FOOD CHEMISTRY

Module 1. Water

Lesson 1
STRUCTURE OF WATER

1.1 Introduction

Water is relatively small inorganic molecule, but organic life is highly dependent on this tiny molecule. It is the only substance on the earth that occurs abundantly in all three physical states (gas, liquid and solid).

Water is essential for life as:

(1) regulator of body temperature

(2) solvent

(3) carrier of nutrients and waste products

(4) reactant and reaction medium

(5) lubricant and plasticizer

(6) stabilizer of biopolymer conformation

(7) facilitator of the dynamic behavior of macromolecules (e.g. catalytic activity)

Most of the fresh foods contain large amounts of water. It is one of the major component in composition of many foods. Each food has its own characteristic amount of this component. Effect of water on structure, appearance and taste of foods as well as their susceptibility to spoilage depends on its amount, location, and orientation. Therefore, it is essential to know its physical properties.

Water has unusually high melting point, boiling point, surface tension, permittivity, heat capacity, and heat of phase transition values. Other unusual attribute of water include expansion upon solidification, large thermal conductivity compared to those of other liquids, moderately large thermal conductivity of ice compared to those of other nonmetallic solids.

1.2 Water Molecule

Some the unusual properties of water are due strong intermolecular attractive forces among molecules of water. (Fig. 1.1)

The unusual properties of water can be explained from nature of water molecules. In formation of water molecule, two hydrogen atoms form covalent bonds with oxygen. The highly electronegative oxygen of the water molecule pulls the single electron from each of the two covalently bonded hydrogen atoms towards its self, as a result each hydrogen atom becomes partially positively charged and oxygen becomes partially negatively charged. Consequently, resultant covalent bond formed between oxygen and hydrogen atoms acquires partial ionic character. The bond angle of individual water molecule in vapor state

1.3 Association of Water Molecules

The shape of water molecule and the partial polar nature of the O-H bond in the water molecule create intermolecular attraction force. Such inter molecular attraction, results in to formation of hydrogen bonds between the water molecules. Therefore, water molecules associate with considerable tenacity.

Each water molecule involves in four hydrogen bonds with neighboring water molecules. Multiple hydrogen bonding between water molecules, forms a structure of three-dimensional network .

Existence of three-dimensional hydrogen bonded structure of water is responsible for many of its unusual properties. The extra energy needed to break intermolecular hydrogen bonds. This leads to large values for heat capacity, melting point, boiling point, surface tension, and enthalpies of various phase transitions of water. The dielectric constant (permittivity) of water is influenced by hydrogen bonding. Hydrogen-bonded multi-molecular dipoles increase the permittivity of water. The hydrogen bonded arrangement of water molecules is highly dynamic, allowing individual molecules to alter their hydrogen-bonding relationships with neighboring molecules. This phenomenon facilitates mobility and fluidity of water.

The open, hydrogen-bonded, tetrahedral structure of water molecules in ice is responsible for low density of water in ice form . The extent of intermolecular hydrogen bonding among water molecules depends on temperature.

With input of heat melting of ice occurs; that is, some hydrogen bonds are broken distance between nearest neighbor increases. The latter factor predominates at temperatures between 0 and 4°C, which causes net increase in density. Further warming increasing distance between nearest neighbors (thermal expansion) predominates above 4°C, which causes net decrease in density.