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While canning tomato sauce today using the water bath method, three lids "pushed outward" while in the water bath. After they were removed at the end of the canning time, they all seemed to seal, but still have a dent where they were pushed out. What caused this and is it safe to used this sauce?

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The cause of the lids bulging during processing is pressurized air trapped in the jar. This happens when the lids are too tight (not leaving sufficient head space can also be a factor). The bands should only be finger tight - From FreshPreserving.com

CLEAN mason jar rim and threads of jar using a clean, damp cloth to remove any food residue. Center lid on jar allowing sealing compound to come in contact with the jar rim. Apply band and adjust until fit is fingertip tight. Place filled jars in canner until recipe is used or canner is full. Lower rack with jars into water. Make sure water covers jars by 1 to 2 inches.

Creased or dented lids have a high failure rate, as such, storing them on the shelf would not be considered safe.

To be safe, the jars with buckled lids can be reprocessed with new lids within 24 hours, refrigerated and treated as if open, or the contents could be frozen.

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