5 Water Chemistry and Biochemistry 115supercritical water. In the 17th century, Denis
Papin (a physicist) generated high-pressure steam
using a closed boiler, and thereafter pressure canners
have been used to preserve food. Pressure cookers
were popular during the 20th century. Pressure cook-
ing and canning use subcritical water. Plastic bags
are gradually replacing cans, and food processing
faces new challenges.
Properties of subcritical and supercritical water
such as dielectric constant, polarity, surface tension,
density, viscosity, and others, differ from those of
normal water. These properties can be tuned by ad-
justing water temperature. At high temperature,
water is an excellent solvent for nonpolar substances
such as those for flavor and fragrance. Supercritical
water has been used for wastewater treatment to
remove organic matter, and this application will be
interesting to the food industry if companies are
required to treat their wastes before discharging
them into the environment. The conditions for su-
percritical water cause polymers to depolymerize
and nutrients to degrade. More research will tell
whether supercritical water will convert polysaccha-
rides and proteins into useful products.
Supercritical water is an oxidant, which is desir-
able for the destruction of substances. It destroys
toxic material without the need of a smokestack.
Water is a “green” solvent and reagent, because it
causes minimum damage to the environment. There-
fore, the potential for supercritical water is great.
AQUEOUS SOLUTIONS
Water dissolves a wide range of natural substances.
Life began in natural waters, aqueous solutions,
and continuation of life depends on them.
Water, a polar solvent, dissolves polar substances.
Its polarity and its abilities to hydrogen bond make it
a nearly universal solvent. Water-soluble polar sub-
stances are hydrophilic, whereas nonpolar sub-
stances insoluble in water arehydrophobicorlip-
ophilic.Substances whose molecules have both polar
and nonpolar parts areamphiphilic. These sub-
stances include detergents, proteins, aliphatic acids,
alkaloids, and some amino acids.
The high dielectric constant of water makes it an
ideal solvent for ionic substances, because it reduces
the attraction between positive and negative ions in
electrolytes: acids, bases, and salts. Electrolyte solu-
tions are intimately related to food and biological
sciences.COLLIGATIVEPROPERTIES OFAQUEOUS
SOLUTIONSVapor pressure of an aqueous solution depends only
on the concentration of the solute, not on the type
and charge of the solute. Vapor pressure affects the
melting point, the boiling point, and the osmotic
pressure, and these are colligative properties.
In a solution, the number of moles of a compo-
nent divided by the total number of moles of all
components is the mole fractionof that component.
This is a useful parameter, and the mole fraction of
any pure substance is unity. When 1 mol of sugar
dissolves in 1.0 kg (55.56 mol) of water, the mole
fraction of water is reduced to 0.9823, and the mole
fraction of sugar is 0.0177. Raoult’s law applies to
aqueous solutions. According to it, the vapor pres-
sure of a solution at temperature Tis the product of
mole fraction of the solvent and its vapor pressure at
T. Thus, the vapor pressure of this solution at 373 K
is 99.53 kPa (0.9823 atm). In general, the vapor
pressure of a solution at the normal boiling point is
lower than that of pure water, and a higher tempera-
ture is required for the vapor pressure to reach 101.3
kPa (1 atm). The net increase in the boiling temper-
ature of a solution is known as the boiling point ele-
vation.
Ice formed from a dilute solution does not contain
the solute. Thus, the vapor pressure of ice at various
temperatures does not change, but the vapor pres-
sure of a solution is lower than that of ice at the
freezing point. Further cooling is required for ice to
form, and the net lowering of vapor pressure is the
freezing point depression.
The freezing and boiling points are temperatures
at ice-water and water-vapor (at 101.3 kPa) equilib-
ria, not necessarily the temperatures at which ice
begins to form or boiling begins. Often, the temper-
ature at which ice crystals start to form is lower than
the melting point, and this is known as supercool-
ing. The degree of supercooling depends on many
other parameters revealed by a systematic study
of heterogeneous nucleation (Wilson et al. 2003).
Similarly, the inability to form bubbles results in
superheating. Overheated water boils explosively
due to the sudden formation of many large bubbles.