Two-piece lids, and why the flat is single use
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What each part does
The two parts do completely different jobs and only one of them seals anything. Understanding which is which explains every instruction that follows, including the one about not reusing the flat and the one about taking the band off.
- The flat lid. A ring of sealing compound that softens against the glass under heat and sets as the jar cools. Once it has been compressed into the shape of one rim it will not reliably do it again.
- The band. Holds the flat in position while air leaves the jar. Once the vacuum has formed it is doing nothing, which is why it comes off for storage.
Fingertip tight, and why
This is the instruction most often improved upon and it should not be. Air has to escape during processing for a vacuum to form afterwards, and a band tightened properly prevents exactly that.
Air has to leave the jar during processing. A band tightened hard can stop it, and a jar that could not vent is a jar that may not seal. The instruction in the guide is to adjust the lids, not to tighten them.
Why the band comes off after cooling
Leaving the band on is harmless until it is not, and the reason to take it off is diagnostic rather than mechanical: a band can hold a failed seal in place and hide it. A jar that is holding a vacuum needs nothing to keep the flat in place; a jar that is not will tell you so within a day once the band is off, which is much better than finding out on a customer.
Because a band left on can hold a failed lid flat against the rim and hide a jar that never sealed. You want to find that in your own kitchen.
Continuous thread lids, and where they belong
A standard threaded lid is not a canning lid and will not make a vacuum seal. It is the right closure for a refrigerated product, for a bulk container that is not being processed, and for repacking.
