Lynne Wong's PhD thesis

What was actually determined was not hygroscopic water but the fibre saturation point at
20 o C (see Section 6.9).
Table 6.9 compiles the values of fibre saturation point bound water of various cane
components at 30, 45, 55 and 60 o C. Data at 20 o C were not available, but results calculated
for 30 o C were comparable with the 25% value obtained by Foster (1956) at 20 o C,
erroneously taken as Brix-free water value. Table 6.9 also shows the Brix-free water values
determined in this work. These are in general lower than the fibre saturation point values.
Hence, the generally accepted value of 25% is much too high for Brix-free water of cane.
6.10 SUMMARY AND CONCLUSIONS
The various parameters calculated have allowed for a better understanding of 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% depending on
the cane components. 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.
Treatment of the experimental sorption data according to the GAB, Caurie I and
BET equations allowed the evaluation of monolayer moisture content in nine cane
components aged 52 and 36 weeks, and in reconstituted cane stalk, dry leaf and
green leaf of R 570 aged 52 and 36 weeks.
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