Bringing the stock back up to a boil for one minute will kill any active bacteria, and holding it at a boil for 10 minutes will inactivate the botulism toxin. According the expert McGee consulted, soup or stock left to cool overnight, then reboiled for 10 minutes and properly refrigerated in the morning is still safe to eat because it isn't cool long enough for the bacteria to germinate and reproduce up to dangerous levels. But a stock left out for two days "'almost certainly has high levels of infectious Clostridium perfringens cells, or Clostridium botulinum or Bacillus cereus cells and their toxins, or some combination thereof.'"
And though even a stock left out for days at a time might not technically be toxic after a thorough boiling, its flavor will certainly be compromised:
A reboiled three-day-old stock may be safe to eat, but it is now seasoned with millions to billions of dead bacteria and their inactivated toxins. It's conceivable that they might add an interesting flavor, but more likely that the bacteria have feasted on the stock's sugars and savory amino acids, the air has oxidized and staled the fat, and the stock has become less tasty.
→ Read more: Bending the rules on bacteria at Curious Cook
What about you? Do you know anyone who bends the rules when it comes to food safety?
(Image: Emma Christensen)