Eli5, why do Mason jar lids typically come in 2 parts, threads and seal, as opposed to nearly every other commonly used household container?
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There are plastic one-piece tops you can buy for mason jars for non-canning purpose, they’re amaaaaazing
If it was a one piece contraption, you would be dragging the sealant material (it’s kind of rubbery) over the glass rim as it tightened. This would lead to more cans with a faulty seal and then ruined food.
The two piece set up lets the rotating part tighten down on the lid that remains stationary, so the seal remains intact more often.
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It’s not just a container, it’s made for canning. They do sell one-piece plastic screw-on lids if you’re just using it as a container. But if you’re canning you use the two-piece as such:
* Put stuff in jar
* Put on lid and loosely screw on the ring so that it’s just keeping the lid on
* Heat the jar, usually by boiling, which will heat contents and create steam under the lid, which escapes from the snug-but-not-tight seal. This also kills the bad bugs.
* Remove from heat and tighten the ring to press lid against jar
* After jar cools down, the steam condenses, and there will be suction against the lid if you’ve done it right (overfilling is a common reason you didn’t).
* You can now remove the ring and reuse it.
* Months or even years later when you open the jar and break the suction, you can use a ring to seal the lid against it again when in the refrigerator. Or you replace it with a plastic screw top.
* Discard the lid when you’re finished because it may not seal right if used again
And the whole time you’re doing this you’re going to feel like a surgeon because you have to keep everything sterile, like boiling the jars, lids, and rings before you use them, and not touching the lids or insides of rings with your hands. They make a nice stick with a flat magnet on it to help pick up the lids and set them on the jars.
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.
In the olden days, people would place a piece of decorative cloth between the threads and the seal.
Home canning in a nutshell:
While you’re preparing the food to be canned (a batch of my jelly, for instance) you put on a large pot of water to boil, with the jars inside to preheat. When you’re ready to pack the jars, you remove them from the hot water, pack in your food being careful to remove any trapped air, and you leave some air space at the top, then you put on the flat lid and just finger tighten the ring.
Once all jars are filled, you put them in the water and boil the jars for several minutes. This accomplishes three things:
1. The boiling heat sterilizes the contents of the jar. Any microorganisms that would cause the food to spoil or would create dangerous toxins, including botulism, is killed.
2. It causes the air inside the jar to expand, and some of it farts out between the seal and the jar.
3. It softens the sealing compound on the rim of the lid, which causes it to conform perfectly to the rim of the jar.
When the jars are removed from the hot water, the air inside cools and contracts, forming a vacuum on the inside, strongly forcing the lid onto the jar, forming a hermetic seal. Everything in side is dead, and nothing alive can get in.
You let the jar cool for awhile, undisturbed, and then remove the rings to inspect the seal. You shouldn’t be able to easily brush the lid off, to declare the contents safe for room temperature storage, the vacuum MUST be strong enough to form a tight seal. With a 1-piece lid, there is no way to perform this inspection; removing the screw thread breaks the seal.
For more information, I recommend reading Ball’s Complete Book of Home Preserving. It gives good, in-depth explanations of home canning technology and technique, as well as hundreds of recipes that have been scientifically verified as safe for canning at home.
The ring holds flat disk on during canning. Take the ring off after canning so you can see if the seal ever fails on the disk. The ring is reusable, the disk is not.
You’re focusing on the lid. Yes it’s two parts but as a whole, the jar only works with the lids and vise versa. The glass can be slowly heated up and filled and then apply your two favorite parts. The seal and the threaded capper. As it cools, it seals the contents inside creating a vacuum. combine these three and you have yourself food for up to two years
The point is to remove the oxygen from the container so that nothing inside spoils.
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