Modernist-Cuisine-Vol.-1-Small

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The concept of efficiency also
applies to the power output of
motors. Some unscrupulous
man ufacturers call a motor that
draws 746 watts a “one- horsepower
motor.” But because no motor is
100% efficient, about 1,250 watts of
electrical power are needed to
generate 1 hp of usable mechanical
energy at the shaft—a value
sometimes specified as “shaft
horsepower.”
Efficiency
Like the water bath in the preceding example,
most electrical appliances are rated in watts. The
ratings refer to the maximum amount of electricity
they draw when operating, not the amount of
power they deliver during use. It’s important to
distinguish between those two quantities because
no appliance is 100% efficient. Not all of the
electrical power drawn by a water bath, for
example, actually gets converted into heat, and
not all of the heat that is created ends up in the
food being cooked. Some of the power may be
diverted to create mechanical action, such as
driving a pump. And some of the heat is lost to
the walls of the bath and the surrounding air.
The fraction of the input power that a device
converts to useful heat and mechanical work is
known as its efficiency. Automobile engines are
typically just 25% efficient, but small electric
motors such as the pump in a water bath or the
motor in a blender can have efficiencies as high as
60%. A pot sitting above the gas burner of a stove
is not nearly so efficient at transferring power
into the food it contains (see next page). The heat
you feel when standing next to the stove comes
from thermal energy that has escaped without
doing its job.
Other types of burners, such as electric coils or
glass-ceramic stoves heated by halogen lamps,
may expend fewer watts to heat the pan. Just how
efficiently a burner operates depends on the shape
of the burner, the materials of which it is made,
and other factors.
Induction burners are far more efficient than
gas burners or all other electric heating elements
because they heat only the pots and pans placed
on them, not the surrounding air or intervening
surfaces. For all kinds of burners, the size, shape,
and material of the pan being heated counts as
well. Shiny pans, for example, heat more efficiently
than black ones (see Why Good Griddles are
Shiny, page 284).
Facts on Friction
When your hands are cold, you can warm them by
simply rubbing them together quickly. The force
known as friction opposes the movement, and the
energy you expend overcoming the friction turns
to heat. Any time two surfaces move against one
another, friction puts up resistance. And if motion
then happens anyway, heat follows.
Friction creates heat in the kitchen, too, although
the amount of heat is often too small to
notice. When you cut food with a knife, for
example, friction is generated as the knife slides
past the cut sides of the food, and this movement
heats the food a tiny bit. You can’t perceive this
effect; it’s too slight, and it happens too fast.
In a blender or a rotor-stator homogenizer
(see page 2·412), however, the “knife” spins at
such a high speed that the food inside can get
quite hot as a result of the mechanical work
against friction. Indeed, you can overheat and
accidentally cook some foods in this fashion if
you are not careful.
What counts as efficient or inefficient in the
kitchen thus depends on our objectives. We want
our blenders to be efficient at the mechanical work
of turning blades; heat is an inefficiency. But we
want our water baths and ovens to heat food, not
move it around. In that case, the mechanical work
required to run a pump or a fan is part of the
cooker’s inefficiency.
THE TECHNOLOGY OF
Heating Food Efficiently
There’s a difference between the amount of energy a kitchen
heater draws from an outlet and the amount that actually gets
delivered to the food you’re heating. The less efficient the
heating device, the larger the difference. The energy efficiencies
of burners, ovens, and water baths vary considerably
depending on the design of the heater and the size, shape,
and composition of the container holding the food, as well as
on other factors, such as how quickly the energy is applied. If
some of those differences in variables are minimized, how do
the cookers compare?
We ran tests in our laboratory to check the efficiency of
several heating or cooling appliances. In each case, we
needed to know both the amount of electrical power the
device consumed and the heat it delivered to (or removed
from) a given amount of water. We monitored the power
consumption of each device with an instrument designed for
that purpose. To determine the heating or chilling power, we
placed a measured amount of water in the device and timed
Electric coil
42% efficient
Induction burner
56% efficient
Chilling water bath, stirred and covered
64% efficient
Water bath, unstirred and covered
85% efficient
Water bath, stirred and covered
87% efficient
how long the device took to raise or lower the water temperature
by a predetermined number of degrees. With that
information, we could use the specific heat of water—that is,
the amount of energy required to raise a kilogram of it by
1 °C—to calculate the heating or cooling power and the
efficiency precisely.
Our figures did not always match the claims of manufacturers.
That discovery is perhaps not surprising because
appliance-makers sometimes define efficiency a little differently:
as the fraction of power that generates heat (anywhere)
rather than the fraction that actually heats the contents of the
cooker.
Our experiments did not account for the fact that electricity
is generated by power stations that have rather low
efficiencies. Gas burners have woeful efficiencies—as low as
30%. But natural gas is a cheaper source of power than
electricity, and it’s delivered without substantial losses
en route, so cooking with gas is still a bargain.
Our experimental setup for measuring thermal efficiency used a
pot of water over the heating source under study, a thermal
probe, and a data acquisition device connected to a computer.
274 VOLUME 1 · HISTORY AND FUNDAMENTALS HEAT AND E NERGY 275