Vol. XII · Food Safety Plate 11 of 12leftovers, reheating, 74 °C, 165 °F, three to four days, fridge temperature, Listeria, microwave cold spots, freezing leftoversS

Leftovers and Reheating.

A fridge at 8 °C instead of 4 °C lets Listeria grow several times faster. The rule of three to four days assumes a cold fridge.

Cooked food that goes into the fridge within two hours (XII.3) is low in bacteria, but not sterile: spores, and anything picked up after cooking, remain. In the fridge only cold-tolerant organisms grow — Listeria above all (XII.4) — and slowly. That is the basis of the USDA guidance: eat refrigerated leftovers within three to four days, or freeze them, where nothing grows. The guidance assumes a fridge at 5 °C or below; many domestic fridges run warmer, especially in the door and the top shelf.

Reheating should be thorough: to 74 °C (165 °F) throughout, steaming hot in the middle, which kills Listeria and any other vegetative cells. Microwaves heat unevenly (II.10), so stir halfway and let the dish stand a minute or two. Reheat only the portion to be eaten, and only once — each cool-down passes through the danger zone again. Remember that heat-stable toxins are not removed (XII.9). The workshop stores a dish in the fridge.

Below: the growth of Listeria in a refrigerated cooked dish against days at the fridge temperature you choose, compared with a cold fridge and a warm one (microbe bench, starting from a trace), with the USDA's four days marked.

What you will learn
Workshop
Microbe bench: Listeria from a trace of 0 log per gram (one cell per gram) with the cardinal-temperature model and a lag of about four doubling times. USDA FSIS: leftovers 3–4 days refrigerated, reheat to 74 °C; FDA Food Code: reheating for hot holding to 74 °C within 2 hours.

Cold, short and once

In this model Listeria doubles about every 27 hours at 4 °C but every 8 hours at 8 °C. Starting from about one cell per gram, a dish in a 3 °C fridge is still at that level after four days and has reached only 0.4 log — a few cells per gram — after ten, while at 8 °C it reaches 2.6 log, hundreds per gram, in four days. The three-to-four-day rule has a margin built in for a cold fridge; a warm one eats it.

Practical rules. Keep the fridge at 5 °C or below — check it with a thermometer — and do not overfill it. Label leftovers with the date. Eat them within three to four days, or freeze them in portions for two to three months (quality, not safety, limits frozen storage). Reheat to 74 °C throughout, stirring; bring soups and sauces to a rolling boil. Reheat once. And for the groups most at risk from Listeria — pregnant women, the elderly, the immunocompromised — be strict about all of it.

At the stove

Put a thermometer in a glass of water on the top shelf of your fridge and another in the door. Read them after a few hours. If either is above 5 °C, turn the fridge colder and keep leftovers on the coldest shelf.

Domestic fridge surveys in several countries have found that a large share of household fridges run above 5 °C, many above 7 °C — one reason food-safety campaigns tell people to buy a fridge thermometer.

Terms

Leftovers
Cooked food kept for later.
Reheating
Heating cooked food again; to 74 °C throughout for safety.
Cold spot
A part of a dish that stays cool when reheated, especially in a microwave.
Fridge thermometer
A thermometer kept in the fridge to check it stays at or below 5 °C.
Leftovers: fridge at 5 °C or below, eat within 3–4 days or freeze; reheat once to 74 °C throughout.

Sources

  1. USDA FSIS, "Leftovers and Food Safety": refrigerate within two hours, eat within 3–4 days or freeze; reheat to 74 °C (165 °F). FDA Food Code 2022, 3-403.11: food reheated for hot holding reaches 74 °C within two hours.
  2. 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.