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This question is the flip-side of the commonly asked food temperature safety concerns. I'd like to know, if a meat-based soup stock (chicken/pork/beef/etc) were to be held constantly at a safe hot temperature of 165F/73C-180F/82C, how long could it stay at this temperature and still be considered safe to eat?

Obviously, it would reduce over time, but suppose it's reconstituted with water regularly so that it could continue being held... Is there ANY sort of time limit (even completely unreasonable spans like, months, years, etc)?

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    This would've been the old-school (medieval & American colonial times) way of 'preservation' by peasants ... just keep the stew hot at all times. Of course, they also didn't have the same population density and global transportation making hygeine quite the issue it is today. (as they'd have built up immunities to the local bugs, and could keep away from sick neighbors) – Joe Nov 4 '14 at 20:32
  • oh, and it's safer than putting it in the fridge each night : cooking.stackexchange.com/q/20978/67 – Joe Nov 4 '14 at 20:33
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    This is a practical concern, as I'm considering implementing a "perpetual stock pot" that would continue for years. I'm concerned that heat-stable exotoxins or similar sort of horrible things might creep in. I'm wondering if it's possible to periodically sample and test for this kind of thing? What possible issues COULD arise? What kind of problems could happen, despite a heated environment? – Troy Howard Nov 4 '14 at 23:54
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Just to provide an official source, the USDA's National Advisory Committee on Microbiological Criteria for Foods (NACMCF) says this (unit conversion added):

Question 4: What minimum time/temperature parameters for hot holding would ensure food safety?

. . . For non-continuous temperature and time monitoring, a minimum hot holding temperature of 130°F / 54.4°C for a maximum time of 4 hours, based on information provided by FDA regarding the limitation of growth of Clostridium perfringens to no more than 1 log_10 in food, would be adequate to ensure food safety. In addition, the Committee concluded that a minimum temperature of 135°F / 57°C for a maximum of 8 hours, or a minimum temperature of 140°F / 60°C indefinitely also would be adequate to ensure food safety. . . .

You can read further details in the link and the detailed report which is linked on that page. But, basically as long as your food stays above 140°F / 60°C, according to this report, it should remain safe "indefinitely."

I would say that the main issue arises in the fact that you might add things to this endless soup/stock. It would only become potentially dangerous if the additions are below 140°F / 60°C when added (and thus cool parts of the food into the danger zone before they recover) or if the additions already contain toxins that might then be concentrated as the liquid evaporates off. Food above 140°F / 60°C should not be producing new toxins (from bacteria at least), but whatever is in there already may not necessarily be destroyed. Adding water though shouldn't pose a problem as long as it's heated before adding or is only added in a small quantity which wouldn't reduce the temperature below 140°F / 60°C.

My main other concern would be that any cooking vessel or utensils used be VERY non-reactive. Elevated temperatures will make it easier for metals or other substances to dissolve more quickly. A pot or pan or other vessel that might be fine for simmering for a few hours could leach more significant quantities of metals or coatings over days or weeks. If your cooking vessel AT ALL reacts with the liquid inside of it, this could pose a long-term issue.

(It's possible that some foods themselves might break down into less desirable components, but this will mostly be a quality issue than a safety issue at temperatures below boiling. At much higher temperatures -- which would be impossible in a liquid stock -- you could also potentially produce carcinogens over time as things break down, but this is probably unlikely for food kept in your proposed temperature range. If you were to let the stock reduce at some point to the point that it went past the demi-glace stage and became drier and temperature began to rise significantly, this could theoretically pose a problem. But in that case you'd probably get smoke and bad smells that would be a clue that your endless soup should be tossed because it had... well, met its end.)

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    Safe from pathogens? Yes. Good to eat? No. I had a college roommate who kept a perpetual slow cooker stew going, adding new ingredients every day to replenish it. It started to look and smell pretty bad after a couple of weeks. Even though it was too hot for pathogens to grow, the food was breaking down and turning into a rather nasty mess. – mrog May 30 '18 at 19:48
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I am trying to think of anything that would refute this notion, but I cannot -- there is no reason you should not be able to hold a liquid at those temperatures for an indefinite period of time. So long as you are reconstituting the mixture with water that is not contaminated, and doing so slowly enough to drop the temperature of the system below 130ºF / 54.4ºC, you should be able to maintain perpetually safe conditions.

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