Canning season sparks calls about cracks

Posted: Sunday, July 08, 2001

We are approaching an intense fish canning period here on the Kenai Peninsula. One of the most frequently asked questions during this time has to do with jars breaking during processing in the canner. Whenever callers phone with this problem, I ask them to describe the appearance of the crack. The appearance of the crack itself will give clues as to the possible causes.

Thermal shock breakage is characterized by a crack running around the base or lower part of the jar and sometimes extending up the side. Possible causes are: 1) filling cold jars with boiling liquid and food; 2) filling hot jars with cold food, water or syrup; 3) not using a rack in boiling water bath canner or pressure canner; 4) setting hot jars on a cold surfaces; 5) setting hot jars to cool in a cold draft; 6) splashing hot jars with cold water; 7) cleaning jars with soap impregnated steel wool; or 8) using metal utensils in the jars. The latest Ball Blue Book recommends after the canner has depressurized and the lid has been removed you let the jars set in the canner 5 to 10 minutes to adjust to the lower temperature. This may also help to prevent thermal shock breakage.

Impact breakage is characterized by cracks radiating from the point of contact. Some of the possible causes are: 1) rough handling of jars; 2) dropping, hitting or bumping glass jars; 3) damage by cleaning with soap impregnated steel wool; 4) glass scratched by dirt or sand on food; 5) glass scratched by jewelry; or 6) using metal utensils to pack food or to remove food for serving.

Pressure breakage is characterized by the origin of the break on the side. It is a vertical crack, which divides into two forks. Possible causes are: 1) using the oven to process home canned foods; 2) not enough headspace to allow food to expand during canning or freezing; 3) fluctuating heat during pressure canning; 4) moving petcock or weight before all pressure has been released; or 5) force-cooling pressure canner with cold water or air.

Handle jars carefully. Use non-abrasive sponges and cleaners when cleaning jars. Use non-metallic utensils to pack food in jars, remove air bubbles from jars prior to canning and to remove food from jars. Repeated contact with metal spoons or knives can weaken jars. Remove jewelry such as rings and bracelets when handling jars.

People also call our office with questions regarding canning jar lids. The lids should be placed in hot water, which means not quite simmering, for at least 10 minutes. Thirty minutes isn't too long for lids to set in hot water. Do not boil. The screwbands do not need to set in hot water but they should be clean and free of rust.

"Fingertip tighten" as directed on many lid packages means to tighten the screwbands with your fingertips to just where there is a slight resistance and then turn about 1 to 1-1/4 inch. Do not over-tighten because the lids will buckle which can cause sealing failure.

Kerr Glass Manufacturing produced an excellent fact sheet on jar breakage. For a free copy of this fact sheet or for more information about food preservation contact the Cooperative Extension Service, 262-5824, or 1-800-478-5824. Our office is located at 43961 K-Beach Rd., Suite A, Soldotna, AK.

Linda Athons is an agent at the Alaska Cooperative Extension office on Kalifornsky Beach Road in Soldotna. She is a home economist and involved in the 4-H/Youth Development programs.

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