Abstract
Arabinoxylans (AXs) with high ferulic acid (FA) content (7.18 µg/mg AXs) were cross-linked using laccase. Storage (G’) modulus of AX solutions at 1% (AX-1) and 2% (AX-2) (w/v) registered maximum values of 409 Pa and 889 Pa at 180 min and 83 min, respectively. Atomic force microscopy revealed the grained and irregular surface of the AX-1 gel and the smoother surface without significant depressions of the AX-2 gel. Cured AX gels exhibited a liquid phase surrounding the samples indicating syneresis. The syneresis ratio percentage (% Rs) of the gels was registered over time reaching stabilization at 20 h. The % Rs was not significantly different between AX-1 (60.0%) and AX-2 (62.8%) gels. After 20 h of syneresis development, the dimers of the FA in the AX-1 and AX-2 gels significantly increased by 9% and 78%, respectively; moreover, the trimers of the FA in the AX-1 and AX-2 gels, by 94% and 300%, respectively. Scanning electron microscopy showed that, after syneresis stabilization, AX gels presented a more compact microstructure. Syneresis development in the gels of highly ferulated AXs could be related to the polymer network contraction due to the additional formation of dimers and trimers of the FA (cross-linking structures), which may act like a “zipping” process, increasing the polymer chains′ connectivity.
Highlights
Arabinoxylans (AXs) are hemicellulose polysaccharides constituted by a linear backbone of β-(1–4)-linked D-xylopyranosyl units to which α-L-arabinofuranosyl units are attached trough α-(1–3)and/or α-(1–2) linkages (Figure 1)
Covalent cross-link content in these AX gels increased during syneresis development
After initial gelation, AX chains crosslinked through di-ferulic acid (FA)
Summary
Arabinoxylans (AXs) are hemicellulose polysaccharides constituted by a linear backbone of β-(1–4)-linked D-xylopyranosyl units to which α-L-arabinofuranosyl units are attached trough α-(1–3)and/or α-(1–2) linkages (Figure 1). Ferulic acid (FA) (3-methoxy, 4 hydroxy cinnamic acid) molecules may be ester-linked on the (O)-5 position to some of the arabinose residues. Some other components like galactose, glucoronic acid, and p-coumaric acid may exist in AX side chains [1]. AXs show interesting functional properties as thickeners, stabilizers, and emulsifiers [2], they form highly viscous solutions, and one of the most important properties is their ability to form chemical gels through covalent cross-linking of FA upon oxidation by some chemical (ferric chloride or Polymers 2017, 9, 164; doi:10.3390/polym9050164 www.mdpi.com/journal/polymers ammonium persulfate) or enzymatic (peroxidase/H2 O2 or laccase/O2 ) agents [2]. AX chains, e.g., hydrogen bonds, may be or ammonium persulfate) or enzymatic
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