How Canning Preserves Food
To can means to heat process food in a glass jar with a lid in place. Processing kills microorganisms -- bacteria, yeasts, and molds -- that contaminate food and cause food spoilage and/or food borne illness. Processing can be done in a water bath canner or a pressure canner, depending on the food's acidity.
Acid foods (all fruits except unacidified figs) can be safely processed in a water bath canner. Acidified tomatoes and figs can also be safely processed in a water bath canner. Microorganisms in or on acid foods are easily killed at 212 degrees F (the temperature of boiling water). Low-acid foods (vegetables and tomatoes and figs that are not acidified) must be processed in a pressure canner. The bacteria that produces botulinum toxin cannot grow in acid foods but can grow in low acid foods. These bacteria (Clostridium botulinum) have spores that survive hours of boiling water temperature. However, these spores are destroyed within a reasonable time at 240oF (the temperature reached inside a pressure canner set at 10 pounds pressure).
If low-acid food is processed in a water bath canner, botulinum spores on the food will survive. In the absence of air, a condition found inside a jar after processing, the spores become living bacteria. As the bacteria grow, they form toxin. Eating even a drop of this potent toxin can be fatal to humans and animals. Over 70% of the cases of botulism have been caused by low-acid foods that were improperly canned at home.
To make sure your home canned foods are safe, carefully follow the canning instructions in this bulletin. Process acid foods in a water bath canner and low-acid foods in a pressure canner. Never process any foods in a conventional oven, microwave oven, steamer or dishwasher, as these methods do not kill microorganisms that cause food spoilage and/or foodborne illness.
Recommended Canning Equipment
Before each canning season, assemble and examine all canning equipment.
Canning jars. Use only standard canning jars (also called Mason jars) with the manufacturer's name printed on the side. These jars can withstand the temperature extremes of canning. And, the sealing edge is smooth and flat so lids will seal properly.
Never use commercial jars, such as mayonnaise and pickle jars, for home canning. These jars are not very resistant to temperature extremes; they break easily. Also, lids may not seal on these jars because their sealing edge may be rounded rather than flat. Finally, the neck of the jar may be so short that the screw band will not hold the lid firmly in place during processing.
Canning jars must be in perfect condition. Check all jars, new and used, for hairline cracks, chips or nicks on the sealing edge. Such defects can result in breakage or failure to seal.
Canning lids. The only safe way to seal a canning jar is with a two-piece canning lid. The set consists of a flat metal lid and a screw band. The lid has a sealing compound around the edge and is enameled on the under side to prevent food from reacting with the metal. The screw band holds the lid in place during processing. A vacuum seal forms during cooling, after the jar is removed from the canner. Screw bands that are in good condition may be reused, but always use new lids. Do not use screw bands that are bent or badly rusted.
Two types of canners. Use a water bath canner to process acid foods. A water bath canner is a large deep kettle that has a cover and a rack to hold jars. You can also use a big, covered pot that is deep enough to allow water to extend 1 to 2 inches over the tops of the jars with enough room for the water to boil briskly. Also add a rack to keep the jars off the bottom of the pot.
Use a pressure canner to process low acid foods. A pressure canner is a deep, heavy kettle that has a rack on the bottom for jars to stand on. It also has a tight-fitting lid with a gasket, and a pressure gauge. The gasket keeps steam from leaking out around the cover. If the gasket is worn, stretched, or hardened, replace it. There are two types of pressure measuring gauges, dial gauge and weighted gauge.
A dial gauge has a needle that moves along a numbered scale to indicate the pressure inside the canner. Each year check the dial gauge, old or new, for accuracy and during the canning season if heavily used. Call your extension agent, Family and Consumer Sciences, to find out where testing can be done.
A weighted gauge fits over the air vent tube. It permits pressure in the canner to rise to the desired point and then releases excess steam by "jiggling" or "rocking" to keep the pressure from going higher. Weighted gauges do not need testing for accuracy, but they do need to be kept clean. Check the vent tube to be sure it hasn't been bent or damaged during use.
Getting Canning Equipment Ready
Wash canning jars in a dishwasher or in hot soapy water, and rinse well. Keep jars hot by leaving them in the dishwasher or hot water until you are ready to fill them. Jars do not need to be sterilized, as this will be accomplished during processing. Wash and rinse canning lids and screw bands. Follow the manufacturer's directions for preparing lids. They may need to be boiled in water for a few minutes before use.
Preparing Fruits and Vegetables For Processing
Select high quality, unblemished fruits and vegetables for canning. Canning will not improve quality. Can them as soon as possible after harvesting. If you must hold foods before canning, keep them in the refrigerator. If you buy fruits or vegetables to can, get them fresh from local farmer's markets, roadside stands or pick-your-own farms.
Thoroughly wash fruits and vegetables before canning even if they will be peeled. Garden soil contains bacteria. NOTE: Potatoes must be peeled before canning. Potato skins contain a high bacteria count increasing the chance of botulinum toxin formation.
Wash by scrubbing with a vegetable brush and rinsing thoroughly. Or, if more practical, soak in water for several minutes. Lift out of the water so the soil that has been washed off won't settle back on the food. Peel, pit, and/or slice only as much food as you can process at one time.
Some fruits and vegetables (apples, apricots, nectarines, peaches, pears and potatoes) darken when cut. To prevent darkening, keep raw, prepared produce in a solution of 1 teaspoon ascorbic acid to one gallon of cold water. Check among the canners' supplies in the supermarket to get this product.
Sugar and Salt
Sugar helps retain the color, shape and texture of canned fruits. Sugar is usually added as a syrup. To make syrup, pour 4 cups of water into a saucepan and add:
* 2 cups of sugar to make 5 cups of thin syrup OR
* 3 cups of sugar to make 5-1/2 cups of medium syrup OR
* 4-3/4 cups of sugar to make 6-1/2 cups of heavy syrup.
Heat until the sugar dissolves. Make 1 to 1-1/2 cups of syrup for each quart of fruit. Up to half the sugar used in making syrup can be replaced with light corn syrup or mild-flavored honey. Fruits also can be safely canned without sugar. Pack the fruit in extracted juice, in juice from another fruit (such as bottled apple juice, pineapple juice, or white grape juice) or in water. Salt may be added to vegetables and tomatoes before canning. Since its only function is flavor, it can be safely omitted. Canning fruits and vegetables without adding sugar or salt does not affect processing times or microbiological safety.
The two methods of packing food into canning jars are raw pack and hot pack. Raw pack is packing raw, prepared food into clean, hot jars and then adding hot liquid. Fruits and most vegetables need to be packed tightly because they will shrink during processing. However, raw corn, lima beans, and peas should be packed loosely, as they will expand. For hot pack, heat prepared food to boiling or partially cook it. It should be packed loosely while boiling hot into clean, hot jars. Hot pack takes more time but has been found to result in higher quality canned foods.
For either packing method, pack acid foods including acidified tomatoes and acidified figs to within 1/2 inch of the top of the jar. Low acid foods to within 1 inch of the top of the jar.
After food is packed into jars, wipe the jar rims clean. Put on the lid with the sealing compound next to the jar rim. Screw the band down firmly so that it is hand-tight. Do not use a jar wrench to tighten screw bands. There must be enough "give" for air to escape from the jars during processing. Process food promptly after packing it into jars and adjusting lids. Processing times are given for pints and quarts. If you are using half pint jars, use processing times for pints. For one-and-one-half pint jars, use processing times for quarts. Fruit juices are the only product that may be canned in half-gallon jars.
Canning at Altitudes Above 1,000 Feet
If you live at an altitude of more than 1,000 feet, you will need to modify the processing time for acid foods and the pounds pressure you use to process low-acid foods. The processing instructions presented in this bulletin are for altitudes of 0-1000 feet.
To determine your altitude, contact the North Carolina Geological Survey Office. Their address is: 512 North Salisbury Street, P.O. Box 27687, Raleigh, NC 27611. Their telephone number is 919-733-2423. After determining your altitude, your local extension center can help you to determine changes you need to make to your canning instructions.
Processing in a Water Bath Canner
Use a water bath canner to process acidified tomatoes, acidified figs and all other fruits. A pressure canner can be used to process acid foods but the quality will not be as good.
1. Fill the canner half full with water; then cover and heat. For raw-packed food, have the water hot but not boiling. For hot-packed food, have the water boiling.
2. Using a jar lifter, place jars filled with food on the rack in the canner. If necessary, add boiling water to bring water 1 to 2 inches over the tops of the jars. Do not pour boiling water directly on jars. Cover.
3. When water comes to a rolling boil, start counting the processing time. Keep water at a rolling boil for the entire processing time. Add more boiling water to keep water 1 to 2 inches above jars.
4. As soon as the processing time is up, use a jar lifter to remove jars from canner. If liquid has boiled out of the jars during processing, do not open them to add more. Do not retighten screw bands, even if they are noticeably loose.