Modernist-Cuisine-Vol.-1-Small

WATER AS A SOLVENT
6
For more on enfleurage, see page 2·323.
Salt will not dissolve in a non-polar liquid
like oil. Hervé This exploits this effect to
prevent salt from dissolving when put on
the surface of a tomato or other wet food.
The salt is tossed in oil first, which
protects it from melting and gives the salt
a nice crunch when you eat it.
We all know that certain solid materials, like salt
and sugar, dissolve in water. The scientific term for
a substance into which other substances dissolve is
solvent. The substance that dissolves into the
solvent is called the solute, and the homogeneous
mixture of solvent and solute is called a solution.
Both solvents and solutes can be in any state of
matter: solid, liquid, or gas. But in the kitchen, the
solvent is usually a liquidmost often water but
sometimes oil.
It’s rare that we use a solid as a solvent for
culinary purposes, but that’s what perfumers do in
the technique of enfleurage, where lard or another
solid fat is used as a solvent to dissolve and trap
volatile aromatic substances that give flowers their
characteristic aromas. These aromatics dissolve in
fats (even solid fats) but not in water.
Gaseous solvents are also rare, but the air
around us is one example. Air can be described as
a gaseous solution of oxygen, carbon dioxide, and
other gases dissolved in gaseous nitrogen, although
that’s stretching the concept a bit. All
gases mix with or “dissolve in” all other gases. That
is certainly not true of liquids and solids.
Broadly speaking, liquid solvents are of two
types. Polar solvents are made of molecules in
which the electrons are unevenly distributed, so
that the molecule has a negative end and a relatively
positive end. This dipole nature affects the
behavior of polar molecules. Water is a highly
polar solvent because its molecules’ electrons are
localized at the oxygen-atom ends, leaving the
hydrogen-atom ends relatively positive (see Why
Water Is Weird, page 298).
Nonpolar solvents are made of molecules that
are not dipoles. Fats and oils are the classic kitchen
examples of nonpolar solvents.
Like liquids, solid compounds can be either
polar or nonpolar. In general, like dissolves in like.
Sucrose and many other sugars are strongly polar
compounds, and they dissolve only in a highly
polar solvent because dipoles in the solvent (water)
attract the dipoles of the solute (sugar). Put another
way, polar solids are soluble in polar solvents.
Because it is polar, sucrose will not dissolve in
oil or other nonpolar solvents. Oils and waxes, by
the same token, dissolve in nonpolar (oily) solvents
but not in water. Polar solvents are insoluble
in nonpolar solvents and vice versa.
Ethanol, the common form of alcohol in the
kitchen, is also a polar solvent, but it is a weaker
dipole than watera bit less than half as strong,
by one common measure chemists use to measure
polarity. As a result, ethanol dissolves some
water-soluble compounds but not all of them or
not very much of them. Sucrose, for example, does
not dissolve in pure ethanol.
Cooks often talk about adding wine to a dish as
“adding alcohol.” But it’s important to realize that
wine, at perhaps only 13% ethanol, is more water
than alcohol. Because its molecules are hindered
by their hydrogen bonding to the water, wine does
not dissolve substances that pure ethanol would.
Sweet and Salty Solutions
Not every substance is polar or nonpolar. Ionic
compounds, like table salt (sodium chloride), are
composed not of molecules but of ions: atoms or
groups of atoms that carry whole positive or
negative electric charges, not merely the partial
charge of a dipole. The charge attraction of the
dipoles in a polar solvent can pull ions apart from
one another, so ionic solids usually dissolve in
polar solvents. Salt, for example, dissolves readily
in water. As that happens, the dipolar water
molecules pull the salt molecules apart into
positively charged sodium ions (Na + , in the
notation of chemistry) and negatively charged
chlorine ions (Cl − ). Although we say that the salt
has dissolved, in reality there is no sodium chloride
as such in the solutiononly separated ions
of sodium and ions of chlorine.
Nonionic compounds, such as sugar, are made
of electrically neutral molecules whose atoms are
bonded together by covalent bonds that form
when the molecules share pairs of electrons.
Dipoles can’t tear covalent bonds apart, partly
because they can’t get an electric “grip” on them as
they can on ions, so nonionic molecules remain
intact when they dissolve. Sucrose is nonionic, so
when you dissolve sugar in water, there really are
intact sugar molecules in the water.
When a solid dissolves completely in a solvent,
the mass of the resulting solution is the sum of the
twoas it must be by the law of conservation of
mass. The volume of the solution, however, is
typically not the sum of the volumes of the solute
and solvent prior to mixingit is less.
The fact that volumes don’t add when a solution
forms makes sense if you envision the solute
molecules fitting into spaces between the solvent
molecules and vice versa. Because there are more
molecules in each bit of space, the density of the
resulting solution is greater than that of the
solvent prior to mixing. If you dissolve salt in
water, for instance, the mass of the solution will
equal the mass of the water plus the mass of the
salt. But the volume of the solution will be 2.5%
less than the sum of the volumes of the salt and the
water. The effect is even more startling in sugar
solutions. With heating, you can actually dissolve
two cups of sugar in one cup of water!
What’s the limitjust how much sugar can you
cram into a syrup that is fully saturated with
sugar? The answer depends on the temperature
and purity of the water, as well as other factors, but
the concentration of the saturated solutiontypically
expressed as a percentage or as grams of
solute per 100 g of solventis called its solubility.
If the solubility is zero, the two substances are
completely immiscible: like oil and water, neither
dissolves in the other. Water and alcohol, in
contrast, do mix homogeneously in any proportions;
they are said to be fully miscible with each
other. Other pairs of substances are misciblebut
only up to a certain concentration. There is a limit,
for example, to how much salt will dissolve into
even very hot water. Add more salt than that and
further stirring or heating will not make any more
dissolve; the extra salt just piles up on the bottom
of the pot. The compound has reached its solubility
limit. Another way of saying this is that you
have made a saturated solution. A saturated
solution of sodium chloride (table salt) in water
contains just under 269 g / 9.5 oz of salt per liter of
water at 50 °C / 122 °F.
In virtually all cases relevant to the kitchen,
the higher the temperature, the higher the
solubility. Salt is an unusual case, in that temperature
makes very little difference in its solubility
in water. Sugar, on the other hand, behaves more
typically, in that solubility increases substantially
with temperaturesee the graphs on the next
page. That is why you must heat a sugar-water
solution to make a syrup.
When you do make a hot sugar syrup, an
interesting thing happens: the boiling point of the
water in the solution rises from boiling point
elevation (see page 318). So you can keep adding
sugar to water even above 100 °C / 212 °F. When
the temperature reaches 140 °C / 284 °F, the sugar
is in what a confectioner would call the “soft
crack” stage and the concentration is 95%an
amazing 19 kg / 42 lb sugar per liter of water.
What happens if you make a saturated solution
When you boil water, the first bubbles to
appear at the bottom of the pan are not
steam but gas escaping from the water.
Two blocks of ice illustrate the point. The
left block shows the gas still trapped. In
the right block, the gas has escaped,
leaving the water gas-free and the cube
clear.
The primary gases in air, nitrogen
and oxygen, are not very soluble in
water: at normal atmospheric
pressure, only a fraction of a gram
dissolves per liter. Carbon dioxide
is quite a bit more soluble. And
unlike solids, all three of these
gases become less soluble in water
as temperatures rise. For example,
3.4 g / 0.12 oz of CO 2
dissolves in
a liter of water at 0 °C / 32 °F,
whereas at 60 °C / 140 °F, the
solubility is 0.55 g / 0.19 oz, only
about a sixth as much. This is why
carbonated drinks are served cold.
330 VOLUME 1 · HISTORY AND FUNDAMENTALS
THE PHYSICS OF FOOD AND WATER 331