Lynne Wong's PhD thesis

This is attributable to possible residual sucrose in the fibre sample, despite repeated
washing and pressing during the fibre extraction process, resulting in an over-estimation of
the Brix increase in the sucrose contacting solution, and hence an over-estimation of the
Brix-free water content.
Reports exist on the use of detergent to clean plant tissue prior to certain analyses, e.g. Van
Soest (1963) experimented with detergents to dissolve forage nitrogen in the preparation of
fibrous portions of plant tissue of low nitrogen content for analysis such as lignin, and
found cetyl trimethyl ammonium bromide in strongly acid solution or sodium lauryl sulfate
in slightly alkaline solutions most effective. However, one must ensure that any residual
reagent does not affect the subsequent Brix-free water determination. It would be much
simpler to effect a blank determination in distilled water, and subtract the Brix value of the
sample blank from that of the test solution. Since it would be difficult to adjust to have the
same mass of distilled water in the blank as that of the contact solution in the test solution,
let alone to have the same mass of fibre sample in the blank and test solution, bearing in
mind the hygroscopic nature of cellulose fibre; a corrected blank is needed to compensate
for the differences in masses. The advantage of the corrected blank is that if one sample is
used several times in the same experiment for different test purposes, only one blank will
be required.
Thus, if w 1 and w 3 are the mass of sample in the blank and test solution respectively, and
w 2 is the mass of distilled water in the blank and w 4 , the mass of the contacting solution in
the test solution; p 1 and p 2 are the Brix of the sample blank before and after equilibrium,
and p 3 and p 4 are the Brix of the test solution before and after equilibrium, the corrected
blank, b, is given by b = p 2 w 2 w 3 /(w 1 w 4 ) and the net Brix increase by p = p 4 – b.
Hence the Brix-free water in the sample is given by BFW = [100 w 4 (1 – p 3 p -1 )]/w 3 .
Results obtained from almost sucrose-free fibre samples (Table 4.3) confirmed that the
samples still had high residual sucrose content as evidenced by the high Brix p 2 after
equilibration with distilled water, and the Brix-free water results are now 30-40% lower
than those obtained had the sample blank not been incorporated in the method. Most other
workers performed their studies on bagasse which probably has a low residual sucrose.
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