Results and discussion
A gelation of PGG is a function of polymer concentration, temperature, and the presence of monovalent and divalent cations
in solution. At low temperature, gellan forms an ordered helix of double strands. At high temperatures, single-stranded polysaccharides
occur, which significantly reduce the viscosity of solution. PGG possesses high viscosity and gelling ability, which can be
useful as release retardants in the preparation of a controlled-release formulation.
In our preliminary studies, we observed that the viscosity of PGG was reduced when it was exposed to high temperature. Hence,
a modified form was prepared by exposing PGG in a microwave oven at higher temperature for different time periods. The results
of physical changes attributed to PGG upon modification shows a marked decrease in viscosity as well as gelling ability. The
MGG possesses excellent swelling property but lesser viscosity; this may be a result of the formation of single-stranded polysaccharides
at higher temperature (35–37).
A 32 full-factorial design was constructed. The amount of distilled water (X
1) and time of exposure in microwave (X
2) were selected as independent variables. The swelling ratio in distilled water, HCl buffer (pH 1.2) and phosphate buffer
(pH 6.8) were selected as dependent variables. A statistical model incorporating interactive and polynomial terms was used
to evaluate the response.
in which Y is the dependent variable, b
0 is the arithmetic mean response of the nine runs, and b1 is the estimated coefficient for the factor X
1. The main effects (X
1 and X
2) represent the average result of changing one factor at a time from its low to high value. The interaction terms (X
2) show how the response changes when two factors are simultaneously changed (38). Factorial design was calculated with the
help of Microsoft Excel 2007.
Swelling ratio in distilled water, HCl buffer, and phosphate buffer value for the nine batches (Batches A to I) showed a wide
variation: 12.5–22, 9–18, and 7.5–16, respectively (see Table I). The data clearly indicate the values are strongly dependent
on the selected variables. The value of correlation coefficient was found to be 0.9859, 0.9729, and 0.9571, respectively,
indicating a good fit. It may be used to obtain a reasonable estimate of the response because a small error of variance was
noticed in the replicates. The polynomial equation can be used to draw conclusions after considering the magnitude of the
coefficient and the mathematical sign it carries, i.e., positive or negative. The data demonstrate that both the factors (X
1 and X
2) affect the swelling characteristics of MGG. The low value of the interaction between X
1 and X
2 indicates that it is not significant. A summary of regression output for dependent variables is shown in the following equations.
The following equations for Y represent the regression output of dependent variables swelling ratio in distilled water, HCl buffer (pH 1.2), and phosphate
buffer (pH 6.8), respectively.
in which DF is degree of freedom and F is Fischer's ratio.
A higher value of X
2 (time of exposure in microwave) was observed as compared with X
1 (amount of distilled water) in all three equations. It can be concluded that time of exposure in microwave (X
2) had more effect on swelling ratio than the amount of distilled water (X
1) in the modification of PGG. In all three media, X
1 and X
2 carried positive signs, meaning there was a positive effect of both factors on the swelling ratio. Batch I showed maximum
swelling in all three media as compared with other batches. Therefore, Batch I was selected as the optimized batch and was
used for further study. Higher values of X
1 (amount of distilled water) and X
2 (time of exposure in microwave) favors swelling ratio of MGG in distilled water, HCl, and phosphate buffer.
The purpose of treating PGG in microwave was to modify it. There was a no usage of any other material or chemical for microwave
treatment except distilled water. MGG (optimized Batch I) was subjected to DSC, X-ray diffractomy, and FT-IR for confirmation
of modification as well as any chemical changes afterward. Figures 1a and 1b depict the physical-state characteristic of PGG
and MGG. The DSC thermogram of PGG exhibited an endothermic peak at 88.16 °C and 270.5 °C; however, it was slightly shifted
to temperatures of 94.35 °C and 284.29 °C in MGG. In addition, a single exothermic peak was observed at 262.8 °C in PGG and
was slightly shifted to a lower temperature of 259.73 °C in MGG. DSC analysis showed that the energy requirement was different
for PGG and MGG, thereby indicating that the modification or change is only in the physical state. There were no major changes
in peaks observed in PGG (see Figure 1a) and MGG (see Figure 1b), which indicated there were no chemical changes after microwave
treatment. Moreover, the energy requirement of the treated sample was different, which indicates that the exposure time in
microwave and the amount of distilled water exhibit significant influence on the modification of the physical properties of
Figure 1: Differential scanning calorimetry (DSC) thermogram of (a) pure gellan gum and (b) modified gellan gum.
Figures 2 and 3 show the XRD patterns of PGG and MGG. The PGG pattern shows no peaks, which indicates its amorphous nature
(see Figure 2). On the other hand, four different peaks were observed at different angles in MGG diffractogram (see Figure
3). These peaks suggest that PGG was converted from an amorphous to a crystalline structure. An X-ray diffractogram showed
there was only a change in the physical state of PGG after microwave treatment.
Figure 2: X-ray powder diffractogram of pure gellan gum.
Microwave high energy is used for any treatment. There may be a chance of chemical degradation or changes of PGG during microwave
treatment. FT-IR studies were performed to determine whether the samples had undergone any chemical degradation or modification.
The FT-IR spectra of PGG (see Figure 4a) show characteristics peaks at ~1736 cm–1 for the carbonyl group, indicating C=O stretching; a strong band at ~3400 cm–1 for the OH group; a band at 2800–3000 cm–1 for C–H stretching; a band at ~1380 cm–1 for methyl C–H; and a band at ~1060 –1150 cm–1 for C–O stretching for alky ether. Figure 4b shows the FT-IR spectra of MGG, which has similar characteristics peaks. The
authors observed the same functional group present before and after microwave treatment. Hence, the gellan gum had not degraded
during the microwave treatment, or there was no chemical degradation, change, or modification observed in MGG.
Figure 3: X-ray powder diffractogram of modified gellan gum.