Modernist-Cuisine-Vol.-1-Small

6
T HE PHYSICS OF
Carbonated Drinks
Gas solubility plays a major role in how we experience
carbonated water, Champagne, beer, and soft drinks. When
a can of soda is filled at the factory, it is generally pressurized
with a volume of carbon dioxide that, at normal atmospheric
pressure, would occupy a volume three to four times larger
than the can. The exact amount of carbonation is considered
part of the recipe and varies for each brand of bottled water
or soda, but the liquid is usually less than saturated with
carbon dioxide. (In some parts of the world, such as India,
they prefer more carbonation. Indian soft drinks contain so
much CO 2
that, when poured into a glass, they look like
they’re at a rolling boil.)
The pressure of the gas inside the can varies with the temperature
but is typically 3–4 bar / 44–58 psi) at room temperature.
The pressure can be much higher if the can gets very hot.
When you open the can, the pressure drops to atmospheric
pressure. The liquid is now a supersaturated solution of
carbon dioxide, so the excess gas comes out of solution as
tiny bubbles. If the soda is served cold, it can still hold
a substantial amount of CO 2
, which then slowly comes out of
solution in steady streams of bubbles as the soda warms.
Caveat emptor: there is a gadget on the market that is
claimed to restore the fizz in bottles of flat soda. You use it to
pump air into the bottle; when you open it later, it makes a
sat isfying pfft! sound. But that’s just the compressed air
escaping. You have added no more carbon dioxide (be yond
the tiny amount in the air) to either the air space or the liquid,
so it is just as flat as before. Trust your tongue, not your ears.
For more on carbonation as a cooking technique, see page 2·456.
Carbon dioxide is a unique gas for many reasons. If it were
less soluble than it is, you couldn’t dissolve enough of it in the
soda to make it bubble out when the pressure is released.
And if its solubility changed more drastically with changing
temperatures than it does, it would depart from the soda too
quickly as it warms. And, of course, CO 2
gives a pleasant
acidic tang to the beverages.
When soda is poured into a glass, the physical agitation
causes more carbon dioxide to bubble out of solution.
Typically, the glass is warmer than the soda, so nucleation
bubbles form on the sides of the glass and feed the
turbulence.
As you drink a cold carbonated soda, it comes in contact
with your mouth and tongue, which are at about 37 °C /
98.6 °F. These surfaces warm the soda, reducing the solubility
of the carbon dioxide and forcing more of it to come out of
solution as bubbles in your mouth, thus creating the unique
sensation characteristic of carbonated beverages.
WATER QUALITY A ND PURITY
Pure water is an excellent solventindeed, it’s
sometimes called the universal solvent, because it
dissolves more substances than any other liquid,
including strong acids. That’s due in part to its
polarized structure and in part to its hydrogen
bonds. Add a little carbon dioxide from the
atmosphere, and water becomes an even better
solvent, as the properties of carbonic acid augment
its native abilities.
Because water dissolves things so well, it’s often
full of minerals collected from its surroundings:
particularly calcium and magnesium but also iron,
copper, aluminum, manganese, bicarbonates, and
sulfates, depending on the geographical location.
Hard water is the term for water containing large
quantities of dissolved minerals.
Most kitchens use tap water for cooking, and
recipes that call for water don’t specify what kind to
use. But the quality and purity of tap water can have
a big impact on cooking processes. Hard water is
a cooking variable that comes out of your faucet.
Hard water toughens some vegetables cooked in
it, for example, as the minerals in the water
combine with the pectin in plant cell walls. Hard
water can interfere with gelling and thickening
processes, too, because the dissolved minerals are
in the form of charged ions and the hydrocolloids
used in these applications are very sensitive to
ionic concentration. The minerals in hard water
can also leave troublesome deposits on equipment
that boils water, such as espresso machines and
combi ovens.
In addition to minerals, municipal tap water in
most parts of the world contains both a form of
chlorine to kill parasites and fluoride to prevent
tooth decay. These compounds also can affect
cooking processes, as well as the flavors and
textures of cooked food.
How can you determine the quality of your
water supply? Very hard water has an off-taste and
a slippery or slimy feel. If you are on a municipal
water system, you can contact your water provider
to get a complete analysis of what’s in your tap
water. If you have a private supply, you can have
your water tested or get a testing kit and do it
yourself. Some manufacturers of water softeners
will even give you a free kit.
Once you know more about the contents of
your water, you can pick the right strategy to
purify it. There are a number of water-softening
and purification methods, varying in cost, capacity,
and the kinds of contaminants they remove.
The simplest method is an ion-exchange filter,
which uses special resins to capture the ions of
dissolved minerals. Often referred to simply as
“water softeners,” these filters make deionized
water, which works best for cooking vegetables
and hydrating hydrocolloids.
You may want an even higher level of purity if
your water tests high for contaminants. Distillation
removes impurities by boiling the water and
condensing the steam in a separate container.
Distilled water makes a fine substitute for deionized
water, but it’s more expensive.
Reverse osmosis uses pressure to pass water
through a membrane that screens out contaminants.
It makes extremely pure water and is
cheaper than distillation, but it generates a large
volume of wastewater and doesn’t remove chlorine
or other dissolved gases.
Carbon filtration, on the other hand, is the best
way to remove chlorine and the dissolved organic
compounds that can be a health issue in some
areas. But it won’t soften the water, so many
household treatment systems utilize more than
one approach: pressurized water passes through
carbon filters and reverse-osmosis membranes
before being irradiated with ultraviolet light to
kill any lingering microorganisms.
Microporous filtration yields water of the
highest purity for use in laboratory experiments.
But it’s overkill for the kitchen.
If you’re overwhelmed by these options or
don’t want to spring for your own waterpurification
system, you can always buy bottled
water for critical cooking applications: deionized
water and distilled water are widely available.
A word of caution, however. Although very pure
water may be appropriate for combining with
food in cooking, it doesn’t taste very good. We’re
used to water flavored by dissolved gases and
minerals, and some of these substances contribute
essential nutrients as well. Without them, the
water tastes flat.
Water softened by an ion
exchange filter contains a higher
concentration of sodium, which is
exchanged for the calcium and
magnesium in hard water. For that
reason, it may be unsuitable for
some cooking uses.
The food industry uses reverse
osmosis extensively to concentrate
fruit juices, maple syrup, and milk
and to isolate whey proteins. It is
even used in making wine, including
many of the more elite vintages.
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VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 335