Beginning food preservation requires several decisions. Water bath vs. pressure canning: how to choose between these two popular methods?
Water bath canning is easier and well suited for most fruits and pickles, but it can’t be used for low-acid foods such as meat and some vegetables. Pressure canning; on the other hand, can seem intimidating to beginners, particularly, in the light of, old tales describing the horrors of exploding pressure canners in the kitchen.
To help clarify the difference between pressure canning and water bath canning, the United States Department of Agriculture has published the Complete Guide to Home Canning, considered by many home canners to be the most respected information source. It is worth visiting the site and reading these helpful guides.
1. Food acidity and processing methods
Whether food should be processed in a pressure canner or boiling water canner depends on the acidity of the food. Acidity may be natural, as in most fruits, or added, as in pickled foods. Low-acid canned foods are not acidic enough to prevent the growth of bacteria. High-acid foods contain sufficient acid to block bacterial growth, or destroy them rapidly when heated. The term “pH” is a measure of acidity, the lower its value, the more acid the food. 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 meat, seafood, poultry, milk and all fresh vegetables except for tomatoes. Most mixtures of low-acid and high-acid foods also have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar to make them high acid foods. Acidic foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades and fruit butters.
Tomatoes are usually considered a high-acid food. Some varieties are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. These products must be acidified to a pH of 4.6 or lower with added lemon juice or citric acid. Properly acidified tomatoes and figs are considered acid foods and can be safely processed in a boiling water canner. This sounds complicated, to the beginner, but that is precisely why it’s important to follow tested recipes from reliable and trusted sources.
Botulin spores are very hard to destroy at boiling water temperatures; the higher the canner temperature, the more easily they are destroyed. All low-acid foods should be sterilized at temperatures of 240 to 250 degrees Fahrenheit, with pressure canners operated at 10 to 15 PSIG. PSIG or PSI refers to pounds per square inch of pressure as measured by gauge.
2. Recommended types of canners
Equipment for heat processing home canned food is of two main types: boiling water canners and pressure canners. Most canners are designed to hold seven quart jars or eight to nine pint jars. Small pressure canners hold four-quart jars; some large pressure canners hold 18-pint jars in two layers, but still hold only seven quart jars.
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 high-acid foods, boiling water canners are recommended for this purpose because they are faster.
3. Boiling water canners
Modern canners are made of aluminum or porcelain-covered steel. Most 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 canner 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.
4. Using boiling water canners
Before starting, fill the canner halfway with clean water. This is approximately the level needed for a canner load of pint jars. For other sizes and numbers of jars, the amount of water in the canner will need to be adjusted so it will be 1 to 2 inches over the top of the filled jars.
Preheat the water to 140 degrees for raw-packed foods and to 180 degrees for hot-packed foods. Food preparation can begin while this water is preheating.
Load the filled jars, fitted with lids, into the canner rack and use the handles to lower the rack into the water; or fill the canner with the rack in the bottom, one jar at a time, using a jar lifter. When using a jar lifter, make certain it is securely positioned below the neck of the jar, which is below the screw band of the lid. Keep the jar upright at all times. Tilting the jar can cause food to spill into the sealing area of the lid.
Add more boiling water, so the water level is at least 1 inch above jar tops. For process times over 30 minutes, the water level should be at least 2 inches above the tops of the jars.
Turn heat to its highest position, cover the canner with its lid, and heat until the water in the canner boils vigorously.
Set a timer for the total minutes required for processing the food by the recipe.
Keep the canner covered and maintain a boil throughout the process schedule. The heat setting may be lowered slightly as long as a complete boil is maintained for the entire process time. If the water stops boiling at any time during the process, bring the water back to a vigorous boil and begin the timing of the process over, from the beginning.
Add more boiling water, to keep the water level above the jars. Important tip: when adding water, add boiling water from a second container.
When jars have been boiled for the recommended time, turn off the heat and remove the canner lid. Wait 5 minutes before removing jars.
Using a jar lifter, remove the jars and place them on a towel, leaving at least 1-inch spaces between the jars during cooling. Let the jars sit undisturbed to cool at room temperature for 12 to 24 hours.
5. Pressure canners
Pressure canners for use in the home have been extensively redesigned in recent years. Modern pressure canners are lightweight, thin walled kettles; most have turn-on lids. They feature a jar rack, gasket, dial or weighted gauge, an automatic vent/cover lock, a vent port or 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 will 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, a canner operated at a gauge pressure of 10.5 lbs. provides an internal temperature of 240 degrees.
Serious temperature errors can occur in pressure canning because the internal canner temperatures are lower at higher altitudes. To correct this error, canners must be operated, at the increased pressures specified in the recipe, for appropriate altitude ranges.
Air trapped in a pressure canner lowers the temperature obtained at 5, 10, or 15 pounds of pressure and results in under processing. To be safe, all types of pressure canners must be vented 10 minutes before they are pressurized.
To vent a pressure canner, leave the vent port uncovered on newer models or manually open the petcock on older models. Heating the filled canner with its lid locked into place boils water and generates steam, which 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 do not need watching during processing. The sound of the weight rocking or jiggling indicates that the canner is maintaining the recommended pressure. 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. Gauges that read high cause under-processing and may result in unsafe food. Low readings cause over-processing. Replace gauges that differ by more than 2 pounds. Gauges may be checked at many county Cooperative Extension offices or contact the pressure canner manufacturer for other options.
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.
Use only canners that have the Underwriter’s Laboratory (UL) approval to ensure their safety.
6. Using Pressure Canners
Put 2 to 3 inches of hot water in the canner. Always follow the directions with USDA processes for specific foods if they require more water added to the canner. Place filled jars on the rack, using a jar lifter. When using a jar lifter, make sure it is securely positioned below the neck of the jar just below the screw band of the lid. Keep the jar upright at all times. Tilting the jar could cause food to spill into the sealing area of the lid. Fasten canner lid securely.
Leave the weight off the vent port or open petcock. Heat at the highest setting until steam flows freely from the open petcock or vent port.
Maintain the high heat setting, let the steam flow continuously for 10 minutes, and then place the weight on the vent port or close the 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 as the canner manufacturer describes.
Regulate the 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. Follow the canner manufacturer’s directions for how a weighted gauge should indicate it is maintaining the desired pressure.
If at any time pressure goes below the recommended amount, bring the canner back to pressure and begin the timing of the process over, from the beginning, using the total original process time from the recipe. This is important for the safety of the food.
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. Forced cooling may result in unsafe food or 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.
Standard-size heavy-walled canners will 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 canner is depressurized, remove the weight from the vent port or open the petcock. Wait 10 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 jar lifter, and place them on a towel, leaving at least 1-inch spaces between the jars during cooling. Let jars sit undisturbed to cool at room temperature for 12 to 24 hours.
7. Selecting 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: a) process jars for the correct number of minutes in boiling water, b) cool the jars at room temperature.
The food may spoil if you fail to add process time in pressure canners for altitudes above 1,000 feet, process for fewer minutes than specified, or cool jars in cold water. To destroy microorganisms in low-acid foods processed with a pressure canner, you must: a) process the jars using the correct time and pressure specified for your altitude, b) allow canner to cool at room temperature until it is completely depressurized.