Modernist-Cuisine-Vol.-1-Small

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James Watt was a Scottish inventor with
an insatiable curiosity. He advanced the
design of the Newcomen steam engine,
a vital element of the industrial revolution
of the 18th and 19th centuries. The watt
was first named for him in 1889. In 1948 it
became the standard international unit of
power in recognition of his numerous
contributions to the efficiency of the
modern world.
Grill and oven manufacturers
commonly misuse BTU, a unit of
energy, to describe the power
rating of their wares. For more on
the difference—and on the proper
use of BTU—see Lies, Damn Lies,
and BTUs, page 2·10.
The joule is the basic unit of energy
in the system of metric units most
commonly used in science, which
are known as SI units (abbreviated
from the French Système International
d’Unités). Because SI units
are based on the fundamental
quantities of a meter, a kilogram,
and a second, they are also
sometimes known as MKS units.
A second metric system is called
CGS, for the centimeter, gram, and
second. The CGS unit of energy is
the erg. Because it takes 10 million
ergs to make just one joule, chefs
are unlikely to encounter the
tiny erg.
ENERGY, POWER, AND
EFFICIENCY
Watts, British thermal units (BTUs), calories, and
horsepowerthese are familiar terms that most
people, including chefs, rarely pause to consider.
All of us know they relate to energy or power, but
we may be a bit unclear about the difference
between the two or about which units refer to
energy and which to power. That’s not surprising
given that some conventions for using these units
seem to have been established purposefully to
mislead.
So to clear up confusion at the outset, on one
hand we have energy, a pure quantity untouched
by time. Power, on the other hand, is a rate of
change in energy: an amount of energy per unit of
time. All units of energy and power are ultimately
related to one another. And in many cases their
numerical values are defined in relation to the
specific heat capacity of liquid water.
The BTU, for example, is defined as the amount
of energy that heats a pound of water from 60 °F to
61 °F. Despite having “British” in its name, the
unit today is used mainly in the United States
often incorrectly as a measure of power.
A more common unit is the joule, which is the
fundamental unit of energy in the metric system.
Named for the physicist James Prescott Joule, the
joule is defined as the amount of energy required
to accelerate a one-kilogram mass from zero to
one meter per second (3.6 kph / 2.2 mph) in one
second, over a distance of one meter. Compared
with a BTU, a joule is a pretty small amount of
energy; it takes 4,190 J (but only 4 BTU) to raise
the temperature of one kilogram (1 l / 1 qt) of
water by 1 °C / 1.8 °F.
In the world of food, the most commonly used
unit of energy is the calorie, which has been the
source of endless confusion because two different
definitions have been in simultaneous use. For
years chemists and other scientists used calorie to
mean the amount of energy that will warm one
gram of water by 1 °C / 1.8 °Fa bit more than 4 J,
in other words. But then food scientists, nutritionists,
and others took to using calorie to mean the
amount of thermal energy necessary to raise the
temperature of one kilogram of water by 1 °C
1,000 times as much energy as in the older definition.
Sometimes people capitalize Calorie to make
clear that they mean the larger unit, but often they
do not. This nonsensical custom has become so
widespread that it is impossible to fight. To avoid
misunderstanding, some people distinguish
between a “gram-calorie” and a “kilogramcalorie.”
A kilocalorie, meaning 1,000 calories,
always refers 1,000 of the smaller unit.
Power in the Balance
In the kitchen, cooks usually don’t concern
themselves with energy as much as they do with
power: the rate at which energy flows from one
thing to another. The basic unit of power is the
watt, named for James Watt, a 17th-century
scientist and inventor. One watt equals one joule
of energy per second. A 500 W induction burner
thus sucks 500 J through its electrical cord every
second.
In the metric system, the prefix kilo- means
1,000, so a kilowatt is simply 1,000 watts. People
often mistake kilowatts as a measure of energy
because household electricity meters usually
record energy consumption in kilowatt-hours.
A kilowatt-hour (kWh) is the amount of energy
expended at a rate of 1,000 W for one hour (3,600
seconds) so 1 kWh equals 3.6 million J. The
kilowatt-hour is indeed a unit of energy, but the
kilowatt alone is a unit of power. Why not simply
use joules on the meters? These practices seem
designed to confuse the casual observer.
One of the stranger units of power that still
persists is horsepower, which was created as a
marketing slogan for steam engines in the 19th
century. To sell a steam engine, you had to tell
people how many horses it could replace. So an
average horse workload was computed for each
engine. To this day, car engines, lawnmowers,
chain saws, shop tools, and even occasionally
vacuum cleaners are rated in horsepower. As you
might assume from the name, a horsepower is
a big unit, equivalent to about 746 watts.
It’s usually pretty straightforward to apply
measures of energy and power when cookingas
long as you use metric units. Suppose you have a
liter of cold water at 5 °C, and you want to heat it to
55 °F. You need to raise the temperature by 50 °C.
Use the specific heat of water (about 4.2 J/g · °C) to
figure how much energy you need. You’ll need to
put 4.2 J of energy into each gram of water for each
1 °C increase in temperature. A liter of water
weighs 1,000 g. Multiply 4.2 by 1,000 g then by
50 °C, and you find that about 210,000 J of energy
must enter the water to heat it to 55 °C.
Converting Among Units of Energy and Power
To convert
from into multiply by
erg joule 0.0000001
calorie joule 4.1868
kilocalorie joule 4,186.8
BTU joule 1,055.06
joule 3,600,000
joule erg 10,000,000
joule calorie 0.238846
joule kilocalorie 0.000238846
joule BTU 0.000947817
joule
kilowatthour
kilowatthour
27,777,800
Power ratings on appliances and light bulbs are commonly given in watts, the fundamental
unit of power in scientific terms. A kilowatt (kW) is 1,000 watts.
If you know the power your heat source can
deliver (which is less than its electrical rating, as
explained on the next page), you can divide that
wattage into 210,000 J to estimate how long the
water will take to heat. A typical 1,800 W water
bath, for example, can actually deliver about
1,200 W to the water. At that rate of 1,200 J/s, the
liter of water will reach 55 °C in about three
minutes. Of course, your water bath probably
holds something closer to 20 liters, in which case it
could take an hour to heat.
To convert
from into multiply by
horsepower watt 745.7
BTU/hour watt 0.293071
kilocalorie/
second
watt 4,186.8
watt horsepower 0.00134102
Energy is a pure quantity unrelated to time. The fundamental unit
of energy in scientific units is the joule; in the U.S. and in older
British literature, BTU is common. You can use the multiplication
factors in the table at left to convert almost any quantity of
energy into joules then into other units.
Power is energy per unit of time. The watt is the most common
unit of power. Use the multiplication factors in the table above to
convert quantities of power into watts.
James Prescott Joule, a 19th-century
physicist and brewery manager from
England, helped to clarify the relationship
between mechanical and thermal energy.
In a brilliantly simple experiment, he
measured the heat produced by paddles
churning a vat of water and compared it
with the mechanical energy produced by
a falling weight that turned the paddle
wheel. His work led to the first law of
thermodynamics, one of the cornerstones
of classical physics.
272 VOLUME 1 · HISTORY AND FUNDAMENTALS
HEAT AND E NERGY 273