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My in-laws have a practice in which they make soup with the lid on the pot, turn off the heat, and leave the pot on the stove without lifting the lid until the next day when they eat it.

Their claim is the heat destroys any existing pathogens, and the lid prevents outside contamination, and that the contents are thus safe to consume.

I found this all a bit dubious. It seems to me that if the lid did have such a good seal, there would be a pressure differential giving significant resistance when trying to remove it.

Is this a safe means of short-term room-temperature preservation?

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This seems to be close enough to cooking.stackexchange.com/questions/19183 to consider it a dupe. –  Peter Taylor Dec 5 '11 at 16:50
    
There definitely would be a pressure differential, and it'd be in the several PSI range. -2 PSIg on a 10-inch lid gives ~160lbs (assuming I'm doing my math right). You'd need to pry the lid off to break the seal. Look also at @PeterTaylor's link. –  derobert Dec 5 '11 at 16:56

4 Answers 4

The seal, the pot, none of it really matters.

You don't sterilize food when you cook it, you pasteurize.

Even if it was perfectly sealed, there's still baddies that didn't get killed. Pasteurization brings food bateria to a safe level, not a 0 level. Once food gets back in the 'danger zone', it can start growing pretty quick again.

This is the same reason you can't cook a sous vide meal and then leave it on the counter forever.

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The reason you find it dubious is because you're right.

The lid will not keep pathogens from entering the system. And even if it did, cooking the soup is not sterilizing the system, but just killing enough pathogens in it for it to be safe for consumption. There will be bacteria in the pot that survived the cooking, and once the temperature drops into the danger zone, they will begin multiplying in earnest.

Sure, there are factors that may make this scenario safer than, say, leaving a roast on the counter overnight. It's obvious that having a lid on the pot will mean fewer organizms are circulated over the soup. And the soup is presumably a high-salt environment, which will inhibit bacterial growth. Perhaps those factors conspire to make it safe to eat, or at least extend the "danger zone window" a bit, but I wouldn't recommend putting any faith in it.

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I'll +1 anybody that says I'm right ;) –  Ray Dec 5 '11 at 23:30

The quick answer is: No, the lid doesn't seal the pot.

With a pressure cooker, there is a seal, and as mentioned, there would be a pressure difference between the pot and the air outside it. Normal pots have no such seal. Therefore, the soup isn't sealed off from the outside world.

It is true, though, that the heat of cooking should destroy any pathogens that were in the soup's ingredients before cooking. This means that anything to spoil the soup needs to come in from the air, which takes a relatively long time, as there is no pre-existing culture. In my experience, a soup left out at night after being prepared in the evening is fine in the morning, which is when I usually put it in the fridge.

It may be worth a note that the government (FDA, I think) would like any foodstuff to be kept at above 60 degrees or below 4 degrees Centigrade, which is outside of what they call the danger zone. In my opinion, the government is a bit on the panicky side of these matters.

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"the heat of cooking should destroy any pathogens that were in the soup's ingredients before cooking. This means that anything to spoil the soup needs to come in from the air." This is NOT true. You'll destroy active bacteria by thorough cooking, but there are quite a few nasty things that leave behind spores that can survive even boiling. Those can reactivate in the "danger zone," even in a sealed pot. Bacillus cereus, for example, is one of the most common causes of food poisoning, where its spores survive in things like cooked rice (common cause of illness from leftover Chinese food). –  Athanasius Apr 10 '13 at 23:32

You could ask your in-laws to keep the lid on for a few more days and see if they still want to eat it :D

Actually, leaving the pot on the stove top instead of putting in the fridge when it has cooled enough will give the bacteria/mold perfect growth conditions. It is a humid and warm environment. After a couple of days you wont be able to recognize the food.

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