Lynne Wong's PhD thesis

A similar procedure was followed to calculate the net isosteric heat of adsorption and the
sorption entropy from the GAB model.
The GAB net isosteric heat of sorption q st is plotted against moisture level for all nine cane
components of R 570 aged 52 and 36 weeks in Fig 6.8, and corresponding entropy of
sorption in Fig 6.9.
From Figs 6.6 and 6.8, it can be seen that q st values for all cane components are large at
low EMC, they reach a maximum between 0 – 5% EMC and then decrease sharply
reaching a plateau at about a moisture content of 15%. The rapid increase in the net
isosteric heat of sorption q st at low moisture content is due to the existence of highly active
polar sites on the surface of the sugar cane components, which are covered with water
molecules forming a monomolecular layer (Hossain et al., 2001). As the moisture content
increases, the available sites for sorption of water decrease, resulting in lower values of the
isosteric heats. The net isosteric heat of sorption approaches the latent heat of vaporisation
of pure water (H L ) at the moisture content of 15% (on a dry basis). At moisture contents
greater than 15% (on a dry basis), there is no significant difference between the latent heat
of vaporisation of pure water and the net isosteric heat of sorption. Iglesias and Chirife
(1976) explained that at this point, the existence of water in free form in the product is
indicated. This moisture level has been termed the ‘free water point’, and seems to
correspond numerically to the Brix-free water value determined in Chapter 4.
The maximum net heat of sorption and entropy of sorption for nine cane components aged
52 and 36 weeks as calculated by the GAB model are shown in Table 6.4.
Stalk pith of 52 weeks showed the highest net isosteric heat of sorption q st value calculated
from the GAB model (Table 6.4 and Fig 6.8), as well as from the Hailwood-Horrobin
model (Fig 6.6); inversely, it had the lowest entropy of sorption S d calculated from the
GAB model (Table 6.4 and Fig 6.9), as well as from the Hailwood-Horrobin model (Fig
6.7).
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