Tuning the molar mass and substitution pattern of complex xylans from corn fibre using subcritical water extraction

Abstract

Glucuronoarabinoxylan (GAX) is a structurally complex hemicellulose abundant in the cell wall of corn kernels that constitutes a valuable target for its valorisation from corn processing side streams. However, the crosslinked and recalcitrant nature of corn cell walls hinders its fractionation through mild green processes. In this study, we propose the extraction of GAX using subcritical water, where temperature, pH and time have been optimised to tune the extraction performance (yields and purity of the GAX) and the molecular structure of the extracted GAX (molar mass distribution, substitution pattern and presence of covalently bound phenolic moieties). Higher temperatures under unbuffered conditions caused a prominent drop in pH and autohydrolysis, resulting in a decrease of the molar mass (∼10⁴ Da) and the cleavage of arabinose substitutions. Mitigating the pH drop using mild buffered neutral and alkaline conditions provided higher molar masses of the extracted GAX (∼10⁵ Da), protecting as well the labile arabinose substitutions and resulting in a higher abundance of more complex glycan side chains. Subcritical water extraction preserved the phenolic acid moieties (mainly ferulic acid) covalently bound to polymeric GAX. Several forms of ferulic acid dehydrodimers (di-FA) were detected and identified by liquid chromatography–tandem mass spectrometry (HPLC-LC-MS²) and these di-FAs were particularly enriched in the mild alkaline extracts. We demonstrate that solely by carefully adjusting the operational parameters during subcritical water extraction we can tune the molar mass and complex substitutions of GAX, i.e. the degree and pattern of monomeric and oligomeric glycan side chains and ester-linked phenolic acid substitutions, without the use of additional catalysts. This molecular control over the production of corn GAX can invaluably benefit subsequent development of agroindustry-based biorefineries towards their conversion into novel bio-based materials for food and biomedical applications.