Modernist-Cuisine-Vol.-1-Small

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For an illustration of how microscopic cracks
and roughness in cookware serve as nucleation
sites for vapor bubbles in boiling—just as
particles in solution serve as nucleation sites
for ice crystals in freezing—see page 2·64.
For more on slug-and-column boiling in thick
sauces, see Burning a Thick Sauce, page 2·68.
Normally we think of the temperature
of boiling water as being 100 °C
/ 212 °F, and in general, that’s true
for pure water at sea level. But on
the hot bottom of a pot where
bubbles of steam are forming, the
water can be superheated beyond
its normal boiling point by 2– 6 °C /
4–11 °F.
Reevaporation zone.
The fog dissipates as
the water droplets
turn to vapor again.
sites so thickly that they join to form big columns
of steam. The columns coalesce into “superbubbles,”
or slugs of vapor. You can see this
so-called slug-and-column boiling most prominently
in thick sauces and stews, which belch up
huge bubbles that splatter everything in the
immediate vicinity.
Pure water and other thin liquids won’t belch
up on a stove top because convection prevents
heat from building up on the pan’s bottom to the
levels needed to create vapor slugs. But power
plants have special high-heat-transfer equipment
that keeps the water in slug-and-column boiling to
maximize the production rate of steam.
The temperature at which pure water boils
depends on several factors. One is the atmospheric
pressure, which makes small changes in
the boiling point as the weather varies. But if you
move to a kitchen at a much higher altitude
above sea level, you will see a bigger difference in
Cloud zone. Water
vapor condenses to
fog. Relative humidity
drops. Air temperature
is approximately
95 °C / 203 °F.
Turbulent zone. Steam
mixes with air.
Humidity is still 100%.
Air temperature drops
to 99 °C / 210 °F.
BLOWING OFF STEAM
Picture a classic tea kettle with water beginning to boil in it. The first inch or so
past the end of the spout is pure water vapor, or steam, and because steam is
invisible, that space appears to be empty. But beyond that region, the steam
mixes with air, causing that air to expand. Any gas cools when it expands, and
as the air cools, the water molecules in it slow down so much that some of
them join together into tiny droplets, forming a visible fog or cloud. The plume
you see spouting from your tea kettle is thus, in essence, a turbulent cloud.
atmospheric pressure and therefore a bigger
change in the boiling point: about a 1 °C / 2 °F
decrease in boiling point for every 300 m /
1,000 ft increase in altitude. In Denver, Colorado
(altitude about 1,600 m / 5,249 ft), water boils at
only 93–95 °C / 199–203 °F, depending on the
weather. At the top of Mount Everest, water boils
at just 69 °C / 156 °F.
The boiling point also depends on what is
dissolved in the water. Whereas you can lower the
freezing point of water by dissolving salt or some
other substance in it, dissolving a solute in water
will raise its boiling point because it lowers the
water’s activity (see page 307), so fewer molecules
are free to evaporate and the vapor pressure drops.
This is called boiling point elevation. For example,
seawater, which is 3.5% salt, boils at 103 °C /
217 °F at sea level. A very concentrated (95%)
sugar solution, the kind used in candymaking,
boils at 135–145 °C / 275–293 °F.
Pure steam zone.
Water vapor exits
the spout at 100 °C
/ 212 °F. Relative
humidity is 100%.
Steam
Steam is a constant presence in the kitchen, but it’s
often confused with its close relative, fog. Understanding
the difference can save you from serious
injury, because steam and fog can exist at very
different temperatures.
Any liquid produced by a phase transition
from the gaseous state is called a condensate; if
a condensate is in the form of droplets so tiny that
they remain suspended in the air, it’s a fog, sometimes
referred to as a mist or cloud, depending on
the size and dispersion of the droplets. Cooks may
call the clouds that rise above kettles and pans
“steam,” but those clouds are not steam, which is
invisible; they’re fog: suspended drops of liquid
water. In short, if you can see it, it’s not steam
(a synonym for vapor); it’s either fog or a mixture
of steam and fog.
The crucial difference for a cook is that fog can’t
be any hotter than the boiling point of waterif it
were, its droplets would vaporize. Steam, in
contrast, can be superheated almost without limit
and can cause serious burns. Its invisibility only
adds to the hazard. Not only is steam typically
hotter than fog, but it also releases a terrific
amount of heat (the heat of vaporization) when it
condenses to liquid water, which it is likely to do if
it comes in contact with your skin. In fact, almost
everything that steam comes in contact with can
be heated by condensation.
When you steam food, water vapor condenses
on the food’s surface, creating a thin liquid layer
called a film condensate, which insulates the food
and inhibits it from further cooking. In vegetables
and other plant foods, the insulating layer of
condensate also traps some of the air that has been
forced out of the spaces between the cells, adding
even more insulation.
For many vegetables, therefore, steaming can be
a slower cooking method than boiling. Steam has
less trouble cooking meat, which doesn’t contain
much air and has very different surface properties.
When the food is in a jar, can, or sous vide bag,
on the other hand, steaming is actually much
faster than boiling. The containers do develop film
condensates, but the water traps no air and tends
to drain off the smooth surfaces. The heat transfer
rate depends on how the film forms and drains.
Flat horizontal surfaces, like the top of a jar in
a pressure steamer, will have a slower heat transfer
rate than the vertical sides of the jar because of the
puddle of condensate it retains. Commercial
canneries often counter this tendency by using
pressure steamers (called retorts) that rotate or
otherwise keep the cans moving during the
steaming process.
When cooking big foods, however, it doesn’t
matter whether you boil or steam: the bottleneck
is the rate of heat transfer through the body of the
food rather than through its surface.
There’s much more to cooking than heat
transfer. Steaming doesn’t dissolve sugars, nutrients,
and other soluble components the way
boiling does. As a result, steamed vegetables are
often more flavorful and nutritious than their
boiled counterparts.
Water In—and Out of—Air
Even in the driest desert climates, the air contains
some water vapor. Put another way, all air has
some degree of humidity. Humidity is not visible,
of course, but you can tell it’s there because it
makes a hot kitchen feel even hotter.
Humans feel humidity the way we do because
we maintain our normal body temperature
partially by evaporative cooling of our skins.
Even when we are not actually sweating, our skin
is always moist. Some of the moisture continually
evaporates, absorbing a lot of energy (again, the
Dry-bulb (top) and wet-bulb (bottom)
thermometers measure quite different
properties. The dry-bulb temperature
does not take into account the effect of
humidity; the wet-bulb temperature
reflects the effect of evaporative cooling.
You can improvise a wet-bulb thermometer
by wrapping the bulb of an ordinary
thermometer in a piece of wet cheesecloth
or muslin (see page 322).
For more on the difference in cooking speed
between steaming and boiling, see Why
Steaming Is Often Slower than Boiling,
page 2·70.
318 VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 319