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I am talking about the process, where you cook food, for example, a vegetable and conserve it in a jar. The jar can then be stored in a non-refrigerated area.

Many recipes include adding salt, vinegar or some other preservative to the jar.

The process I am talking about only uses water and heat.

I am trying to do this for a third year, each time I have multiple jars going bad.

Most tutorials include the filled jar being heated up (in a water bath) and then (or even while heating) being turned on its head and being let to cool in this position until it is ready to be stored away.

Most tutorials mention, that there is a vacuum being formed, and the jar being on its head somehow helps the process.

It looks something like this:

enter image description here

The way I understand it is that during the heating process the air in the jar expands and escapes the jar, which reduces the air.

What I don't understand is why this is necessary, obviously there is no perfect vacuum and why the jar needs to be turned on its head?

Also, since the jar is cooked and all bacteria killed, why is air a problem?

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Generally turning a jar upside down after filling is a alternative to a proper hot-water bath, not an addition to it. This is known as the inversion method, and is used by many cooks of jams, jellies, mustards, and other very-low-risk canned foods.

The idea of inversion canning is that, by forcing the air in the jar through the hot liquid, you can kill and bacteria in the air (and on the inside of the lid). It is not considered safe by experts, because there are numerous ways that bacteria can survive the process.

If you are doing "proper" canning, that is heating up the lidded jar to 85C or above, the sealing, inverting the jars is in fact a bad idea as it could result in liquid preventing the jar lid from sealing properly. The Ball jar corporation says “Do not invert. move or store jars while cooling, as this may cause seal failure.” At the least, it's unnecessary.

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    Incidentally, the photo you picked for this is an example of completely-totally-unsafe canning. – FuzzyChef Oct 2 '18 at 3:05
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    @ FuzzyChef, to clarify your comment: the reason you say the photo represents unsafe canning is ... the re-use of lids that originally came on commercially canned foods instead of using fresh sealing-caps and rings to tighten them down ? – Lorel C. Oct 2 '18 at 4:25
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    It should be made clear that inversion canning (instead of) proper waterbath processing of canned food is NOT a safe method of canning. It is an old outdated method that will not reliably result in a safe and shelf stable product. Inversion AFTER waterbathing is unnecessary and is more likely to cause seal failure as the jar contents can seep under the lid. – BunnyKnitter Oct 2 '18 at 15:46
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    @LorelC. the contents appear to be largely tomatoes, which are a low-acid food and therefore high-risk for botulism when canned. They can't even be safely prepared in a conventional water bath; you need a pressure canner which can raise the exterior temperature past the normal boiling point. On top of that, these are re-using lids as you mentioned and they appear to be barely cooked at all. It somehow combines almost every single potentially-lethal mistake you can make. – logophobe Oct 2 '18 at 19:35
  • Logophobe is exactly right. – FuzzyChef Oct 2 '18 at 19:44
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I agree with the above response, in that it is safer to sterilise by immersing the entire jar in boiling water. However, in answer to your question, the idea is to sterilise the lid and inner rim of the jar after sealing by getting them to >85°C for 10 mins. This only works if you make sure you bottle your preserve when far hotter than 85°C so that it takes longer than 10 mins to drop below that temp. Assuming you have steam or oven sterilised your jars before bottling, you are only killing the stray bug that floats into the jar in the brief moment when you open it and pour in the preserve before replacing the lid.

  • I did immerse the entire jar, but heads down. Guess that's unnecessary? – user1721135 Oct 2 '18 at 6:56
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    @user1721135 Waterbathing upside down is a new one to me. That would heat okay but would result in no vaccum being able to form and contents likely seeping out. Waterbath them upright and ensure the water is at least 1" above the jars. – BunnyKnitter Oct 2 '18 at 15:47

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