Characterization of solubilized arabinoxylo-oligosaccharides by MALDI-TOF MS analysis to unravel and direct enzyme catalyzed hydrolysis of insoluble wheat arabinoxylan

Hydrolysis of arabinoxylan is an important prerequisite for valorization of wheat endosperm materials, but the water insoluble arabinoxylan fraction of wheat endosperm cell walls has proven difficult to hydrolyze enzymatically. In this study, enzymatic hydrolysis of purified, water insoluble wheat arabinoxylan was examined and improved by combining detailed substrate analysis with analyses of product monosaccharides and oligosaccharides after treatments with rationally composed enzyme mixtures. Treatment of purified, water insoluble wheat arabinoxylan (substrate concentration 1.0%, w/w) for 48 h at pH 5, 50 °C with a combination of β-xylosidase from Trichoderma reesei and a hemicellulolytic fungal enzyme preparation from Humicola insolens, Ultraflo L, liberated 1.6 mg ferulic acid, 24 mg acetic acid, 51 mg arabinose, 167 mg xylose, and furthermore, solubilized 244 mg oligosaccharides per gram substrate dry matter. This yield was equivalent to solubilization of 45% by weight of the insoluble wheat arabinoxylan. Analysis of the solubilized oligomers by matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS) analysis revealed presence of different feruloylated and acetylated pentose oligomers. The data thus signified the presence of acetyl groups in wheat arabinoxylan. Addition of acetyl xylan esterase from Aspergillus aculeatus on top of the Ultraflo L and β-xylosidase addition did however not facilitate additional degradation of the insoluble wheat arabinoxylan, whereas, addition of feruloyl esterase obtained from Aspergillus niger on top of the β-xylosidase and Ultraflo L blend solubilized an additional 1.9% (w/w) dry matter from the substrate and significantly changed the MALDI-TOF MS profile of the solubilized oligomers which became significantly less feruloylated after this enzyme treatment.