Food Biochemistry and Food Processing

110 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

promote nucleation and help reduce supercooling
for ice and frost formation.
At ambient conditions,hexagonal ice (Ih) is
formed. Snowflakes exhibit the hexagonal symme-
try. Their crystal structure is well known (Kamb
1972). Every oxygen atom has four hydrogen bonds
around it, two formed by donating its two H atoms,
and two by accepting the H atoms of neighboring
molecules. The hydrogen bonds connecting O atoms
are shown in Figure 5.5. In normal ice, Ih, the posi-
tions of the H atoms are random or disordered. The
hydrogen bonds O–H⎯O may be slightly bent,
leaving the H–O–H angle closer to 105° than to
109.5°, the ideal angle for a perfect tetrahedral ar-
rangement. Bending the hydrogen bond requires
less energy than opening the H–O–H angle.
Bending of the hydrogen bond and the exchange of
H atoms among molecules, forming H 3 O
and OH
in the solids, give rise to the disorder of the H
atoms. These rapid exchanges are in a dynamic
equilibrium.
In the structure of Ih, six O atoms form a ring;
some of them have a chair form, and some have a
boat form. Two configurations of the rings are
marked by spheres representing the O atoms in
Figure 5.5. Formation of the hydrogen bond in ice
lengthens the O–H bond distance slightly from that
in a single isolated water molecule. All O atoms in
Ih are completely hydrogen bonded, except for the
molecules at the surface. Maximizing the number of
hydrogen bonds is fundamental to the formation of
solid water phases. Pauling (1960) pointed out that
formation of hydrogen bonds is partly an electro-

static attraction. Thus, the bending of O–H⎯O is

expected. Neutron diffraction studies indicated bent

hydrogen bonds.

Since only four hydrogen bonds are around each

O atom, the structure of Ih has rather large channels

at the atomic scale. Under pressure, many other

types of structures are formed. In liquid water, the

many tetrahedral hydrogen bonds are formed with

immediate neighbors. Since water molecules con-

stantly exchange hydrogen-bonding partners, the

average number of nearest neighbors is usually more

than four. Therefore, water is denser than Ih.

Other Phases of Ice

Under high pressures water forms many fascinating

H 2 O solids. They are designated by Roman numer-

als (e.g., ice XII; Klug 2002, Petrenko and Whit-

worth 1999). Some of these solids were known as

early as 1900. Phase transitions were studied at cer-

tain temperatures and pressures, but metastable

phases were also observed.

At 72 K, the disordered H atoms in Ih transform

into an ordered solid called ice XI. The oxygen

atoms of Ih and ice XI arrange in the same way, and

both ices have a similar density, 0.917 Mg m^3.

Under high pressure, various denser ices are

formed. Ice II was prepared at a pressure about 1

GPa (1 GPa  109 Pa) in 1900, and others with den-

sities ranging from 1.17 to 2.79 Mg m^3 have been

prepared during the 20th century. These denser ices

consist of hydrogen bond frameworks different from

Figure 5.5.The crystal structures of ice Ih and Ic. Oxygen atoms are placed in two rings in each to point out their
subtle difference. Each line represents a hydrogen bond O–H⎯O, and the H atoms are randomly distributed such
that on average, every O atom has two O–H bonds of 100 pm. The O–H⎯O distance is 275 pm. The idealized tetra-
hedral bond angles around oxygen are 1095°.