WATER IN MILK AND DAIRY PRODUCTS 297Figure 7.1 Schematic representations (a-c) of a water molecule and hydrogen bonding between
water molecules (d).maintained only over short distances. The structure is dynamic; molecules
can rapidly exchange one hydrogen bonding partner for another and there
may be some unbonded water molecules.
Water crystallizes to form ice. Each water molecule associates with four
others in a tetrahedral fashion as is apparent from the unit cell of an ice
crystal (Figure 7.2). The combination of a number of unit cells, when viewed
from the top, results in a hexagonal symmetry (Figure 7.3). Because of the
tetrahedral arrangement around each molecule, the three-dimensional struc-
ture of ice (Figure 7.4) consists of two parallel planes of molecules lying
close to each other ('basal planes'). Basal planes of ice move as a unit under
pressure. The extended structure of ice is formed by stacking of several basal
planes. This is the only crystalline form of ice that is stable at a pressure of
1 atm at O'C, although ice can exist in a number of other crystalline forms,
as well as in an amorphous state. The above description of ice is somewhat
simplified; in practice the system is not perfect due to the presence of ionized