Vol. I · What Cooking Does Plate 4 of 10thermometer, probe, calibration, coldest point, lag, ice water, boiling pointHTS

Trusting a Thermometer.

The number on the screen is the temperature of the thermometer's tip, a few seconds ago. Put the tip in the right place and give it time.

Doneness and safety are decided by the temperature at the coldest point of the food — for a steak or a breast, the centre of its thickest part. A probe a few millimetres off centre reads hotter than the centre, because temperature climbs steeply towards the surface; a probe pushed right through reads the pan side. And every thermometer lags: its tip must itself warm up, and a cheap dial thermometer can take half a minute to settle.

The workshop sears a 40 mm steak and lets you place the probe and choose how quickly it responds. It also checks the thermometer the way a cook should: in ice water, which is 0 °C, and in boiling water, which is 100 °C only at sea level.

Below: the true centre temperature and what your thermometer shows, through eight minutes in a hot pan; and the boiling point of water at your altitude.

What you will learn
Workshop
A 40 mm beef steak from 5 °C in a pan at 200 °C, turned every minute. Solid: the true centre. Dashed: the reading of a probe at the depth you choose (measured from the pan side), after its response time.

The coldest point, and a little patience

With the tip at the centre (20 mm) a quick probe is off by only 0.1 °C; at 12 mm it reads 13.5 °C high at the end of cooking, and at 6 mm from the pan side 41.2 °C high — enough to take a steak you think is medium-rare off the heat while its centre is still raw. A slow thermometer (25-second response), even at the centre, reads 1.3 °C low at the end, because the meat is heating faster than the thermometer can follow. Push the probe in from the side, aim for the middle of the thickness, and wait until the number stops moving.

Check it once in a while. Crushed ice topped up with water reads 0 °C anywhere. Boiling water reads 100 °C only at sea level: at 2,500 m it boils at 91.6 °C (Vol. III.3). A thermometer that is out by two degrees can be corrected in your head; one that is out by ten should be replaced.

At the stove

Fill a glass with crushed ice, add cold water to the top, stir and wait a minute. Put the probe in the middle, away from the glass. It should read 0 °C (within 0.5 °C for a good digital probe, 1 °C for a dial one). Many digital thermometers can be recalibrated; dial thermometers usually have a nut under the head.

When you probe meat, insert it from the side into the thickest part, avoid bone and fat, and for whole birds check the thigh as well as the breast.

Fahrenheit chose the temperature of a brine of ice, water and salt as his zero; Celsius proposed the freezing and boiling of water as his two fixed points in 1742 — with boiling at 0 and freezing at 100, a scale reversed after his death.

Terms

Coldest point
The point of a food that heats last — usually the centre of its thickest part — where doneness and safety are judged.
Response time
How long a thermometer takes to reach most of a new temperature.
Calibration
Checking a thermometer against known temperatures: 0 °C in ice water, the local boiling point.
Probe thermometer
A thermometer whose sensor is in the tip of a thin metal stem.
A thermometer reads its own tip: put the tip at the coldest point, wait for it to settle, and check it in ice water.

Sources

  1. ISO 2533:1975, Standard Atmosphere (the troposphere barometric formula).
  2. IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).
  3. USDA Food Safety and Inspection Service, "Safe Minimum Internal Temperature Chart" (foodsafety.gov), 2024.