Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech Steam explosion pre-treatment of alkali-impregnated lignocelluloses for hemicelluloses extraction and improved digestibility Gezahegn T. Mihiretu, Annie F. Chimphango, Johann F. Görgens ⁎ Stellenbosch University, Process Engineering Department, Stellenbosch 7602, South Africa ARTICLE INFO Keywords: Xylan extraction Alkaline steam explosion pre-treatment Sugarcane trash Aspen wood Lignocellulosic biorefinery ABSTRACT The application of steam explosion pre-treatment to extract xylan-rich biopolymers from alkali-impregnated lignocelluloses, while simultaneously increasing the enzymatic digestibility of cellulose, was investigated. Steam-enhanced extraction of xylan from sugarcane trash (SCT) and aspen wood (AW) was performed at varying temperatures (176–204 °C) and retention times (3–17 min) after the impregnation of biomass samples with sodium hydroxide at 1:20 (w/w) solid loading ratio. Xylan extraction and cellulose digestibility results were statistically analysed to fix the condition/s for significantly enhanced values. Accordingly, maximum xylan yields of 51 and 24%, and highest cellulose digestibility of 92 and 81%, were attained for SCT and AW re- spectively following their pre-treatment at 204 °C for 10 min. At this most-severe condition, neither xylose nor furfural – a degradation product from xylose – were observed in the hemicellulose extract, indicating steam explosion pre-treatment with alkali impregnation of lignocelluloses as viable biorefinery approach to co-produce xylan biopolymers and bioethanol. 1. Introduction The development of lignocellulosic biorefinery systems for an in- tegrated production of biofuels and other bio-based co-products is widely regarded for its potential role towards decarbonized economic growth with improved environmental sustainability (Mohan et al., 2016; Ahorsu et al., 2018). Such bioindustrial approaches essentially imply increased use of fibrous plant biomass where those major lig- nocellulosic components (cellulose, hemicellulose and lignin) patently become of immediate economic interest (Silveira et al., 2015; Serna- Loaiza et al., 2019). Hemicelluloses are hetero-polysaccharides gen- erally comprising 20–35% of dry lignocellulosic biomass (Isikgor and Becer, 2015; Anwar et al., 2014); however, they have largely remained underutilized despite their abundance and potential applications (Zhang et al., 2014; Liu et al., 2018). Hemicelluloses in hardwoods and herbaceous residues are particularly rich in xylan – a natural biopo- lymer which, if extracted in its polymeric and/or oligomeric form, can serve as a precursor for developing functional biomaterials having po- tential applications in the food, agricultural, pharmaceutical, cosmetic and biomedical areas (Hu et al., 2018; Mihiretu et al., 2017; Norstrom et al., 2015; Zhang et al., 2014; Chimphango et al., 2012). A number of biomass fractionation techniques have been reported in view of enhancing extraction of xylan and improving digestibility of solid residues for subsequent bioconversion steps (Cantero et al. 2019; Baruah et al., 2018; Mihiretu et al., 2017; Carvalho et al., 2016). Al- kaline pretreatment is one widely studied chemical route for the ex- traction of xylan-rich hemicelluloses from hardwoods and agricultural residues (Carvalho et al., 2016; Kim et al., 2016; Longue Junior et al., 2013). Alkaline extraction of hemicelluloses with minimal depolymer- isation and degradation typically takes place under temperatures below 100 °C, hours-long retention times and alkali loadings of 1 to 5% (w/w) per dry biomass (Kim et al., 2016; Singh et al., 2015; Karp et al., 2014). Although thermally and/or chemically more severe conditions may lead to higher hemicellulose yield, they are very often associated with in- creased depolymerisation and degradation of extracts (Xu and Sun, 2016; Longue Junior et al., 2013). For instance, in Longue Junior et al. (2013), where the effect of different alkali (NaOH) loadings on the yield of hemicellulose extraction from Eucalyptus globulus and degree of polymerization (DP) of the extracts was investigated, it was shown that increasing the alkaline charge from 10 to 50% (w/w, ODW) led to xylan yield increase from 39.3 to 58.6%, whereas a decrease in DP (from 250 to 97) was observed for the respective xylan extracts. Steam explosion pre-treatment (SEPT) is another widely studied biomass fractionation technique suitably applied for selective removal of hemicelluloses from lignocelluloses – a process primarily catalysed by the hydrolytic action of organic acids (such as acetic acid) that form https://doi.org/10.1016/j.biortech.2019.122121 Received 12 July 2019; Received in revised form 3 September 2019; Accepted 4 September 2019 ⁎ Corresponding author at: Process Engineering Department, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7602, South Africa. E-mail address: jgorgens@sun.ac.za (J.F. Görgens). Bioresource Technology 294 (2019) 122121 Available online 11 September 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved. T