Could I make an eternal pot of soup?

January 9, 2006 1:20 AM

Theoretical-food-safety-question-filter.

Let's say I keep a big pot of soup in the fridge. Every few days I take it out, heat it to boiling, allow it to cool and replace it in the fridge.

Is there any real reason why this would ever be rendered unsafe to eat? Not that I plan on trying this, but a large pot of soup in the fridge made me think of this hypothetical scenario.

posted by tomble to food & drink (24 answers total)

Are there meat and/or dairy products in this soup? There will be a direct correlation between that answer and how long the soup will last.

Regardless of the answer, it definitely would be smarter to ladle out the amount you plan to eat before heating, thus keeping the rest refrigerated.
posted by allen.spaulding at 1:43 AM on January 9, 2006

Well, let's say it's a soup with some meat in it, but not much, maybe a meat based stock. If I keep wiping out the bacteria that grows with a good boiling, would it ever become dangerous?

And yes, I alway do remove only that which I plan to consume immediately, leaving the rest nice and cold.
posted by tomble at 1:50 AM on January 9, 2006

We once left a big pot of turkey-soup in the fridge, telling our house-sitting friend to help himself to it, little knowing he would still be returning to it after it had been in there for about twelve days. He didn’t get sick, but that’s a while longer than I’d feel comfortable keeping soup on hand.
posted by misteraitch at 2:07 AM on January 9, 2006

No, it is unhealthy.
Boiling at 100C will not kill spore forming organisms, this is why microbiolgy labs use an autoclave (15 psi, 120C, 15 minutes) to sterilise their equipment.
Boiling for several minutes, or in a covered pot is more effective, as shown here.
In addition, it is not only live bacteria that can make you sick. Most cases of botulism (for example) result from eating the toxin made by bacteria before they are killed by heat treatment.
posted by scodger at 2:17 AM on January 9, 2006

I do this and I'm still here...
posted by CunningLinguist at 2:53 AM on January 9, 2006

When I was a kid we had a pot of minestrone, permanent, during Winter. Got boiled, covered, every day and it was fine.

Then one year we got a microwave, and the minestrone went blue....
posted by pompomtom at 3:44 AM on January 9, 2006

The rule of thumb that I've always read (and followed) with any meat-based stock is that you should boil it for several minutes no less frequently than every other day. I find this to be somewhat of a pain when you have a giant pot of soup, plus after a several days I would start to question the flavor quality after so many reheats. What I normally do when I have a big pot of stock or soup is only keep enough in the fridge to last a day or two, and portion off the rest and freeze it to reheat later.
posted by boomchicka at 5:30 AM on January 9, 2006

And yes, I alway do remove only that which I plan to consume immediately, leaving the rest nice and cold.

So is this the correct thing to do, or should you boil the whole thing every couple of days?
posted by smackfu at 5:59 AM on January 9, 2006

I've always read that the whole thing should be boiled, to stop anything that might have started growing in the fridge. I don't have any scientific data to back it up though. Maybe someone can find some?
posted by boomchicka at 6:12 AM on January 9, 2006

Why don't you just freeze the soup? If you put it in quart-size Ziplocs, you can take out about 2 portions at a time.
posted by mkultra at 6:20 AM on January 9, 2006

bugs like nice warm temperatures. that's why we use refrigerators, right? when you heat something up, especially a large quantity of water, it takes a long time to cool - that means a long period at the temperature bugs like.

so you have two choices:

1. trust in refrigeration. heat only as much as you need and keep only for as long as you feel confident in refrigerated food.

2. hope that boiling destroys all previous bacteria (and any poisons they have produced). in this way you hope to constantly re-sterilize food and keep it indefinitely.

if you don't think (2) is going to be completely effective then you really shouldn't do it, because each heating cycle will include a warm phase that is going to really enourage those bugs.

i personally wouldn't trust boiling on a stove top to sterilize food reliably. especially given the need to also destroy toxins. but it sounds like people do it (which amazes me!).
posted by andrew cooke at 6:37 AM on January 9, 2006

for example, say that during the cooling phase (while the soup is still warm) each bug can have 10 generations (where it splits into two each time). and lets say that boiling kills 99% of bugs.

if we start with 1 bug after first boiling (say it just fell in!), that breeds 1024 baby bugs. boiling reduces that to 10 bugs, but they then breed 10,000 new babies. boiling again reduces that to 100, but they then breed 100,000. so each time you boil, the population of bugs goes up by a factor of 10.

on the other hand, if you have 100% kill rate, you're ok.

so the process is extremely sensitive to how well the boiling kills things. 100% is ok, 99% dead might kill you. technically, this is because population growth in bugs is exponential in warm, tasty environments.

obviously exactly how this plays out depends on the numbers. but what this illustrates is how delicate the balance can be. it's easy to find a scenario in which you need 100% sterilisation - where "pretty good" isn't good enough. and this is because of the bug-friendly conditions of warm soup.
posted by andrew cooke at 6:46 AM on January 9, 2006

There's so much bacteria in the food that you get from grocery stores, as opposed to back in the day, when you might get home-grown food from a fairly clean farm. Vegetables may come from the next state, or from Mexico, Chile, or wherever they're cheaply grown. They may be harvested by workers with no access to adequate sanitation in the fields, which includes workers in the U.S. So even if Grammy used to keep a pot of soup going, it's not worth the risk.

Try to use up a pot of soup in under a week. If you like adding new food to the pot, you can add new soup materials to a container in the freezer and have a new soup experience every week or so.

I'm not a food safety expert, I read Fast Food Nation, and it scared me.
posted by theora55 at 6:56 AM on January 9, 2006

I do this all the time. It works, boil every couple of days, and you're golden. It won't work forever, but it will work for a week or 10 days, as opposed to the four or so days you'd get without the re-boiling.

In situations like pompomtom mentioned, where you are constantly adding new stock and ingredients to a pot of soup, and boiling it frequently, you can keep it going indefinitely. You're boiling it often enough and long enough to keep down bacteria growth, and you're diluting toxins that do result before they rise to a level that would cause harm. This isn't a casual process, you've got to sure that you're evenly taking material out and properly sanitizing everything before it goes in.

These methods have worked for many generations... but break the chain, and it goes south quickly.
posted by voidcontext at 7:08 AM on January 9, 2006

In addition, it is not only live bacteria that can make you sick. Most cases of botulism (for example) result from eating the toxin made by bacteria before they are killed by heat treatment.

This really got my attention, but everyone seems to be skimming right over it. Just wanted to make sure you'd seen this excellent argument for limiting the length of time you allow the soup to hang.
posted by Miko at 7:45 AM on January 9, 2006

There were 8 cases of food-borne botulism in the entire US in 2003. In case you thought it was at all likely.
posted by smackfu at 8:25 AM on January 9, 2006

Any of those 8 result in death?
posted by spicynuts at 8:33 AM on January 9, 2006

Food-borne botulism is usually a result of bad sanitation in home canning. CDC info on botulism.
posted by voidcontext at 8:43 AM on January 9, 2006

There were 8 cases of food-borne botulism in the entire US in 2003. In case you thought it was at all likely.

I don't. But none of the fatality rates for food-borne illnesses are particularly high, as the table in this article shows. Heaven knows I've eaten enough 5-second-rule food and raw cookie dough to earn food-borne illness many times over; fortunately, our systems are resilient. It's not the frequency of food-borne illness, but its preventability, that is worthy of note.
posted by Miko at 9:04 AM on January 9, 2006

In college, my roommates and I once had a "bottomless pot o' chili" that was in a continuously running crockpot. The rule was that whenever you took a bowl or two, you had to add something back in to replenish. We kept it going for about 2 weeks or a little longer with no health problems.
posted by achmorrison at 12:25 PM on January 9, 2006

I've had listeria, and while it didn't kill me, I wouldn't wish it on anybody. You might get away with poor food handling 99 times out of 100, but it's still worth avoiding food poisoning.
posted by theora55 at 12:55 PM on January 9, 2006

From the information on the CDC page linked above, botulism is not very likely in soup as Clostridium botulinum grows best in low oxygen conditions (a sealed can rather than a stock pot), and heat (10 minutes of boiling recommended) destroys the toxin. I think, given the proposed regular boiling regime, you can dismiss the botulism worries as scare-mongering.

There may well be other food born pathogens to worry about though.
posted by Quinbus Flestrin at 1:50 PM on January 9, 2006

No, I'm not trying to scare-monger either, but the mention of botulism was given as an example. What other toxins might be in soup as byproducts of bacteria that are eventually killed by boiling?

Also, probably worth noting that the CDC report I linked to says

Our analysis suggests that unknown agents account for approximately 81% of foodborne illnesses and hospitalizations and 64% of deaths.

I don't panic irrationally about the risk of food poisoning in my own life. But it's easy enough to follow the guidelines.
posted by Miko at 2:13 PM on January 9, 2006

If this scares you, try looking up a traditional home-cooked recipe for chicken adobo sometime.

I do this with a big pot of vegetarian hot and sour soup on occasion, but never more than a week. (whole pot in refrigerator, whole pot to a rolling boil, serve, re-refrigerate)

And the time I tried it with my former favourite curry lentil soup, I got blue mold pancakes on day one. I wouldn't do this with anything beanish like chili.
posted by Sallyfur at 6:41 PM on January 9, 2006