Home canning basics
How to can your own safe, high-quality food. Learn about processing time and methods, recommended canners, and cleaning and maintaining equipment.
Ensuring safe canned foods
How canning preserves foods
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 such as bacteria, molds and yeasts.
- Activity of food enzymes.
- Reactions with oxygen.
- Moisture loss.
Microorganisms live and multiply quickly on the surfaces of fresh food and on the inside of bruised, insect-damaged, and diseased food. Oxygen and enzymes are present throughout fresh food tissues. These proper canning practices can minimize the effects of microorganisms:
- 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.
When these practices are followed along with recommended methods, they control potential spoilage by:
- Removing oxygen.
- Destroying enzymes.
- Destroying and/or preventing the growth of undesirable bacteria, yeasts, and molds.
- Helping form a high vacuum in jars. Good vacuums form tight seals which keep the food in the jars and keep air and microorganisms from re-entering.
The most critical step in ensuring safety in canning is processing in a boiling-water bath or pressure canner. Processing destroys microorganisms and creates the desired vacuum for a good seal. Both a high temperature and sufficient time are required to be certain of adequate heat processing. This ensures that all parts of the food have received enough heat to reduce the number of microorganisms to an extremely small level. A safe food with a long storage life is then produced. The complete destruction of every microorganism would result in a product with unacceptable quality and little nutritional value. The best process is that which has a maximum effect on spoilage organisms and minimal effect on quality.
Reducing bacteria through canning
Growth of the bacterium Clostridium botulinum in canned food may cause botulism, a deadly form of food poisoning (Clostridium botulinum, University of Nebraska-Lincoln). These bacteria exist either as spores or as vegetative cells. The dormant spores can survive harmlessly in soil and water for many years. When ideal conditions exist, the spores produce vegetative cells which multiply rapidly. These cells may produce a deadly toxin within three to four days of growth in an environment consisting of:
- Moist, low-acid food.
- Temperature between 35 and 122 degrees F (depending on strain).
- Less than 2 percent oxygen.
Botulism spores are on most fresh food surfaces. Because they grow only in the absence of air, they are harmless on fresh foods.
Most bacteria, yeasts, and molds are difficult to remove from food surfaces. Washing fresh food reduces their numbers only slightly. Peeling root crops, underground stem crops, and tomatoes reduces their numbers greatly. Blanching also helps, but the vital controls are canning method and using the recommended research-based process times.
The correct processing time destroys the largest number of heat-resistant microorganisms. Properly processed, canned food will be free of spoilage if lids seal and jars are stored below 95 F. To retain the best quality, store jars at 50 to 70 F.
To further reduce the risk of botulism, home canned low-acid and tomato foods should be boiled even if you detect no signs of spoilage. Boil foods for 10 minutes at altitudes below 1,000 feet. Add an additional minute of boiling time for each additional 1,000 feet elevation. In Minnesota, boil food for 11 minutes.
Food acidity and processing methods
Deciding whether to process food in a pressure canner or a boiling-water bath to control botulism bacteria, depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Acid foods contain enough acidity to block bacteria growth, or destroy them more rapidly when heated. low-acid canned foods contain too little acidity to prevent the growth of these bacteria. To increase the acidity level of foods, add lemon juice, citric acid, or vinegar.
The term pH is an index of acidity. The lower its value, the more acid in the food.
- 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.
- Most mixtures of low-acid and acid foods also have pH values above 4.6 unless enough lemon juice, citric acid or vinegar is included to make them acid foods.
- Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades and fruit butters.
Processing foods with an unknown pH
If you’re canning products with unknown pH as acid foods, they must be acidified to a pH of below 4.6 with lemon juice or citric acid. 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. When properly acidified, tomatoes and figs can be safely processed in a boiling-water bath.
Producing the best canned foods begins with selecting high quality, fresh foods. Quality varies among different fruits and vegetables. Examine food carefully for freshness and wholesomeness. Discard diseased and moldy food. Trim small diseased or discolored areas from food.
Can fruits and vegetables picked from your garden or purchased from nearby producers when the products are at their peak of quality. This is 6 to 12 hours after harvest for most vegetables. For best quality, apricots, nectarines, peaches, pears and plums should be ripened one or more days between harvest and canning. If you must delay the canning of other fresh produce, keep it in a shady, cool place.
Fresh, home-slaughtered red meats and poultry should be chilled and canned without delay. Do not can meat from sickly or diseased animals. Put fish and seafoods on ice after catching or buying. Clean (gut, devein, etc.) immediately and can within two days.
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.
Follow these guidelines to ensure that your canned foods retain optimum colors and flavors during processing and storage:
- Use only high quality foods which are at the proper maturity and are free of diseases and bruises.
- Use the hot-pack method, especially with acid foods to be processed in boiling water.
- Don't unnecessarily expose prepared foods to air. Can them as soon as possible.
- While preparing a canner load of jars, keep apples, apricots, nectarines, peaches, and pears in a solution of ascorbic acid or Vitamin C. 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. Use 1 teaspoon per gallon of water as a treatment solution.
- Vitamin C tablets: Economical and available year-round in many stores. Buy 500-milligram tablets; crush and dissolve 6 tablets per gallon of water as a treatment solution.
- Commercially prepared mixes of ascorbic and citric acid: Seasonally available among canners' supplies in supermarkets. Sometimes citric acid powder is sold in supermarkets, but it is less effective in controlling discoloration. If you choose to use these products, follow the manufacturer's directions.
- Fill hot foods into jars and adjust headspace as specified in recipes.
- Tighten screw bands securely, but if you are especially strong, not as tightly as possible.
- Process and cool jars.
- Store the jars in a relatively cool, dark place, preferably between 50 and 70 F.
- Can no more food than you will use within a year.
How hard or soft water affects food preservation
Water is an important ingredient in successful food preservation. Hard water contains larger amounts of minerals than soft water. A certain amount of calcium and magnesium salts is desirable to set the pectins in fruits and vegetables such as in canning peaches and pears.
Although not harmful, the large amount of minerals in hard water:
- Can toughen peas, beans and shrivel pickles.
- Can cause cloudy liquid in canned fruits and vegetables as the high temperatures cause the minerals to settle out of the liquid.
A good example of the water dilemma is green beans. When canning, very soft water can cause mushy green beans, hard water is preferred. But, when blanching green beans for freezing, hard water will toughen them - softer water is then preferred.
General water guidelines:
- Avoid excessively soft or hard water in canning, freezing and pickling.
- Distilled bottled water is preferred to chemically softened water.
- Soft water is preferred over hard water, in most cases, because it causes fewer problems.
- Hard water can be made more acceptable by boiling it for 15 minutes and allowing the calcium and magnesium salts to settle out.
Advantages of hot-packing over raw-packing
Many fresh foods contain from 10 percent to over 30 percent air. The length of time 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. The entrapped air in and around the food may cause discoloration within two to three months of storage. 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 3 to 5 minutes, and promptly filling jars loosely with the boiled food. 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. This practice helps to remove air from food tissues, shrinks food, helps keep the food from floating in the jars, increases vacuum in sealed jars, and improves shelf life. Pre-shrinking food permits filling more food into each jar.
Hot-packing is the best way to remove air and is the preferred pack style for foods processed in a boiling-water canner. At first, the color of hot-packed foods may appear no better than that of raw-packed foods, but within a short storage period, both color and flavor of hot-packed foods will be superior.
The unfilled space above the food in a jar and below its lid is called headspace. Directions for canning specify leaving ¼ inch for jams and jellies, ½ inch for fruits and tomatoes to be processed in boiling water and from 1 to 1¼ 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. The extent of expansion is determined by the air content in the food and by the processing temperature. Air expands greatly when heated to high temperatures; the higher the temperature, the greater the expansion. Foods expand less than air when heated.
Food may be canned in glass jars or metal containers. Metal containers can be used only once. They require special sealing equipment and are much more costly than jars.
The best containers for home canning are threaded, Mason canning jars with self-sealing lids. These come with either standard or wide-mouth openings. The standard mouth opening is about 2⅜ inches. Wide-mouth jars have openings of about 3 inches, making them more easily filled and emptied. Jars are available in different sizes including ½ pint, pint, 1½ pint, quart and ½ gallon. To be certain of adequate processing, it is important to use only the size(s) of jar listed in the recommended method. With careful use and handling, Mason jars may be reused many times requiring only new lids each time. When lids are used properly, jar seals and vacuums are excellent. Mason jars designed for use in home canning ensure a safe product and no breakage during processing.
Before reuse, wash empty jars in hot water with detergent and rinse well by hand, or wash in a dishwasher. Unrinsed detergents may cause unnatural flavors and colors. These washing methods do not sterilize jars. Scale or hard-water films on jars are easily removed by soaking jars several hours in a solution containing 1 cup of vinegar (5 percent) per gallon of water.
Sterilization of empty jars
All jams, jellies and pickled products processed 10 minutes or less 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 inch above the tops of the jars. Boil 11 minutes when at Minnesota altitudes. At higher elevations, boil 1 additional minute for each additional 1,000 feet elevation. Remove and drain hot sterilized jars one at a time. Save the hot water for processing filled jars.
Empty jars used for vegetables, meats and fruits to be processed in a pressure canner need not be presterilized. It is also unnecessary to pre-sterilize jars for fruits, tomatoes, and pickled or fermented foods that will be processed 10 minutes or longer 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 band during processing. The flat lid is crimped around its bottom edge to form a trough, which is filled with a colored gasket compound. When jars are processed, the lid gasket softens and flows slightly to cover the jar-sealing surface, yet allows air to escape from the jar. The gasket then forms an airtight seal as the jar cools. Gaskets in unused lids work well for at least five years from date of manufacture.
To prepare lids, wash them with soapy water and keep at room temperature until ready to use. According to research done by Jarden Home Brands quality assurance team, preheating metal canning lids is not necessary.
The gasket compound in older unused lids may fail to seal on jars. Buy only the quantity of lids you will use in a year. To ensure a good seal, carefully follow the manufacturer's directions in preparing lids for use. Examine all metal lids carefully. Don’t use lids that are old, dented, deformed or have 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. Slowly turn the jar and move the spatula up and down to allow air bubbles to escape. 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. Uncleaned jar-sealing surfaces may cause seal failures.
Fit the metal screw band over the flat lid. Follow the manufacturer's guidelines enclosed with or on the box for tightening the jar lids properly.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. Overtightening also may cause lids to buckle and jars to break, especially with raw-packed, pressure-processed food.
Screw bands are not needed on stored jars. They can be removed easily after jars are cooled. When removed, washed, dried and stored in a dry area, screw bands may be used many times. If left on stored jars, they become difficult to remove, often rust, and may not work properly again.
- Pressure canner with rack, lid and a dial gauge or weighted gauge.
- Water-bath canner with rack and lid.
- Jars, lids and screw bands.
- Jar lifter for easy removal of hot jars from canner.
- Funnel or jar filler to pack small food items into jars.
- Bubble freer, plastic knife or spatula. Do not use metal because it scratches glass which could cause it to break more easily.
- Lid wand with magnet on the end to transfer lids to top of jars.
- Clean cloths for wiping jars rims and general cleanup.
- Knives for product preparation.
- Timer or clock to determine end of processing time.
- Clean towels, rack, or board on which to cool the hot jars after processing.
- Hot pads.
- Cutting board.
Pressure canners are required for canning low-acid vegetables, meats, fish and poultry. The large kettle has a jar rack, a lid that locks in place, a safety valve, a vent and gauge. Gauges indicate inside pressure and are either dial gauges or metal weighted gauges
Dial gauges must be tested for accuracy each canning season.
Check the cover's gasket and make sure it is flexible and soft. If it is brittle or cracked, replace it. Make sure vent openings are clean and open. Follow manufacturer instructions, because new canners have new instructions.
Boiling Water Canners
Fruits, pickles, jellies and jams are processed in boiling water canners. The canner should be deep enough to allow at least 1 to 2 inches of water to boil over the jar tops. It must have a tight-fitting lid and a rack to keep jars off its bottom.
Use only standard, home canning jars that seal properly, are durable for repeated use and safe to use in pressure canners. Inspect for nicks, cracks or chips, especially around the sealing edge.
Jars weaken if:
- Banged against each other.
- A metal knife touches the bottom or side.
- If jars were scoured with steel wool.
- If used in the freezer.
To check their condition, immerse them in water and boil for 15 minutes. If they are not good, they will break! Remove hard-water film by soaking jars in a solution of 1 cup vinegar per gallon water for several hours.
Always use NEW, two-piece, self-sealing, metal lids. Throw away used lids. The lid's air-tight seal keeps a vacuum when it comes in contact with the jar rim. Lids may need to be boiled or held in boiling water before placing on jar rim; follow manufacturer's directions.
Write year purchased on lid boxes and use lids within 3 years. Store in a cool, dry place.
Screw bands are reusable if they are not bent, dented or rusted.
Canning on a smooth or ceramic cooktop
A smooth or ceramic cooktop is easy to clean and looks nice, but it may not be right for home canning. For water bath canners or pressure canners to work successfully on a ceramic cooktop, the canner bottom must be:
- In contact with the cooktop.
- Extend no more than 2 inches beyond the design on the cooktop surface.
Concave canner bottoms
The lighter weight stamped aluminum canners usually have a concave bottom. If the concavity is greater than 1/8 inch, the canner will not perform well.
Scratches and cracking of cooktop
If a heavy canner is slid or pulled across the surface, the cooktop may scratch and lead to major cracking.
Auto shut-off or heat-cycling causes under-processing
A food safety issue results when many ceramic cooktops have automatic shut-offs or heat-cycling when the heat gets very high. If the burner shuts off during processing, the food may be under-processed.
Many manufacturers do not recommend using a ceramic cooktop for canning.
The size and weight of the pan and extended cooking times can damage the cooktop and may void the warranty if the manufacturer advises against it. Check your cooktop manufacturer recommendations.
Heat transfer in canning jars
The time required for sufficient heat to penetrate all parts of the food in the jar must be considered in addition to the acidity of the food and the heat resistance of the microorganism. Heat is transferred from the outside of the jar through the food.
Heat transfer is affected by many things:
- The size and shape of the container. Smaller jars heat faster than wider or taller jars.
- Amount of liquid. Food containing a large amount of free liquid heats much more quickly than a more solid product.
- Piece size. Smaller pieces of food (corn, peas) heat much more quickly than large chunks.
- Amount of fat. Fat insulates the food and slows heat transfer.
- The type of heating medium being used. Wet steam heats faster than dry air.
Use approved methods of canning
The many factors involved make it impossible to estimate the correct processing conditions for any food product, so it is important to use a recipe from a trusted source. This is especially true for items which are mixtures of food with differing water content, piece size, fat content or acidity as well as types and numbers of microorganisms present. The establishment of a correct, safe process requires laboratory research by trained scientists.
Individuals who can food at home and do not use methods approved by University of Minnesota Extension, other state extension services, United States Department of Agriculture (USDA) or other reputable sources (Kerr, Ball, etc.) are taking a great risk with the health of those who eat the canned product.
Any of the following can cause canned food to spoil:
- Failure to add process time for lower boiling water temperatures at altitudes about 2,000 feet.
- Processing for fewer minutes than specified.
- Cooling jars in cold water.
- Failure to exhaust canners properly.
- Processing at lower pressure than specified.
- Cooling the canner with water.
Principles of 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.
Process times for 1½ pint and pint jars are the same, as are times for 1½ pint and quart jars. For some products, you have a choice of processing at 5, 10, or 15 PSI. In these cases, choose the canner pressure (PSI) you wish to use and match it with your pack style (raw or hot) and jar size to find the correct process time.
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 for the correct number of minutes at 240°F (10 PSI) to 250°F (15 PSI) as the method indicates.
- Allow canner to cool at room temperature until it is completely depressurized.
The effect of temperature on foods:
- -10 to zero F - Best storage temperatures for frozen foods.
- Zero to 32 F - Freezing temperatures stop growth of microorganisms, but may allow some to survive.
- 32 F to Temperature water freezes.
- 32 to 40 F - Cool temperatures permit slow growth of some bacteria, yeast and mold.
- 50 to 70 F - Best storage temperatures for canned and dried foods.
- 40 to 140 F - DANGER ZONE: Allows rapid growth of bacteria, yeast and mold.
- 95 F - Maximum storage temperatures for canned food.
- 140 to 165 F - Warming temperatures prevent growth, but may allow survival of some microorganisms.
- 180 to 212 F - Canning temperatures are used to destroy most bacteria, yeasts, and mold in acid foods. Time required to kill these decreases as temperatures increases.
- 212 F - Temperature water boils at sea level. Canning temperature for acid fruits, tomatoes, pickles, and jellied products in a boiling-water canner.
- 240 to 250 F - Canning temperatures for low-acid vegetables, meat and poultry in a pressure canner.
There are two main types of heat-processing equipment for home canned food; 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.
Steps for boiling-water canning
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 1-2 inches 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 1 inch between the jars during cooling.
Watch our 5-minute module on the boiling water canning method.
Pressure canners for home use 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.
Avoiding temperature errors in pressure canners
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 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.
Venting a pressure canner
To vent a canner:
- Leave the vent port uncovered on newer models or manually open petcocks on some older models.
- Heat the filled canner with its lid locked into place. This 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 gauges yearly
Check dial gauges for accuracy before use each year. Replace the gauge if it reads high by more than 1 pound at 5, 10, or 15 pounds of pressure. Low readings cause over-processing and may indicate that the accuracy of the gauge is unpredictable.
See Testing dial pressure canner gauges for more information.
Maintain canner lid gaskets
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.
Take care with safety fuses
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.
Steps for pressure canning
Follow these steps for successful pressure canning outlined by the National Center for Home Food Preservation:
- 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 recommended pressure has been reached on the dial gauge 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. Don’t 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 canner is depressurized, remove the weight or open the petcock. Wait for 10 minutes; then unfasten the lid and remove it carefully. Lift the lid away from you so that the steam doesn’t burn you.
- Remove jars with a lifter, and place on towel or cooling rack, if desired.
Watch our 5-minute module on the pressure canning method.
When you remove hot jars from a canner, don’t re-tighten their jar lids. Re-tightening of hot lids may cut through the gasket and cause seal failures. Cool the jars at room temperature for 12 to 24 hours. Jars maybe 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, don’t 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 seal. 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.
Reprocessing unsealed jars
If a jar fails to seal, remove the lid and check the jar sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time.
Headspace in unsealed jars may be adjusted to 1½ inch and jars could be frozen instead of reprocessed. Foods in single unsealed jars could be stored in the refrigerator and consumed within several days.
If lids are tightly vacuum-sealed on cooled jars:
- Remove screw bands.
- Wash the lid and jar to remove food residue.
- Rinse and dry jars.
- Label and date the jars.
- Store them in a clean, cool, dark, dry place.
- Don’t store jars above 95 F. Don’t store near hot pipes, a range, a furnace, in an uninsulated attic, or in direct sunlight. Under these conditions, food will lose quality in a few weeks or months and may spoil.
- Dampness may corrode metal lids, break seals, and allow recontamination and spoilage.
Accidental freezing of canned foods will not cause spoilage unless jars become unsealed and recontaminated. However, freezing and thawing may soften food. If jars must be stored where they may freeze, wrap them in newspapers, place them in heavy cartons, and cover with more newspapers and blankets.
Identifying and handling spoiled canned foods
Do not taste food from a jar with an unsealed lid or food which shows signs of spoilage. You can more easily detect some types of spoilage in jars stored without screw bands. Growth of spoilage bacteria and yeast produces gas which pressurizes the food, swells lids and breaks jar seals.
Check each jar thoroughly
- As each stored jar is selected for use, examine its lid for tightness and vacuum. Lids with concave centers have good seals.
- Next, while holding the jar upright at eye level, rotate the jar and examine its outside surface for streaks of dried food originating at the top of the jar.
- Look at the contents for rising air bubbles and unnatural color.
- While opening the jar, smell for unnatural odors and look for spurting liquid and cotton-like mold growth (white, blue, black or green) on the top food surface and underside of lid.
Carefully detoxify and discard spoiled food
Carefully discard any jar of spoiled food to prevent possible illness to you, your family and pets. You must detoxify the container, lid and all the contents before disposal. Follow this process if you suspect spoiled food:
- Place container with contents and lid on their sides in an 8-quart or larger stock pot, pan or boiling-water canner.
- Wash your hands thoroughly.
- Add water to the pot so it is 1 inch or more above everything in the pot. Avoid splashing the water.
- Place a lid on the pot and heat the water to boiling.
- Boil 30 minutes to ensure detoxification of the food, container and lids.
- Cool and discard the food, container and all container components, such as lids, in the trash or bury in soil.
- Take care that animals or children can’t get in contact with the disposed food and containers.
- Thoroughly wash all counters, containers and equipment including can opener, clothing and hands that may have been in contact with the food or containers.
- Discard any sponges or washcloths used in cleaning.
- Place everything in a plastic bag and discard in the trash where they’re unreachable by animals and children. This will prevent accidental poisoning.
Estimating the amount to can
The amount of food to can or freeze for your family should be decided by your family. The following formula might be helpful in calculating your needs:
- Determine serving size (see suggested serving size list below).
- Multiply serving size by the number of family members who will be eating this food.
- Then multiply this by your estimate of the number of servings per week, per person.
- Multiply this figure by 52 weeks to get a total amount necessary to preserve in one year.
- To get this amount in quarts, divide the number by 4.
Fruit - ½ cup suggested serving.
X 4 family members = 2 cups.
2 cups X 3 servings per week per person = 6 cups/week.
6 cups X 52 weeks/year = 312 cups.
Divided by 4 = 78 quarts.
Suggested serving size for different foods:
- Fruits - ½ cup.
- Juices - 1 cup.
- Vegetables - ½ cup.
- Meat and seafood - ½ cup.
- Soups - 1 cup.
- Sauces - ½ cup.
The high cost of commercially canned, special diet food often prompts interest in preparing these products at home. Some low-sugar and low-salt foods may be easily and safely canned at home. However, the color, flavor, and texture of these foods may be different than expected and be less acceptable.
Canning without sugar
When canning fruits without sugar, it is very important to select fully ripe but firm fruits of the best quality. Prepare these as hot packs, but use water or regular unsweetened fruit juices instead of sugar syrup. Juice made from the fruit being canned is best. Blends of unsweetened apple, pineapple and white grape juice are also good for filling over solid fruit pieces. Adjust headspaces and lids and use the processing recommendations given for regular fruits. Pack in half-pint, preferably, or pint jars and use the following processing times for all Minnesota altitudes. Add sugar substitutes, if desired, when serving.
Canning without salt
To can tomatoes, vegetables, meats, poultry, and seafood simply omit the salt. In these products, salt seasons the food but is not necessary to ensure its safety. Add salt substitutes, if desired, when serving.
Canning baby foods
You may prepare any chunk-style or pureed fruit with or without sugar. Pack in half-pint, preferably, or pint jars and use the following processing times for all Minnesota altitudes.
Recommended process: Boiling-water bath, half-pints or pints, hot pack, 25 minutes.
Caution: The National Center for Home Food Preservation warns; do not attempt to can pureed vegetables, red meats, or poultry because proper processing times for pureed foods have not been determined for home use. Instead, can and store these foods using standard processing procedures. Puree or blend them at serving time. Heat the blended foods to boiling, simmer for 10 minutes, cool, and serve. Store unused portions in the refrigerator and use within two days for best quality.
Why people can foods
Canning can be a safe and personally rewarding way to preserve quality food at home. Canning special products to be enjoyed by family and friends often is a fulfilling experience and a source of pride for many people. Canning may not be the least expensive way of obtaining every type of food. The cost of equipment, energy, and time must be considered.
However, the main objective of canning is to preserve the food by the application of heat so that it can be safely eaten at a later time. Safety of the consumer is the primary concern when food is canned. It’s also important to achieve acceptable quality in the final product and to retain as much of the nutritive value of the food as possible. The potential advantages of home canning are lost:
- When you start with poor quality fresh foods.
- When jars fail to seal properly.
- When food spoils.
- When prolonged storage in warm, bright light conditions causes flavors, texture, color and nutrients to deteriorate.
The nutritional value of canned food
Many vegetables begin to lose vitamins when harvested. Nearly half the vitamins may be lost within a few days unless the fresh produce is cooled or preserved. Within one to two weeks, even refrigerated produce may lose half of its vitamins.
The heating process during canning destroys from one-third to one-half of vitamins A and C, thiamin, and riboflavin. Once canned, additional losses of these sensitive vitamins are from 5 to 20 percent each year depending on storage conditions. The amounts of other vitamins, however, are only slightly lower in canned compared with fresh food. Vegetables handled properly and canned promptly after harvest may be more nutritious than fresh produce held many days after harvest under abusive conditions.
|Method||Vitamin A||Vitamic C||B vitamins||Minerals|
|Drying||Minimal loss||Destroyed by blanching||Some loss during blanching; use water to rehydrate||Some loss if soaking water is not used; no loss of iron|
|Refrigeration||1/2 loss within 1-2 weeks||1/2 loss within 1-2 weeks||1/2 loss with 1-2 weeks|
|Canning||1/3 to 1/2 loss (5-20% each year)||1/3 to 1/2 loss (5-20% each year)||1/3 to 1/2 loss (5-20% each year)|
|Freezing||None||Some loss if blanched||Some loss if blanched||No loss of iron|
Reviewed in 2018