Lynne Wong's PhD thesis

5.6.1 Materials
Water sorption isotherms were determined for sugar cane variety R 570. Since the water
activity of a material can be greatly affected by maturity, two ages were examined: 52 and
36 weeks. For each age, nine cane component parts, namely: stalk fibre and pith, rind fibre
and fines, top fibre, dry leaf fibre and fines, and green leaf fibre and fines obtained as
described in Section 3.4.3.2 were studied.
The three replicates of stalk fibre aged 52 weeks were well mixed separately, a
representative sample (30 g) was taken from each replicate to obtain a mixture of 90 g.
Subsamples of one gram of this mixture were weighed out in plastic bags, and the bags
sealed ready for use. The other eight cane component parts of R 570 aged 52 weeks were
treated in the same way. Subsamples of the nine component parts of R 570 aged 36 weeks
were similarly obtained.
An atmosphere of constant relative humidity can be created with sulfuric acid solutions of
varying concentrations. Published data exist for the various concentrations of sulfuric acid
solutions corresponding to particular water vapour pressures, e.g. Perry and Chilton (1973)
and Perry et al. (1984); or to water activity (Rüegg, 1980). An extract of relevant data
from the former is reproduced in Table 5.3, and from the latter in Table 5.4.
Table 5.3. Vapour pressure of aqueous sulfuric acid solutions at selected
concentrations and temperatures (Perry and Chilton, 1973).
Concentration of
sulfuric acid
solution/% (m/m)
Vapour pressure/mm Hg
30 °C 45 °C 55 °C 60 °C
Latent heat of
pure water
/kJ mol -1
5* 32.0 72.0 118.0 146.0 -
10 30.0 68.1 113.0 143.0 43.24
20 27.8 63.3 106.0 133.0 43.40
30 23.8 54.7 91.0 116.0 43.47
40 18.0 41.0 69.0 87.3 44.00
50 11.3 26.7 45.5 58.0 45.05
60 5.4 13.0 22.7 29.3 47.01
* The vapour pressure values for 5% sulfuric acid at all temperatures have
been extrapolated from the higher concentration data.
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