A Safe Kitchen.
Bacteria in a kitchen do not need much: food, water, warmth and time. Take away the warmth or the time and they cannot multiply.
A few bacteria on raw chicken are normal and are killed by proper cooking. The danger comes when they are given time to multiply — on a chopping board, in a dish left on the counter, in a stock cooling slowly overnight — or when they travel from the raw food to something that will not be cooked again. Food-safety rules exist to break one of those links: keep it cold, keep it hot, keep raw apart, cook it through.
The workshop leaves raw chicken out at room temperature and shows how the number of Salmonella grows: slowly at first, while the bacteria adjust (the lag), then doubling every half hour or so in a warm kitchen. In the fridge they hardly grow at all.
Below: the number of Salmonella (as a power of ten per gram) on raw chicken left at room temperature for the hours you set, beside the same chicken kept in the fridge.
- Name the four things bacteria need to multiply.
- See why the two-hour rule exists.
- Explain cross-contamination and how to prevent it.
- Know the first rules for burns, cuts and a grease fire.
Warmth plus time
At 25 °C, once its lag is over, Salmonella doubles about every 36 minutes. Two hours on the counter leave the count almost where it started (0.0 powers of ten higher) — the lag protects the food at first, which is where the two-hour rule comes from — but four hours raise it by 0.8, a 6-fold increase, and six hours in a 32 °C kitchen by 3.4. In the fridge the rise over the same time is close to zero. Cooking will still kill most of them, but some bacteria leave toxins that heat does not destroy (XII.9), and a bigger starting number means more survivors.
The other kitchen dangers have equally short rules. Wash hands with soap after handling raw meat, eggs or soil, and before ready-to-eat food. Use separate boards for raw meat and for food that will not be cooked. Keep knives sharp and cut away from your hand (IV.2). Turn pan handles inward. Never put water on burning oil: turn off the heat and cover the pan with a lid or a fire blanket (IX.8).
Put a fridge thermometer on the middle shelf of your fridge and read it after the door has been shut overnight. It should read 5 °C or below (the U.S. Food Code limit; 4 °C is better). Many home fridges run warmer than their owners think.
Put leftovers in the fridge within two hours of cooking — sooner on a hot day — in shallow containers so they cool quickly (XII.7).
Terms
- Lag phase
- The time bacteria spend adapting to new conditions before they begin to multiply.
- Doubling time
- The time for a growing population of bacteria to double.
- Cross-contamination
- Bacteria carried from raw food to food that will not be cooked again, by hands, boards, knives or cloths.
- Two-hour rule
- Perishable food should not be left between 4 and 60 °C for more than two hours (one hour above 32 °C).
Sources
- World Health Organization, Five Keys to Safer Food Manual, 2006.
- USDA Food Safety and Inspection Service, "Safe Minimum Internal Temperature Chart" (foodsafety.gov), 2024.
- ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.
- 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.