Lynne Wong's PhD thesis

value determined could be corrected for the residual moisture after the vacuum drying
procedure.
4.4.7 Concentration of the contact solution
By varying the concentration of the contact solution from 5 to 10 and 15° Brix sucrose
solution, it was found that the Brix-free water in a rind fibre and stalk pith varied with a
change in sucrose concentration (Table 4.12), and to increase with increased concentration
of sucrose solution (Table 4.13) in a later experiment with a stalk fibre and a rind fibre.
This is in contradiction to the findings of Qin and White (1991), Van der Pol et al. (1957)
and Kelly and Rutherford (1957), who found that the Brix-free water values decreased as
the concentration of the contact solution increased.
With a 5° Brix sucrose solution, the increase in Brix after contacting would be smaller than
0.1 unit and hence difficult to detect, whereas with a 15° Brix solution, the stability of the
sucrose would be doubtful after the long procedure of determination. It was therefore
decided to adopt a 10° Brix sucrose solution as the contact solution, since it was also
adopted by Mangion and Player (1991).
4.4.8 Ratio of the contact solution to sample size
In this experiment, a 10° Brix sucrose solution was used and four samples were examined:
two rind fibres and two stalk piths. The ratio of contact solution to sample mass was
varied from 25 to 10. At a high ratio, e.g. 25, the errors became large as the increase in
Brix after equilibration was less than 0.1 unit, and at a low ratio, e.g. 15, the volume of the
solution obtained from pith samples after equilibration was not sufficient for the Brix
determination. At a ratio of 5, pith samples were not completely wetted. It was therefore
found that a ratio of 20 was most appropriate. The higher surface area of stalk pith as
shown by scanning electron microscopy (Fig 3.24f) may explain its higher liquid
absorption than sample of other cane components.
Two samples showed a decrease in Brix-free water with decrease in the ratio of contact
solution/sample, while one stalk pith showed the opposite (Table 4.14). Qin and White
(1991) with their press cell method, however, found that this ratio does not affect the Brixfree
water results, although the errors become larger at the higher juice/fibre ratios.
139