Home canning is a popular technique for storing food in times of abundance. It is a relatively inexpensive form of preservation, since most of the equipment required may be used repeatedly over a period of years. There have been a variety of methods used to seal canning jars over the years, but the safest and most common is the modern two-part self sealing lid.
Prepare the foods to be canned, as directed in your recipe or in the USDA canning guide. Spoon or ladle the foods into canning jars, allowing room at the top, or "headspace," as directed. A canning funnel makes this easier.
Tip the jars and look for any large bubbles that may be caught in the food. These will promote spoilage, if they are not eliminated. Release bubbles from the jar with a plastic or silicon spatula, by sliding it down to the bubble and giving the air a path to the surface.. Do not use metal implements, which may react with the food, or wooden implements, which are difficult to sterilize. Some canning kits include a utensil for this specific purpose.
Carefully wipe the rims of the jars with a clean, damp paper towel to remove any soil or food debris that might interfere with making a seal. Set the lids on top of the jars. Screw the sealing rings loosely onto the jars, to prevent the lids from moving. Do not tighten them completely, as air must be able to escape from the jars.
Place the jars in a water bath or pressure canner. Process as directed in your recipe, or in the USDA's canning guide.
Remove the jars from the canner when the appropriate time has elapsed. If your canner is not fitted with a lift-out rack for the jars, you may wish to purchase a set of canning tongs for the purpose. Allow the jars to cool for at least twelve hours. Do not tighten the sealer rings during this time, as the lid gaskets are still soft and may fail.
Remove the rings after cooling, and test the jars to ensure they have made a proper seal. A sealed jar will be concave in the middle, and when tapped with a spoon will make a clear ringing sound, rather than a dull thump. The lid should not give, when pushed with a fingertip, and should not pop back when the finger is lifted. Store sealed jars in a protected place, out of direct sunlight.
Clean and sterilize all jars and implements before canning. Examine the rims of the jars closely, as even a small nick or crack can prevent a proper seal.
Jars and sealer rings may be reused many times, but lids are intended for a single use.
If a jar does not seal properly, check the rim once more for any cracks or food soil that may have prevented a good seal. Process the jar once again in the canner. If it fails to seal on the second attempt, transfer the food to another sterile jar and discard the damaged one.
Read the USDA Home Canning Guide or an equivalent publication before canning. Failure to follow instructions may lead to serious food-borne illness. Discard any canned foods that ferment or are visibly spoiled. Discard any canned foods if the seal on the jar fails in storage.