Lynne Wong's PhD thesis

4.2 METHODS OF DETERMINATION OF BRIX-FREE WATER IN CANE
FIBRES
Published literature shows that Brix-free water of sugar cane fibre may be determined by
two main methods:
1) a contacting method where dry sugar-free cane fibre is placed in a sucrose (or any
suitable compound) solution of known concentration, is allowed to equilibrate and
the hygroscopic water calculated from the change in sucrose concentration. The
concentration change needs to be determined with great accuracy and it is uncertain
whether the fibre is absolutely dry, due to the extremely hygroscopic nature of
cellulose.
2) a vapour sorption method, where water is sorbed by dry sugar-free cane fibre from an
atmosphere of controlled relative humidity. This method requires that the
experiment be carried out at constant temperature. Since in the contacting method, a
small amount of sucrose may be sorbed by the fibre in addition to the water, this
method measures the differential sorption of water over sucrose (Kelly and
Rutherford, 1957). According to Downing and McBain (2000), the difference
between the two methods is somewhat indistinct. The two methods may differ due to
the contribution of capillary condensation; however, condensation of water in
preformed spaces within dry fibres is not expected to be more than 2 – 3% by mass
on cellulose (Valko, 1943).
As early as the end of the 19 th century, Prinsen Geerligs (1897) described experiments
designed to determine directly Brix-free water, which he called colloidal water in fibre. He
added known masses of completely washed bagasse, of known moisture content to
solutions of sodium chloride of a known concentration. After a mixing time of 24 hours,
the salt concentration was re-determined. From the increase in salt concentration, he was
able to calculate the Brix-free water associated with the fibre by subtracting the amount of
water in which the salt was dissolved from the total amount of water in the system. He
obtained a value of 35% Brix-free water in bagasse. It was evident that absorption of
sodium chloride on the fibre also took place, especially at high salt concentrations, leading
to negative results at times. With sucrose solutions, he found the value of 20% Brix-free
water on fibre.
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