Two Types of Canning: What's The Difference?
Boiling Water Method
Acid foods are processed in a boiling-water canner. The heat is transferred to the product by the boiling water which completely surrounds the jar and two-piece cap. A temperature of 212oF (100oC) is reached & must be maintained for the time specified. This method is adequate to kill molds, yeasts, enzymes and some bacteria. This method never reaches the super-high temperatures needed to kill certain bacterial spores and their toxins, which can produce botulism, therefor, this method cannot be used for processing low-acid foods.
In order to kill all bacteria, their spores and the toxins they
produce, low-acid foods must be super-heated to a temperature
of 240oF and held there for a specified time. Because the steam inside
the canner is pressurized, it's temperature exceeds the boiling
point of water.
Food Preservation: Selection, Use and Care of Canning Equipment
More than 30 different brands of home canning lids are sold. Do not use any home canning closure that has not been scientifically proven. Don't use any questionable closure or technique recommended in newspaper, magazines, books or write-in advertisements.
Do not reuse flat metal lids in home canning. Gasket materials are designed to soften sufficiently to provide an air-tight seal and maintain a vacuum in the jar when in contact with the jar rim. Variations in the shape of jar tops may prevent an adequate seal when lids are reused.
Be sure all closures are perfect. Don't use any with dents or rust because these prevent airtight seals.
Wash all lids and bands. Metal lids with sealing compound may need to be boiled or held in boiling water for a few minutes; follow the manufacturer's directions.
Use only standard home canning jars. They are made to seal properly, to be durable with repeated use and to be used safely in a steam pressure canner. The manufacturer's name is usually blown in the side of the jar.
Commercial jars, also known as packers' jars, are not made for home canning. They contain such products as coffee, mayonnaise or peanut butter. Even though standard home canning lids may seem to fit these jars, the lids may not seal because the jar mouth is too small or the finish too irregular. The jar neck may be too shallow for a standard home canning band to hold the lid tightly against the jar.
In addition to sealing problems with commercial jars they may also be dangerous. Most of them are made of thin glass and are not heat tempered, as regular home canning jars. When you open the canner, the jars may still be under pressure. The quick drop in temperature could cause the recycled jar to explode in your face.
Inspect jars for any cracks or chips before you use them because defects prevent airtight seals.
Wash glass jars in hot, soapy water and rinse well. When you can most food by the boiling-water-bath method and all foods by the pressure-canner method, it is not necessary to sterilize jars before canning. The jars, as well as the food, are sterilized during processing.
You should sterilize jelly containers in boiling water for 10 minutes before using them. Then keep the containers hot -- either in a slow oven or in hot water -- until you use them. This will keep them from breaking when you fill them with hot jelly.
Boiling Water-Bath Canner
Water-bath canners are available on the market. You can use any big metal container, however, if it is deep enough for the water to cover the tops of jars and have space to boil freely. Allow 2 to 4 inches (5 to 10cm) above jar tops for brisk boiling. The canner must have a tight-fitting lid and a wire or wooden rack. If the rack has dividers, jars will not touch each other or fall against the sides of the canner during processing.
Fruits, tomatoes, pickled vegetables, relishes, jams and preserves can be processed safely in a boiling water-bath canner.
Put filled glass jars into canner of hot or boiling water. For jams, preserves and raw packed fruits or tomatoes, have water in canner hot but not boiling. For pickles and hot packed fruits and tomatoes, have water boiling.
Add boiling water until it is an inch or two over tops of containers; don't pour boiling water directly on glass jars. Place cover on the canner.
For most foods, you start to count processing time when water in the canner comes to a rolling boil. Processing methods for some pickles, such as fresh-pack dill pickles, are slightly different from the usual water-bath procedures. For these products, start to count the processing time as soon as you place the filled jars in the actively boiling water. Follow instructions carefully for the food you are canning.
Keep water boiling gently and steadily during the time recommended for each food. Add boiling water during processing if needed to keep containers covered.
Remove jars from canner immediately when processing time is up.
Meat, fish, poultry and all common vegetables except tomatoes should be canned in a steam-pressure canner. NEVER can these foods in a boiling water-bath canner, an oven, a steamer without pressure or an open kettle. The pressure canner is the only method of home canning that will heat these foods enough to kill the dangerous bacterial spores of Clostridium botulinum within a reasonable time.
Essential Parts of Pressure Canners
Covers of pressure canners are locked in place so that they cannot be lifted by steam. Most new canners have covers that slide into a locked position.
A gauge, whether a dial or weight, is essential to control pressure. The weight type permits pressure to rise to a definite point and then releases excess steam to keep pressure from going higher. With either type, you must adjust the heat to keep the pressure steady.
Gaskets keep steam from leaking out around the cover. Replace the gasket when it becomes stretched or allows steam to escape.
Safety plugs go into action only if pressure or temperature becomes dangerously high. One type of plug melts when pressure gets too high or utensil boils dry. Another type of plug is blown out by excessive pressure. Both types are replaceable.
Vents are provided to allow air to be exhausted from the utensils and to permit the release of steam as needed. A petcock, safety valve or weight on the vent is used to control the escape of air or steam. Most new dial gauge canners have a weight on the vent.
Use of Pressure Canners
Keep the manufacturer's instruction book that comes with your pressure canner. Reread the directions at the beginning for each canning season and follow them carefully. If you have lost the manufacturer's book, write for a new one. Give the model number and any other information you find on the canner.
Before the canning season, put water in the canner and bring it up to pressure in the usual way to see that it is in good working order. Allow time for repairs, if needed. Have a dial gauge checked before the canning season, and also during the season if you use the canner often. Ask your county Extension home economist, dealer or manufacturer about checking it.
Basic Steps in Using a Pressure Canner
Put 2 to 3 inches (5 to 8cm) of boiling water in the canner. The amount depends on the size and shape of the canner.
Place rack in canner. Set filled jars in canner so that steam can flow around each container. Fasten canner cover securely so that no steam can escape, except through the vent (petcock or weighted gauge opening).
Leave petcock open or weighted gauge off at the beginning of the heating time. Watch until steam flows steadily from the vent. Let it escape for 10 minutes or more to drive all air from the canner. Then put on weighted gauge or close petcock.
Let pressure rise to 10 pounds (116oC or 240oF). The moment this pressure is reached, start counting processing time. Keep pressure constant by adjusting heat under the canner. Keep drafts from blowing on canner.
Remove canner from heat as soon as processing time is completed.
With glass jars, let canner stand until pressure is zero. Never try to rush the cooling by pouring cold water over the canner or setting it in water. Wait a minute or two after pressure reaches zero; then slowly take off the weighted gauge or open the petcock. Open the cover and tilt the far side up so steam escapes away from you. Take jars from canner.
Care of Pressure Canners
Wash thoroughly after each use, but don't put the cover in water because this will damage a dial gauge and may cause vents to become clogged. Never run water over the dial gauge. Wipe the cover with a soapy cloth and then with clean damp one.
Clean the vent pipe by drawing a pipe cleaner or string through. Wash gasket and replace in cover.
Store a canner carefully. Make sure it is clean and dry before you put it away at the end of the season. Crumple newspapers inside the canner to absorb moisture and odors. Some manufacturers recommend turning the cover upside down on the canner. This is designed to prevent odors in the canner and to protect the valves and gauge.
Distributed by Barbara P. McLaurin, R.D., Extension Food and Nutrition Specialist
Information Sheet 845
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture.
This document is public information and may be reproduced in part or in total. It should not be used to imply endorsement of any specific brand or product. Mississippi residents may get a printed copy of this publication through their county Extension offices.
Mississippi State University Extension Service
Last modified: Wednesday, 12-Aug-98 13:07:48
LINKS FOR PRESSURE CANNERS AND ACCESSORIES
U.G. White Hardware Company, All American Pressure Canners
Back to Basics Home - Canning Accessories and water bath canners
Lehman's Non-Electric Hardware and Appliances Catalog - All American Pressure Canners in various sizes, accessories
Presto - Pressure Canners
Kitchen Krafts - Canning Accessories
Wells Can Company Ltd - All American Canner
Happy Hovel - All American Canner and steamer canners
Kitch 'N Canning Cellar - All American canner is several sizes and Presto and Mirro gauges, sealing gaskets, and miscellaneous parts