Lynne Wong's PhD thesis

particular fibre is systematically higher at 52 weeks (48 – 49 kJ mol -1 ) than at 36 weeks
(46 – 47 kJ mol -1 ), whereas the heat of sorption of monolayer is very variable.
The net isosteric heat of sorption-entropy relationship for water adsorption in fibres of nine
cane components of R 570 aged 52 and 36 weeks, satisfy the enthalpy-entropy
compensation theory. The adsorption process in the fibres of sugar cane plant is
essentially enthalpy-controlled (isokinetic temperature > harmonic mean temperature) and
non-spontaneous (+∆G).
The various quantities calculated provide an insight into the nature of the water sorbed
onto the sugar cane component parts.
As described in literature the bound water occurs in three regions.
From the monolayer moisture content and the amount of “hydrated water” as calculated
from the Hailwood-Horrobin model it is clear that at EMC values between 0 and 5% the
water is tightly bound to the surface of the fibre. The corresponding water activities are
0 – 0.3. This constitutes what has been termed non-freezable water.
The second region starts at EMC values from about 5% and ends at 10 – 15%. The
corresponding a w values are 0.3 to 0.6 – 0.8. In this region the bound water has a heat of
adsorption, H m , which is 2 to 5 kJ mol -1 greater than the heat of vaporisation H L of bulk
pure water. This is termed the freezable water.
The third type of water is essentially free water, it exists after the second region and ends at
EMC of approximately 25% (where the relative humidity is nearly 100%). This type of
water has the same heat of adsorption as the latent heat of vaporisation of pure water.
From this study it is apparent that Brix-free water as measured in this work measures the
amount of water bound in the first two regions. The heat of sorption of monolayer and the
plot of binding energy against moisture content lead to the primary, secondary and tertiary
bound water, it was found that the secondary bound water corresponds to the Brix-free
water values of sugar cane component parts, except in the case of stalk pith, which shows
similarity to the tertiary bound water.
The value of 25% traditionally accepted as Brix-free water of cane was found to be the
fibre saturation point bound water determined at 20 °C.
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