Modernist-Cuisine-Vol.-1-Small

5
The term “graybody” was coined to
describe the many objects that
excel at absorbing and emitting
light—ceramics and the fire bricks
in pizza ovens among them—but
aren’t ideal blackbody emitters.
temperatures, the object emits long-wavelength
light that carries little energy; heat transfer is such
a small effect that we can effectively ignore it. But
when the object’s temperature (in Kelvin) doubles,
its radiant energy goes up 16 times; when the
temperature triples, its capacity to transfer heat
increases by a factor of 81!
This property of radiant heat shows up in
ordinary cooking ovens. At 200 °C / 392 °F /
473 K or below, most of the heat is transferred by
convection from the cooker’s element. But increase
the temperature to 400 °C / 750 °F / 673 K,
and radiant energy becomes a significant fraction
of the heat transfer that’s occurring.
At 800 °C / 1,470 °F / 1,073 K, the tables are
turned. In such blistering heat, the contribution
from convection is negligible; radiationhaving
increased some 26-fold from the starting point
overwhelms all other means of heat transfer.
That’s why blazing-hot, wood-fired ovens used
to bake pizza or bread really are different from
their conventional domestic cousins. They cook
primarily by radiation, not convection.
Radiation differs from conduction and convection
in yet another way: how it decreases over
distance. As a form of light, heat rays obey the
inverse-square law of light, meaning that intensity
falls off as the square of the distance from a point
source (see illustration below). A light bulb looks
only about a quarter as bright from two meters
away as it does from one meter; the distance
doubled so the brightness fell by a factor of four
(22). Back up to a distance of three meters, and
now the brightness is down to a ninth of its
intensity at one meter.
Most people grasp this property of radiative
heat transfer intuitively but tend to overestimate
its importance in the kitchen. The heating elements
used in grills or broilers aren’t point
sources like light bulbs; instead they tend to be
linear bars (like an oven element) or flat planes
(such as a bed of coals) spread over a relatively
wide area. For more on how radiative heat transfer
from these more complicated heat sources
works, see Grilling, page 2·7; Broiling, page 2·18;
and Roasting, page 2·28.
THE PHYSICS OF
What Makes a Hot Wok Glow?
You may have noticed that your normally deep-black cookware
glows orange or red when heated to extreme temperatures.
The black coils of an electric range or oven also turn
bright orange when cranked up to the high setting. The
source of this color change, thermal radiation, is also the
source of most of the light around us, including illumination
from the sun and from incandescent light bulbs.
In truth, everything has a thermal glow. But most objects
are not hot enough to glow in the visible light range. People
emit infrared light, which has a longer wavelength than
visible light. Food glows in the infrared spectrum, too. Infrared
thermometers work by analyzing the light to determine
the temperature of a person or a piece of food.
As objects are heated, their glow moves from infrared into
shorter and shorter wavelengths. Red light has the longest
wavelength of visible light, so deep red is the first glow we can
see as an object gets hotter. As a pan or electric coil heats
further, the glow turns orange then yellow, white, and finally
blue—hence the terms “red hot” and “white hot.” Eventually
an object can become so hot that it emits wavelengths of light
too short to be seen by the human eye: ultraviolet radiation.
Not all objects emit light equally well. A perfectly black
object absorbs nearly all visible light and reradiates the most
light, too. In practice, there’s no such thing as a pure blackbody,
but some materials, like soot and other forms of carbon,
get pretty close.
A perfect blackbody will start to glow red at 1,000 K, or
near 728 °C / 1,340 °F. At any given temperature, an object
emits a range of wavelengths (see chart on page 285). The
color we perceive is the wavelength that has the peak intensity,
which varies with the temperature of the object.
Josef Stefan, a 19th-century Austrian physicist, discovered
that the energy emitted by an object as thermal radiation is
directly proportional to its temperature (in Kelvin) raised to
the fourth power. So the hotter an object, the more energy it
radiates as light. Stefan was able to use this principle, along
with previous work that calculated the sun’s radiant energy, to
correctly estimate that the temperature of the surface of the
sun is about 5,800 K, or 5,527 °C / 9,980 °F, which gives it a
white-hot color.
A sphere with a radius r has
a surface area of 4πr2
Central source
of intensity S
If the intensity at the
surface of the sphere is X…
X
S
4πr2
= X
The inverse square law states that the intensity of radiation is
inversely proportional to the square of the distance from its source.
That means radiant energy falls off steeply as you move away from
its source. The law applies only to point sources of radiation; light
from heating sources commonly found in the kitchen, such as bars
or coils, behaves somewhat differently.
…then the intensity at a
distance of 2r is ¼X…
S
=
4π(2r)²
X
4
…at a distance of 3r,
it is ¹⁄9X, and so on
X
9
1r
2r
3r
286 VOLUME 1 · HISTORY AND FUNDAMENTALS