Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 111

Ih and XI, but each O atom is hydrogen-bonded to
four other O atoms.
Cubic ice, Ic, has been produced by cooling va-
por or droplets below 200 K (Mayer and Hall-
brucker 1987, Kohl et al. 2000). More studies
showed the formation of Ic between 130 and 150 K.
Amorphous (glassy) water is formed below 130 K,
but above 150 K Ih is formed. The hydrogen bond-
ing and intermolecular relationships in Ih and Ic are
the same, but the packing of layers and symmetry
differ (see Fig. 5.5). The arrangement of O atoms in
Ic is the same as that of the C atoms in the diamond
structure. Properties of Ih and Ic are very similar.
Crystals of Ic have cubic or octahedral shapes,
resembling those of salt or diamond. The conditions
for their formation suggest their existence in the
upper atmosphere and in the Antarctic.
As in all phase transitions, energy drives the trans-
formation between Ih and Ic. Several forms of amor-
phous ice having various densities have been ob-
served under different temperatures and pressures.
Unlike crystals, in which molecules are packed in an
orderly manner, following the symmetry and period-
ic rules of the crystal system, the molecules in
amorphous iceare immobilized from their posi-
tions in liquid. Thus, amorphous ice is often called
frozen water or glassy water.
When small amounts of water freeze suddenly, it
forms amorphous ice or glass. Under various tem-
peratures and pressures, it can transform into high-
density (1.17 Mg/m^3 ) amorphous water, and very
high-density amorphous water. Amorphous water
also transforms into various forms of ice (Johari and
Anderson 2004). The transformations are accompa-
nied by energies of transition. A complicated phase
diagram for ice transitions can be found in Physics
of Ice(Petrenko and Whitworth 1999).
High pressures and low temperatures are required
for the existence of other forms of ice, and currently
these conditions are seldom involved in food pro-
cessing or biochemistry. However, their existence is
significant for the nature of water. For example, their
structures illustrate the deformation of the ideal
tetrahedral arrangement of hydrogen bonding pre-
sented in Ih and Ic. This feature implies flexibility
when water molecules interact with foodstuffs and
with biomolecules.

Vapor Pressure of Ice Ih

The equilibrium vapor pressure is a measure of the

ability or potential of the water molecules to escape

from the condensed phases to form a gas. This po-

tential increases as the temperature increases. Thus,

vapor pressures of ice, water, and solutions are im-

portant quantities. The ratio of equilibrium vapor

pressures of foods divided by those of pure water is

called thewater activity, which is an important

parameter for food drying, preservation, and storage.

Ice sublimes at any temperature until the system

reaches equilibrium. When the vapor pressure is

high, molecules deposit on the ice to reach equilibri-

um. Solid ice and water vapor form an equilibrium

in a closed system. The amount of ice in this equilib-

rium and the free volume enclosing the ice are irrel-

evant, but the water vapor pressure or partial pres-

sure matters. The equilibrium pressure is a function

of temperature, and detailed data can be found in

handbooks, for example, the CRC Handbook of

Chemistry and Physics(Lide 2003). This handbook

has a new edition every year. More recent values

between 193 and 273 K can also be found in Physics

of Ice(Petrenko and Whitworth 1999).

The equilibrium vapor pressure of ice Ih plotted

against temperature (T)is shown in Figure 5.6. The

line indicates equilibrium conditions, and it sepa-

rates the pressure-temperature (P-T)graph into two

domains: vapor tends to deposit on ice in one, and

ice sublimes in the other. This is the ice Ih—vapor

portion of the phase diagram of water.

Figure 5.6.Equilibrium vapor pressure (Pa) of ice as a

function of temperature (K).