Figures 1 and 2 show the profiles of saccharides leaving the
fixed-bed column for runs 1 and 4, respectively, which presented
good efficiency of separation using different gradient volumes (60
and 100 mL) respectively, during the elution. From these Figures,
it can be seen that the maximum of FOS concentration peak was
achieved when the ethanol gradient concentration reached about
80% of its value (around 16% v/v), indicating that the use of a
gradient with a narrow range of concentration could efficiently
elute the FOS.
This finding is demonstrated in Fig. 3, where the profile of
saccharides leaving the column is shown, with the use of an
ethanol gradient concentration in the range of 7–15% (run 6 of
Table 1). The FOS was eluted from the column when the ethanol
concentration reached 100%. In agreement with these results,
some authors reported that glucose does not have much affinity
for activated charcoal. Nobre et al. [7] observed that the sugars
from a fermentative broth which were absorbed the least were
fructose and glucose. In this study, the same behavior was
observed, since glucose is the first sugar to leave the column
and the excess of this saccharide is eluted with water, after about
200 min.
In Fig. 3, the importance of ethanol gradient concentration on
the separation is demonstrated. The monosaccharides (glucose
and fructose) can be recovered using lower ethanol concentration
in water, until 10%, while the FOS was recovered using a higher
concentration, around 15–50% [6,7,16].
The use of 7% ethanol (run 4) eluted only the excess of the
monosaccharides (glucose and fructose) up to 240 min, and only
after 300 min was the remaining glucose and fructose eluted.
Lower concentrations of ethanol lead to lower affinity between
the activated charcoal and monosaccharides making them more
easily desorbed [7,16]. In Fig. 3, 7% ethanol was sufficient to desorb only part of the FOS, and a higher concentration (15% v/v) was
required to elute all the FOS remaining in the column, after
600 min. Boon et al. [11] also observed that the activated charcoal
has more affinity for FOS than for others sugars (glucose and
fructose), as found here.