Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 109

cules, their properties would be similar to those of
clusters.

CONDENSED WATER PHASES

Below the critical temperature of 647 K (374°C) and
under the proper pressure, water molecules con-
dense to form a liquid or solid—condensed water.
Properties of water, ice, and vapor must be consid-
ered in freezing, pressure-cooking, and microwave
heating. In food processing, these phases transform
among one another. The transitions and the proper-
ties of condensed phases are manifestations of mi-
croscopic properties of water molecules. However,
condensation modifies microscopic properties such
as bond lengths, bond angles, vibration, rotation,
and electronic energy levels. The same is true when
water molecules interact with biomolecules and
food molecules. All phases of water play important
parts in biochemistry and food science.
Water has many anomalous properties, which are
related to polarity and hydrogen bonding. The melt-
ing point (mp), boiling point (bp), and critical tem-
perature are abnormally high for water. As a rule, the
melting and boiling points of a substance are related
to its molecular mass; the higher the molar mass, the
higher the melting and boiling points. Melting and
boiling points of water (molar mass 18, mp 273 K,
bp 373 K) are higher than those of hydrogen com-
pounds of adjacent elements of the same period,
NH 3 (molar mass 17, mp 195 K, bp 240 K) and HF
(molar mass 20, mp 190 K, bp 293 K). If we com-
pare the hydrogen compounds of elements from the
same group (O, S, Se, and Te), the normal boiling
point of H 2 O (373 K) is by far the highest among
H 2 S, H 2 Se, and H 2 Te. Much energy (21 kJ mol^1 ) is
required to break the hydrogen bonds. The strong
hydrogen bonds among water molecules in con-
densed phase result in anomalous properties, includ-
ing the high enthalpies (energies) of fusion, subli-
mation, and evaporation given in Table 5.1. Internal
energies and entropies are also high.
Densities of water and ice are also anomalous. Ice
at 273 K is 9% less dense than water, but solids of
most substances are denser than their liquids. Thus,
ice floats on water, extending 9% of its volume
above water. Water is the densest at 277 K (4°C).
Being less dense at the freezing point, still water
freezes from the top down, leaving a livable environ-
ment for aquatic organisms. The hydrogen bonding

and polarity also lead to aberrant high surface ten-

sion, dielectric constant, and viscosity.

We illustrate the phase transitions between ice,

liquid (water), and vapor in a phase diagram, which

actually shows the equilibria among the common

phases. Experiments under high pressure observed

at least 13 different ices, a few types of amorphous

solid water, and even the suggestion of two forms of

liquid water (Klug 2002, Petrenko and Whitworth

1999). If these phases were included, the phase dia-

gram for water would be very complicated.

SOLIDH 2 O

At 273.16 K, ice, liquid H 2 O, and H 2 O vapor at

611.15 Pa coexist and are at equilibrium; the tem-

perature and pressure define the triple-point of

water. At the normal pressure of 101.3 kPa (1 atm),

ice melts at 273.15 K. The temperature for the equi-

librium water vapor pressure of 101.3 kPa is the

boiling point, 373.15 K.

Under ambient pressure, ice often does not begin

to form until it is colder than 273.15 K, and this is

known as supercooling, especially for ultrapure

water. The degree of supercooling depends on vol-

ume, purity, disturbances, the presence of dust, the

smoothness of the container surface, and similar fac-

tors. Crystallization starts by nucleation, that is,

formation of ice-structure clusters sufficiently large

that they begin to grow and become crystals. Once

ice begins to form, the temperature will return to the

freezing point. At 234 K (39°C), tiny drops of

ultrapure water would suddenly freeze, and this is

known as homogeneous nucleation(Franks et al.

1987). Dust particles and roughness of the surface

Table 5.1.Properties of Liquid Water at 298 K

Heat of formation 
Hf 285.89 kJ mol^1

Density at 3.98°C 1.000 g cm^3

Density at 25°C 0.9970480 g cm^3

Heat capacity 4.17856 J g^1 K^1


Hvaporization 55.71 kJ mol^1

Dielectric constant 80

Dipole moment 6.24  10 ^30 C m

Viscosity 0.8949 mPa s

Velocity of sound 1496.3 m s

Volumetric thermal 0.0035 cm^3 g^1 K^1

expansion coefficient