**3.2. Rheological and structural characteristics of AX gels**

AX1 and AX2 solutions at 10% (w/v) presented gelling capability under laccase exposure (Figure 3). The kinetics of gelation of these solutions was rheologically monitored by small amplitude oscillatory shear. Figure 4 shows the development of storage modulus (G'), loss (G") modulus, and tan δ (G"/G') versus time of 10% (w/v) AX1 and AX2 solutions undergoing oxidative gelation by laccase.

The gelation profiles followed a characteristic kinetics with an initial increase in G' and G", followed by a plateau region for both gels. This behavior reflects an initial formation of covalent linkages between FA of adjacent AX molecules producing a three-dimensional network [19]. At the end of gelation, G' and G" were 78 and 13 Pa for AX1, respectively, while for AX2 they were 32 Pa and 8 Pa for G' and G", respectively (Table 1). Similar kinetics of gelation have been Gelation of Arabinoxylans from Maize Wastewater — Effect of Alkaline Hydrolysis Conditions on the Gel... http://dx.doi.org/10.5772/61022 107

**Figure 2.** Elution profiles of AX1 and AX2. Pullulan molecular weight markers (kDa) used as calibration scales are shown at the top.

**Figure 3.** AX1 and AX2 gels at 10% (w/v).

grain) are low NaOH concentration (2.0%), temperature of 120°C, and a short reaction time (90 minutes) [26]. In the present work, maize grains cooking in a lime solution was performed during 90 minutes and 30 minutes for AX1 and AX2, respectively; but after heating, long soaking periods were used (24 hours and 4 hours for AX1 and AX2, respectively), which could explain the lower FA content in the polysaccharide. This is congruent with a previous study

where the FA content in AX was dependent of the time of alkaline hydrolysis [20].

**Figure 1.** FT-IR of AX1 and AX2. The arrows indicate the characteristic absorption bands.

AX1 and AX2 solutions at 10% (w/v) presented gelling capability under laccase exposure (Figure 3). The kinetics of gelation of these solutions was rheologically monitored by small amplitude oscillatory shear. Figure 4 shows the development of storage modulus (G'), loss (G") modulus, and tan δ (G"/G') versus time of 10% (w/v) AX1 and AX2 solutions undergoing

The gelation profiles followed a characteristic kinetics with an initial increase in G' and G", followed by a plateau region for both gels. This behavior reflects an initial formation of covalent linkages between FA of adjacent AX molecules producing a three-dimensional network [19]. At the end of gelation, G' and G" were 78 and 13 Pa for AX1, respectively, while for AX2 they were 32 Pa and 8 Pa for G' and G", respectively (Table 1). Similar kinetics of gelation have been

**3.2. Rheological and structural characteristics of AX gels**

oxidative gelation by laccase.

106 Wastewater Treatment Engineering

previously reported for maize bran AX gels [17, 27, 28]. AX1 gel presented higher elasticity value in comparison to AX2 gel, which can be attributed to its higher FA content.

Gelation time (tg) at crossover point (G' > G") was 26 min and 40 min for AX1 and AX2, respectively. The tg value indicates the sol/gel transition point and at this point G' = G". The lower FA content in AX2 compared with AX1 could have affected the cross-linking of AX chains and retard the gel formation. The tan δ (G"/G') values decreased during the AX1 and AX2 gelation indicating the formation of a more elastic covalent system (Figure 4) [17]. The tan δ calculated at the end of the test, were 0.16 for AX1 and 0.24 for AX2, indicating than AX1 gel is more elastic than AX2 gel [29].

Niño-Medina et al. [21] reported nejayote AX gels (4% and 8%, w/v) with smaller G' values (2 and 4 Pa, respectively) and a higher crossover point (150 min) than those found in the present work. On the other hand, Ayala-Soto et al. [30] reported nejayote AX gels (4 % w/v) that showed a fluid-like behavior with G' of 5.9 Pa. Such differences might have its origin in the structural and/or conformational characteristics of these macromolecules [21, 31]. Possible differences in structure such as arabinose and FA distribution throughout the AX molecule could explain the variance in the rheological characteristics of the gels formed.

**Figure 4.** Monitoring the storage (G') and loss modulus (G") of AX1 and AX2 solutions (10% w/v) during gelation by laccase at 0.25 Hz and 25°C.


**Table 1.** FA content in AX1 and AX2 and rheological characteristics of the gels formed at 10% (w/v).

The mechanical spectra of AX1 and AX2 after gelation (Figure 5) exhibited a solid-like behavior with G'>G". The mechanical spectra of AX gels with a linear G' independent of frequency and G" much smaller than G' and dependent of frequency have been previously reported [11, 16, 17, 27]. AX1 and AX2 gels G' increase at frequency values, this may indicate the presence of physical interactions in the polymer network in addition to the covalent bonds induced by laccase.

and/or conformational characteristics of these macromolecules [21, 31]. Possible differences in structure such as arabinose and FA distribution throughout the AX molecule could explain

**Figure 4.** Monitoring the storage (G') and loss modulus (G") of AX1 and AX2 solutions (10% w/v) during gelation by

AX1 24 0.012 ± 2.7 x 10-5 26 78 13 0.16 AX2 4 0.008 ± 1.4 x 10-4 40 32 8 0.24

The mechanical spectra of AX1 and AX2 after gelation (Figure 5) exhibited a solid-like behavior with G'>G". The mechanical spectra of AX gels with a linear G' independent of frequency and G" much smaller than G' and dependent of frequency have been previously reported [11, 16, 17, 27]. AX1 and AX2 gels G' increase at frequency values, this may indicate the presence of physical interactions in the polymer network in addition to the covalent bonds induced by

**Gelation time, tg (min)**

**G' (Pa)**

**G" (Pa)** **Tan delta (δ, G"/G')**

**Ferulic acid, FA (µg/mg AX)**

**Table 1.** FA content in AX1 and AX2 and rheological characteristics of the gels formed at 10% (w/v).

laccase at 0.25 Hz and 25°C.

108 Wastewater Treatment Engineering

**AX Hydrolysis time (h)**

laccase.

the variance in the rheological characteristics of the gels formed.

**Figure 5.** Mechanical spectra of AX1 and AX2 gels at 4 h. Rheological measurements made at 25 °C and 5% strain.

The images from SEM of the lyophilized gels AX1 and AX2 are shown in Figure 6. Both gels present many connections and resemble an imperfect honeycomb. In general, the microstruc‐ tural characteristics of AX1 and AX2 are similar to those previously reported for lyophilized wheat and maize bran AX gels [16, 17, 22]. Nevertheless, AX2 gel appears to have a more fragmented morphology with a rougher and heterogeneous surface (Figure 6c, d). These microstructural dissimilarities between AX1 and AX2 gels could explain the differences in G' values of the gels, since a more compact and defined microstructure could give stronger gels.

The average inner diameter of the AX1 and AX2 cells were approximately 30 μm and 50 μm, respectively. Higher cell dimensions were reported in lyophilized AX gels (>200 μm) [17, 19], and this difference could be related to the method used to freeze the gels before lyophilization. In the present study AX gels were frozen by immersion in liquid nitrogen (fast congelation), while previous studies [17, 19] reported AX gel congelation at -20°C for several hours (slow congelation).

An important aspect of achieving a high-quality frozen material, particularly with high water content such as gels, is the freezing rate. Fast congelation results in a better-preserved structure (i.e., finer ice crystals). The microstructural characteristics of AX1 and AX2 gels were similar to those reported in AX gels frozen by fast congelation [32, 33] under similar conditions to those used in the present study. In those previous studies [32, 33], AX gels can be also compared with an irregular honeycomb structure in which pore diameters ranged from 12 μm to 50 μm.

**Figure 6.** The SEM of lyophilized AX1 (a, b) and AX2 (c, d) gels. a and c at 500x magnification; b and d at 2000x magni‐ fication.

The entrapment of biomolecules or microorganisms in high cell dimension (>200μm) AX gels has been previously reported [14, 19, 31], but reducing cell dimensions of AX gels could increase the possibility of carried out smaller compounds or cells. Therefore, AX1 and AX2 gels microstructural characteristics could be of interest for the development of designed delivery systems, which could allow alternative uses for maize wastewater.
