Michigan State University Extension
Preserving Food Safely - 01600498
10/13/97
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. Gaskets in unused lids work well for at
least 5 years from date of manufacture. The gasket compound
in older unused lids may fail to seal on jars.
Buy only the quantity of lids you will use in a year.
To ensure a good seal, carefully follow the manufacturer's
directions in preparing 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. Uncleaned jar-sealing
surfaces may cause seal failures.
Then fit the metal screw band over the flat lid.
Follow the manufacturer's guidelines enclosed with or on the
box 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 screw bands are too loose, liquid may escape
from jars during processing, and seals may fail.
* If screw bands are too tight, air cannot vent
during processing, and food will discolor
during storage. Overtightening also may cause
lids to buckle and jars to break, especially
with raw-packed, pressure-processed food.
Screw bands are not needed on stored jars. They can be
removed easily 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.