Production of arabinoxylan from sorghum by-products and its application in agroindustry

Lignocellulosic residues comprise heterogeneous materials that attract much research interest as renewable resources. In Indonesia, sorghum-derived biomass is estimated at around 30-40 tons/hectare on an annual basis. There is an issue in sorghum plantation concerning the discard of the upper parts of the plant (stalks and panicles) after harvesting the grain. In large amounts, these residues can raise environmental, financial and management problems. The aim of the current study was to investigate the fractionation of sorghum processing by-products, and the utilisation of the subsequent hemicellulose, lignin and cellulose fractions in agricultural and packaging applications. Sorghum bran, stalk, and panicles were investigated as suitable starting materials, originating from an Indonesian variety of Sorghum bicolor, namely Kawali. A sequential alkaline extraction with varying concentrations of NaOH (0.75, 1, and 1.5 M) was applied, resulting in the generation of three fractions, those of residue (R), hemicellulose (H), and alkali lignin (L). The fractionation of the sorghum stalks with 1 M NaOH at 50oC for 3 hours obtained ~64% (w/w) glucose in residue fraction. Moreover, 52 % (w/w) of xylose was predominant in the hemicellulose fraction of the stalks, which was further characterises as low branched xylan, and ~2% (w/w) of lignin was present in the liquid lignin fraction. On the other hand, in the bran, the fractionation with 0.75 M NaOH at 50oC for 3 hours obtained ~40% (w/w) glucose in the residue fraction which corresponded to the cellulose. Approximately 76% of the glucose was available in hemicellulosic fraction that was further characterised as xyloglucan (XG), while nearly 2% (w/w) lignin was extracted in the lignin fraction. Thermal analysis determined by Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) showed that the hemicellulose fraction exhibited lower thermal stability and higher crystallinity compared to that of cellulose. Subsequently, the sorghum-derived hemicellulosic fractions were used for the development of films and their potential use in agricultural applications as fertilising agents and food packaging materials was investigated. The addition of plasticiser (glycerol) improved the mechanical properties of bran-extracted hemicellulose films. v Moreover, the incorporation of long chain anhydride groups (2-octenyl succinic anhydride, 2-OSA) to sorghum hemicellulose extracts improved the performance (no brittleness), mechanical properties (tensile strength and total work), and barrier properties of the developed films. The films were tested as carriers of fertiliser (NPK) in soil trials and demonstrated quick release of the fertiliser and biodegradable properties. Furthermore, the application of 2-OSA-HS films as sealed-wrapping materials demonstrated adequate moisture barrier properties and reduced the weight loss of fruits after 7 days of storage in room temperature and under chilled conditions. Finally, biodegradable food trays were developed via thermopressing, using modified and unmodified corn starches with the addition of extracted fractions (cellulose, hemicellulose and lignin) from sorghum by-products (stalk and bran). The inclusion increment of cellulose fraction from 2% to 5% and hemicellulose fraction from 1% to 10% had a significant impact on the trays’ colour and mechanical properties, water absorption capacity, microstructure and crystallinity. All trays exhibited no colour and structural changes during storage at 25 oC and 4 oC for 7 days. The food tray formulation that exhibited the best properties contained 2% of cellulose and 1% of hemicellulose, had a maximal resistance of 0.77 MPa and 9.46 mJ of total work, attributes which corresponded to a compact, homogenous, and dense microstructure. Overall, the extracted fractions derived from sorghum by-products have the potential to be used as starting materials for the production of a range of agricultural and packaging products. Further development of this process to improve the purity of each fractions, technology, and the properties of end products could contribute in the development of agroindustry and establish a viable route for the valorisation of sorghum-derived by-products.