Lynne Wong's PhD thesis

The experiment was repeated with the remaining sub-samples after drying at 80 °C in a
vacuum oven under 875 mbar vacuum for 3, 4, 5, 7 and 16 hours.
Results obtained (Table 4.8) after vacuum drying at 80 °C for 3, 4, 5 and 7 hours are all
comparable to those obtained after drying overnight for 16 hours at 65 °C (average of
17.4% c.f 16.9%). However, after overnight drying at 80 °C, the Brix-free water value
obtained was much lower at 15.7%.
Air-oven drying was tried on subsamples of a rind fibre at 65 °C and 105 °C for various
lengths of time up to 21 hours. Results (Table 4.8) show that 65 °C was insufficient to dry
the samples even after 21 hours whereas 105 °C yielded the highest Brix-free value of the
sample after 6-20 hours. This means that at 105 °C after 6-20 hours, the Brix-free water
originally in the sample has been completely driven out of the sample, enabling ‘new’
water to be adsorbed during analysis and determined as Brix-free water. However, it
would be unwise to adopt this drying method prior to analysis, since many workers have
warned against the use of high temperatures, notably Kollmann and Schneider (1963) who
indicated that elevated temperatures reduce timber water adsorption capacity.
From the data in Table 4.8, it was evident that after one hour of drying at 65 °C, the Brixfree
water result of 9.9% represented more than half of the maximum value of 17.37%
achieved after 20 hours of drying at 105 °C. Hence it was thought reasonable to use one
hour air-oven drying at 65 °C prior to vacuum drying to boost the drying process. Results
for one hour air-oven drying at 65 °C followed by 65 °C or 80 °C vacuum-oven drying for
various lengths of time are shown in Table 4.9.
It appears that more consistent results are obtained after air-oven drying at 65 °C for one
hour followed by overnight vacuum-oven drying for 16 hours at 65 °C under 875 mbar
vacuum. (See also Section 4.4.6.).
4.4.6 Residual moisture in fibre samples prior to Brix-free water determination
The Brix-free water values obtained for certain sugar cane component parts were in
general lower than those obtained by Mangion and Player (1991), for example, 13 – 15%
both for cane top and dry leaf fibres compared to those of Mangion and Player of 23 – 24%
for cane top and 20 – 21% for dry leaf. It was therefore thought opportune to check the
remaining moisture content of fibres after the method of vacuum oven drying at 65 °C for
16 hours under 875 mbar vacuum prior to the Brix-free water determination. Should there
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