Modernist-Cuisine-Vol.-1-Small

D
Freezing Point
Triple point
SUBLIMATION
A ND DEPOSITION
32 212 705.2˚F
The triple point is the place where solid,
liquid, and gas can all coexist. Every
0
material has at least one triple Temperature point, and (˚C)
100 374
complicated materials with multiple
phases can have more. At temperatures
and pressures below the triple point,
a solid sublimates directly into a gas
without melting into a liquid first.
212 705.2˚F
Critical point
Melting and freezing, steam and fog, dew and water vapor that was then whisked away by the
humiditythese are all relatively familiar phenomena
involving phase changes in water. The frost-free freezer, the process takes longer, but it
unit’s humidity controller. If you don’t have a
SUPERchanges
from solid to liquid to gas and back again
CRITICAL
still happensthe water vapor drawn from the ice
from gas to liquid to solid happen Critical daily point right FLUID ends up deposited again as frost on the freezer
before our eyes. But there are two other phase interior.
changes that we don’t often see. They take shortcuts:
moving directly from solid to gas or from gas of the common phase transitions. It uses up a lot of
Sublimation is the most energetically expensive
LIQUID
to solid. Liquids need not apply.
heat, called the heat of sublimation: 2,594,000 J / kg
The direct transition of a solid to a gas with no (1,115 BTU/lb). As you might suspect, this equals
intermediate liquid phase is called sublimation; the heat of fusion plus the heat of vaporization
Boiling Point
the reverse transition, from gas straight to solid, is because first the ice structure has to be broken
called deposition.
down and then the resulting liquid water has to be
Most cooks would be reasonably confident that evaporated. The energy balance is the same
they have never seen these things occur in the whether this takes place in two stages or in the
kitchen. But if you haven’t actually caught sublimation
or deposition in action, you SUPER- have undoubt-
Think of it this way: when you hike from a valley to
single, liquid-free transformation of sublimation.
edly witnessed their effects, perhaps
HEATED
more
GAS
often a mountaintop, the net gain or loss of altitude is the
than you would have preferred. Sublimation is same no matter how many hills and valleys you
GAS largely responsible for freezer burn, the damage traverse along your way.
dealt to frozen foods by dehydration and subsequent
oxidation. Deposition, on the other hand, is thermal energy, how can it happen at freezing
If the sublimation of ice demands so much
how the same water vapor that came out of your temperatures or even colder ones? And it does:
freezer-burned food winds up as thick deposits of you may have noticed that snow on the ground
ice and frost that cover the insides of your freezer, and icicles on trees slowly disappear, even when
if it’s not a self-defrosting model.
the air temperature remains well below freezing.
In a high school chemistry class or at a rock The answer has to do with vapor pressure, which,
concert, you may have seen the “smoke” that pours you’ll recall from the discussion above, arises from
off dry ice (solid carbon dioxide). That smoke is the difference between the rate at which water
evidence the solid has sublimed to a chilly gas that molecules depart from the ice into the air and the
cools the surrounding air enough to cause a fog of rate at which they arrive from the air and freeze to
condensed water to form. You can’t see the CO2 the ice. When the air is very dry or when the
gas itself, but the fog is quite noticeable, especially atmospheric pressure is very low (or both), the
if you put the dry ice in a bucket of water. Before vapor pressure of water in the air can be so low
modern fog machines came into use, that’s how that even molecules held tightly to the surface can
special effects designers on scary movies and rock escape, one by one, to enter the air as water vapor.
concerts made the “smoke.” (You could always tell In fact, so many more can depart from the ice
it was made with dry ice because the air holding than arrive from the air that the ice shrinks. It is
the fog was so cold and dense that it hugged the a sublime process.
ground.)
The main practical implication of this phenomenon
is that cold, dry air alone can dehydrate
Regular ice sublimes, too, but at colder temperatures
than dry ice does. If you have a frost-free food. The ice crystals in frozen food can sublime,
freezer, you may have noticed that your ice cubes leading to freezer burn. You can protect most
slowly vanish from their trays after a few months, foods from this withered-looking conditionor
if left undisturbed. The ice has turned directly to at least prolong the period of stable frozen
SUPER-
CRITICAL
FLUID
At temperatures and pressures above the
critical point, a material becomes
a supercritical fluid, which shares some
properties of both liquids and gases.
Dry ice sublimates under water, thereby
creating cold CO 2
gas that pours out of
the container. Water vapor in the air is
cooled below its dew point, thereby
creating a dense, cold fog.
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SUPER-
HEATED GAS
326 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR CHEFS 327