Interactive lesson

Cooling without electricity, step by step

Interactive lesson: the danger zone, five sources of cold, four food zones, what needs 0–4 °C, the thermometer, fast cooling and keeping the cold in.

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The whole lesson as text

1. Why food spoils

There are always bacteria in food, and warmth wakes them up.

The danger zone — Bacteria multiply fastest between 5 and 60 °C (41–140 °F). At 20–45 °C their number doubles every 20–30 minutes.
The 2-hour rule — Cooked food stands in the warm for 2 hours at most. Above 32 °C (90 °F), only 1 hour.
Your nose won’t help — Salmonella, listeria and other bacteria that poison often change neither smell nor taste. Chicken soup that smells good can be poisonous.
Cooking won’t rescue it — Staphylococcus and Bacillus cereus leave a toxin that survives cooking. A dish that stood lukewarm overnight is not rescued by boiling it again.

Why: At low temperatures bacteria rest or multiply slowly. In the warm they divide, so a piece of meat with a thousand bacteria per gram in the morning can have millions by evening.

Safety is decided by temperature and time, not by your nose.
When in doubt, throw it out. Smell and taste do not reveal the bacteria that poison.

2. Five sources of cold

Without power, cold comes from the ground, water, evaporation, night air and ice.

Ground — At 1.5–2 m deep, roughly 10–15 °C in summer. The ground lags behind, so a cellar is warmest in August and September.
Water — A well, spring or stream gives 8–14 °C. Water carries heat away fast, but it cannot cool anything below its own temperature.
Evaporation and night — Wet clay and hessian give 5–12 °C below the air in dry, breezy weather, and almost nothing when it is muggy. Summer night air is often 10–15 °C colder than the day.
Ice — The only source that gives 0–4 °C in summer without power. It is gathered in winter and kept in an ice house until summer.

Why: Each source has its own lowest temperature, season and cost. If you know which source works when, and how cold it gets, you know what you can put in it.

Only ice gives 0–4 °C in summer. Everything else is cool, but it is not a fridge.
A good system has two or three parts: fast cooling, everyday storage and habits that reduce the need for cold.