How do I know a preserving jar has sealed properly?

The seal is not decoration. Hot contents in a hot jar contract as they cool, the air above them contracts too, and the resulting partial vacuum pulls the lid down and holds it there. That vacuum is what keeps air and its passengers out for a year, so knowing whether you have one is the last step of every preserving session.

Step by step

1. Leave the jars completely alone until cold.Twelve to twenty-four hours, undisturbed, on a wooden board or a folded tea towel rather than a cold worktop. Moving or tightening a jar while the seal is forming can break it. You may hear the lids ping as they pull down, which is a good sign.
2. Press the centre of each lid.A sealed lid is already dished down and does not move at all. An unsealed one gives, clicks, and springs back up. This is the primary test and it takes two seconds a jar.
3. Lift the jar by its lid, over the worktop.A properly sealed jar can be picked up by the lid alone. Do it low over a surface in case it is not. This confirms the press test on jars where the lid is stiff.
4. Look at the lid from the side.A sealed lid is visibly concave. One that stands flat or bulges upwards has either not sealed or has something going on inside the jar. A bulging lid on a stored jar means the contents are producing gas and the jar goes in the bin without opening.
5. Get the headspace right in the first place.About 5 to 10mm below the rim for jam and chutney. Filled to the brim there is no air to contract, so no vacuum forms. Filled too low there is a lot of air, which oxidises the top of the preserve and darkens it.
6. Wipe every rim before the lid goes on.Clean kitchen paper, or a cloth dipped in just-boiled water, run right round the rim and the threads. A single drip of jam under the lid keeps the seal from closing and is the most common reason a jar fails.
7. Use lids that can seal.New lids, or lids whose sealing compound is intact and undeformed. Reused metal lids from shop-bought jars often will not pull down a second time because the compound has already taken the shape of another rim. Reuse the jars freely; be much more sparing with the lids.
8. Deal with a failed jar immediately.Either put it in the fridge and eat it within a few weeks, or tip the contents back into a pan, bring them back to a boil, and re-jar into a fresh sterilised jar with a fresh lid. Do not put an unsealed jar in the cupboard and hope.
9. Label and check again in a fortnight.Contents and date on every jar. Then look over the batch two weeks later — a seal that was marginal will often have let go by then, and that is much better found in a fortnight than in a year.

Tips

• Kilner-style clip jars with a rubber ring seal by the same principle but do not have a poppable lid. Test them by unclipping the metal bail once cold and lifting the jar by the glass lid — a sealed one holds.
• Renew rubber rings on clip jars regularly. A hardened, cracked or previously used ring is the usual reason those jars stop sealing.
• Any leakage, crust or stickiness down the outside of a stored jar means it has been venting. Bin it without opening it.

Common mistakes

• Tightening the lids again once the jars have cooled — It achieves nothing on a sealed jar and can break the seal on one that is still forming. Tighten firmly once, while hot, and then leave them alone.
• Treating a ping as proof — The ping is encouraging but it is not the test — some lids seal silently and some ping and then let go as the jar finishes cooling. Press every lid when cold.

If it doesn't work

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