Eli5, why do Mason jar lids typically come in 2 parts, threads and seal, as opposed to nearly every other commonly used household container?

Question

Eli5, why do Mason jar lids typically come in 2 parts, threads and seal, as opposed to nearly every other commonly used household container?

In: 7899

Answer 1

Aside from everything else you’ve read on here about reusability, there’s another, bigger reason that the lid assembly is in two parts. The seal (disc) has to basically float in place above the mouth of the jar. When you’re boiling your contents, there liquid and gas will expand, and that gas will escape past the loose seal. The threaded ring does help keep the seal in place so it doesn’t move too far to the side, but the ring isn’t completely necessary at this point.

Once the contents start to cool, the liquid and gas will begin to contact once again, taking up less space. The seal begins to suck down the lid against the jar mouth, creating a vacuum effect, keeping out a lot of the nasties from the outside environment. You could probably turn the jar upside down without the ring, and the vacuum would keep the lid on in most cases. Now, you can tighten the ring, and the contents will be safe from the outside environment, and the lid won’t accidentally come loose if it gets bumped.

With a one-piece lid, the seal would lift with the ring, and it wouldn’t allow the seal to come to rest on the jar mouth to get a vacuum seal when the contents cool.

That said, I have boiled stuff and screwed on a one-piece lid right after I turned off the burner, and the vacuum effect did happen.