Franklin County

                                   May 2009


 

Choose two-piece lids when canning

Although both 1-piece and 2-piece canning lids are appearing on home canning supply web sites, only the 2-piece ring and flat is currently recommended.

The design of 1-piece lids does not allow enough air to escape under home canning conditions, and may result in a weak seal.

The common self-sealing lid consists of a flat metal lid held in place by a metal screw band during processing. The flat lid is crimped around its bottom edge to form a trough, which is filled with a colored gasket compound. When jars are processed, the lid gasket softens and flows slightly to cover the jar-sealing surface, yet allows air to escape from the jar. The gasket then forms an airtight seal as the jar cools.

For sealing success, use flats within one year of manufacture. Flats may be good up to 5 years from date of manufacture if stored in tight, cool containers.

However, expect more sealing failures in older unused lids.

Buy only the quantity of lids that can be used within one year. Follow the manufacturer's directions in pre-paring lids for use. Examine all metal lids carefully. Do not use old, dented, or deformed lids, or lids with gaps or other defects in the sealing gasket.

After filling jars with food, release air bubbles by inserting a flat plastic (not metal) spatula between the food and the jar. Slowly turn the jar and move the spatula up and down to allow air bubbles to escape. Adjust the headspace and then clean the jar rim (sealing surface) with a dampened paper towel. Place the lid, gasket down, onto the cleaned jar-sealing surface. Un-cleaned jar-sealing surfaces may cause seal failures.

Then fit the metal screw band over the flat lid. Follow the manufacturer's guidelines for tightening the jar lids properly.

Do not retighten lids after processing jars. As jars cool, the contents in the jar contract, pulling the self-sealing lid firmly against the jar to form a high vacuum. If rings are too loose, liquid may escape from jars during processing, and seals may fail. If rings are too tight, air cannot vent during processing, and food will discolor during storage. Over tightening may also cause lids to buckle and jars to break, especially with raw -packed, pressure-processed food.

Remove screw bands after jars are cooled. When removed, washed, dried, and stored in a dry area, screw bands may be used many times. If left on stored jars, they become difficult to remove, often rust, and may not work properly again.  

Source: USDA Guide 1, Principles of Home Canning
 

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		University of Missouri Extension Franklin County Site maintained by: Billie Jo Brenner, brennerbj@missouri.edu Last revised: 07/02/08