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Carter County OSU Extension |
Principles of
Home Canning
Why Can Foods?
Canning can be a safe and economical way to preserve quality food at home. Many vegetables begin losing some of their vitamins when harvested. Nearly half the vitamins may be lost within a few days unless the fresh produce is cooled or preserved.
How Canning Preserves Food
The high percentage of water in most fresh foods makes them very perishable. They spoil or lose their quality for several reasons:
* growth
of undesirable microorganisms-bacteria, molds, and yeasts
* activity of food
enzymes,
* reactions with
oxygen,
* moisture loss.
Proper canning practices include:
*
carefully selecting and washing fresh food,
* peeling some
fresh foods,
* hot packing many
foods,
* adding acids
(lemon juice or vinegar) to some foods,
* using acceptable
jars and self-sealing lids,
* processing jars
in a boiling-water or pressure canner for the correct period of time.
Collectively, these practices remove oxygen; destroy enzymes; prevent the growth of undesirable bacteria, yeasts, and molds; and help form a high vacuum in jars. Good vacuums form tight seals which keep liquid in and air and microorganisms out.
Food Acidity and Processing Methods
Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods contain too little acidity to prevent the growth of these bacteria. Acid foods contain enough acidity to block their growth, or destroy them more rapidly when heated. The term "pH" is a measure of acidity; the lower its value, the more acid the food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.
Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.
Ensuring High-Quality Canned Foods
Begin with good-quality fresh suitable for canning. Discard diseased and moldy food. Trim small diseased lesions or spots from food.
Can fruits and vegetables picked from your garden or purchased from nearby producers when the products are at their peak of quality--within 6 to 12 yours after harvest for most vegetables.
Maintaining Color and Flavor in Canned Food
To maintain good natural color and flavor in stored canned food, you must:
* Remove
oxygen from food tissues and jars,
* Quickly destroy
the food enzymes,
* Obtain high jar
vacuums and airtight jar seals.
* Use the hot-pack
method, especially with acid foods to be processed in boiling water.
* While preparing a
canner load of jars, keep peeled, halved, quartered, sliced, or diced apples,
apricots,
nectarines, peaches, and pears in a solution of 3 grams ascorbic acid to 1
gallon of cold
water. This procedure is
also useful in maintaining the natural color of mushrooms and potatoes,
and for
preventing stem-end discoloration
in cherries and grapes. You can get ascorbic acid in several forms:
Pure powdered form--seasonally available among canners' supplies in supermarkets. One level teaspoon of pure powder weighs about 3 grams.
Commercially prepared mixes of ascorbic and citric acid--seasonally available among canners' supplies in supermarkets.
Advantages of Hot-Packing
Many fresh foods contain from 10 percent to more than 30 percent air. How long canned food retains high quality depends on how much air is removed from food before jars are sealed.
Raw-packing is the practice of filling jars tightly with freshly prepared, but unheated food. Such foods, especially fruit, will float in the jars. Raw-packing is more suitable for vegetables processed in a pressure canner.
Hot-packing is the practice of heating freshly prepared food to boiling, simmering it 2 to 5 minutes, and promptly filling jars loosely with the boiled foods. Whether food has been hot-packed or raw-packed, the juice, syrup, or water to be added to the foods should also be heated to boiling before adding it to the jars.
Hot-packing is the best way to remove air and is the preferred pack style for foods processed in a boiling-water canner.
Controlling Headspace
The unfilled space above the food in a jar and below its lid is termed headspace. Directions for canning specify leaving 1/4-inch for jams and jellies, 1/2-inch for fruits and tomatoes to be processed in boiling water and from 1- to 1 1/4-inches in low-acid foods to be processed in a pressure canner. This space is needed for expansion of food as jars are processed, and for forming vacuums in cooled jars.
Jars and Lids
Regular and wide-mouth Mason-type, threaded, home-canning jars with self-sealing lids are the best choice. They are available in 1/2 pint, 1 1/2 pint, quart, and 1/2 gallon sizes. The standard jar mouth opening is about 2 3/8 inches. Wide-mouth jars have openings of about 3 inches, making them more easily filled and emptied. Regular-mouth decorator jelly jars are available in 8 and 12 ounce sizes. With careful use and handling, Mason jars may be reused many times, requiring only new lids each time.
Jar Cleaning
Before every use, wash empty jars in hot water with detergent and rinse well by hand, or wash in a dishwasher. Un-rinsed detergents may cause unnatural flavors and colors.
Sterilization of Empty Jars
All jams, jellies, and pickled products processed less than 10 minutes should be filled into sterile empty jars. To sterilize empty jars, put them right side up on the rack in a boiling -water canner. Fill the canner and jars with hot (not boiling) water to 1 inche above the tops of the jars. Boil 10 minutes at altitudes of less than 1,000 ft. Remove and drain hot sterilized jars one at a time. Save the hot water for processing filled jars. Fill jars with food, add lids, and tighten screw bands.
Empty jars used for vegetables, meats, and fruits to be processed in a pressure canner need not be pre-sterilized. It is also unnecessary to pre-sterilized jars for fruits, tomatoes, and pickled or fermented foods that will be processed 10 minutes or linger in a boiling-water canner.
Lid Selection, Preparation, and Use
The common self-sealing lid consists of a flat metal lid held in place by a metal screw bad during processing. Gaskets in unused lids work well for at least 5 years from date of manufacture.
Buy only the quantity of lids you will use in a year. Do not use old, dented, or deformed lids, or lids with gaps or other defects in the sealing gasket.
After filling jars with food, release air bubbles by inserting a flat plastic (not metal) spatula between the food and the jar. Adjust the headspace and then clean the jar rim (sealing surface) with a dampened paper towel. Place the lid, gasket down, onto the cleaned jar-sealing surface. Unclean jar-sealing surfaces may cause seal failures.
Then fit the metal screw bad over the flat lid. Do not re-tighten lids after processing jars. As jars cool, the contents in the jar contract, pulling the self-sealing lid firmly against the jar to form a high vacuum.
*
If rings are too loose, liquid may escape from jars during processing, and seals
may fail.
* If rings are too
tight, air cannot vent during processing, and food will discolor during
storage,
Over-tightening
also may cause to buckle and jars to break, especially with raw-packed,
pressure-processed food.
Screw bands are not needed on stored jars.
Recommended Canners
Equipment for heat-processing home-canned food is two main types--boiling-water canners and pressure canners. Pressure sauce pans with smaller volume capacities are not recommended for use in canning.
Low-acid foods must be processed in a pressure canner to be free of botulism risks. Although pressure canners may also be used for processing acid foods, boiling-water canners are recommended for this purpose because they are faster.
Boiling-Water Canners
These canners are made of aluminum or porcelain-covered steel. They have removable perforated racks and fitted lids. The canner must be deep enough so that at least 1 inch of briskly boiling water will be over the tops of jars during processing. Some boiling-water canners do not have flat bottoms. A flat bottom must be used on an electric range. Either a flat or ridged bottom can be used on a gas burner.
Using Boiling-Water Canners
Follow these steps for successful boiling-water canning:
1.
Fill the canner halfway with water.
2. Preheat water to
140 degrees F for raw-packed foods and to 180 degrees F for hot-packed
foods.
3. Load filled
jars, fitted with lids, into the canner rack and use the handles to lower the
rack into
the water; or fill
the canner, one jar at a time, with a jar lifter.
4. Add more boiling
water if needed, so the water level is at least 1 inch above jar tops.
5. Turn heat to its
highest position until water boils vigorously.
6. Set a timer for
the minutes required for processing the food.
7. Cover with the
canner lid and lower the heat setting to maintain a gentle boil throughout the
process schedule.
8. Add more boiling
water, if needed, to keep the water level above the jars.
9. When jars have
been boiled for the recommended time, turn off the heat and remove the
canner
lid.
10. Using a jar lifter
remove the jars and place them on a towel, leaving at least 1-inch spaces
between the jars
during cooling.
Pressure Canners
Pressure canners for use in the home have been extensively redesigned in recent years. Models made before the 1970's were heavy-walled kettles with clamp-on or turn-on lids. They were fitted with a dial gauge, a vent port in the form of a petcock or counterweight, and a safety fuse. Modern pressure canners are lightweight, thin-walled kettles; most have turn-on lids. They have a jar rack, gasket, dial or weighted gauge, an automatic vent/cover lock, a vent port (steam vent) to be closed with a counterweight or weighted gauge, and a safety fuse.
Pressure does not destroy microorganisms, but high temperatures applied for an adequate period of time do kill microorganisms.
To vent a canner, leave the cent port uncovered on newer models or manually open petcocks on some older models. Heating the filled canner with its lid locked into place boils water and generates steam that escapes through the petcock or vent port. When steam first escapes, set a timer for 10 minutes. After venting 10 minutes, close the petcock or place the counterweight or weighted gauge over the vent port to pressurize the canner.
Weighted-gauge models exhaust tiny amounts of air and steam each time their gauge rocks or jiggles during processing. They control pressure precisely and need neither watching during processing nor checking for accuracy.
Check dial gauges for accuracy before use each year and replace if they read high by more than 1 point at 5, 10, or 15 pounds of pressure.
Handle canner lid gaskets carefully and clean them according to the manufacturer's directions. Nicked or dried gaskets will allow steam leaks during pressurization of canners. Gaskets on older model canners may require a light coat of vegetable oil once per year. Gaskets on newer model canners are pre-lubricated and do not benefit from oiling.
Using Pressure Canners
Follow these steps for successful pressure canning:
1.
Put 2 to 3 inches of hot water in the canner. Place filled jars on the
rack, using a jar lifter.
Fasten canner lid
securely.
2. Leave weight off
vent port or open petcock. Heat at the highest setting until steam flows
from
the petcock or
vent port.
3. Maintain high
beat setting, exhaust steam 10 minutes, then place weight on vent port or close
petcock. The canner will pressurize during the next 3 to 5 minutes.
4. Start timing the
process when the pressure reading on the dial gauge indicates that the
recommended pressure
has been reached, or when the weighted gauge begins to jiggle or rock.
5. Regulate heat
under the canner to maintain a steady pressure at or slightly above the correct
gauge pressure.
Quick and large pressure variations during processing may cause unnecessary
liquid losses from jars. Weighted
gauges on Mirro canners should jiggle about 2 or 3 times per
minute. On
Presto canners, they should rock
slowly throughout the process.
6. When the timed
process is completed, turn off the heat, remove the canner form heat if
possible,
and let the
canner depressurize. Do not force-cool the canner.
Forced cooling may result in
food spoilage. Cooling the
canner with cold running water or opening the vent port before the
canner is
fully depressurized will cause loss of
liquid from jars and seal failures. Force-cooling
may also warp the canner
lid of older model canners, causing
steam leaks. Standard size heavy-
walled canners require about 30 minutes
when loaded with pints and 45
minutes with quarts.
Newer thin-walled canners cool more rapidly and are
equipped with vent locks. These
canners are depressurized when their vent lock piston drops to a normal
position.
7. After the canner
is depressurized, remove the weight from the vent port or open the
petcock.
Wait 2 minutes,
unfasten the lid, and remove it carefully. Lift the lid away from you so
that the
steam does not burn your face.
8. Remove jars with a lifter,
and place on towel or cooling rack, if desired.
Select the Correct Processing
Time
When canning in boiling water, more processing time is needed for most raw-packed foods and for quart jars than is needed for hot-packed foods and pint jars.
To destroy microorganisms in acid foods processed in a boiling-water canner, you must:
*
Process jars for the correct number of minutes in boiling water.
* Cool the jars at
room temperature.
To destroy microorganisms in low-acid foods processed with a pressure canner, you must:
*
Process the jars using the correct time and pressure specified for your
altitude.
* Allow canner to
cool at room temperature until it is completely depressurized.
The food may spoil if you fail to select the proper process times for specific altitudes, fail to exhaust canners properly, process at lower pressure than specified, process for fewer minutes than specified, or cool the canner with water.
Cooling Jars
when you remove hot jars from a canner, do not retighten their jar lids. Retightening of hot lids may cut through the gasket and cause seal failures. Cool the jars at room temperature for 12 to 24 hours. Jars may be cooled on racks or towels to minimize heat damage to counters. The food level and liquid volume of raw-packed jars will be noticeably lower after cooling. air is exhausted during processing and food shrinks. If a jar loses excessive liquid during processing, do not open it to add more liquid. Check for sealed lids as described below.
Testing Jar Seals
After cooling jars for 12 to 24 hours, remove the screw bands and test seals with one of the following options:
Option
1: Press the middle of the lid with a finger or thumb.
If the lid springs up when you release
your finger the
lid is unsealed.
Option
2: Tap the lid with the bottom of a teaspoon. If it
makes a dull sound, the lid is not sealed.
If food is in
contact with the underside of the lid, it will also cause a dull sound. If
the jar
is sealed correctly, it will
make a ringing, high-pitched sound.
Option
3: Hold the jar at eye level and look across the lid.
The lid should be concave (curved
down slightly in the
center). If center of the lid is either flat or bulging, it may not be
sealed.
When preparing foods for home canning, select new
researched based recipes. These may be
obtained by contacting your local OSU Cooperative Extension Office.
