The Temperature Danger Zone.
Below 5 °C most pathogens barely grow; above 60 °C they die. Between, the clock is running.
Combine the growth ranges of the common pathogens and they cover roughly 5 to 60 °C: the danger zone. Food-safety agencies draw it slightly differently — 4.4–60 °C (40–140 °F) for the USDA, 5–57 °C (41–135 °F) in the FDA Food Code for cold and hot holding, 8–60 °C in China's catering standard — but the idea is the same: perishable food should spend as little time there as possible.
The rule for home cooks is simple: do not leave perishable food — cooked meat, rice, dairy, cut fruit, anything moist and not acid — at room temperature for more than two hours, or one hour above 32 °C (a hot day, a car, a picnic). Then refrigerate it or throw it away. The workshop shows how fast the common pathogens double across the zone, and how many logs they gain in the hours food sits out.
Below: the doubling time of four pathogens against temperature on a logarithmic scale, with the danger zone shaded, and the logs Salmonella gains in food left out for the hours and at the temperature you choose (microbe bench).
- State the danger zone and its sources.
- Apply the two-hour rule.
- Explain why the middle of the zone is worst.
- Hold hot and cold food safely.
Two hours, then the fridge
In this model a lightly contaminated dish at 25 °C is still in its lag after two hours, the bacteria adapting rather than multiplying; after six hours it has gained 1.8 logs. On a hot day at 35 °C it gains 0.5 in two hours and 2.2 in four. That is the logic of the rule: two hours at room temperature, one hour when it is hot, then refrigerate or discard. C. perfringens is the special danger of large cooked meat dishes: near 43 °C it can double every 9 minutes, so a big pot of stew cooling slowly on the stove spends hours in its best range (XII.7).
Keep hot food hot and cold food cold. Hold hot food at or above about 60 °C (a low oven, a slow cooker on warm, a chafing dish); hold cold food at or below 5 °C, on ice at a buffet. Carry picnic food in an insulated box with ice packs. Thaw in the fridge, not on the counter (XIII.3). And when in doubt about how long something has sat out, throw it out: no cooking, smelling or tasting makes it safe again if toxins have formed (XII.9).
Put a thermometer in your fridge for an hour. It should read 5 °C or below; if not, turn it colder. Then check what is left on your counters after dinner and how long it has been there.
Terms
- Danger zone
- Roughly 5–60 °C, where common pathogens can grow.
- Two-hour rule
- Perishable food out of refrigeration no more than two hours, one above 32 °C.
- Hot holding
- Keeping cooked food hot, at or above about 57–60 °C.
- Cold holding
- Keeping food cold, at or below 5 °C.
Sources
- USDA Food Safety and Inspection Service, "Safe Minimum Internal Temperature Chart" (foodsafety.gov), 2024.
- U.S. Food and Drug Administration, Food Code 2022 (sections 3-401.11, 3-403.11, 3-501.14, 3-501.16, 3-402.11 and the definition of time/temperature control for safety food).
- China: GB 31654-2021 (National Food Safety Standard — General Hygiene Specification for Food Service), which requires cooked food held hot above 60 °C and cold below 8 °C, and food cooked so that its core reaches at least 70 °C.
- Bacterial growth: cardinal-temperature model (L. Rosso et al., J. Theor. Biol. 162, 1993) with cardinal temperatures and optimum rates rounded from ICMSF (1996); teaching values that put each organism in its known range.