Camp Pasta Salad: Bacterial Growth and the Reheating Claim
On the last day of school camp, the group packs leftover pasta salad containing mayonnaise, tuna and cooked pasta into a chilly bin. The chilly bin is left in the boot of a car from 9 am to 2 pm; the inside temperature is measured at 24 °C for most of that time. The salad is then put in a fridge at 4 °C overnight and served for lunch the next day. A teacher says, 'It smells fine and we're reheating it, so it's safe.' A student counts an initial 200 bacteria per gram at 9 am and knows that at 24 °C the bacteria double every 30 minutes. The camp has no thermometer for the salad and no food safety adviser. The group must decide whether to eat the salad or throw it out, and explain the reasoning.
1. Calculate the number of bacteria per gram at 2 pm. Show the number of doubling periods.
Read the worked answer
- Time from 9 am to 2 pm = 5 hours = 300 minutes.
- Doubling time = 30 minutes, so number of doublings = (300/30=10).
- Final number per gram = (200\times2^{10}=200\times1024=204{,}800).
- This is about 1000 times the starting number. For a 200 g serving, the total would be about (204{,}800\times200=40{,}960{,}000) bacteria, although the per-gram figure is enough to judge the risk.
- The calculation assumes unlimited growth at a constant 24 °C with no lag phase, so it is an estimate; real growth may be slightly different but the order of magnitude is clear.
2. Evaluate the teacher's claim that the salad is safe because it smells fine and will be reheated. Use the growth calculation and food-safety principles.
Read the worked answer
- 24 °C is inside the temperature danger zone (roughly 5–60 °C), where bacteria grow rapidly. Five hours is far longer than the usual 2-hour limit for perishable food at room temperature.
- The calculation gives about 204,800 bacteria per gram, a very high load. Some food-poisoning bacteria, such as Staphylococcus aureus or Bacillus cereus, produce toxins that are heat-stable.
- Reheating may kill many bacteria, but it does not reliably destroy toxins that have already formed. Smell is also unreliable: pathogenic bacteria can grow without making food smell or look bad.
- Judgement: throw the salad out. The combination of time, temperature and a high calculated bacterial load makes the risk unacceptable, and reheating does not guarantee safety.
3. Describe two changes to the camp plan that would have reduced the risk, and explain why each works.
Read the worked answer
- Keep the salad at 4 °C or below using ice packs or a properly insulated chilly bin, and check with a thermometer. Cold temperatures slow bacterial growth, so the doubling time becomes much longer and the final number stays low.
- Shorten the time out of refrigeration: serve within 2 hours, or discard the food after 2 hours in the danger zone. Because growth is exponential, time is critical: 5 hours allowed 10 doublings, while 2 hours would allow only 4 doublings, giving (200\times2^4=3200) per gram under the same assumptions.
- Also avoid leaving food in a car boot, separate ready-to-eat food from raw food, and use a thermometer rather than relying on smell. These reduce the chance of contamination and give evidence for a safe-or-discard decision.