Recovery of xylan from Acacia mearnsii using ultrasound-assisted alkaline extraction

Abstract

Novel and effective biorefinery methods are necessary to fractionate lignocellulosic biomass into separate components for the valorization of these components into value-added products. For example, the xylan fraction can be used as an adhesive, a thickener, an additive to plastics, an emulsifier, a drug carrier, and as a precursor of prebiotic xylooligosaccharides. Unfortunately, conventional xylan extraction procedures require lengthy extraction periods and are characterized by poor product purity and yields. In this study, a non-conventional ultrasound-assisted alkaline extraction method was developed for the effective extraction of xylan from Acacia mearnsii, an industrially relevant hardwood in South Africa. The effect of the additional ultrasonication step on the conventional alkaline extraction of xylan was evaluated. After this, basic structural analysis was conducted on both acacia xylan extracts and commercially available beech-derived xylan. Composition analysis data showed the acacia-derived glucuronoxylan to have an Ara:GluA:Xyl ratio of 0.5:1:5, while that from beechwood had a 0:1:4 ratio. Ultrasound-assisted alkaline extraction also gave a better yield (24%) than conventional alkaline extraction (21%) and had a quicker extraction period of 1 h versus 4 h. Finally, the acacia-derived xylan exhibited antioxidant potential, with an EC50 value of ≈1.5 mg/mL against 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH). In conclusion, ultrasound-assisted alkaline extraction was successfully employed to extract high-purity, bioactive xylan from acacia. The purified acacia xylan is a suitable replacement for birchwood glucuronoxylan, which is no longer commercially available, or for expensive beechwood substitute for numerous applications.