## 1 Introduction
Wheat straw (WS) is the third most produced cereal in the world, after maize and rice, being a promising feedstock for high value-added products [42]. It was shown to have good xylan/lignin ratios, be biodegradable and have low-cost, but owing an inherent complex structure of the biopolymers cellulose, hemicellulose and lignin in different proportions, a strong native recalcitrance is formed, which blocks its enzymatic hydrolysis [10; 21]. Therefore, an integrated approach that would allow selective fractionation of the wheat straw seems a crucial step. Generally, XOS can be produced from xylan, which is the main hemicellulose in wheat straw, consisting of a linear backbone of \(\beta\)-14-linked xylopyranose (Xyl) residues. A first pretreatment step of the wheat straw, to break down the complex polymeric structure, and enhancing the accessibility of enzymes to the substrate during a enzymatic hydrolysis step were shown to be effective for generating fermentable sugars and XOS [6; 10]. Previous studies showed that autohydrolysis, or hydrothermal pretreatment (HTP), might be an attractive method, since it was low-cost and used only water as a reaction media, effectively depolymerizing hemicelluloses by the hydrolytic action of hydronium ions (generated from water autoionization and from in situ generated organic acids) into soluble sugars and XOS and enhancing the accessibility of enzymes to the solid fraction (cellulose and lignin) [10; 15; 34]. The extent of depolymerization depends on the treatment severity (temperature and time), with studies showing a varied range of temperatures as effective (130-230 \({}^{\circ}\)C), but also on other factors such as particle size, pH and liquid-solid ratio [18]. To remove the lignin from the pretreated residues, post-treatment technologies are used, such as alkaline, acidic and enzymatic processes or a combination thereof. Alkali treatment was shown to improve enzymatic digestibility, by degrading the lignin structure and swelling the cellulose fibers [10; 49]. However, the chemical and autohydrolysis methods showed shortcomings, such as degradation of pentoses to contaminants like furfural and hexoses to hydroxymethylfurfural (HMF), low control over the degree of polymerization (DP) and high downstream costs [50]. A second step consisting of applying an enzymatic treatment for xylan hydrolysis, using endo-1,4-\(\beta\)-xylanases (EC 3.2.1.8) and endo-1,3-\(\beta\)-xylanases (EC3.2.1.32), affected the yield of XOS, depending on various factors such as enzyme activity and incubation conditions (pH, hydrolysis time and temperature) [4; 12]. The enzymatic process was reported to be more environmentally friendly, since no use of chemicals is needed, and it can be conducted at milder temperatures; however, the cost of the various enzymes needed might be quite high. Against this background, the main objectives of this paper were to characterize the liquid fractions of wheat straw obtained during the autohydrolysis, alkali and enzymatic treatment and the combinations thereof in terms of XOS production (xylobiose, xylotriose and xylotetraose) and fermentable sugars. A downstream process method consisting of filtration, decolorization, and an- and cation exchange resins was tested in order to obtain a purified fraction of oligosaccharides and fermentable sugars for potential use in food applications (Figure 1).