Modernist-Cuisine-Vol.-1-Small

5
Warm air expands considerably
when it’s heated, a fact captured
mathematically in an equation
known as the ideal gas law. The law
informs us that the volume of air in
an oven will increase by about half
when it’s heated from room
temperature to 177 °C / 350 °F. In
a typical domestic oven that holds
140 liters / 5 ft³ of air, some 70 liters
/ 2.5 ft³ will go out the vent.
THE TECHNOLOGY OF
Cooking In Silico
If you hold your hand over a gas burner, you can
feel the warmth without touching the flame. The
flame heats nearby molecules of air, which then
rise from the flame, carrying some of its heat
toward your hand.
The air near the flame rises because it is hotter
than surrounding air. Nearly all solids expand
when they warm and in doing so become a little
less dense. This effect is more pronounced for
liquids and is quite dramatic for air and other
gases. As fluids heat and expand, they become
more buoyant; as they cool, their densities increase
so they tend to sink.
In the kitchen, convection almost always leads
to turbulence: the roiling boil, the swirls of steam
and fog, the billowing of oil in a deep fryer. The
flow is so turbulent in large part because cookers
usually apply heat unevenly, such as to the bottom
of a pot or deep fryer. The heated fluid cools as it
moves away from the source so its density increases,
its buoyancy drops, and it falls, only to
Because the mathematics of heat flow is so well understood,
computer programs such as COMSOL (below left)
and Mathematica (below right) can model it with terrific
accuracy—to within a fraction of a second or a fraction of
a degree. Food presents special challenges to heat-flow
models, however, because it’s not usually made of uniformly
conducting materials but instead is a sloppy mixture of fats,
be heated and rise again. In natural convection,
in which heat alone is the driving force, the fluid
thus tends to circulate in a pattern of loops called
convection cells.
In the world at large, natural convection kicks
up winds, drives ocean currents, and even slowly
moves the earth’s crustal plates, which rise from
the planet’s molten center, creep across the
sur face, then cool and sink toward the core again.
Even though the warmed walls of an oven apply
heat from every side, the heating is not perfectly
even, so natural convection happens inside an
oven, too. Large baking platters or pieces of food
disrupt the flow of air, however, which reduces
efficiency, creates hot spots, and makes cooking
less predictable.
Forced convection ovens (often simply called
convection ovens) attempt to overcome the
drawbacks of natural convection by using fans to
blow the air around the oven interior. Although
the fanned air can accelerate drying and thereby
sugars, and proteins, solids and liquids, and muscle and bone.
Nevertheless, simple models can give results that are
accurate enough to be useful. By augmenting off-the-shelf
programs with custom software, we’ve been able to do virtual
cooking experiments in silico that would be physically difficult
or would simply take too much time in the kitchen. The results
are highly informative—if not edible.
speed cooking for certain kinds of foods, the
results vary widely depending on the size, shape,
and water content of the food.
Convection is also at work when foods are
cooked in water, wine, broth, or other liquids.
Convection in liquids moves heat much more
efficiently than convection in air does because
the density of water or other cooking liquids is
a thousand times higher than that of air. Far
higher density translates into far more collisions
between hot molecules and food. That’s why you
can reach your hand into an oven without burning
it, but if you stick your hand in a pot of boiling
water you’ll get scaldedeven though the oven
may be more than twice as hot as the water.
Efficient as natural convection in liquid is,
forced convectionalso known as stirringis
still worthwhile. Stirring helps disrupt a thin
sheath of fluid called the boundary layer, that
surrounds the food and insulates it somewhat
from the heat. A boundary layer forms when
friction slows the movement of fluid past the
rough surface of the food.
The boundary layer can be the most important
factor that determines how quickly your food
bakes or boils at a given temperature. Add a circulating
pump to your water bath, or stir a simmering
pot of food, and you can disturb the boundary
layer and greatly hasten cooking.
To quantify just how quickly convection moves
heat from source to food, we need a measure that
takes into account the density, viscosity, and flow
velocity of the fluids involvedmuch as thermal
diffusivity incorporates the analogous information
for heat conduction in solids. The heat transfer
coefficient is just such a quantity; it conveys in a
single number just how quickly heat passes from
one medium or system to another. Convection
ovens cook some foods faster because they have
a higher heat transfer coefficient than conventional
ovens do. In general, forced convection increases
the heat transfer coefficient by tenfold or more.
For more on the actual effects of forced
convection during baking, see Convection
Baking, page 2·108.
Heat rises from a hot griddle through
convection. As the hot air expands, it
becomes more buoyant, then lifts and
churns the surrounding air. The resulting
turbulence is captured in this image by
using a photographic technique that
reveals variations in the density of fluids.
The same scintillation is visible to the
naked eye in the air above a hot, paved
road on a sunny summer day.
The wind chill factor takes into
account the effect of circulating air
on the temperature we perceive.
Wind disturbs the boundary layers
enveloping our bodies, creating a
cooling effect.
It Matters How You Heat
Some cooking methods move heat into the food faster than others.
The heat transfer coefficient is a measure of the speed of heat flow
from the cooking medium to the surface of the food.
Heating method
natural convection from air 20
forced convection from air 200
Heat transfer
coefficient
(W/m² · K)
water bath 100–200
condensing steam 200–20,000
deep-frying 300–600
You can put your arm in a 260 °C / 500 °F
oven for a moment or two without getting
hurt. But you can’t hold your arm over
a pot of boiling water for even a second
(left). The reason is the difference in the
heat transfer coefficient, a measure of how
readily thermal energy will pass between
a fluid (the air in the oven or the steam
above the pot) and a solid (your arm). In
an oven, it’s 20 W/m2 · K; in boiling water,
it’s 100–1,000 times higher because of
the terrific amount of heat released when
water changes from vapor to liquid
(see table at left).
282 VOLUME 1 · HISTORY AND FUNDAMENTALS
HEAT AND E NERGY 283