Modernist-Cuisine-Vol.-1-Small

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For more on units of energy and power, see
Converting Among Units of Power, page 273.
Point of No Return
During cooking, subtle irreversible changes, both
physical and chemical, occur in foods. These
changes can alter the specific heat capacity of the
food. As the name suggests, irreversible changes
are the sort that cannot be undone.
Freeze some warm water, melt it again, boil it
to steam, and recondense it; at the end, you’ll
have the same substance you started with, and its
specific heat will be just as it was at the beginning.
In other words, those changes are completely
reversible. If you take a steak from the refrigerator
at 5 °C / 41 °F and warm it to 20 °C / 68 °F, that is
also a reversible change. You’ve made the meat
15 °C / 27 °F hotter. But not long after you return
the steak to the refrigerator, it will be essentially
the same as before, aside from some subtle
changes due to enzymatic activity and aging.
If you instead heat a steak from 40 °C to 55 °C
/ 104 °F to 131 °F, however, the appearance,
texture, and taste of the meat all change profoundly.
As in the previous example, the temperature
of the meat rises just 15 °C / 27 °F. But in
this case the heating elicits chemical changes
that transform the meat from raw to mediumrare.
You can cool the steak back to 40 °C /
104 °F, but it will never again be raw. This transformation
is what we mean when we refer to an
irreversible change.
Most cooking is about achieving such irreversible
changes in a controlled way. Changes of this
kind typically occur within narrow bands of
temperature, and very little heat energy is needed
to make them. Much of the difficulty in cooking is
getting and keeping food within those narrow
bands of temperature where miniscule amounts of
THE TECHNOLOGY OF
Measuring Temperature
Small changes in temperature can make all the difference in
cooking. Cooks should thus measure temperature as accurately
as they can. One famous chef argues that by touching
a skewer to his lip he can judge temperature better than any
digital thermometer. We beg to differ—although few of these
devices are accurate to better than 1 °C / 1.8 °F, they all outperform
the human lip.
Thermometers always trade accuracy for expediency; the
truest thermometers tend to be the slowest. The probes
below are ranked roughly in order of their accuracy from least
to most. Bear in mind that accuracy depends on how skillfully
thermometers are used as well as on their inherent capabilities.
THE TECHNOLOGY OF
Measuring Specific Heat
To measure the specific heat capacity of a food, which
affects how long it takes to cook, researchers use a tool
called a differential scanning calorimeter (DSC).
Because the DSC can measure specific heat at a wide
range of temperatures, it can help to identify the irreversible
chemical changes that occur as cooking alters the
structure and properties of the food. In the right hands,
a DSC can reveal the precise temperatures at which crystals
break down, proteins unravel, fats melt, or juices evaporate.
In a DSC, a small sample of the food rests on a metal
platform inside the device’s chamber. The machine slowly
heats the sample, degree by degree, while keeping careful
track of exactly how much electrical energy it has expended.
The more energy that goes in before the sample temperature
rises one degree, the higher the specific heat at
that temperature.
Thermocouple K
A temperature difference
between two metals in the
probe produces a voltage that
is translated into temperature.
Accuracy: ±2.8 °C / ±5.0 °F
Analog dial
A probe connects to a metal strip
that expands when heated,
rotating a needle over a scale.
Accuracy: ±2.5 °C / ±4.5 °F
Infrared
A sensor measures the spectrum
of long-wavelength light emitted
by an object, which varies
with temperature.
Accuracy: ±2.0 °C / ±3.6 °F
Thermistor
A metal bead encased in glass
measures electrical resistance,
which changes in proportion
to temperature.
Accuracy: ±1.5 °C / ±2.7 °F
Temperature (°F)
130 150
170 190
We placed a piece of Kobe beef cheek into a differential scanning calorimeter (left) and
gradually heated it to typical cooking temperatures. Energy input per degree of heating
Specific heat
Collagen
shrinks
Tightly bound water desorbs
and proteins unravel
Collagen
turns to gelatin
60 70
80
Temperature (°C)
(right) rose near 63 °C / 145 °F and again near 78 °C / 170 °F, signaling the irreversible
changes in protein chemistry that transform meat from raw to cooked.
90
Thermocouple T
This device works like
a thermocouple K, but uses
different metals as sensors.
Accuracy: ±1.6 °C / ±2.9 °F
Analog liquid
A compound such as mercury or
ethanol expands readily with heat
inside a graduated glass capillary.
Accuracy: ±1.0 °C / ±1.8 °F
Platinum resistance
temperature detector
Electrical resistance in a coil
of platinum wire changes in
proportion to temperature.
Accuracy: ±0.1 °C / ±0.2 °F
Don’t be fooled by all the decimal
places on your thermal probe’s
readout. A thermometer can be
precise (as implied by all those
digits) without being accurate.
Precision means that the instrument
will give the same value each
time you repeat the same measurement.
Accuracy means it gives
a number that accords with the
actual temperature—something no
amount of digits can ensure.
268 VOLUME 1 · HISTORY AND FUNDAMENTALS HEAT AND E NERGY 269