Pressure Canning of Vegetables and Meat
Safely Canning Low Acid Foods
by: Val Hillers, Ph.D. , Extension Food Specialist
Foods that are low in acid require the high heat of a pressure canner to be safely canned at home. These are the highest risk foods for home canning because there is a potential of botulism poisoning if the canning process is not done correctly. The following are low-acid foods:
- meats, poultry, seafood
- legumes such as beans
- mixtures which contain any of the above foods, unless the recipe is developed as a high-acid recipe and is tested in a research laboratory for acidity.
Before you being canning any low-acid food, obtain up-to-date canning instructions from Washington State University or another source of the current U.S. Department of Agriculture home canning recommendations. Follow these directions exactly -- it is important that you make no changes. WSU home canning bulletins are available online,click here for a listing.
Low-acid foods (meat, fish, poultry, vegetables) not canned according to USDA recommendations should be destroyed.
Low acid foods have a pH of 4.6 or higher. All low-acid foods must be canned using a temperature higher than boiling water (212°F) to destroy the bacteria that causes botulism, so theymust be processed in a pressure canner at 240° to 250°F. (A canner operated at 10.5 pounds per square inch of pressure (PSI) at sea level obtains an internal temperature of 240°F.)
Canner pressuresmust be increased above sea level. At 5,000 feet, a pressure of 12 1/2 pounds is required to reach 240°F.
The time needed to destroy bacteria in low-acid canned food ranges from 20 to 120 minutes. The exact time depends on the kind of food being canned, the way it is filled into jars, and the size of the jars.
As a safety precaution, properly canned, home-canned foods can be boiled before eating to be certain any botulism toxin that might be present is destroyed. To assure that every part of the food reaches the boiling temperature, heat food for 10 minutes at full rolling boil at altitudes below 1,000 feet. Add an additional minute of boiling for each additional 1,000 feet elevation.
Using Pressure Canners
Follow the directions carefully that come with your pressure canner. You can get additional information in the bulletinUsing and Caring for Your Pressure Canner -PNW 421, which is availablehere.
When using a pressure canner be certain to follow all directions for processing time, size of jar, and elevation adjustment (if you live above 1000 feet elevation). Also exhaust the air for 10 minutes from the pressure canner before the pressure builds up in the canner
Two serious errors in temperatures obtained in pressure canners occur because:
- Internal canner temperatures are lower at higher altitudes. To correct this error, canners must be operated at increased pressure as given in recommendations for processing food at high altitudes.
- Air trapped in a canner lowers the temperature and results in under-processing. The highest volume of air trapped in a canner occurs in processing raw-packed foods in dial-gauge canners. These canners do not vent air during processing. To be safe, all types of pressure canners must be vented for 10 minutes before they are pressurized, regardless of what their instructions may say.
To vent a canner, leave the vent port uncovered on newer models or manually open the petcocks on some older models. Heating the filled canner with its lid locked in place boils water and generates steam that escapes through the petcock, vent tube, or port. Vent steam 10 minutes and then close the petcock or place the counterweight or weighted gauge over the vent tube 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 more accurately and do not need to be watch during processing nor checked for accuracy. As the weight rocks or jiggles, it makes an audible sound to remind you that it is maintaining the recommended pressure and needs no further attention until the load is processed for the set time. The single disadvantage of weighted-gauge canners is that they cannot correct precisely for higher altitudes. They must be operated at canner pressures of 15 instead of 10 at altitudes above 1,000 feet.
Dial gauges need to be checked for accuracy before use each year andreplaced if they read high by more than 1 pound at 5, 10, or 15 pounds of pressure. Low readings cause overprocessing and may indicate that the accuracy of the gauge is unpredictable.
Using Pressure Canners
- Add two to three inches of hot water to the canner. Place filled jars on the rack, using a jar lifter. If two layers of jars are put in, use a second rack between them, staggering the second layer. Fasten canner lid securely.
- Leave weight off vent port or open petcock. Heat at the highest setting until steam flows from the petcock or vent port.
- Maintain high heat setting and exhaust steam for 10 minutes, then place weight on vent port or close petcock. The canner will pressurize during the next three to five minutes.
- Start timing the process when the pressure reading on the dial gauge indicates the recommended pressure is reached or when the weighted gauge begins to jiggle or rock.
- Regulate heat under the canner to maintain a steady pressure at or slightly above the correct gauge pressure. Quick and large pressure variations during the processing often cause unnecessary liquid losses from jars.
- The correct gauge pressure must be maintained for the entire processing time. If pressure drops below the required level, turn up the heat and bring the canner up to the target pressure level. Reset the timer for the full processing time; continue processing at the required pressure for an additional 30 or 40 minutes.
- When the timed process is completed, turn off the heat, remove the canner from heat (if possible), and let the canner depressurize as it cools to room temperature. Do not force-cool the canner because the "cool-down time" is an essential part of the processing time. The food may be underprocessed if the canner is cooled by running water over it, laying wet towels over it, or opening the vent port before the canner depressurizes by itself. In addition, liquid will spurt from jars, causing low liquid levels and jar seal failure. Forced cooling may also warp the canner lid, causing steam leaks.
Depressurization of older models should be timed. Standard-size, heavywalled canners require about 30 minutes when loaded with pints and about 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.
- After the vent port or petcock has been open for two minutes, unfasten the lid and remove it carefully. Lift the lid away from you so that the steam does not burn your face.
- Remove the jars with a lifter and place on a towel or cooling rack.
Half-gallon jars should never be used for pressure canning foods. Some foods should only be canned in pint jars (or jars smaller than pints) because these foods pack firmly in jars and the penetration of heat into these foods is slow. Foods that should not be canned in quart jars are fish, creamed corn, mushrooms and chili peppers.
Purchasing a new or used pressure canner
Before you buy a pressure canner, check to see that it has all these features:
____ rack for jars
____ lid which can be locked or clamped down to make kettle steam tight
____ accurate pressure gauge which will produce steam at 10 and/or 15 pounds pressure per square inch
____ a gasket around the rim of the lid to keep steam from leaking
____ a vent port which can be opened or closed to permit exhausting of air from the pressure canner
____ replacement parts are available from the company
If you purchase a canner with a dial gauge, remember that the gauge needs to be tested on a yearly basis.