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In the poor student or youth worker days it was popular to have never ending soup; a large pot on the stove that was topped up with new and leftover ingredients when available.

Normally the pot was stored in the fridge overnight.

Food items added were sometimes plate leftovers from fellow flat (share house) mates, who would in turn be eating the soup, too.

Most days other ingredients would be fresh(ish) vegetables, beans, grains, and fresh meat products sourced from the local butcher as money allowed.

Brussels sprouts where banned, and so were strong spices (you added those to your bowl before serving).

Also, for irony, a large (cleaned) stone is always left in the pot.

Fresh bread was sometimes baked in the oven while cracked bones were browning, but the oven was often an electrical and environmental death trap.

The pot kept going till the summer holidays, and no one got food poisoning in my time, or from any student stories I have ever heard.

This sort of activity is recorded in history as being a popular resource-saving idea.

Are there any real food safety implications with this method of cooking? And have any actual studies of this technique ever been published?

I still recommend this money-saving idea to current students, not keen on bad practices being continued though.

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That's hilarious... and kind of disgusting. I assume that the pot was actually heated and not just sitting there, right? It's a bit similar to the question Can one preserve food by periodically re-heating it?. –  Aaronut Feb 1 '12 at 13:42
    
I think there are some modern day inexpensive buffet restaurants that still work sort of like this. There may be one piece of pasta in the mac n cheese that has been traveling in there for months... Shudder. –  renegade Feb 1 '12 at 17:20
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Put was pulled out of fridge (it was usually in there), and then put on the stove to boil. If we got too tanked and no one put it away, the cat would start eating it when it had cooled sufficiently by the middle of the night. The cat ate at our table anyway, so no biggie –  TFD Feb 1 '12 at 19:56
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We had stone soup as a kid, but the soup was always made fresh. We had a "soup stone" that was re-used, a nice smooth river rock. –  thursdaysgeek Feb 2 '12 at 23:02
    
If you'd left it going all the time on the stove, it would be perfectly safe... ironically the refrigeration is what makes it unsafe. Also... the cat eating from the pot. Nasty! –  BobMcGee Jul 6 '12 at 19:22
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3 Answers

up vote 14 down vote accepted

It takes quite a while for a pot of hot soup to cool down to 40°F in the fridge. Several hours, sometimes, depending on the shape of the pot and the volume of soup. If you're heating and re-chilling the same soup daily, it's going to spend a lot of time in the danger zone. From a safety perspective, you'd be much better off making a pot of soup every few days and then reheating just the portion that you're actually going to eat.

I still recommend this money saving idea to current students

Soup is a great food for stretching a dollar, but I don't see how it's any more expensive to make a fresh pot twice a week and it shouldn't take much time either. I understand that you were adding scraps each night, but you could as easily save those scraps for a day or two until you make the next pot of soup.

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Good point about the amount of time in the 'danger zone'. If someone insisted on doing this it would be a good idea to use a "cold paddle" or "ice wand" to speed up the cooling process. But I agree that just being in a routine of starting over again every so often is the best policy. Saving up scraps and even peelings is not at all unusual. I do this myself with chicken parts for example and make a big batch of stock whenever I have material and time. –  renegade Feb 10 '12 at 18:39
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Officially, its unsafe.
This is largely due to the fact that the time spent in "the danger zone" is cumulative. You may be killing off bacteria, but during their lifespan they may release toxins and spores that you may not kill. Every time you reheat the bits of leftovers, they're adding up time in the 'danger zone'.

Addionally, soup is only good in the fridge 3-4 days - so unless it was fully reheated every 3-4 days, its unsafe that very first 5th day , irrespective of any other debate. Bacteria still grow in the fridge, just slower.

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Are you sure about this? Assuming the food is being reheated to cooking temperature each time - we're talking about the very bottom of an exponential growth curve, combined with regular consumption and replacement (i.e. dilution). –  Aaronut Feb 1 '12 at 19:05
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@Aaronut - I'm definitely sure that the danger zone is cumulative and there is food that is being reheated well beyond 2 hours in it. The problem is you never know if you're consuming the toxins or just making it more concentrated. You then re-dilute it, but you could be just bring it back to the strength it was. Would I personally do it? Sure. Is anything likely to happen? Probably not. Could I in good conscious call it 100% safe to a stranger on the Net? No. –  rfusca Feb 1 '12 at 19:12
    
@Aaronut - The difference here between this and Bob's answer on cooking.stackexchange.com/questions/16872/… is that they're keeping it out of the danger zone by periodically reheating it and they might cool it down and reheat it again for a day or maybe two. Not indefinitely. –  rfusca Feb 1 '12 at 19:29
    
@rfusca It was boiled most week days –  TFD Feb 8 '12 at 6:51
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The good thing about soup is it is hot enough to kill all potential pathogens. It is also unlikely to be a low-acid low-oxygen environment long enough for botulism to rear its head. Sounds completely practical and the safest possible way to make use of left overs and questionable cuts (the other way is curries I guess).

Seems to me a "cup of brown" was available from soup vendors in the distant past. Maybe that's just in fantasy novels :-).

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I'm not entirely sure that that soup vendors in the distant past would hold up to modern food safety standards... –  KatieK Feb 8 '12 at 2:10
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