# To check if a jar is air tight
**URL:** https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155
**Category:** Factual Questions
**Created:** [October 20, 2004, 2:44am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155 "2004-10-20T02:44:26Z")
**Posts on this page:** 17
**Page:** 1
### Author:  [@saluki\_fan](https://boards.straightdope.com/u/saluki_fan)
#### Post date: [October 20, 2004, 2:44am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/1 "2004-10-20T02:44:26Z")
What is the best way to check and see if a jar is completely air tight before storing any food in it? Would it be best to buy special jars or can I use old tea and spaghetti jars? Thanks.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [October 20, 2004, 2:56am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/2 "2004-10-20T02:56:43Z")
You should google on home canning. For perishable foods there is little point in a jar being air tight unless a) most of the air is removed and b) you kill the anerobic bacteria (the nasty ones that don’t need air) with heat.
FWIW I use a vacuum sealer with a lid attachment and have had no jars fail to hold a seal as long as I use new, undamaged mason jar lids. Some sauces come in mason jars but don’t use regular jars for heat canning.
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### Author:  [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)
#### Post date: [October 20, 2004, 3:02am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/3 "2004-10-20T03:02:31Z")
one way to do it would be to put some water in there and carefully heat it in the microwave until it steams up and see if any steam is leaking. You have to be careful because if it goes too much overpressure, then it explodes which is nasty.
In general though, a slightly un-air tight jar is not going to worry you to any significant extent for food preservation.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [October 20, 2004, 4:19am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/4 "2004-10-20T04:19:35Z")
**Shalmanese** , I think that is an incredibly bad idea for the reason you already stated. In any event even if you don’t end up with a broken glass bomb which mutilate/blind/maim you it’s an invalid test. Food is never stored in jars under pressure but vacuum. If you put positive pressure inside the jar it will push the lid outwards and likely cause it to leak. With a partial vacuum inside the jar the lid forced against the mouth of the jar by atmosphere pressure.
**Unless you have a death with do not heat up a sealed glass jar!!!** The SDMB will not be responsible if you do this.
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### Author:  [@Gary\_Robson](https://boards.straightdope.com/u/Gary_Robson)
#### Post date: [October 20, 2004, 5:07am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/5 "2004-10-20T05:07:02Z")
> [@Shalmanese](#):
>
> one way to do it would be to put some water in there and carefully heat it in the microwave until it steams up and see if any steam is leaking. You have to be careful because if it goes too much overpressure, then it explodes which is nasty.
Canning jars generally have metal lids. Are you _really_ suggesting that a jar with a metal lid should be heated in a microwave?
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [October 20, 2004, 5:49am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/6 "2004-10-20T05:49:29Z")
If you’re storing macaroni, flour or sugar in a jar, a tiny leak won’t hurt it. If you’re storing cooked green beans, peaches, or tomato sauce, you really need to read up on [home food preservation](http://www.uga.edu/nchfp/index.html).
That said, you can get a good idea of whether a jar is air tight by filling the jar with water, capping it, and letting the jar sit upside down. If water leaks out, air will leak in.
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [October 20, 2004, 6:00am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/7 "2004-10-20T06:00:54Z")
> [@InvisibleWombat](#):
>
> Canning jars generally have metal lids. Are you _really_ suggesting that a jar with a metal lid should be heated in a microwave?
You could. I wouldn’t recommend it because of the pressure issue, but the the metal lid in and of itself would pose no danger as long as it’s not too close to the cavity wall. Smooth metal objects with no gaps in them are generally ok in the microwave. Of course, food in metal containers won’t get very hot, since metal reflects microwaves.
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### Author:  [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)
#### Post date: [October 20, 2004, 6:18am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/8 "2004-10-20T06:18:32Z")
> [@Padeye](#):
>
> **Unless you have a death with**
…or a lithp…
> [@](#):
>
> **do not heat up a sealed glass jar!!!**
Heh. Can’t you just fill your sink with water, hold the sealed jar under the surface and look for air bubbles?
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [October 20, 2004, 7:59am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/9 "2004-10-20T07:59:14Z")
Put some hot(NOT boiling) water inside the jar - to about one-third full - and put the lid on.
Leave overnight - as the contents cool down (particularly the air that the water has heated up), they will contract, creating a partial vacuum.
The next day, open the lid - if it makes a ‘suck’ sound, it’s good.
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### Author:  [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)
#### Post date: [October 20, 2004, 8:26am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/10 "2004-10-20T08:26:03Z")
> [@Mangetout](#):
>
> The next day, open the lid - if it makes a ‘suck’ sound, it’s good.
So ‘suck’ is good?
Ah, you crazy kids with your bebop slang. Now get off my lawn!
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [October 20, 2004, 8:27am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/11 "2004-10-20T08:27:58Z")
Suck is always good. Oooh yeah baby.
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### Author:  [@t-keela](https://boards.straightdope.com/u/t-keela)
#### Post date: [October 20, 2004, 8:46am UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/12 "2004-10-20T08:46:22Z")
> [@saluki\_fan](#):
>
> What is the best way to check and see if a jar is completely air tight before storing any food in it? Would it be best to buy special jars or can I use old tea and spaghetti jars? Thanks.
First of ALL… what are you planning on canning? What kind of food are you gonna store? Is it something that needs actual canning or are you simply gonna put noodles or beans and stuff in them. That’s the most important thing to know.
If it is something you need to be airtigh and not necessarily canned just buy a new stack of lids and rings. But for most nonperishable items, they don’t need to be in an vacuum sealed container. If you simply wany to keep dust, moisture, “bugs” or whatever out a well fitting lid’ll do the trick.
To seal them…with new lids and rings you should fill the jars with whatever is gonna be stored and sit them in a large pot of water. The water should only be high as it will sit win the pot without tipping over. If the content were fluid you’d be able to fill it more. (BTW be sure to sterilize the jars if you are attempting to can something)
Bring the water to a low boil and turn the heat off. Put the lids on the jars. As the jars cool off it’ll provide a suction which will cause the lids to “pop” when they do. You’ve got an airtight seal.
If you’re gonna do any real canning however…go get a canning book. _Ball_ mason jars usually has one available where you buy their jars and lids. They’re real cheap. Most good cookbooks include a section on canning. Matter of fact I canned a gallon of jalapenos yesterday. (4 quarts actually)
🙂
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### Author:  [@missbunny](https://boards.straightdope.com/u/missbunny)
#### Post date: [October 20, 2004, 2:10pm UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/13 "2004-10-20T14:10:18Z")
What do you mean by “storing” food? Do you mean you’re canning? Or do you mean you open a can of peaches and want to save the ones you don’t eat for a day or two? If it’s the latter, I think you can use any old jar whether it’s 100% airtight or not. If you are actually doing canning, you need to get special jars and follow the canning instructions exactly or you’re at danger of contracting botulism.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [October 20, 2004, 3:06pm UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/14 "2004-10-20T15:06:32Z")
> [@Bryan Ekers](#):
>
> …or a lithp…
>
> Heh. Can’t you just fill your sink with water, hold the sealed jar under the surface and look for air bubbles?
I had a speech impediment as a child and I’m a little sensitive about it so I’d ask you not to call attention to it. \
If there is no pressure differential you probably won’t see bubbles. I think **Mangetout’s** method is best.
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### Author:  [@saluki\_fan](https://boards.straightdope.com/u/saluki_fan)
#### Post date: [October 20, 2004, 9:07pm UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/15 "2004-10-20T21:07:05Z")
What I am canning is some freshly dehydrated fruits and vegetables. I am thinking that since they are already dry putting them in an old Nestea or Prego jar would be fine. The reason why is, because I once heard that putting dehydrated fruits and vegetables in the cold (not necessarily the refrigerator due to the high humidity levels) made them really good. Well I don’t have a really cold room so I want to use my refrigerator, but I need to make sure the jars are airtight and that’s why I ask.
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### Author:  [@missbunny](https://boards.straightdope.com/u/missbunny)
#### Post date: [October 20, 2004, 9:15pm UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/16 "2004-10-20T21:15:38Z")
You’re just storing them, then, right? Not “canning” them? You mean you took fresh fruit and dehydrated it in the oven or in a fruit dehydrator? If so, then you just need a jar then with a screw-on lid. An old jelly jar is fine. Or a Rubbermaid or Tupperware container. Or a Ziploc bag.
“Airtight” for this purpose is just airtight as much as possible. It doesn’t mean the jar has to have a vacuum seal like you’d need if you were actually canning something. If you are concerned about the jar, make sure it’s dry on the outside, put some water in it, put the lid on, and then turn it over and see if your hand gets wet. If not, then it’s airtight enough for purposes of storing dried fruit.
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### Author:  [@t-keela](https://boards.straightdope.com/u/t-keela)
#### Post date: [October 20, 2004, 10:38pm UTC](https://boards.straightdope.com/t/to-check-if-a-jar-is-air-tight/270155/17 "2004-10-20T22:38:19Z")
Another way of storing stuff like you’re talking about that I do sometimes. I’ll take a ziplock storage or freezer bag and put my food in it. Then I’ll take a straw and put it down the inside corner of the bag and zip the bag closed up to the straw. Then I’ll suck the air out of the bag and zip it shut as I pull the straw out the rest of the way. It works pretty good. If you do this with your bread, it’ll stay fresh a lot longer.