Canning basics 7: Recommended canners and their use
Equipment for heat-processing home-canned food is of two main types, boiling-water canners and pressure canners. Most are designed to hold 7 quart jars or 8 to 9 pints. Small pressure canners hold 4 quart jars. Some large pressure canners hold 18 pint jars in two layers, but hold only 7 quart jars. Pressure saucepans with smaller volume capacities are not recommended for use in canning. Small capacity pressure canners are treated in a similar manner as standard larger canners, and should be vented using the typical venting procedures.
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, it is faster to do so in boiling-water canners. A pressure canner would require from 55 to 100 minutes to can a load of jars whereas the total time for canning most acid foods in boiling water varies from 25 to 60 minutes.
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. To ensure uniform processing of all jars with an electric range, the canner should be no more than 4 inches wider in diameter than the element on which it is heated. Follow these steps for successful boiling-water canning:
- Fill the canner halfway with water.
- Preheat water to 140°F for raw-packed foods (lower temperature reduces jar breakage) and to 180°F for hot-packed foods.
- 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.
- Add more boiling water, if needed, so the water level is at least 1 inch above jar tops.
- Turn heat to its highest position until water boils vigorously.
- Set a timer for the minutes required for processing the food.
- Cover with the canner lid and lower the heat setting to maintain a gentle boil throughout the process schedule.
- Add more boiling water, if needed, to keep the water level above the jars.
- When jars have been boiled for the recommended time, turn off the heat and remove the canner lid. Wait 5 minutes before removing jars from canner to prevent liquid from boiling out of the jars.
- Using a jar lifter, remove the jars and place them on a towel, leaving at least one inch between the jars during cooling.
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Pressure canners for use in the home have been extensively redesigned in recent years. Models made before the 1970s 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 which usually 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. High temperatures applied for a certain period of time kill microorganisms. The success of destroying all microorganisms capable of growing in canned food is based on the temperature obtained in pure steam, free of air, at sea level. At sea level, a canner operated at a gauge pressure of 10 pounds provides an internal temperature of 240°F.
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 the increased pressures specified in this publication for the appropriate altitude.
- Air trapped in a canner lowers the temperature obtained at 5, 10, or 15 pounds of pressure 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 10 minutes before they are pressurized.
To vent a canner, leave the vent 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. The sound of the weight rocking or jiggling indicates that the canner is maintaining the recommended pressure and needs no further attention until the load has been processed for the set time. The single disadvantage of weighted-gauge canners is that they cannot correct precisely for higher altitudes. At altitudes above 1,000 feet they must be operated at canner pressures of 10 instead of 5, or 15 instead of 10 PSI.
Check dial gauges for accuracy before use each year. Contact your local county extension office for gauge testing times and places and replace the gauge if it reads 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.
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. Keep gaskets clean between uses. Older canner models may require a light coating of vegetable oil once a year. Newer models are pre-lubricated and do not need oiling. Check your canner's instructions if you are unsure whether or not your canner lid has been pre-lubricated.
Safety fuses found in the lid are thin metal inserts or rubber plugs designed to relieve excessive pressure from the canner. Do not pick at or scratch fuses while cleaning lids. Use only canners that have the Underwriter's Laboratory (UL) approval to ensure their safety. Avoid purchasing pressure canners made in foreign countries or old used canners which are no longer manufactured. Replacement parts are difficult or impossible to obtain and the canner may be unsafe to operate.
Replacement gauges and other parts for newer canners are often available at stores offering canner equipment or from canner manufacturers. When ordering parts, give your canner model number and describe the parts needed. Follow these steps for successful pressure canning:
- 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.
- 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, exhaust steam 10 minutes, and then place weight on vent port or close petcock. The canner will pressurize during the next 3 to 5 minutes.
- 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.
- 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.
- When the timed process is completed, turn off the heat, remove the canner from heat if possible, and let the canner depressurize. Do not force-cool the canner. If you cool it with cold running water in a sink, or open the vent port before the canner depressurizes by itself, liquid will spurt from jars, causing low liquid levels and jar seal failures. Force-cooling may also warp the canner lid of older model canners, causing steam leaks. Depressurization of older models should be timed. 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.
- After the vent port or petcock has been open for 2 minutes, unfasten the lid and remove it carefully. Lift the lid away from you so that the steam does not burn your face.
- Remove jars with a lifter, and place on towel or cooling rack, if desired.
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