Modernist-Cuisine-Vol.-1-Small

6
The solubilities of table salt (sodium
chloride; black curve) and ordinary sugar
(sucrose, blue curve) in water depend on
temperature. Sugar has a much higher
solubility than salt, and its solubility varies
much more with temperature. At 30 °C /
86 °F, at most 361 grams of salt will
dissolve in a liter of water to make a 26.5%
solution; at 80 °C / 176 °F, the solubility
rises only slightly, to 380 g (27.5%). Only
19 g / 0.7 oz more salt dissolves in a liter
of water at the higher temperature.
In contrast, you can dissolve 2.2 kg /
4.9 lb of sugar in a liter of water at 30 °C,
and at 140 °C / 284 °F, that amount rises
to more than 10 kg / 22 lb! (Note that
dissolved sugar raises the boiling point of
water.) In water at 100 °C / 212 °F, sugar is
roughly 12 times as soluble as salt is.
Highly soluble gases used in
cooking include carbon dioxide
(CO 2
), used in carbonated drinks
and many other contexts, and
nitrous oxide (N 2
O), used in
whipping siphons. They are much
more soluble than oxygen or
nitrogen at one atmosphere (1 bar)
of pressure.
For more on culinary techniques that exploit
carbon dioxide’s ability to dissolve into water,
see Dry Ice, page 2·456.
Solubility (grams / liter)
10,000
5,000
2,000
1,000
500
20 60 100 140 180 220 260
Sugar
Salt
−20 0 20 40 60 80 100 120 140
Temperature (°C)
at one temperature, then lower the temperature or
evaporate out some of its solvent? You then have
a supersaturated solution that wants to rid itself of
the amount of solute that exceeds its solubility. The
excess solute generally precipitates out by reverting
to the solid state as crystals. Initially rather
small, these crystals of the solute can grow to be
quite large, especially if you allow solvent evaporation
to continue in an uncovered container.
It’s easy to make a supersaturated solution of
sugar in water, simply by cooling a saturated
solution or allowing it to evaporate. You can make
those huge sugar crystals called rock candy in this
way by adding thousands of crystallization nuclei
(in the form of several suspended strings) and
letting the setup stand around, evaporating away,
for a couple of weeks.
When cooks blend two liquids together, they
often think of that as mixing. Sometimes, however,
what they are really doing is making a solution. As
when dissolving a solid in a liquid, the polarities of
the components often govern what happens.
Most people know that alcohol (ethanol) is
miscible in water; mix any proportions of the two
liquids and they stay mixed, hence the wide range
of wine and spirits at the liquor store. Ethanol thus
dissolves in waterand water dissolves in ethanol.
But you may not know that if you mix 1 l / 34 oz
of water with a 1 l / 34 oz of ethanol, the resulting
solution has a volume of only 1.92 l / 64.92 oz.
One plus one, in this case, does not equal two but
Temperature (°F)
90%
80%
70%
60%
50%
40%
30%
Solubility (mass of solute / mass of solution)
rather about 4% less than two. That’s because
ethanol and water molecules form hydrogen bonds
that draw them tightly to one another.
Oil and water, in contrast, are immiscibleas
anyone who has made a vinaigrette knows. Still,
with enough shaking you can break the oil and
vinegar (which is essentially water) into droplets
small enough that, for a while, they look like
a homogeneous mixture. But the lighter oil
droplets inevitably float to the top, and eventually,
you’re back to two separate layers.
To slow that natural separation, you need an
emulsifier, a substance that induces the oil and
water droplets to adhere to each other so tightly
that they never, or almost never, separate. Add
that ingredient and some vigorous agitation, and
you can make an emulsion, which is so useful in
cooking that we have devoted an entire chapter to
the topicsee Emulsions, page 4·196.
Tiny Bubbles
Champagneand fishare possible because
gases, too, dissolve in many liquid solvents.
Marine creatures need oxygen just as land animals
do, but instead of extracting it from the air via
lungs, they extract it from the water by means of
gills and other organs. Aquariums have air pumps
and bubblers to provide a constant supply of
dissolved oxygen. Without this, the fish would
soon exhaust the oxygen and suffocate, just as
a person would in a small, airtight room.
Some gases are highly soluble in water, others
much less so. Oxygen is a relatively poor dissolver.
At 25 °C / 77 °F and normal atmospheric pressure,
only 40 mg / 0.0014 oz of oxygen will dissolve in
1 l / 34 oz of waterfar lower than the solubilities
of salt and sugar. Nitrogen, which constitutes 78%
by weight of our air, is even less soluble: only about
16 mg / 0.0006 oz per 1 l / 34 oz at the same
temperature and pressure. Carbon dioxide is very
much more soluble in water than either of these:
about 1,500 mg / 0.05 oz per 1 l / 34 ozbut
that’s a slightly different situation because CO2
actually reacts chemically with water.
The solubility of gases in water depends on
temperature, but in the opposite way from that of
most solids: gases become less soluble as the
temperature increases. When the water reaches its
boiling point, all dissolved gas molecules are
carried off along with the steam bubbles. So boiling
a pot of water for several minutes will completely
remove any dissolved air or other gases.
Conversely, the colder the water becomes, the
more soluble gases become, all the way down to
the freezing point. When the water freezes,
dissolved gas molecules are expelled from the
developing crystal latticeexcept for those that
are trapped with no way out. These often appear
as tiny bubbles in ice cubes.
The solubility of gases also depends on pressure.
At normal atmospheric pressures of around 1 bar /
14.7 psi, the solubility varies in a pretty straightforward
way: double the pressure, double the
solubility. But at very low pressures, such as in
a partial vacuum, the dissolved molecules are
essentially pulled out, and the water degasses.
You can exploit this effect to make clear ice
cubes. Just boil the water for several minutes and
let it cool without stirring (which could encourage
air to dissolve in it) before you freeze it. If heat
would alter the flavors in the liquid you want to
freeze into clear ice cubes, you can boil it in a
partial vacuum, which makes the boiling point
lower. The setup used for vacuum reduction,
described on page 2·379, is ideal for this purpose.
An ultrasonic homogenizer can also work; in
effect, it shakes the gases out of the liquid. Because
most dissolved gas molecules, such as the nitrogen
and oxygen molecules in air, are not chemically
bound to the water, they are easy to dislodge.
332 VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 333