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Food Safety Feature Articles


Canning jars, two-piece lids & potPrevent buckling canning lids

Janet Hackert, Nutrition and Health Education Specialist, Harrison County, University of Missouri Extension


In recent years canning lids have changed several times so the recommendations for using them and sealing jars successfully have changed as well. Perhaps the most common concern is buckling lids, but this problem can be prevented.


The USDA recommends using two-piece lids. One piece is commonly referred to as a flat. It is a relatively flat circle of metal with an inner liner and a ring of sealing compound that resembles rubber. The other piece is a metal ring that is screwed down over the flat and jar to secure the flat to the jar during the canning process.


USDA guidelines say to fill the jar according to the procedure for the particular food that is being canned, wipe the rim of the jar with a clean paper towel to remove any liquid or particles that may be present, and place the flat on the rim of the jar. Then tighten the ring. Although one might want to tighten the lid as tightly as it will go, manufacturers recommend finger-tip tight. This means that once the ring stops turning freely, it is tight enough.


The lid needs to be adjusted properly in order for the canning process to be effective. During canning, air trapped in the headspace between the bottom of the lid and the top of the food is forced out of the jar. When lids are too tight, the air cannot easily escape so it forces its way out by deforming the lid. This leads to the buckling or crinkling effect. The seal on a jar with a buckled lid has a very high failure rate so this food would not be safe to store.


Also, lids sold these days should be warmed but not boiled. The sealing compound is warmed in steaming hot (NOT boiling) water. Softened, it is then ready to conform to the shape of the lip of the jar. Boiling water, on the other hand, causes that compound to melt so that it has already begun to change shape before being placed on the jar. This often leads to the jar not sealing properly.


When canning, follow USDA and manufacturers recommendations to increase the likelihood that all jars have sealed properly and no food goes to waste.


After canning is complete and jars are cool, remove the ring band for storage. Ring bands left on in storage may rust shut.


Store jars in a dry, dark, cool place, ideally 50-70 F. Dampness may corrode metal lids, break seals and spoil food.

For more information, refer to the MU Extension publication Quality for Keeps: Steps to Success in Home Canning (GH 1452). You can also contact Janet Hackert at 660-425-6434 or or contact your local MU Extension office.


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Last update: Tuesday, August 12, 2014