Why A Canning Jar Lid Will Come Unsealed

A canning jar with a failed lid is something I’ll come across once in a blue moon. It’s a fairly rare occurrence and is always a rude surprise.




When I find a jar of home canned food in my pantry with a breached lid I dispose of the food carefully. Well out of the way of people or animals.

Unsealed Lid

A Failed Lid On A Jar Of Lentil Soup

After the offending jar is emptied, it’s thoroughly washed and then closely examined for nicks around the rim or hairline cracks. A Mason jar must be near perfect for a lid to make a good strong seal.

With a faulty jar the vacuum inside the jar is eventually breached and allows spoilage to occur. Bacteria and yeast will begin to grow inside a jar causing fermentation and gas. Sometimes the force of the gas will blow a lid off and sometimes knock the lid to the side of a jar.
Slime and  mold on top of any food product tells me that the jar has been opened for a while.

Failed Lid

The Lid Failed On This Jar

Usually if there is a sealing failure with a jar I’ll catch it with my “lift test” after the jars cool from processing.
My lift test consists of me lifting a jar about 2″ above a table or countertop by the rim of the lid. If the lid holds and doesn’t come loose the seal is good.

Lift Test

Testing A Jar Seal

There are many reasons for a faulty seal on home canned foods.

  • Sometimes jars will not seal properly if the rim of the jar is not wiped perfectly clean before the lid and band are applied.
  • Also if the one trip lids are not simmered long enough problems can occur. The red rubber lining on the lid must soften up a bit for a really strong seal. I simmer lids 3 -5 minutes.
  • If grease or some other material from the food product becomes forced under the lid while processing, many times the lid will not give a good seal. This past summer I had 4 or 5 quart jars of ham & bean soup, from 2 different batches, that gave that kind of trouble. Grease had been forced between the lids and rim of the jars and prevented sealing. When I get food that doesn’t seal or pass the “lift test” I simply freeze it. It’s too much trouble and a waste of LP gas or wood to re-process it.
  • Canning jars that have not been kept hot enough before packing or suffer inadequate processing time will sometimes cause a lid failure. This is especially true when processing cold pack food.
  • Sometimes there is an unseen hairline crack in the jar or a nick on the rim. This is I believe what happened to the jar of green beans above.
  • Often the reason will remain unknown and it can be a guessing game.