Vol. XXIII · The Everyday Cook Plate 7 of 12leftovers, reusing food, fridge temperature, Listeria, 3-4 days, reheating, fried rice, frittata, soup, bubble and squeak, 剩菜, food wasteSF

Cooking With Leftovers.

A fridge at 4 °C and a fridge at 8 °C look the same from outside. In four days, one has let Listeria multiply a hundredfold more than the other.

Cooked food that is cooled within two hours and kept below 5 °C stays safe for three to four days (XII.11). The reason for the limit is the few bacteria that grow in the cold, Listeria above all (XII.4): at 4 °C they barely grow for days, but every degree warmer speeds them, and a fridge that has crept to 7 or 8 °C — common when it is packed full or opened often — allows real growth within the same four days. So the fridge thermometer matters as much as the date on the container, and leftovers that will not be eaten in time go into the freezer, where nothing grows.

Within the window, leftovers are ingredients. Day-old rice is the best fried rice (XXI.4); roast vegetables and potatoes become frittatas, hash and bubble and squeak; cooked meat becomes pies, salads, sandwiches, 炒饭 and noodle soups; stale bread becomes breadcrumbs, croutons, bread pudding and panzanella; vegetable trimmings and bones become stock (XXII.1). Reheat once, until steaming hot throughout (74 °C), and do not put reheated food back in the fridge (XII.11). The workshop runs a fridge.

Below: the growth of Listeria in cooked food kept in a fridge at the temperature you choose for the days you choose (microbe bench, course growth parameters, including the lag before growth starts), against a hundredfold rise.

What you will learn
Workshop
Microbe bench: Listeria monocytogenes with the course cardinal-temperature growth model and lag, starting at 10 per gram. A rise of 2 logs (a hundredfold) is taken as the level to avoid (teaching). Leftovers guidance: eat within 3–4 days at 5 °C or below (USDA).

A cold fridge, a short window, a new dish

With the course parameters, cooked food in a fridge at 4 °C shows Listeria growth of about 0.0 logs over four days and 0.6 over a week; at 7 °C about 1.7 logs in four days; at 10 °C about 4.5 — far past a hundredfold. That is the whole case for a fridge thermometer: keep the fridge at 5 °C or below, cool food quickly before it goes in (XII.7), do not overfill it, and eat or freeze leftovers within three or four days. Most foodborne illness from leftovers comes from slow cooling and warm fridges, not from the food's age alone.

Ideas for leftovers: rice — fried rice, congee (XXI.5), rice balls; roast meat — 炒饭, noodle soup, pies, curry, sandwiches; vegetables — frittata or Spanish omelette, soup blended with stock, bubble and squeak, a hash with an egg on top; pasta — baked with cheese, or fried; bread — breadcrumbs, croutons, French toast, bread pudding, panzanella. Cook leftovers until steaming hot right through, or use them cold in salads on the first or second day; cooked rice, seafood and anything for the vulnerable are the most cautious cases.

At the stove

Put a fridge thermometer in the middle shelf of your fridge for a day. If it reads above 5 °C, turn the setting down and check again tomorrow.

Many cuisines are built on leftovers: Spanish ropa vieja, Italian ribollita and panzanella, British bubble and squeak, Chinese 炒饭 and 杂烩, Japanese chazuke and Indian "khichdi" — dishes that began as ways to use yesterday's food and became classics.

Terms

Leftovers
Cooked food kept for a later meal.
Fridge thermometer
A thermometer to check the fridge is at 5 °C or below.
Listeria
A bacterium that grows in the cold (XII.4).
Reheat once
Heat leftovers to steaming once; do not chill and reheat again.
Leftovers keep 3–4 days when cooled fast and held at 5 °C or below; a warmer fridge lets Listeria grow sharply faster. Freeze what you will not eat, reheat once to steaming, and turn leftovers into new dishes.

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. Heat resistance of Listeria monocytogenes in meat: D(60 °C) of a few minutes, z about 6–7 °C (summarised in ICMSF 1996 and Food Code annex 3). Teaching values within the range.
  3. 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.
  4. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.