Learn How to Tell If a Canning Jar is Sealed

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, you need to test each jar to ensure that it has sealed properly. The question is, how do you know whether your jars have sealed? Here are the steps recommended by the USDA.

Cooling the Jars

Don't make the rookie mistake of tightening the bands when you remove your jars from the canner and they are still hot. That can 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 will 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 at this point and add more food. That would break the seal, and you would have to reprocess any jar that you opened.

Testing Canning Jar Seals

Once at least 12 hours has passed, you can take off the screw bands. Now is the time to test the seals of each jar. There are 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 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.

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 and there is a vacuum, the jar should give a clear, high-pitched ringing sound when tapped. Give the jar lid a tap with the bottom of a metal spoon.

  • 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

As a jar is sealed, a vacuum is created, and the lid will be drawn downward, forming a shallow, concave depression from the outer edge to the center. To test this, hold the jar level with your eye, and look to ensure the lid has that dip towards the center.

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

Reprocess or Use any Jars Suspected of Being Unsealed

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

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

Once you've verified that your jars have sealed properly, they can 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. It's not worth the risk.