How long food can sit out, and the growth rate behind the limit

1. 01

   The two numbers: two hours, one hour above 90 degrees

   Perishables should never sit at room temperature for more than 2 hours, and the limit is 1 hour if the air temperature is above 90 degrees Fahrenheit. The same window applies to leftovers, to takeaway and doggie bags, and to groceries in a car, which is usually hotter than a typical room (FDA). The clock starts when the food leaves temperature control, not when the meal finishes.

2. 02

   Where the number comes from

   At room temperature the numbers of bacteria that cause foodborne sickness can double every 20 minutes, and chilling foods to proper temperatures is one of the best ways to slow that growth (FDA). Two hours is therefore about six doubling intervals, which is why the guidance is a cut-off rather than a gradient: exponential growth stays visually unremarkable and then is not.

3. 03

   Cooking does not undo the delay

   If left unrefrigerated, some organisms can create toxins that will survive the cooking process even if the food is cooked to temperatures that kill the bacteria themselves (FDA). This is the clause that defeats the argument that a hot oven fixes a long afternoon. Cooking kills cells; it does not remove what the cells produced, which is why the limit is on the delay rather than on the final internal temperature.

4. 04

   In the refrigerator is not the same as cold

   A deep mass of hot food loses heat slowly from the centre, so a full stockpot moved straight into a fridge can hold its middle in the growth range for hours while the surface reads cold. The guidance addresses this directly: refrigerate or freeze leftovers within 2 hours, divide them into smaller containers before refrigerating to cool faster, and note that putting hot food in the refrigerator does not harm the appliance (FDA).

5. 05

   What a slow cooling curve costs, measured

   Noncured ground pork cooled from 54.4 to 7.2 degrees Celsius over 20 hours showed a 2.8 log colony-forming unit per gram increase in Clostridium perfringens, roughly a six-hundred-fold rise, with nothing ever left on a counter (Marquez-Gonzalez et al. 2012). Cooling is a step with its own exposure window, and depth is the variable that sets how long that window stays open.

6. 06

   Formulation changes the outcome

   In cured ground pork under the same abusive cooling, C. perfringens decreased by 1.1 log from 54.4 to 36.3 degrees Celsius and then increased by 0.9 log until the product reached 7.2 degrees, and even from an initial 5 log colony-forming units per gram the final counts did not exceed 3.4 log, leading the authors to conclude no risk under the tested conditions (Marquez-Gonzalez et al. 2012). Curing salts change the answer, which is why laboratory results do not transfer uniformly.

7. 07

   The field version: cooking fine, holding not

   A hazard analysis of cooked food vending found cooking usually gave time-temperature exposures lethal for vegetative pathogens, while holding often provided exposures conducive to growth, particularly overnight. Ten million Clostridium perfringens per gram came from leftover beef stew, above 100,000 Staphylococcus aureus from leftover chicken, and above 10 to the seventh Bacillus cereus from leftover rice (Jermini et al. 1997).

8. 08

   Reheating did not close the gap

   In that same analysis, time-temperature exposures during reheating had variable effects in terms of killing the microorganisms that germinated from surviving spores or reached the foods after cooking, and heat-stable toxins would not have been inactivated (Jermini et al. 1997). Once the delay has happened, the safe response is to discard rather than to reheat harder, because the surviving component is the one heat cannot reach.

What people ask us next.

Q01

How long can cooked food sit out before it is unsafe?

Two hours at room temperature, and one hour if the air temperature is above 90 degrees Fahrenheit (FDA). The limit is derived from a growth rate rather than chosen arbitrarily: at room temperature the bacteria that cause foodborne sickness can double every 20 minutes, so two hours is roughly six doubling intervals. The clock starts when the food leaves temperature control, which for a served dish is when it comes off the heat.

Q02

Can I just reheat it thoroughly instead?

No, and this is the most important correction on the topic. If left unrefrigerated, some organisms can create toxins that will survive the cooking process even if the food is cooked to temperatures that kill the bacteria themselves (FDA). A field hazard analysis of foods held overnight found reheating had variable effects on organisms germinating from surviving spores and that heat-stable toxins would not have been inactivated (Jermini et al. 1997). Discard rather than reheat.

Q03

Should I let food cool before putting it in the fridge?

No. Putting hot food in the refrigerator does not harm the appliance, and leftovers need to be refrigerated or frozen within 2 hours, so waiting on the worktop spends the budget on nothing (FDA). What does need attention is depth: divide leftovers into smaller, shallower containers before refrigerating, because that is what allows the centre to reach a safe temperature quickly rather than sitting warm inside a cold appliance.

Q04

Does slow cooling in the fridge actually matter?

Measurably. Noncured ground pork cooled from 54.4 to 7.2 degrees Celsius over 20 hours showed a 2.8 log colony-forming unit per gram increase in Clostridium perfringens, roughly a six-hundred-fold rise, without ever being left at room temperature (Marquez-Gonzalez et al. 2012). Cooling is its own exposure window, and the depth of the container is the variable that determines how long the centre stays in the growth range.

Q05

Does the two-hour rule apply to groceries?

Yes. Raw meat, poultry, seafood, eggs and produce that requires refrigeration should never sit at room temperature for more than 2 hours, with a 1-hour limit above 90 degrees Fahrenheit, and a car is usually hotter than a typical room, so groceries should not be left in it longer than absolutely necessary and never more than 2 hours, or 1 hour on a hot day (FDA). The shopping trip is part of the same budget as the meal.

Q06

What about food left out overnight that looks and smells fine?

Appearance is not informative here, because the organisms of concern do not necessarily spoil the food. In a field analysis, leftover beef stew held overnight yielded ten million Clostridium perfringens per gram, leftover chicken above 100,000 Staphylococcus aureus and leftover rice above 10 to the seventh Bacillus cereus, and heat-stable toxins would not have been inactivated by reheating (Jermini et al. 1997). Overnight at ambient temperature is a discard regardless of how it looks.