Lynne Wong's PhD thesis

CHAPTER 5. ADSORPTION ISOTHERMS OF SUGAR CANE FIBRES
In the previous chapter the Brix-free water content of the cane components are determined
by means of a contact method. This chapter describes the experiments performed to
determine the equilibrium moisture content, and hence adsorption properties, by means of
a vapour sorption method of these cane component parts which enabled the determination
of a number of thermodynamic parameters that provide insight into the microstructure of
the fibre-water interface.
5.1 THE CONCEPT OF BOUND WATER IN FIBRE
All biological systems have the ability to retain molecular hydration as a fundamental
defensive mechanism against dehydration (Quioco et al., 1989). The structure, mobility
and function of biological molecules are affected by the water molecules bound to the
ionic, polar and hydrophilic sites of macromolecules (Vertucci and Leopold, 1987).
Rascio et al. (1992) demonstrated the important role played by bound water in the plant's
adaptation to the moderate stress of dehydration and related its tolerance towards
dehydration to the quantity of bound water, the strength of binding and its ability to
tolerate the removal of bound water without damage. However, the relationship between
the quantity of bound water and water binding strength of plant tissues was not elucidated.
In their review of moisture sorption isotherm characteristics of food products, Al-Muhtaseb
et al. (2002) quoted that food preservation consisted of controlling the moisture content
during the processing of foods, achieved either by removing it or binding it such that the
food becomes stable to both microbial and chemical deterioration (Labuza, 1980). Later,
the concept of water activity was introduced to indicate the ‘quality’ of the water content
of food. It describes the degree of ‘boundness’ of water and hence, its availability to
participate in physical, chemical and microbiological reactions. In a biological system,
three aspects of water can be distinguished (Rizvi and Benato, 1984):
1) Structural, the position and orientation of water molecules in relation to each
other and to macromolecules.
2) Dynamic, molecular motions of water molecules and their contribution to the
hydrodynamic properties of the system.