Modernist-Cuisine-Vol.-1-Small

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Frost forms via deposition; water vapor
leaves the air and directly forms ice
crystals. Snowflakes grow in the same way
while suspended in the air inside clouds.
Pressure (bar)
storageby sealing the food in vapor-tight
pouches, containers, or wrappings. So-called
freezer paper is made for that purpose, but
vacuum sealing in a sous vide bag works better. It
also helps to remove air (and the water vapor in
it) from the package before freezing. Any empty
space in the package creates a comfortable
surface on which water vapor can refreeze, so it
encourages sublimation.
Cold temperatures and dehydration are useful
separately for preserving food, and together, they
can make a powerful combination. Freeze food
10 4 –400 –200 0 200 400 600
10 3
10 2
10
10 -1 1
10 -2
SOLID
Melting curve
Triple point
Temperature (˚F)
LIQUID
Sublimation curve
SUPERCRITICAL FLUID
Critical point
Saturation curve
GAS
–200 –100 0 100 200 300
Carbon Dioxide
Temperature (˚C)
Triple point: −57 °C / −71 °F at 5.2 bar / 75 psi
Critical point: 31 °C / 88 °F at 74 bar / 1,073 psi
rapidly, then put it under vacuum to speed sublimation,
and you have freeze-drying.
You might associate freeze-dried food mainly
with instant coffee, the trail chow consumed by
backpackers, and astronaut food. But it’s much
more useful than that. Freeze-drying came upon
the scene in the 1960s as something of a technological
marvel, but believe it or not, the Peruvian
Incas used to freeze-dry their crops by taking
them to the top of Machu Picchu, where both the
temperature and the atmospheric pressure were
low. Modern freeze-drying can preserve the
appearance, flavor, aroma, and nutritional value of
food, which can then be stored nearly indefinitely
at room temperature. With its moisture gone, both
microbial growth and chemical spoilage reactions
in the food are substantially slowed.
In the freeze-drying process, the temperature of
the food is first brought below the triple point of
water, where only ice and vapor can exist, so that
no liquid will form in subsequent steps. Ice
crystals form; then the ice sublimes when the food
is exposed to a vacuum, taking most of the food’s
water content with it. The absence of melting
avoids many of the pitfalls of freezing food discussed
earlier in Freezing and Melting. For more
on the equipment and techniques involved, see
Freeze-Drying, page 2·438.
Deposition is a more familiar phenomenon than
sublimation is. You’ve seen deposition put frost on
your windshield on a cold morning. You’ve seen it
make frost in your freezer. You’ve seen the snowflakes
it has grown in the clouds.
In all these cases, ice has been deposited
straight from water vapor in the air, with no
intermediate liquid state. Deposition is the reverse
of sublimation, and as such, it releases a lot of heat,
equal to the heat absorbed in sublimation.
The frost that forms on the inner walls of
a freezer and that coats some frozen foods can
come from loosely wrapped food itself, which
releases water vapor, or, if the freezer is frequently
opened, from the influx of humid
kit chen air. Deposition in a freezer is never a
good thing, because it signals adverse conditions
for frozen storage. In order to preserve the
qual ity of your frozen food, you need to store it
in air-tight containers and avoid opening the
freezer too often.
T HE P HYSICS OF
Freeze-Drying
The words “freeze-drying” may prompt loathing in the
hearts of cooks, but the technique can actually yield exquisite
results if it’s done right. The trick is to know which
foods to freeze-dry. Many don’t do well: juices, fruits, and
sweet vegetables such as onions, for example, are too high
in sugar. When these dried foods are subsequently exposed
to air, they absorb water from it, which combines
with the sugars to make them tacky and sticky. Meats and
cheeses go soft and rubbery when their proteins absorb
moisture, and fatty foods go rancid quickly when dried
because they’ve lost the water that normally protects them
from oxidative spoilage. Starches, in contrast, absorb water
more slowly, so starchy foods make great candidates for
freeze-drying.
The other trick is to know just the right way to do it. Freezedrying,
as the name implies, occurs in two stages: first you
The cold trap of a freeze dryer is extremely cold, often as low as −70 °C / −94 °F. It must
be cold enough to freeze out water vapor that sublimated from food in the freeze dryer.
freeze, then you dry by pulling a vacuum. It’s important to
freeze food slowly to very low temperatures if you don’t want
to damage its texture. Specifically, you must bring the food
below its glass transition temperature, which for most food
means between −70 °C and −40 °C / −95 °F and −40 °F. If any
water remaining in the food is not in a glassy state, it will
vaporize quickly when the pressure is reduced, rupturing
cells. You’ll eventually get to the glass transition temperature,
anyway, as the vaporization cools the water, but your food
will pay the price.
Such low temperatures are outside the reach of domestic
freezers, so proper freeze-drying requires a cryogenic commitment.
If you’re not concerned with preserving the texture
of your freeze-dried food—if you’ll be grinding the food, or
making stock from it—then you need not take such extreme
measures.
For more on the ideal conditions for frozen
storage of food, see Freezing, page 2·256.
328 VOLUME 1 · HISTORY AND FUNDAMENTALS THE PHYSICS OF FOOD AND WATER 329