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I have made some food in my pressure cooker and left it in room temperature for over a week. When I opened it, there was no mold. I threw it out because it might have still been bad, but this got me thinking.

Since canning is basically cooking food in a sealed environment, isn't pressure cooking similar when I do not open the pressure cooker? Could the food be left in the pressure cooker at a room temperature for a longer time, given that I pressure cook it every time I close the lid?

apaderno
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MatthewRock
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    This is an interesting question – I hope someone can answer in terms of the science without getting too distracted by the 'no food safety except for government guidelines' thing. – dbmag9 Jun 24 '22 at 07:12
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    Does the pressure cooker boil the food during the cooking process? Did you eat some of it before leaving it for a week? If the lid was left closed then it sounds fairly sterile but if you put a utensil in there, whether to eat or test consistency, then all bets are off. – MonkeyZeus Jun 24 '22 at 15:51
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    It's important to point out that the presence or absence of mold doesn't really tell you anything about the safety of food. – barbecue Jun 24 '22 at 20:53
  • Note that thorough cooking basically forces bacterial growth to start over. If it's in a container that restricts airflow that will slow it down on getting started even more. But the effectiveness tends to be highly subject to environmental variation, which is why canning goes to such lengths to achieve a consistent internal environment. But there are many traditional stew recipes based around bringing the pot to cooking temperature once or twice a day and otherwise letting it sit. Such recipes do require care and attention to detail though since mistakes will turn them into incubators. – Perkins Jun 24 '22 at 21:34

3 Answers3

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When you take the pressure cooker off the heat, it's filled with high-temperature, high-pressure steam. As it cools, that steam condenses, leaving a vacuum. Pressure cooker valves are designed to allow air to enter to fill the vacuum (to avoid damaging the pressure cooker, and to make it possible to take the lid off). So it isn't really sealed once it's not at high pressure.

Sneftel
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    So the moment you open up the valve, the outside air (and all the bacteria) will get inside the food container. The only way to use pressure cooker for preservation is to never open it up, right? – aaaaa says reinstate Monica Jun 24 '22 at 16:37
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    @aaaaasaysreinstateMonica No because as Sneftel said, the pressure cooker automatically allows pressure to equalize so it's not damaged or dangerous - so there is no way to use a pressure cooker as a preservative tool. – SnakeDoc Jun 24 '22 at 17:01
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    @aaaaasaysreinstateMonica just look at things like instant pots, it should be rather obvious that even when "sealed" there's still going to at least some air exchange, you need a lot more than a cheap rubber gasket to stop that. Even at high pressure there are some losses, but too small to matter for the cooking process – eps Jun 24 '22 at 18:09
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    @eps To be fair, it does seem to work for jam jars. – DKNguyen Jun 24 '22 at 22:11
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    @eps no you don’t: jam jars, mason jars, Jerry cans, screw top wine bottles, beer bottles. Tin cans don’t even need the rubber, just tightly folded metal. – Tim Jun 24 '22 at 22:25
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    Surely this answer is technically right. But still we should think of the details: most of the steam will condense while the food is still at a temperature like 90°C (on the metal lid and walls, because these are cooled from the outside). And the air sucked in will also be heated to a considerable temperature (70°C?) by the food. So, it'll still be _pretty_ close to sterile. It's true that “pretty close to sterile” is not enough to store it _for a long time at warm temperature_ because of exponential growth, but it is certainly enough that you buy a lot more time at a slow-growth temperature. – leftaroundabout Jun 26 '22 at 17:15
  • @leftaroundabout One could make an experiment by placing something molding next to the valve which will serve as an inlet and see whether it starts to mold faster inside. Because even late in the cooling process, when the temperature inside cools from 40C to 20C, some air will still flow in and the spores will find ideal conditions for germination. Just not very much air, so chances are only few spores -- unless we provide them. – Peter - Reinstate Monica Jun 27 '22 at 12:46
  • @DKNguyen Very true. But the seal of a pressure cooker is designed "the wrong way around", sealing against over-pressure. It will readily let air in when the pressue inside is lower, as opposed to a jam jar etc. – Peter - Reinstate Monica Jun 27 '22 at 12:49
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The (old-fashioned Central European) pressure cookers I've worked with have outlet valves which in pressure cooking position let gas out (at defined pressure), but not in. These cookers do have an underpressure after cooling which you manually relieve before opening (it's plain impossible to move the lid unless you do that). Either with a lever pushing the rubber sealing or by twisting/screwing the valve into an "all directions open" position. I don't know whether/how long they keep the full bar of underpressure, but after some days you still get the "ffflump" sound when air goes in and the rubber detaches from pot wall and lid.

I'd certainly not expect this to last years like with a jar, but I do use it for several days or a week. Whether you do, is up to your own judgment.

There's also the question of what is in there: keeping fruit from molding until I have time to finish and jar the jelly a week later => fine. The risk here is to have to throw away the food and work, but it's not a risk of not being able to detect if that stuff went bad.
Personally, I also don't have a problem with, say, a goulash kept that way since (and iff) it's properly heated again before eating (the 2nd heat treatment will destroy any botulinum toxin that may have formed - such a twice-cooked-scheme is btw an officially recommended option over here).

For everything else in terms of microbiological contamination, I think the probability of anything I cannot detect by sight and smell getting in without any detectable microbial contamination is negligible. Note that this is the same heuristic as for canning: underpressure OK and no mold, no smell, no bad taste => everything as it should be.

I wouldn't keep fish that way - but then I don't pressure cook fish anyways.

Note also that

  • the official food safety recommendations for private homes over here contain more "know which food is prone to have undetectable problems" and more "trust your cerebellum" for the rest than what I know about North American recommendataions.
  • The local climate would typically allow me to have a closed pot with cooked quinceys stand in a place of, say, 10 - 15°C. Which is not as cool as the +8°C of the fridge, but also quite different from the "never much below +30°C for weeks" in summer when I was living at the Mediterranean Sea.
    (My latitude in North America would translate to the north end of Newfoundland or Vancouver Island, or Regina)
cbeleites unhappy with SX
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Sterilisation usually is done for longer than pressure cooking. The process will still be pretty bad for bacteria, partly depending on how many solid pieces there are in the cooker. After cooling off, the cooker is no longer sealed: the cooker I know has one valve that closes only after a short time of boiling in order to replace air with steam. This valve will be open again after cooling off.

However, the amount of air exchange through that hole will be small, so both processes driven by oxygen and contamination with mold spores will happen at a significantly lower rate than usual.

I would expect botulism bacteria not to survive pressure cooking, and new infections will only arrive with new air so I consider the anaerobic environment required for them to be not there.

Mold can either take hold or not. If it does, it will be from the surface and relevant contamination should be visible.

Other processes only driven by oxygen will turn into discoloration and/or bad smell and/or acidification (which may not be dangerous but usually spells the end of something tasting well).

In short: if it looks ok and smells ok and the cooker was as closed as it will be after depressurisation, my personal "don't waste good food" reflexes would likely prevail, but I'd not hide history and my personal judgment about it from others who possibly might eat from it.

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    The amount of air exchange will be much higher than your intuition. As the pot cools, the head space pressure goes down, drawing cold (tainted) air in. The cold air then falls to the bottom of the pot (where the food is), heats up, and expands, pushing hot (sterilized) air out through the valve. The process then repeats, with the temperature gradient preventing an equilibrium from being reached until the food reaches room temperature. The mechanism is similar to that of a pop-pop boat. – Sneftel Jun 24 '22 at 16:10
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    @Sneftel Do you have any references? I'd expect the exchange to happen gradually and continually, not in big bursts of swapped air. It feels like by focusing on individual components of the process at a time, you're conjuring effects that wouldn't happen in a continuous simultaneously working system. Do you have any evidence that what you describe is actually what happens? – Erhannis Jun 24 '22 at 17:13
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    @Sneftel Wouldn't it just be a steady draw in of outside air as the interior cools? I can't picture a process that would result in such a cycle. The cooler outside air will be heated as it is drawn in, but the air already inside the pot will be cooled by exactly that amount of total energy. That heat will just be exchanged within the pot rather than it being additional heat being added to the system. – Michael Richardson Jun 24 '22 at 18:13
  • Perhaps the easiest intuition is this: In this system, you’ve got a bunch of heavy stuff (cold air) balanced on top of light stuff (hot air). Clearly that’s an unstable equilibrium. But additionally, at the bottom you have the food, actively turning just-fallen heavy stuff into ready-to-rise light stuff. – Sneftel Jun 24 '22 at 18:22
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    @Erhannis It's not going to be exactly that. Because the pressure cooker at the beginning isn't full of air, it's full of steam. And as it cools, the air that gets drawn in to replace it won't have nearly the same expansion coefficient. So the additional contraction of the incoming cold air cooling more steam will outweigh the expansion of the air warming up. Becoming a pulse jet once all the steam is gone requires a fairly exact geometry, which a pressure cooker probably doesn't have (but could). But you're still going to suck in a full volume of air, and then exchange more over time. – Perkins Jun 24 '22 at 21:24
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    @Sneftel There is no way the air flow works like you describe. I think your mistake is in assuming the air goes in a whole bunch at a time, whereas in reality it's a continuous process. Air goes in and certainly doesn't go out again. – Nobody Jun 24 '22 at 21:34
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    Following our "be nice" policy, I'm pointing out that the lack of upvotes is an indication that the community recognizes that you are giving food safety advice based on opinion, personal practice and a tenuous grasp of the science of food safety. One single bacterium in sucked-in air can become billions in a week. Food doesn't have to smell or taste bad to kill you. Botulism spores need a minimum of 100 minutes at 10 PSI in a pressure cooker/canner, and they ***can*** grow in an air-filled pot if the liquid is thick enough. Please: science-based food safety advice only in the future. – MTA Jun 24 '22 at 22:54