How to test if my preserve jars have a good seal after canning

Canning is one of the best ways to preserve your Fruit, Vegetables, Sauces and more. With the right equipment and technique it can be easy.

Testing if your water bath or pressure canner has made a successful seal on your glass preserve jar, is a very important step in Canning your produce for long term storage. There are a few ways in which you can test if you have a good seal, or if you need to start again with that particular jar.

What is a preserve jar

A preserve jar is any jar that has the ability to create a vacuum seal under heat processing. The most common Preserve Jar in Europe and the UK at least, is made by Kilner. They patented the 2 Lid Vacuum Seal lid system back in 1900, and it has been the best quality Preserve Jar on the market since then.

Kilner Preserve Jar two piece screw top 0.5 litre (0025.400) Copyright © All rights reserved. Kilner Jars / Rayware Group

Kilner Preserve Jar two piece screw top 0.5 litre (0025.400)

The 2 piece lid system consists of a disc or vacuum seal lid that sits directly against the lip of the jar, with a rubber gasket or seal, and a ring or band that threads over the top of the lid, and holds it tightly in place. It is the lid part that will form a Vacuum seal when properly processed.

It is also important to note that Kilner recommends that you replace the vacuum seal Lid after each use.

See the Kilner Preserve Jars in our shop

How to test if you have a good vacuum seal on your preserve jar

Here are three ways to test if you have a good seal on your Preserve Jars after processing
Before you test if your seals have taken properly, you must always wait until the jars have cooled completely. If they are still warm to the touch, wait a little longer, and then complete all or one of the three testes below:

  1. Press the lid - Press the center of the lid, and feel for any movement. If you have a good seal, the lid should not move at all. If there is even the slightest movement or bounce, then your Preserve jar has not sealed properly.
  2. Tap with a Spoon - When properly sealed, and tapped with a spoon you should hear a high pitched ringing sound from the lid. If the sound is dull, you get a thud, or there is any movement in the lid at all, then your preserve jar is not sealed properly.
  3. Inspect the lid for concave shape - When your Preserve jar is properly processed and sealed, the disc of the 2 part lid system will become concave. During processing the air is forced out of the jar, and the lid is sucked in towards the produce. If the lid does not have a concave shape, or is a strange shape in any way, then your preserve jar is not sealed properly.

What to do if my Preserve Jar is not sealed properly

You have a few options in the event that one or all of your Preserve Jars is not sealed properly. The easy option is to place the jar or jars in the fridge, and consume the contents within a few days. If none of your jars have sealed this may not be an option. Your other option at this point is to reprocess the jars in a water bath or pressure canner. See our article on Canning for all the techniques and safety considerations.

Canning and Jarring Food for Long Term Storage

Ian Hunter

Ian Hunter - Father of three, based in Southern Sweden. Author and Co owner of Grow Zone and growing Food all winter. See my full About Page here.

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