If you've ever intentionally or accidentally left a soup or stock in the pot overnight, you've probably wondered if it is still safe to eat after reheating. Harold McGee had the same question, especially when he heard about the food writer Michael Ruhlman's practice of using stock from a pot left out all week. He talked to a food safety expert to find out if reboiled stock is still safe to eat when you leave it out for one night — or even several days.
The logic behind leaving stock out for extended periods of time is that because the liquid was boiled, any bacteria in it has been killed. While this is true for some bacteria like E. coli and salmonella, other dangerous bacteria species — such as the one that causes botulism — can form inactive spores that survive the boiling process. Once the stock cools below 130°F, these spores can germinate and multiply quickly.
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?
Related: Kitchen Shortcuts: An Easy Trick for Straining Stock
(Image: Emma Christensen)