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Most two-piece lids will seal with a "pop" sound while they're cooling, as the lid gets sucked down by the vacuum created by the contents cooling and contracting inside the jar. After cooling jars for 12 to 24 hours, remove the screw bands and test seals with one of the following options (do not test the jars while they are still hot!):
If a jar is not sealed, refrigerate it and use the unspoiled food within two to three days. Other options are to reprocess (see below) the food within 24 hours or to freeze it.
If, by the time that a home canned jar reaches room temperature (normally within 12 to 24 hours of canning), a lid fails to seal on the jar, 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 11/2 inches 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; then rinse and dry jars. Label and date the jars and store them in a clean, cool, dark, dry place. For best quality, store between 50 and 70 °F. Can no more food than you will use within a year.
Do not store jars above 95° F or 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.
Do not taste food from a jar with an unsealed lid or food that shows signs of spoilage.
You can more easily detect some types of spoilage in jars stored with the rings (the screw bands that hold the lids down) removed. Growth of spoilage bacteria and yeast produces gas which pressurizes the food, swells lids, and breaks jar seals. 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.
Spoiled low-acid foods, including tomatoes, may exhibit different kinds of spoilage evidence or very little evidence. Therefore, all suspect containers of spoiled low-acid foods, including tomatoes, should be treated as having produced botulinum toxin and handled carefully in one of two ways:
If you did not add any acid (lemon juice, vinegar, citric acid, etc.) to tomato products and you did not follow an approved recipe or process for the right amount of time, they may be contaminated. Outside of a lab test, there is no way to know with certainty.
See this page for more information about the Acid content of common fruits and vegetables.
Improperly canned low-acid foods can contain the toxin that causes botulism without showing signs of spoilage. Low-acid foods are considered improperly canned if any of the following are true:
Because improperly canned low-acid foods can contain the toxin that causes botulism without showing signs of spoilage, they should also be detoxified as directed above and then discarded.
Surfaces that come in contact with spoiled or questionable food should be cleaned with a solution of one part chlorine bleach to five parts water. Wet the surface with this solution and let stand five minutes before rinsing.
For more information on canning foods at home, request HGIC 3040, Canning Foods at Home; HGIC 3051, Most Frequently Asked Canning Questions; HGIC 3020, Home Canning Equipment; or HGIC 3000, Preserving Foods.
This is what the USDA recommends:
Honestly, the USDA process above looks pretty darn rigorous and time-consuming for the home canner. I usually pour the contents down the garbage disposal, rinse the jars and boil them in water for 10 minutes. I then pour the boiling water into the sink, then spray the sink with a Clorox solution and let it stand for 10 minutes before wiping up with paper towels. I put the lids and rings in a sealed trash container where children and animals cannot come in contact with them.
Adapted from the "Complete Guide to Home Canning," Agriculture Information Bulletin No. 539, USDA (Revised 1994).