Hydrogel Formation of Enzymatically Solubilized Corn Bran Feruloylated Arabinoxylan by Laccase-Catalyzed Cross-Linking

Abstract

In order to upgrade the potential of cereal bran arabinoxylan for advanced hydrogel applications, a deep understanding of its gelation process is required. This work provides a comprehensive and systematic analysis of the laccase-catalyzed cross-linking of feruloylated arabinoxylan (FAX) to establish a clear link between processing conditions and final hydrogel properties. Endo-1,4-xylanase was used to obtain corn bran FAX rich in ferulic acid moieties, and then we demonstrated that gel formation is driven by the oxidative coupling of these feruloyl monomers into diferulic acid bridges, e.g., 8-5', 5-5', 8-O-4', and 8-5' benzofuran diferulic acids. A systematic investigation revealed that hydrogel properties were significantly affected by the processing conditions, i.e., FAX concentration, enzyme dosage, reaction pH, and reaction temperature during the enzymatic gel formation catalyzed by laccase. While gel strength peaked at a FAX concentration of 30 mg/mL, an optimal temperature of 25 °C and pH 6 were identified. Notably, we discovered a critical trade-off with enzyme concentration: higher laccase levels accelerated the reaction but compromised the final hydrogel's mechanical strength and water retention. Gelation failed completely at pH ≥ 9 due to laccase inactivation. Meanwhile, scanning electron microscope analysis revealed that the microstructure of the FAX hydrogels was significantly affected by changes in the processing conditions. These findings offer crucial insights for the rational design of FAX-based hydrogels, enabling their tailored fabrication for food industry applications.

Keywords: arabinoxylan; diferulic acid; ferulic acid; hydrogel; laccase.

Figure 1

(a) SEC chromatogram of FAX; (b) monosaccharide compositions of FAX; (c) chromatographic profiles of ferulic acid and four types of diFAs, i.e., 8-5′, 5-5′, 8-O-4′, and 8-5′ benzofuran diFA of FAX. Color coding is used in all chromatograms: FA, red, m/z 193; FA, 8-5′ diFA (green, m/z 385); 5-5′ diFA (green, m/z 385); 8-O-4′ diFA (green, m/z 385) and 8-5′ benzofuran diFA (blue, m/z 341). The peak appearing at retention time about 7.5 min is unknown but has an m/z of 385 and similar fragments as other diFAs.

Figure 2

(a) Dynamic change of ferulic acid and four types of diFAs during the FAX hydrogel formation (2, 5, 10, 20, and 60 min from top to bottom); (b,c) SEM images showing surface morphology and cross-sectional structures of FAX hydrogels collected at different time points during the hydrogel formation; (d) gelation kinetics of hydrogel formation of FAX hydrogels under a time sweep test; (e) changes of amount of ferulic acid and diFAs during FAX hydrogel formation; (f) appearance of FAX hydrogel.

Figure 3

(a) Gelation kinetics of hydrogel formation of FAX at different substrate concentrations (10–50 mg/mL) under a time sweep test (0–70 min); (b) frequency sweep test of FAX hydrogels at different substrate concentrations; (c) strain sweep test of FAX hydrogels at different substrate concentrations; (d) water holding capacities (WHC) of FAX hydrogels at different substrate concentrations; (e,f) SEM images showing surface morphology and cross-sectional structures of FAX hydrogels at different substrate concentrations.

Figure 4

(a) Gelation kinetics of hydrogel formation of FAX with laccase at different enzyme concentrations (0.625–10 mg/mL) under a time sweep test (0–70 min); (b) frequency sweep test of FAX hydrogels with laccase at different enzyme concentrations; (c) strain sweep test of FAX hydrogels with laccase at different enzyme concentrations; (d) water holding capacities (WHC) of FAX hydrogels with laccase at different enzyme concentrations; (e,f) SEM images showing surface morphology and cross-sectional structures of FAX hydrogels with laccase at different enzyme concentrations.

Figure 5

(a) Gelation kinetics of hydrogel formation of FAX under different reaction pH (3–8) under a time sweep test (0–70 min); (b) frequency sweep test of FAX hydrogels under different reaction pH; (c) strain sweep test of FAX hydrogels under different reaction pH; (d) water holding capacities (WHC) of FAX hydrogels under different reaction pH; (e,f) SEM images showing surface morphology and cross-sectional structures of FAX hydrogels under different reaction pH; (g) Q-ToF-MS chromatograms of bound ferulic acid and diFAs and released ferulic acid and diFAs in FAX hydrogel solutions under reaction pH 9 and 10; (h) appearance of FAX hydrogel solutions under reaction pH 9 and 10.

Figure 6

(a) Gelation kinetics of hydrogel formation of FAX hydrogels under different temperatures (15–55 °C) under a time sweep test (0–70 min); (b) frequency sweep test of FAX hydrogels under different temperatures; (c) strain sweep test of FAX hydrogels under different temperatures; (d) water holding capacities (WHC) of FAX hydrogels under different temperatures; (e,f) SEM images showing surface morphology and cross-sectional structures of FAX hydrogels under different temperatures.

Figure 7

(a) Infrared spectra of the optimal conditioned FAX hydrogel and FAX; (b) XRD plots of optimal conditioned FAX hydrogels; (c) low-field NMR image of optimal conditioned FAX hydrogel.