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Let's say that I have a pot of boiling stew. I then pour this, still boiling, inside a thermos bottle, and close the lid right away.

Everything inside the bottle should be sterile at this point, due to the high temperature which will be maintained for over an hour, correct?

Let's say then that I let this sit for maybe 24h. In this time frame, the temperature of the stew will fall in the danger zone between 60°C and 5°C, and stay there for hours and hours. Usually, that would mean that the meat in the stew is no longer fit for consumption.

However, given that the content of the bottle was earlier sterile, would it still be safe? Can bacteria contaminate the content trough the lid?

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The assumption that it was sterile is wrong. Standard cooking leads to a reduction of bacteria to about log6, so 1 in 100 000 survives. Afterwards, these bacteria multiply exponentially in your soup, potentially achieving levels at which people can get sick.

Your logic will apply to canning. But it is known that the "fill the cooked food into an airtight closing container" is an unsafe canning method even for high acid foods, and a thermos bottle is not guaranteed to be closed airtight. If you want to can soup, you have to follow safe canning procedures, which include a restriction of the ingredients you can use, a restriction in the liquid/solid ratio, the prescription of proper containers, and a sufficiently long sterilization step in a pressure canner.

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  • What makes this 1 in 100000 of bacteria able to survive in 100°C water for extended periods of time?
    – Michael
    Mar 16, 2019 at 18:27
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    @Michael Humans shed about a million particles of biological material per hour, and many of those will be colonized by bacteria. In an ordinary kitchen environment, some of that material will have ended up in the thermos without ever having been in the stew.
    – alephzero
    Mar 16, 2019 at 19:23
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    (a) A 6 log reduction means 1 in 1 000 000 survives. (b) A 6 log reduction of vegetative organisms is often called pasteurisation [en.wikipedia.org/wiki/Pasteurization ] (c) safe storage of low acid foods in the absence of air requires killling spores. See "Introduction for Consumers: A Snapshot" on page 3 of "The Bad Bug Book" [fda.gov/downloads/Food/FoodborneIllnessContaminants/…
    – user20637
    Mar 16, 2019 at 20:12
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    @alephzero that doesn't explain it though, even those bacteria added in from the air would cook inside the hot liquid in the thermos. Of course there are other concerns like spores and the container not being air-tight (as explained in the comment above and the answer).
    – JJJ
    Mar 17, 2019 at 6:08
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    There is no explanation of "how", and it is possibly a different combination of causes for each individual bacterium. This is just a general property of biological systems for to their high complexity. Just like individual people have been known to survive over 6 minutes without oxygen in the brain, or to spontaneously heal from cancer. The same initial conditions lead to different outcomes in different individuals, and sometimes the outcome can be radically different.
    – rumtscho ♦
    Mar 17, 2019 at 7:15

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