Lynne Wong's PhD thesis

The values of K 1 , K 2 and W were used to calculate m h , m s and m for each experimental a w
value at each of the four temperatures for all the cane components. The results are
compared with the experimental values of the equilibrium moisture content, and are shown
in Tables 6.7 for all nine components aged 52 and 36 weeks.
Similarly the results for reconstituted cane stalk, dry leaf and green leaf are compared in
Table 6.8. These results are plotted against water activity a w in Figures 6.13.1 – 6.13.8 for
nine cane components aged 52 and 36 weeks at 30, 45, 55 and 60 °C and are shown on the
CD (File: Fig.6.13.1-6.13.8 Hydrated and dissolved water.xls). Typical plot for nine cane
components aged 52 weeks at 30 °C is shown in Figure 6.13, and similar plots for
reconstituted cane stalk, dry leaf and green leaf aged 52 and 36 weeks at four temperatures
are presented in Figures 6.14 and 6.15 respectively.
From Tables 6.7 – 6.8, we observe that m h decreases with decrease in water activity and
with increase in temperature. The latter is in keeping with the fact that, in general, for a
fixed a w the EMC decreases with increase in temperature at the smaller values of water
activity. At 52 weeks and at 30 °C, dry leaf fines has the highest m h value of 6.47 and rind
fines, the lowest value of 4.20. At 36 weeks and 30 °C, top fibre has the highest m h value
of 5.01 and stalk fibre, the lowest value of 3.99.
From Fig 6.13 it can be seen that the m h curves exhibit a Langmuir-type monolayer
isotherm, and become saturated in the high water activity region, in fact the m h values
obtained from the Hailwood-Horrobin model agree fairly well with the monolayer
moisture content, m o values derived from the GAB model. It therefore appears that this
hydrated water corresponds to the initially bound water that binds directly to the polar
groups on the surface of the fibre. This stronger binding is reflected in the larger heats of
sorption observed when the EMC is between 0 and 5%.
The dissolved water, m s , values decrease with decrease in water activity and increase with
increase in temperature. This is in keeping with the fact that at large a w values the EMC at
given value of a w increases with temperature. At 52 weeks and at 30 °C, stalk pith has the
highest m s value of 27.87 and rind fibre, the lowest value of 16.94. At 36 weeks and at
30 °C, stalk pith has the highest m s value of 24.28 and rind fibre, the lowest value of 16.54.
The m s curves depicted in Fig 6.13 increase sharply within the whole water activity region.
This dissolved water therefore corresponds to multilayer adsorption where water molecules
hydrogen bond to water molecules already attached to the surface of the fibre. The binding
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