Vacuum Seal

• Metal self-sealing lids come in two parts -- a flat metal lid and the metal ring that holds it in place. A gasket around the metal lid contains a compound that softens during processing. As the compound softens it covers the lip of the jar, forming an airtight seal as the jar cools. Once the vacuum seal forms, air and moisture are unable to enter the jar and bacteria are unable to grow. The pop that you hear is the creation of this vacuum seal.

Pop Test

• If by chance you are distracted and miss the popping sound when it happens, try this simple test to check that the jar did indeed seal. Once the jar is completely cool, take a close look at the lid. A sealed lid should have a clearly defined depression in the center. When you press the depressed area with your finger, it should not have any spring to it. If it does, do not put the jar in storage.

No Pop

• A loose lid puts the food inside the jar at risk of spoilage. These jars need to go immediately into the refrigerator for storage. Because air can enter the jar, dangerous bacteria are free to grow. Refrigerating the canning jars slows the growth of the bacteria, keeping it safe to eat for two to three days. Refrigerated canning jars are safe to reprocess within 24 hours. Freezing will keep the food safe to eat indefinitely.

Worn Gaskets

• The gasket of an unused canning lid has a shelf life of about five years before the gasket compound dries out and no longer forms a seal. Examine all lids before canning, throwing away any that contain dents or have gaps between the lid and metal ring -- a sign that the gasket is worn. A simple rule of thumb is to only purchase as many lids as you will use in one year, so you will have no sealing worries.