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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Seals are not reusable, while the threads are. It saves a little bit of money making them separate, instead of having to buy a complete new lid every time you want to use the jar.
TL;DR – the sealant coating on the lid is really only single-use; it’ll never work well a second-time around.
Since mason jars are _intended_ to be re-usable over and over; however the act of sealing the jar + food residue may deteriorate the sealing coat on the bottom of the lid – and then they don’t seal as well and maybe that jar spoils. Wheras, unless you’re not careful with the “rings” (leave food residue on them, let them sit in water and they rust), they’ll last a long time, so you can reuse the rings over and over. So each time you re-use a set you use new lids, reuse the jar and ring. Its a bit cheaper if you’re a serious canner/preserve maker as you can buy the lids in bulk for not a lot.
I don’t can/preserve but a lot of my family does and I get the benefit – so I always have a collection of mason jars in my cupboard for re-use. However if Im storing leftovers, Ill actually use single-piece lids from sauce jars (Prego lids fit nicely) on my mason jars so Im not always burning new mason jar lids everytime I seal something up that I intend to be opening frequently.
The seal bit is single-use when canning, where the contents is heated before sealing to kill bacteria which would cause it to spoil. As it cools the pressure goes down which creates suction holding the seal on to the can.
The threaded bit is reusable and can/should actually be removed once the jar is sealed. If the seal is disrupted then you will know as the seal fails, but if you left the threaded bit on then it could reseal and you wouldn’t know the jar was contaminated.
The Mason jar has three parts: the jar, the lid, and the ring. They are made for preserving food by canning. That process seals the lid to the jar very tightly (there’s a little plastic seal on the lid, and a vacuum in the jar that holds it tightly in place). The ring is there to protect the lid and make sure the seal doesn’t break and to hold the lid after your break the seal to open the jar.
Popping the lid off requires using something (I use the edge of a spoon) to pry up the edge of the lid to break the seal. Doing this can damage the seal material, nick the lid’s metal, or bend the lid — all things that would make it unsafe to use for canning again (microorganisms can sneak in). However, the rings and jars can be reused many times over.
So, the lids are sold separately because they aren’t safe to use over and over, whereas the rings and jars can be reused for years.
I’m thrilled that redditors understand the concepts of jar canning. The part they are leaving out, however, involves the insane mother who demands that father heat the juice, etc., outside, carry the boiling mixture inside to sit on her stove, and as she fills the jars, she removes the lid and seal from a percolating pot of boiling water, affixes both to the full jar, and then hands it to father for final tightening and removal to the only refrigerated room in the house.
This might sound like a lot, and I know it was way over the top, but we never had a canned good go bad and we always had too much to eat.
I was going to mention my father died way too young (59), but it would be gratuitous to attribute his early death to my mother’s canning procedures. 🙂
Edit: By the way, if you carefully scrape the seal, you might be able to reuse it. Not recommended.
Actually in my country the lid is one piece, is either screw on or uses a weird press contraption to get on and if you want to reuse it, you need to boil it before use. That sterilises it and softens the rubber. Once the rubber changes color and/or the lid cracks, you toss it and use new one, but you can safely get 5-10 years of use of a lid.
Also, to open the jar, you have to leverage a knife tip under the edge of the lid to let some air in, otherwise it will literally not budge.
In addition to being a single use item, the lid (flat) has a distinct function when the jars are processed. As the contents of the jars heat up, air in the jars is expelled under the lid and a vacuum is formed (more on this below). At about this time, the sealing of the lid takes place and the ring keeps everything in order until cooling. Then the ring is removed.
When adding the lid and ring to the jars, the instructions are for a “finger tight” seal of the ring. If the ring is too tight the lid won’t let air escape and can jeopardize the seal, and the vacuum.
I’ll sometimes use the containers for short term storage or freezing. In those use cases, I buy reusable plastic 1-piece lids. (Think sauces, stocks and duck fat!)
Their designed for canning.
When you can something you boil the jar under water. The liquid inside starts to evaporate and exits through the seal. You then pull it out of the water and it cools and condenses. This creates that pressure differential that causes the top to pop when you open it.
Before WWII they were one part reusable rings saved metal. You can still purchase one piece lids.
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