Modernist-Cuisine-Vol.-1-Small

5
HOW HEAT IS LOST
The road to hot food is full of wrong turns, and heat takes
HEAT IN MOTION
them all. The sources of heat loss on any given stove top are
legion. Heat streams from below, beside, and above the food
being heated. It leaks from the burner to the frame. It wafts
from the sides of the pot. It radiates from the lid. The exact
sources of heat loss vary from one kind of burner and cookware
to another, but the leaks noted here are ubiquitous.
The sides of the
pot pump heat
into the air.
The lid of a covered
pot radiates heat
and also conducts
heat to the air.
The burner elements
emit radiant heat in
all directions.
An uncovered pot allows
heat to escape from the
top in the form of
evaporating water vapor.
The frame of the
burner conducts
heat away from
the pot.
Hot air flows from the
burner past the sides
of the pot.
The most important ways that frying, boiling,
steaming, baking, grilling, and other methods of
cooking differ from one another are the medium
and mode through which each transfers heat to
food. In any given cooking method, four modes of
heat transfer operate independently and often
simultaneously. But one mode is almost always
dominant.
The most common mode is conduction, which
is how most heat flows within solids and between
solid materials in contact. Conduction carries heat
from an electric burner coil through a skillet and
into a strip of bacon, for example. A second mode,
called convection, dominates in fluids such as
boiling water, deep-frying oil, and the hot air of
a baking oven. A third form of heat transfer,
radiation, consists of waves of pure energy, like
sunlight. Microwave ovens, broilers, and charcoal
grills all work mainly by using radiant heat.
Finally, the condensation of water vapor onto
a cooler surface, such as a snow pea, injects heat
into the food. That process of phase change comes
into play strongly during steaming.
Each of these four modes of heat transfer works
in some ways that are intuitive and other ways that
are surprising. The better you understand how
they convey energy through your cookware and
into your food, the better you will be able to wield
them effectively in cookingand to comprehend,
if not entirely eliminate, those vexing circumstances
in which even science cannot fully predict
the outcome of your cooking efforts.
How Heat Conducts Itself
Conduction is heat transfer by direct contact;
particles bumping into and vibrating against one
another exchange energy and allow it to spread
through a solid or from one object to another it is
touching. (Conduction can also occur in liquids
and gases but usually as a minor effect.)
Conduction doesn’t happen at a distance. You
can hold your hand just above a hot electric burner
for a second or more and pull it away without
getting burned. Touch the burner, however, and
you’ll feel conduction at work right away!
Heating the center of a solid food relies almost
exclusively on conduction to ferry energy from the
food surface to its interior. Stove-top methods
such as panfrying and sautéing also use conduction
to transfer heat from the pan to the food.
Some materials conduct heat more readily than
others, of course; that is why oven mitts work.
Thermal conductivity is a measure of the ease
with which heat moves within a material. An oven
mitt has a very low conductivity, so it is an
insulator.
Metals, in contrast, respond quickly to contact
with a source of heat or cold. A steel counter top
feels cool to the touch because heat readily flows
from your warm fingertips into the cooler counter.
A plastic spatula with the same temperature as the
counter but a lower conductivity doesn’t feel as
cool. Diamonds are called “ice” for a reason; at
room temperature, they conduct heat away from
your fingers about four times as fast as copper does.
Conduction in Cookware
Diamond-coated pans are not yet an option, but
copper pots are quite popular because of a widespread
perception that they cook more efficiently.
In our opinion, the burner you use is much more
important than the cookware. But cooks tend to
obsess about the quality of their pots and pans,
and we don’t expect that to change any time soon.
In particular, some cooks express a keen interest
in the conductivity of cookware. Whether they
know it or not, however, conductivity isn’t the
only quality they’re looking for.
The perfect pan would be made of a material
that not only allows heat to move freely but also
transmits heat very evenly, without developing hot
spots or cool zones. A highly conductive pan will
not achieve both goals if it is too thin because heat
will flow directly from the burner through the pan
and into the food without spreading out sideways
first. In other words, the pan will transmit the
unevenness of the heat sourcetypically a coiled
electric element or a ring of gas flames. Even
heating over an uneven burner thus demands a pot
bottom that is thick enough to allow time for heat
to diffuse horizontally as it rises vertically.
The pan should also respond promptly when the
The silver teapot is a stylish but impractical
solution for storing a hot beverage.
Silver conducts heat better than most
cookware, which is why the handles on
this pot are insulated with hard rubber.
Because of its high conductivity, the pot
will cool quickly. The popularity of the
silver teapot created the market for
insulating tea cozies.
For more on the relative contributions of pans
vs. burners, see page 2·52.
Ceramics make superior baking
dishes because they are poor
conductors and store more
thermal energy than metals do.
Their slow response to heat tends
to buffer the inevitable temperature
fluctuations in ovens.
HEAT AND E NERGY 277