This information has been reviewed and adapted for use in South Carolina by Pamela Schmutz, HGIC Information Specialist, and Elizabeth Hoyle, Extension Food Safety Specialist, Clemson University. (New 07/99. Revised 6/10.)
Canning is an important, safe method of food preservation if practiced properly. The canning process involves placing foods in jars and heating them to a temperature that destroys microorganisms that could be a health hazard or cause the food to spoil. Canning also inactivates enzymes that could cause the food to spoil. Air is driven from the jar during heating, and as it cools, a vacuum seal is formed. The vacuum seal prevents air from getting back into the product bringing with it microorganisms to recontaminate the food.
Clostridium botulinum bacteria are the main reason why pressure canning is necessary. Though the bacterial cells are killed at boiling temperatures, they can form spores that can survive these temperatures. The spores grow well in low-acid foods, in the absence of air, such as in canned vegetables and meats. When the spores begin to grow, they produce the deadly botulinum toxin (poison).
These spores can be destroyed by canning the food at a temperature of 240 °F, or above, for the correct length of time. This temperature is above the boiling point of water so it can only be reached in a pressure canner.
There are two safe ways of canning, depending on the type of food being canned. These are the boiling water bath method and the pressure canner method.
Boiling Water Bath Method: The boiling water bath method is safe for fruits, tomatoes and pickles as well as jams, jellies and other preserves. In this method, jars of food are heated by being completely covered with boiling water (212 °F at sea level).
High-acid foods (pH of 4.6 or less) contain enough acid that the Clostridium botulinum spores can’t grow and produce their deadly toxin. High-acid foods include fruits and properly pickled vegetables. These foods can be safely canned at boiling temperatures in a boiling water bath. Tomatoes and figs have pH values close to 4.6. To can these in a boiling water bath, acid in the form of lemon juice or citric acid must be added.
Pressure Canning Methods: Pressure canning is the only safe method of canning low-acid foods (those with a pH of more than 4.6). These include all vegetables, meats, poultry and seafood. Because of the danger of botulism, these foods must be canned in a pressure canner. Jars of food are placed in 2 to 3 inches of water in a pressure canner and then heated to a temperature of at least 240 °F. This temperature can only be reached in a pressure canner.
Assemble and wash equipment and containers before gathering fruits and vegetables. Gather products early when they are at their peak of quality. Do not use overripe products. Gather or purchase only as much as you can handle within 2 or 3 hours.
Wash the product carefully, handling small amounts at a time. Lift the food out of the water, drain the water and continue rinsing until the water is clear and free of dirt. Dirt contains some of the bacteria that are hardest to kill. Don’t let the food soak; it will lose flavor and nutrients. The cleaner the raw foods, the more effective the canning process. Do not can decayed or damaged food items.
Preparing the Jars & Lids: Examine jars and discard those with nicks, cracks and rough edges. These defects will not permit an airtight seal on the jar, and food spoilage will result. All canning jars should be washed in soapy water, rinsed well and then kept hot. This could be done in the dishwasher or by placing the jars in the water that is heating in your canner. The jars need to be kept hot to prevent breakage when they’re filled with a hot product and placed in the canner for processing.
Jars that will be filled with food and processed for less than 10 minutes in a boiling water bath canner need to be sterilized by boiling them for 10 minutes. NOTE: If you are at an altitude of 1,000 feet or more, boil an additional minute for each 1,000 feet of additional altitude. Jars processed in a boiling water bath canner for 10 minutes or more or in a pressure canner will be sterilized during processing.
Be sure to use new two-piece lids. Follow the manufacturer’s instructions for treating them. Some need to be brought almost to a boil and then left in hot water, while others need to be boiled for a period of time.
Fruits and vegetables may be packed raw or they may be preheated and then packed into canning jars. The hot pack yields better color and flavor, especially when foods are processed in a boiling water bath. For both raw pack and hot pack, there should be enough syrup, water or juice to fill in around the solid food in the jar and to cover the food. If not covered by liquid, food at the top tends to darken and develop unnatural flavors. It takes from ½ to 1½ cups of liquid for a quart jar.
Raw Pack: For this method put raw, unheated food directly in jars. Pour boiling hot water, juice or syrup over the food to obtain proper headspace. Fruits and most vegetables packed raw should be packed tightly because they will shrink during processing; however, corn, lima beans, potatoes and peas should be packed loosely because they expand during canning.
Hot Pack: For this method, heat the food to boiling (or cook it for specified time) and then pack the hot food and boiling hot liquid in jars. Shrinkage has already taken place, so pack foods loosely enough to allow food to be surrounded by liquid.
Use the boiling water bath method only to preserve high-acid foods such as fruits, tomatoes and pickles.
Remember that pressure canning is the only safe method of processing low–acid foods such as vegetables, meat, poultry and fish. Be sure to read your manufacturer’s instructions on the use of your pressure canner. Dial-gauge canners must be tested for accuracy every year before the canning season. Call your local Extension service office to make arrangements to have your dial-gauge canner tested.
Test the Lid for a Proper Seal: Most two-piece lids will seal with a "pop" sound while they’re cooling. When completely cool, test the lid. It should be curved downward and should not move when pressed with a finger. If a jar is not sealed, refrigerate it and use the unspoiled food within 2 to 3 days, reprocess within 24 hours, or freeze.
If liquid has been lost from sealed jars do not open them to replace it, simply plan to use these first. The food may discolor, but if sealed, and the liquid is only a little lower than the food, the food is safe.
Label & Store Jars: Remove screw bands from the sealed jars to prevent them from rusting on. Wash, dry and store screw bands for later use. Wash food residue from the outside of the jars and rinse. Label, showing contents, date and lot number (if you canned more than one canner full that day).
Store in a clean, cool, dark, dry place. The best temperature is between 50 and 70 °F. Avoid storing canned foods in a warm place near hot pipes, a range or a furnace, or in direct sunlight. They lose quality in a few weeks or months, depending on the temperature, and may even spoil. Keep canned goods dry. Dampness may corrode metal lids and cause leakage so food will spoil. For best quality, use canned foods within one year.
If you decide to reprocess food from jars that did not seal, do so within 24 hours. To do this, remove the lid and check the sealing surface on the jar for tiny nicks. Change the jar if necessary, add a new treated lid and reprocess using the same processing time. Label the jars of food that have been recanned and use these foods first. They will be softer in texture and lower in nutritional value than food processed only once.
Don’t taste or use canned foods that show any sign of spoilage! Look closely at all jars before opening them. A bulging lid or leaking jar is a sign of spoilage. When you open the jar, look for other signs such as spurting liquid and off-odor or mold. Spoiled canned foods should be discarded in a place where they will not be eaten by humans or pets.
All suspect containers of spoiled, low-acid foods, including vegetables, meat, seafood and tomatoes, must be treated as having produced botulinum toxin and handled carefully in one of two ways:
Improperly canned, low-acid foods can contain the toxin that causes botulism without showing signs of spoilage. Jars of foods that have not been properly processed must also be discarded, or if they are unsealed, open or leaking they must be detoxified and discarded as directed above even if there are no signs of spoilage. Low-acid foods are considered improperly canned if any of the following are true:
Contact with botulinum toxin can be fatal whether it is ingested or enters through the skin. Be extremely careful not to splash or come in contact with the suspect food or liquid. Wear disposable rubber or heavy plastic gloves. Wear clothes and aprons that can be bleached or thrown out if contaminated.
Step-by-Step Instructions for Detoxification:
How to Clean Up Contaminated Surfaces:
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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.