Canning Lid Anatomy

• Today's canning jars are sealed using a thin steel lid and a separate screw band. The underside of the lid's slightly flanged edges is coated with a grainy rubber compound that creates a seal with the jar's upper rim. The separate screw band keeps the lid in place during processing in a water bath canner or a pressure canner. You can use the lids once for canning purposes, and the screw bands indefinitely, as long as they show no rust, dents or pitting.

Canning Science

• During canning, heat builds up inside the jars, essentially cooking the contents and destroying harmful bacteria. Steam also builds up inside the jars, and is released slowly through tiny pores in the lids' rubber coating. Once all the air has been exhausted from the jars to create a vacuum, the rubber coating on the lids expands, forming a tight seal. New, unused lids are up to this task and form a tight seal readily, while older lids may not hold up as well and might not seal properly.

Testing, Testing

• One of the most reliable ways to know if your canning lids are still good is how they behave once the jars have been removed from the canner. Good lids will give off the characteristic ping or pop as the jars cool, and show a slight depression in the center, indicating that no air remains in the jar and the foods are completely sealed. Old or bad lids, on the other hand, bulge upward and make no sound. In some cases, the lid may not be the culprit. The rim of the jar itself may have a tiny nick in it that wasn't visible, which would contribute to a poor seal even with a good lid.

Environmental Issues

• Canning lids kept in a cool dry place should last indefinitely, but certain environmental conditions may contribute to their deterioration. Fluctuations in temperature, humidity levels or exposure to extreme heat or cold can compromise the rubber coating, causing it to dry, turn brittle and crumble. Always check canning lids before using them by running your finger lightly around the rubber coating. If it appears loose, or bits of it fall off, the lid is not usable for canning. Use it to store foods in the refrigerator or dry goods in the pantry, and use any foods in jars that did not seal properly immediately or refrigerate.