When A Jar Becomes Unsealed - Penn State Extension

The jars from your canner are sitting prettily on the counter, and you have heard some "pings"—some a bit louder than others. You wait until the jars are completely cool, test the seals, remove the screw bands, wash the jars, label them, and put them in storage. The jars appear sealed, and you are looking forward to the tasty contents this winter. What a disappointment to discover that some of the jars have become unsealed while in storage!

Vacuum Seal

• A vacuum seal should be strong enough that you can lift the jar by the lid.
• A vacuum seal occurs when heat applied to the contents during processing causes contents to expand and drives air from the jar.
• During processing, heated air escapes from under the lid, creating a vacuum.
• As the jar cools, the lid contracts and a seal is formed. You see this contraction as the lid curves down.
• Sometimes you will hear a "ping" as the lid seals. However, floating fruit against the underside of the lid may muffle the sound. Other times a jar will seal in the canner and you don't hear the sound.

False Seals

A false seal is a weak seal that can happen for a number of reasons. False seals occur when the products are not canned correctly, when jar rims are not wiped clean before processing, or if jars are not filled correctly.

• One of the most common occasions for a false seal occurs when hot food is poured into jars, lids are applied and the jars of product are not heat processed. This method is called open kettle canning or the hot fill method; it is not recommended for home food preservation and is not safe. A false seal will often occur with applesauce that is canned this way. You may also find this with jams and jellies that are put into jars without processing.
• Wipe rims of jars with a damp, clean paper towel before applying the lids to make sure the rims are clean. Usually, the towel is dipped in water. However, if you are canning a product that is greasy, like meat, a paper towel dipped in white vinegar helps to cut any grease that may have deposited on the rim. Using a wide-mouth funnel to fill jars helps avoid excess product from getting on the rims or jar threads.
• Use a scientifically research-tested recipe to heat process your canned goods for the specified time and by the method appropriate to the food. Heat processing helps to ensure a safe product by destroying microorganisms and forcing air from the jar as indicated above. If the product is not processed for the entire recommended time, air can be trapped in the jar. In a warm storage space that trapped air can expand and pop the lid off the jar. Under-processing may also allow some microorganisms to survive. They can become active at storage temperatures, producing a gas that can pop the lid off the jar.

Headspace and Siphoning

• Correct headspace is important.
• Too much headspace means all the air in the jar may not be removed during processing, creating a weak seal.
• Too little headspace, and the contents may expand and overflow the jar. Siphoning occurs when the product seeps out of the jar between the rim and the lid. It may siphon in the canner, and you notice the water is colored or see tiny fibers of food in the water; for some products, you will even notice the odor of the food. Other times, the jars siphon when the jar is removed from the canner and you will notice a puddle of liquid surrounding the base of the jar on the counter. Jars will often seal when the product siphons, but there will be product stuck on the rim of the jar and under the lid. This can cause a false seal if the product on the jar rim molds and pushes the lid off the jar rim, breaking the seal.
• Another cause of siphoning is erratic temperature fluctuations during processing. In a boiling water bath or atmospheric steam canner, bring the canner to a rapid boil and adjust the stove temperature to maintain a steady, even boil. After processing is complete, remove the canner from an electric burner, remove the lid from a boiling water bath canner, and allow the jars to sit in the hot water for 5 minutes before removing to a counter. Remove an atmospheric steam canner from heat and allow jars to sit in the canner for 5 minutes with the lid still in place before removing jars to a counter. This allows the temperature in the jars to equalize with the surrounding temperatures and reduces siphoning. In a pressure canner, carefully monitor the canner, making small adjustments in the heat to maintain a consistent pressure. Pressure canner directions include a 10-minute wait after the pressure returns to zero and the pressure regulator or weight is removed.
• If your jars siphon, check the jars for a solid seal. Clean jars thoroughly, being careful to remove all food that may be trapped in the threads at the top of the jar. If food is left on the jar, it will mold. Check these jars occasionally in storage to make sure they stay sealed. Mark the jars to use first. Sometimes a jar can become unsealed months after it was canned.

If you find an unsealed jar in storage, do not eat the product. Stay safe and dispose of it.

References:

Andress, E. & Harrison, J. (2020). So Easy to Preserve, Bulletin 989. University of Georgia.

National Center for Home Food Preservation. (n.d.). "Cooling Jars and Testing Jar Seals". University of Georgia.

Wilber, M.  (n.d.). "What Can Cause a False Seal?" (PDF). UCCE El Dorado County Master Food Preserver, University of California.

Zepp, M. (2024, April 30). "Avoid Open Kettle Canning: Always Process Canned Goods." Penn State Extension.

Zepp, M. (2024, February 12). "Why Allow Headspace When Canning and Freezing Foods." Penn State Extension.