*4.2. Carbohydrates in the Pretreatment Liquids*

The carbohydrate profiles of the pretreatment liquids, i.e., relatively high concentrations of xylose, xylo-oligosaccharides (Xylo-OS), glucose, and gluco-oligosaccharides (Gluco-OS) (Table 3), are as expected, considering the polysaccharide content of the raw material (Table 1) and considering that most of the glucan is cellulose, which is more resistant to pretreatment than hemicellulose. The increase of xylose and glucose concentrations with the increase of temperature is typical for hydrothermal pretreatment of wheat straw [31,32] and other herbaceous feedstocks such as sugarcane bagasse [42] and corn stover [43]. The discrepancy between the increase of xylose and the decrease of Xylo-OS for A-HTP at

205 ◦C compared to 190 ◦C point towards degradation reactions. This type of reaction has been discussed elsewhere [14].

The relatively high arabinose formation at 160 ◦C agrees well with the observations regarding the composition of pretreated solids (Section 4.1) and with the literature on the hydrolysis of hemicelluloses [38]. The fast dynamics of arabino- and galacto-oligosaccharides are in agreement with previous studies of HTP under mild conditions. Chen et al. [32] reported maximal arabinose and galactose formation from wheat straw at 160 ◦C, which is lower than the temperature of the maximum concentrations in the current work (175 ◦C), but they applied a longer pretreatment time (30 min). A similar trend has also been reported for hydrothermal pretreatment of sugarcane bagasse, which displayed maximum arabinose formation at 180 ◦C, followed by a sequential reduction of its concentration at 185, 190, and 195 ◦C [42].

It was unexpected that the amount of monosaccharides released by SA-HTP was almost as low as that of A-HTP and that the amount of oligosaccharides was rather high for both pretreatment approaches. Typically, SA-HTP liquids are rich in monosaccharides, and A-HTP liquids are rich in oligosaccharides. For instance, xylose concentrations around 19 g/L were detected in SA-HTP liquids of sugarcane bagasse at 175 ◦C for 3.9 min [35], while a yield of 61.7 g xylo-OS per kg raw material was reported for A-HTP of wheat straw at 180 ◦C for 30 min [32]. That, together with the observation that the yield of pretreated solids was more affected by temperature increase than by the use of sulfuric acid, can be attributed to the low acid-loading, which was not enough to cause any major hydrolysis of hemicelluloses. Although compositional analysis of solids pretreated at 190 ◦C revealed more extensive xylan solubilization for SA-HTP than for A-HTP (Table 1, Figure 2), the total concentration of xylose and Xylo-OS was comparable for both pretreatment approaches (Table 3). That indicates that a fraction of the solubilized xylan was degraded, and, therefore, it could not be quantified in the pretreatment liquids either as xylose or Xylo-OS.
