Vol. XII · Food Safety Plate 7 of 12cooling, two-stage cooling, ice bath, shallow containers, stock, Clostridium perfringens, Food Code, 57 °C, 21 °C, 5 °CSH

Cooling Safely.

Five litres of stock in a covered pot in the fridge takes most of a working day to cool. In an ice bath, stirred, it takes a quarter of an hour.

Cooking kills growing bacteria but not the spores of C. perfringens and B. cereus (XII.4). As a dish cools through the danger zone they germinate and multiply, fastest around 40–45 °C. So the cooling of large batches is one of the main causes of outbreaks from restaurants and large gatherings. The FDA Food Code sets a two-stage rule: from 57 °C to 21 °C within two hours — through the fastest-growth range — and from 21 °C to 5 °C within a further four hours.

A large pot of liquid cools slowly because it holds a lot of heat (its mass) and loses it through little surface (its area). The fridge itself is poor at removing heat: still air carries it away slowly, and a hot pot warms everything around it. The cures enlarge the surface and the heat transfer: divide into shallow containers, uncovered until cool; stand the pot in iced water; stir; add ice as part of the liquid. The workshop cools a pot.

Below: the temperature of a batch of stock cooling from 90 °C by four methods, with the Food Code's two stages marked, and whether the method you choose meets them (a lumped teaching model of cooling).

What you will learn
Workshop
Teaching model, the liquid treated as one well-mixed body: temperature approaches the surroundings exponentially with time constant ρcV / hA. Pot of height equal to its diameter; heat-transfer coefficients 6 W/m² K (covered pot on a fridge shelf), 15 (uncovered trays, with evaporation), 120 (ice bath), 300 (ice bath, stirred); surroundings 4 °C in the fridge, 0 °C in iced water. Real cooling depends on the fridge and the food.

More surface, colder surroundings

In this model five litres of stock in a covered pot in the fridge takes 6 h 48 min to fall from 57 to 21 °C — far beyond the two hours allowed — and 16 h 56 min more to reach 5 °C, warming everything beside it. Shallow uncovered trays 5 cm deep take 2 h 12 min for the first stage, near the limit; an ice bath takes about 27 minutes, and an ice bath with stirring about 11. The pot was the same; only surface and surroundings changed.

At home: cool stocks, soups, stews, rice and large joints quickly, aiming for the fridge within two hours of cooking. Stand the pot in a sink of iced water and stir; or divide into shallow containers and leave them uncovered until cold; cut large joints into smaller pieces; never put a large hot pot straight into a small domestic fridge. Restaurant kitchens use blast chillers and ice wands for the same reason. Rice is a special case: spread it out to cool and refrigerate within an hour if it is not to be eaten at once (XII.9).

At the stove

Next time you make a big pot of soup, put a thermometer in it and note the time it takes to fall below 21 °C on the counter. Then try the same with the pot standing in iced water and stirred.

Outbreaks of C. perfringens are typically traced to meat or gravy cooked in bulk the day before and cooled slowly — so commonly that it has been called the "cafeteria germ" and the "food service germ".

Terms

Two-stage cooling
57 °C to 21 °C within 2 hours, then to 5 °C within 4 more (FDA Food Code).
Ice bath
A sink or bowl of iced water in which a pot is stood to cool.
Blast chiller
A refrigerator with strong air flow for cooling cooked food quickly.
Time constant
The time for a temperature difference to fall to about a third.
Cool 57 → 21 °C within 2 hours and 21 → 5 °C within 4 more. Big pots cool slowly: use shallow containers or an ice bath and stir.

Sources

  1. 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).
  2. Clostridium perfringens grows between about 15 and 50 °C with an optimum near 43–45 °C, where it can double in under ten minutes; its spores survive cooking (ICMSF 1996; FDA Bad Bug Book, 2nd ed., 2012).
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.