Frequently Asked Questions
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Canning -- Miscellaneous
How much should I tighten the jar lid before processing?
Most people over-tighten the screw band when canning. The screw band should be tightened just so it is hand tight. No force is necessary. The purpose of the screw band is to hold the lid in place until the vacuum is formed while the jar is cooling after processing. During processing, air is forced out of the jar. Thus if the lid is tightened forcefully, you will impede this important procedure.
If screw bands are too tight, air does not vent properly and when the contents of the jar are exposed to air they darken quicker. Also, over-tightening can lead to the lid buckling during processing as the air tries to escape from the jar. This is seen as a crimp or dent in the lid and the jars will not seal or, if they do seal, they may come unsealed at a later date.