Bacteria in food – the temperature danger zone
BiologyMicrobiology & VirusesAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Microbiology & Viruses
How long can cooked food sit out before it's unsafe? Pick a bacterium (Salmonella, Staphylococcus aureus, or Bacillus cereus in leftover rice), a storage temperature and a number of hours to watch bacterial counts grow exponentially in the 5–60 °C zone, then reheat to 75 °C: the bacteria die, but staph's heat-stable toxin and Bacillus spores survive – which is why chilling food promptly matters far more than reheating it.
Lesson: Food safety and hygiene
What it shows
Most food-poisoning bacteria multiply between about 5 °C and 60 °C, the temperature danger zone, and fastest near body temperature. After a lag phase, cells divide roughly every 25–30 minutes at the optimum, so counts rise exponentially and can pass the level that causes illness within a few hours. Refrigeration below 5 °C almost stops growth without killing the bacteria, while holding food above 60 °C kills them gradually. Reheating destroys living cells but not the heat-stable toxins of Staphylococcus aureus and Bacillus cereus, nor B. cereus spores, so food left out too long can stay unsafe after reheating.
How to use
Choose a Bacterium, set Temperature and Left out, and read the bacterial count on the graph against the illness level of about 10⁵ CFU/g. Compare the same storage time at room temperature, in the fridge and above 60 °C. Then tick Reheat to 75 °C and check the conclusion for each bacterium: which foods become safe, and which remain unsafe because of toxins or spores.
Parameters you can change
- Bacterium Salmonella (poultry, eggs), Staphylococcus aureus (food handler's hands), Bacillus cereus (cooked rice)
- Storage temperature 0–65 °C
- Time left out 0–24 hours
- Reheat to 75 °C before eating
Questions to explore
- Why can rice left overnight at room temperature cause illness even after it has been thoroughly reheated?
- At which storage temperatures do the bacteria hardly multiply at all?
- How long can food contaminated with Salmonella stay at 30 °C before it becomes unsafe?
