Vol. XIII · Keeping Food Plate 2 of 10refrigeration, fridge temperature, 5 °C, 4 °C, Listeria, shelf life, Q10, fridge zones, door, overfillingSH

Refrigeration.

Going from 8 °C to 3 °C roughly triples how long most chilled food keeps. The thermometer in the fridge is worth more than the date on the pack.

Refrigeration works because microbial growth slows steeply with cold (V.10). Most pathogens stop below about 5 °C (XII.3), and spoilage organisms crawl. But cold-tolerant organisms — Listeria, some spoilage bacteria, yeasts and moulds — keep growing slowly, so a fridge buys days and weeks, not months. Use-by dates assume the fridge is cold: the FDA recommends 4 °C (40 °F) or below, the UK Food Standards Agency below 5 °C.

Fridges are not uniform. The door is warmest, opened most and should hold drinks and condiments, not milk; the back of the lowest shelves is coldest and suits raw meat (sealed, below everything else, XII.8). Overfilling blocks air flow; hot food warms everything near it (XII.7). The workshop shows how keeping time depends on fridge temperature.

Below: doubling time of Listeria and of a typical spoilage organism against fridge temperature (microbe bench; the spoiler is a teaching organism with a minimum of −3 °C), and how many times longer food keeps at your setting than at 8 °C.

What you will learn
Workshop
Microbe bench: Listeria from its published cardinal temperatures; a teaching spoilage organism with Tmin −3 °C, Topt 25 °C, Tmax 34 °C. Keeping time taken as inversely proportional to growth rate. Recommended settings: FDA 4 °C (40 °F) or below; UK FSA below 5 °C.

A thermometer, not a dial

In this model a typical spoilage organism grows about 3.2 times more slowly at 3 °C than at 8 °C, so chilled food keeps roughly that much longer; at 0 °C, 12.2 times. Listeria doubles about every 4 days even at 2 °C: slow, but not stopped, which is why ready-to-eat foods have use-by dates and why the at-risk groups heat them until steaming (XII.4).

Practical rules. Set the fridge so a thermometer in a glass of water on the middle shelf reads 3–4 °C. Keep milk and leftovers on the shelves, not the door; raw meat sealed at the bottom; salad and fruit in the drawers, where humidity is higher. Do not overfill. Cool hot food before it goes in (XII.7). Respect use-by dates on ready-to-eat foods; best-before dates are about quality. And remember that the fridge only delays: smell and look at food, but do not trust them to detect pathogens.

At the stove

Put a thermometer in a glass of water on each shelf and in the door. Read them the next morning. Move milk and leftovers to the coldest shelf and adjust the dial until the middle reads about 4 °C.

Before mechanical refrigeration, ice was cut from frozen lakes in winter and stored in insulated ice houses; Frederic Tudor of Boston shipped New England ice as far as Calcutta in the 1830s.

Terms

Use-by date
The last date a perishable food is safe to eat, if stored correctly.
Best-before date
A date about quality, not safety.
Fridge zones
Parts of a fridge at different temperatures: door warmest, bottom coldest.
Psychrotroph
A microbe able to grow at fridge temperatures.
A fridge slows, not stops: set it to 3–5 °C, check with a thermometer, and remember Listeria grows cold.

Sources

  1. 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.
  2. 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).
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.