Water and Its Relation to Fresh Produce 269
Postharvest uptake of water may occur during washing and hydrocooling and under
conditions of high humidity, but this has minimal effect on maintaining the quality
of the harvested fresh produce during subsequent storage.
9.2 CHEMICAL COMPOSITION, STRUCTURE, AND
PROPERTIES OF WATERWater posses unique physical and chemical properties that enable it to be easily
transportable throughout the plant’s system as well as to act as a solvent. Water aids
in transport of solutes across cells, cell expansion and cell turgor, temperature
regulations due to the high heat of vaporization, high thermal capacity, and high
conductivity (Nobel, 1999). The properties of water are mainly attributed to the polar
structure of the water molecule. Thermal properties of water account for temperature
control of plants by ensuring that temperatures are neither too high nor too low for the
survival of the plant. Similarly, water maintains a liquid state over the wide range of
temperatures at which the majority of biological reactions occur (Steinbeck, 1995).
9.2.1 POLAR NATURE OF WATER
Water is a very simple molecule composed of one oxygen atom covalently bound
to two hydrogen atoms. The water molecule is V-shape with an angle of 105° between
the two O-H bonds (Figure 9.1). Ruan and Chen (1998) attributed the unusual
properties of water to its angled shape and the intermolecular bonds that it can form.
Two orbitals of the oxygen atom participate in covalent bonds with two hydrogen
atoms; the remaining two orbitals contain unpaired electrons. Due to the repulsion
between the unpaired electrons and those in the covalent bonds, the orbitals arrange
themselves in a tetrahedron format around the central oxygen atom.
The oxygen atom is more electronegative than hydrogen and thus it tends to attract
the electrons of the covalent bond. The attraction results in a partial negative charge at
the oxygen end of the molecule and a partial positive charge at each hydrogen (Figure
9.1). Although the partial charges are equal, resulting in a water molecule carrying no
net charge, the combination of the separation of the partial charges and the shape of
the water molecule makes the water molecule positive. This electrostatic attraction
between water molecules is called the hydrogen bond. The energy of the hydrogen bond
is about 20 kJ/mol. Although weaker than covalent or ionic bonds but stronger than the
short-range, transient attractions known as Van der Waals forces (about 4 kJ/mol), it is
responsible for many unique properties of water (Steinbeck, 1995). Hydrogen bonding
can occur within water molecules and between neighboring water molecules, and each
water molecule can form up to four hydrogen bonds.
9.2.2 PROPERTIES OF WATER
The strong hydrogen bonds within and between water molecules contribute to the
unique thermal properties of water, namely, high specific heat, thermal conductivity,
and high latent heat of vaporization (Cybulska and Doe, 2002). The specific heat of
a substance relates to the amount of heat required to raise the temperature of 1 gram