preserving foods. Freezing keeps the natural color, the fresh flavor,
and nutritive qualities of most foods better than any other known methods of preservation.
Although freezing does not sterilize food,
the extreme cold slows down the growth of microorganisms as well as the
chemical changes that affect quality or cause food to spoil. Clostridium
botulinum, the microorganism that causes botulism, the greatest problem in canning
low-acid foods, does not grow and produce toxin at zero degrees Fahrenheit.
Therefore, freezing provides a safe and easy alternative to pressure canning low-
acid foods. If no home canning recommendations exist for a particular
product, freezing is always a safe option. I am Sonya Koukel with the Cooperative
Extension Service, University of Alaska Fairbanks. This module provides
guidelines on the use of freezing to safely preserve vegetables and fruits.
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.
To maintain top quality, frozen vegetables and fruits should be stored
at zero degrees Fahrenheit or lower. Most refrigerator-freezer combinations and
separate chest-style freezer units should have no trouble in maintaining this temperature.
A freezer thermometer can help you determine the actual
temperature of your freezer. These thermometers are relatively inexpensive
and generally available at hardware or grocery stores. If your freezer has
numbered temperature settings, such as from 1 to 9, check the owner's
manual to see what settings are recommended for different uses.
Storing frozen foods at temperatures higher than zero degrees Fahrenheit
increases the rate at which deterioration can take place and can
shorten the shelf life of the foods. For best results, products should be frozen
as quickly as possible. To facilitate more rapid freezing, set the temperature
control of your freezer at the coldest setting several hours before foods will
be placed in the freezer. Some freezer manuals indicate the location of the
coldest shelves in the freezer and suggest placing products on these shelves.
Then, prior to loading the freezer, again check the freezer manual
for instructions on recommended amounts of unfrozen product which can be frozen
at one time. Generally, these guidelines specify 2 to 3 pounds of food to
each cubic foot of freezer space per 24 hours. Overloading the freezer with
unfrozen products results in a long, slow freeze and a poor-quality product.
Properly packaging foods for freezing is important in order to protect their
flavor, color, moisture, and nutritional value from the dry climate of the freezer.
To prevent loss of quality and nutrients, freezing must be rapid.
Therefore, it is best not to freeze fruits and vegetables in containers with
a capacity over 1/2 gallon as the foods freeze too slowly to result in a satisfactory product.
The selection of containers depends on the type of food to be frozen,
personal preference, and availability. There are two types of
packaging materials for home use: rigid containers and flexible freezer
bags or wrappings. Rigid containers are made of plastic or glass and are
suitable for all types of packs. They're especially good for liquid packs. They're
often reusable and their design makes stacking of foods easier.
Covers for rigid containers should fit tightly.
If they don't, freezer tape can be used to reinforce the seal.
When using glass jars, choose the wide-mouth, dual-purpose jars
made for freezing and canning. These jars have been tempered to withstand extremes
in temperatures. Dry-packed products with little or no liquid and foods with
irregular shapes can be preserved using flexible freezer bags or wrappings.
Wrapping materials include plastic freezer wrap, freezer paper, and
heavyweight aluminum foil. Plastic freezer bags are available in a variety
of sizes and also work well for liquid packs. Use bags labeled for freezer use.
Regular storage bags are thinner and suitable for short-term refrigerator
storage only. When packaging make sure to eliminate air pockets.
Dry packs and frozen vegetables may be packaged using a home vacuum sealer.
Vacuum packaging removes more air from the packages than can be pressed out by hand.
Carefully follow the directions that come with the vacuum sealer. To avoid the risk of botulism,
store vacuum-packaged foods correctly by keeping foods frozen until ready to use.
This module provides guidelines for freezing two different products,
vegetables and fruits. This first section focuses on vegetables.
Freezing cannot improve the flavor or the texture of any food, but when
properly done, it can preserve most of the quality of the fresh product.
Freezing actually consists of freezing the water contained in the food. When the
water freezes, it expands and breaks the cell walls. As a result, the texture of
the product will be much softer when the product thaws. Those products that have a
higher water content, mostly foods that are eaten raw, such as celery, will have
the most changes when they thaw. You can see the difference in textures between
this fresh celery and the celery that's been frozen and thawed.
How do you determine which vegetables freeze the best? The general rule is that the
vegetables that are best to freeze are the same ones that are best when cooked before eating.
These include carrots, broccoli, peas, and cauliflower, among others.
Texture changes due to freezing are not as evident in foods that are cooked before they are eaten
because cooking also softens the cell walls.
For freezing, the chosen vegetables should be young, tender, unwilted, and garden fresh.
If vegetables must be held before freezing,
be sure to store them in the refrigerator to prevent deterioration.
Blanching is the first step in preparing vegetables for home freezing. Enzymes in
vegetables are inactivated by blanching. The blanching process involves steaming
or scalding vegetables in boiling water for a short time. Blanching serves many
purposes in the preparation of freezing vegetables. The process stops enzyme
actions that can cause loss of flavor, color, and texture. It cleanses the
surface of dirt and organisms. It brightens the color, helps retard the
loss of vitamins, and softens the vegetables and makes them easier to pack.
For preparing the vegetables for freezing, you may use a blanching kettle,
which has a blanching basket, or a steam basket and a lid. Or you may use a large
pot with a lid and a wire basket that fits inside -- a metal colander would work as well.
And then after blanching, the vegetables will need to be cooled.
So for this step, you'll need a large bowl or pot, a colander
and plenty of fresh, cold water or ice water.
Other equipment you may need includes a knife, a vegetable peeler,
and then you'll want the freezer bags or the rigid freezer containers.
Also a waterproof marker.
Now I'll set up the kitchen for the blanching process and then we'll get started.
We'll begin the blanching process using carrots. Prepare the vegetables as I've
done here. These carrots have been washed, peeled, and the ends have been trimmed.
You'll wash them again thoroughly in cold water as I've already done --
don't let the vegetables soak in the water -- and then sort the vegetables by
size or cut them into uniform-size pieces so that each batch heats evenly.
These carrots have been cut into about 1/4 inch slices. You'll want 1 gallon of
water for every pound of prepared vegetables.
You'll bring the water to a vigorous boil on high heat. I've already got my water going here.
Place the prepared vegetables into the blanching basket, and then lower this
into the boiling water. The vegetables should be completely immersed in the water.
Place a lid on the kettle or the pot and the water should return to
boiling within 1 minute. If this is not the case, then you're using too much
vegetable for the amount of boiling water. You'll want to adjust accordingly.
Start counting blanching time as soon as the water returns to a boil. Keep the
heat on high for the recommended time. Now for these carrots
that have been cut in 1/4 inch slices, the blanching time is 2 minutes,
so set your timer for the recommended time.
Blanching time is determined by the type of vegetable being processed.
Blanching charts are available in this "Home Freezing of Vegetables" publication.
You'll notice that the blanching chart is on the back.
Or you may contact your local Cooperative Extension office.
At this point, it's important to note that over-blanching results in a cooked product
and under-blanching can stimulate enzyme activity. Either of these errors will reduce the quality
of the frozen food. We have a little over 1 minute before our carrots are ready
to be cooled. So we'll be back.
At the end of the recommended time, lift the vegetables out of the boiling water,
drain them momentarily, and then immediately plunge them into a large quantity of cold water.
A rubber spatula or a large spoon helps in getting some of those pieces of
vegetable out of this blanching basket.
This cold water should be 60 degrees Fahrenheit or below, and the vegetables
should be cooled for the same amount of time that they were heated, in this case
2 minutes, so I'll set my timer again for 2 minutes.
When processing several batches of vegetables, you'll want to change this
cooling water often or use ice cubes to keep the water cold.
When the 2 minutes are done, we'll come back to the carrots.
After cooling, lift the vegetables out of the cold water and let them drain in the colander.
Drain the vegetables thoroughly after cooling as extra moisture can
cause a loss of quality when the vegetables are frozen.
These carrots are now ready for packing.
Another method used to prepare vegetables for freezing is to steam
blanch. Here we'll use broccoli. The stalks have been peeled and then
combined with the broccoli heads. All of this has been cleaned, washed, and chopped.
Remember that you'll want to use approximately 1 pound of produce to 1
gallon of water. Turn the heating element to high, bring the water to a vigorous
boil, as I've done here, place your prepared vegetables into the steam basket,
and place this into the kettle.
The vegetables should be above the boiling water, not immersed in. Place the
lid on the kettle or pot and then return the water to boiling. Remember to start
counting the blanching time as soon as the water returns to a boil. Keep the
heat on high for the recommended time. For chopped broccoli, the blanching time
is 1 and 1/2 to 2 minutes. I'll start my timer for 2 minutes.
Some vegetables, such as pumpkin, squash, and sweet potatoes, are mashed prior to freezing.
These are sweet potatoes that we've already cooked and mashed. To cool
them, we've placed them in a shallow pan, and then we will set this into a
container of cold water. You'll want to stir occasionally to hasten the cooling.
Bring the hot product to the top and let the air and the cold water do its work.
It looks as though we have about 50 seconds left on our broccoli.
As soon as it's ready, we'll be back.
So at the end of the recommended time for the broccoli
you're going to lift them out of the boiling water -- oh, these are beautiful.
Let them drain just a little bit because, remember, these have not been down in the water.
And then we're going to put them into this cold water -- 60 degrees Fahrenheit or colder.
Now you can either do it this way or you can just place the blanching basket down
in there. Either way works. We're going to let these cool for 2 minutes, remove
them with a slotted spoon, and then once all of our prepared vegetables are
cooled, they're ready for packing.