Modernist-Cuisine-Vol.-1-Small

6
FREEZING LIQUID NITROGEN IN A VACUUM
At 1 bar of pressure, nitrogen has a boiling point of −196 °C / −321 °F, and its freezing
point is only a few degrees lower: −210 °C / −346 °F. In a vacuum chamber (such as
the one in a chamber vacuum-sealing machine), you can easily decrease the boiling
point enough to freeze the nitrogen into solid ice.
striking. On a summer day, when high humidity
suppresses evaporative cooling, the surface of food
can be as much as 9 °C / 16 °F hotter than it might
be on a winter day. On the other hand, a piece of
fish pan-frying in the dry, drafty air of a kitchen in
winter may take longer to cook through without
being turned than the same-size portion in summer.
Hot foods resting on the counter in a winter
kitchen may cool faster than you might expect
because the air is not only cold, it’s also dry. That’s
why, in cold seasons, it’s a good idea to wrap your
cooling foods in foil, which reflects radiating heat
back toward the food.
When warm air cools, some of its water vapor
may condense out onto solid surfaces. The dewpoint
temperature is the temperature at which the
air, as it cools, begins to produce the familiar
condensate called dew. In a comfortable room at
20 °C / 68 °F and 50% relative humidity, the
dew-point temperature is 9 °C / 48 °F, so when you
pull something from the refrigerator at 5 °C /
41 °F, beads of dew soon form on it.
Some dew-point condensation can also occur
when you’re cooking in air, especially in the
initial stages of heating a cold food. Put a cold
ham into a hot oven, and the moisture in the
oven’s air may condense on the ham’s surface.
Heating then proceeds rapidly until the temperature
of the air exceeds its dew-point temperature,
at which time it will begin reabsorbing the dew,
and evaporative cooling will ensue. That’s just one
reason that it’s a good idea to know the dew-point
temperature as well as the relative humidity in
your kitchen. You can determine both values with
a humidity meter (see How to Measure Relative
Humidity, page 322).
The weather phenomenon known as haze
is neither water vapor nor water droplets.
Meteorologists define it as a visibilitylimiting
suspension in the air of solid
particles—from farming, road traffic,
wildfires, etc.—or of wet particles such as
sulfuric acid formed from sulfurous gases
released by burning fuels. But water is not
among the wet particles that cause haze.
The reactions that form haze are in ten sified
by sunlight, high humidity, and
stag nant air, so they occur more readily in
the summer. But when you see haze, you’re
not seeing the humidity.
324 VOLUME 1 ·· HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 325