How to Ensure a Good Seal
Posted: August 16, 2012
What is a vacuum seal? When you fill a jar with food and apply a lid at room temperature, the atmospheric pressure is the same inside the jar as it is outside the jar. When the jar is heated as in water bath or during pressure canning, the air and food inside the jar expand; this forces air out of the jar. As the jar cools and the contents shrink, a partial vacuum forms. The sealing compound found on the underside of the lid prevents air from re-entering the jar so that no microorganisms can enter the jar to recontaminate the food.
Here are some tips to ensure a good jar seal.
- Use standard mason jars, lids, and screw bands.
- Use new lids.
- Use jars that are free of nicks, cracks or dips on the sealing surface.
- Inspect lids for dents scratches or a narrow or incomplete ring of sealant.
- Use correct headspace—usually ¼ inch for jams and jellies, ½ inch for tomatoes, fruit, pickles, 1 inch for vegetables and meat, and 1 ¼ inch for chicken.
- Use recommended processing methods and times.
- Use a dampened paper towel to remove food particles from the jar sealing surface.
- Turn screw bands firmly tight. This is sometimes described as fingertip tight. If bands are too tight, air cannot vent during processing and lids will buckle; if bands are too loose, vacuum will be low and seals may fail now or later.
- Use a jar lifter to insert and remove jars from the canner. Position the jar lifter on the neck of the jar below the screw band.
- Wait 12 to 24 hours to test if jars are sealed.
- Reprocess jars that did not seal within 24 hours. Use new lids and reprocess for the original processing time. (Or freeze product or refrigerate and use within 3 days.)
- Remove screw bands when jars are cool. Wash jars, rinse and dry before storing. Store jars without replacing screw bands.
- Avoid extreme changes in storage temperatures. Store between 50°F and 70°F.
Testing the Seal
- Press the center of the lid. If it is down and will not move, it is sealed.
- If the lid looks concave (curved down), it is sealed.
- Pick the jar up by the lid and the lid does not come off.
- Tap the lid with a spoon. A clear ringing sound indicates the jar is sealed. However, if food has expanded in the jar and is touching the lid, the lid may still be sealed even with a dull sound.
