Vol. II · Heat Plate 4 of 11radiation, infrared, grill, broiler, embers, view factor, Stefan–BoltzmannHT

Radiation.

Every hot thing glows in infrared. The hotter it is the far more it sends, and the further away the food is, the less of it lands.

Hold your hand near a grill element or a bed of embers and you feel heat on your skin even though the air between is cool. That is radiation: energy carried by infrared light, which needs no air at all. Every surface radiates in proportion to the fourth power of its absolute temperature (the Stefan–Boltzmann law), so an element at 800 °C sends out many times more than an oven wall at 200 °C.

How much of it reaches the food depends on geometry. A small source far away fills little of the food's "sky", and the received heat drops quickly as the gap grows. The workshop grills a 20 mm steak under an element and lets you move the rack.

Below: radiant heat flux reaching the steak's surface as the rack moves, and the steak cooked for six minutes, turned once, at the distance you choose: browning and centre temperature.

What you will learn

State how radiant power depends on temperature.
Explain why distance matters so much under a grill.
Use rack position to control browning against cooking through.
Recognise radiation inside ovens.

Workshop

Left: radiant flux on the food from a 16 cm wide element at the chosen temperature, against distance (view factor of a disc, emissivity 0.9). Right: a 20 mm steak grilled six minutes at your distance, turned once; browning index and centre temperature from the thermal bench.

The fourth power, and the inverse square

An element at 850 °C, 5 cm away, delivers about 58 kW to each square metre of the steak; at 15 cm, 18 kW — 3.2 times less. Close to the element the steak browns fast (index 1.00 after six minutes) while its centre reaches 60 °C; far away it hardly browns (0.00) and its centre has only reached 30 °C — it needs far longer there, and gentler heat is what thick pieces want. The rack position is a grill's heat dial.

Temperature matters even more steeply. Radiation grows with the fourth power of absolute temperature: an element at 850 °C (1 123 K) radiates about 31 times as much per square metre as an oven wall at 200 °C (473 K). That is why a grill or broiler browns in minutes what an oven takes an hour to colour, and why the dark, hot walls of an oven contribute a good part of its heat even when its fan is off. Dark food and dark pans absorb more of it; shiny foil reflects it, which is why foil shields the edges of a pie.

At the stove

Under your oven's grill, put slices of bread on the top rack and on a rack two positions lower. Grill both for the same time. The difference in colour shows how quickly radiant heat falls with distance; use it to choose the rack for cheese on toast (close and quick) and for a chicken thigh (further and longer).

Josef Stefan found the fourth-power law from experiment in 1879, and Ludwig Boltzmann derived it from thermodynamics in 1884; it is why a bed of glowing coals, at about 800 °C, cooks so much more fiercely than a 250 °C oven.

Terms

Thermal radiation: Energy carried by infrared (and, from very hot sources, visible) light from every warm surface.
Stefan–Boltzmann law: Radiated power per area is proportional to the fourth power of absolute temperature.
View factor: The fraction of the radiation leaving a source that reaches the food; it falls with distance.
Emissivity: How close a surface comes to a perfect radiator and absorber; high for dark, matt surfaces, low for shiny metal.

Radiant heat grows with the fourth power of the source's temperature and falls fast with distance: the rack position is a grill's heat dial.

Sources

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