How to Tell If a Canning Jar Has Sealed Properly

Three Ways to Test a Jar Seal

woman testing to see if canning jar is properly sealed
Photo © Erin Huffstetler

Whether you're new to home canning or a seasoned pro, it's important to test each jar to ensure that it's sealed properly. The question is, how do you know whether your jars have sealed? Here are the three ways to check that are recommended by the USDA.

Wait Until the Jars Have Cooled

Don't make the rookie mistake of tightening the bands as soon as you remove your jars from the canner. Adjusting the bands, while the jars are hot will disturb the gasket, and lead to failed seals. It's okay if the bands have loosened a bit during processing.

Let the jars cool at room temperature for at least 12 hours (and up to 24 hours) before you test the seals. You'll probably notice that the level of the contents is lower than when you packed the jars. This is to be expected. Don't be tempted to open the jars and add more food. This'll break the seal, and you'll have to reprocess any jar that you opened.

How to Test Canning Jar Seals

Once at least 12 hours has passed, you can safely remove the screw bands, and test the seal of each jar. Here are the three different methods you can use.

Jar Seal Testing Method 1: The Press

A properly sealed jar lid will not spring up when you press down in the center. Use a finger to press down on the middle of the lid.

  • Sealed: There is no give when you press down in the center. This jar is good for storage.
  • Unsealed: The lid springs up when you lift your finger. This jar will need to be reprocessed before it can be stored safely.

Jar Seal Testing Method 2: The Spoon Tap

This method relies on your ears, so it's best used if you have normal hearing. Because air has been removed from the jar to create a strong vacuum effect, the lid should give a clear, high-pitched ringing sound when you tap it. Give the jar lid a tap with the bottom of a metal spoon, and see how it sounds.

  • Sealed: There is a high-pitched ringing sound when the lid is tapped with a metal spoon.
  • Unsealed: There is a dull sound when you tap the lid with a spoon. However, this could also be caused by food clinging to the bottom of the lid. If you aren't sure, use one of the other two methods to ensure the jar is sealed.

Jar Seal Testing Method 3: Look for a Concave Lid

During the sealing process, a vacuum is created, which draws the lid down, and forms a shallow, concave depression from the outer edge to the center. To see if this has occurred, hold the jar level with your eye, and look for an indention at the center of the lid.

  • Sealed: The lid dips towards the center; giving it a concave shape.
  • Unsealed: The lid is flat, or bulging upward. If you see this, the jar will need to be reprocessed.

Reprocess or Use any Jars Suspected of Being Unsealed

Don't gamble with your health. If you suspect any jar is unsealed, store it in your refrigerator and use it within a couple of days; freeze the contents (in a freezer-safe container); or process the jar again.

Before you reprocess a jar, remove the lid, and check to ensure that it doesn't have any nicks or other defects along the rim that may have caused it not to seal properly. If the jar has an issue, transfer the contents to a new jar. If the jar checks out, stick a new lid on it (canning lids can't be reused), and reprocess the jar within 24 hours. Then, recheck the jar to see if it's sealed properly this time.

Once you've verified that your jars have all sealed, they can safely be stored at room temperature, with or without their bands. Here are some free canning labels that you can use to label your jars. If you notice anything unusual when you open a jar of home-canned food (even one that tested as sealed), discard it, and grab another one. It's not worth taking a chance on questionable food.