*3.2. Carbohydrates in the Pretreatment Liquids*

Carbohydrates in the pretreatment liquids are reported as monosaccharides and oligosaccharides (Table 3), with disaccharides included among the latter. Xylose, arabinose, and glucose were the main monosaccharides identified, whereas there were very little or no galactose and mannose. The concentrations of xylose, glucose, galactose, and mannose increased with increasing pretreatment temperatures. However, for the A-HTP series, the arabinose concentration was highest at intermediate temperatures (175/190 ◦C).


**Table 3.** Concentrations of monosaccharides and oligosaccharides (OS) (g/L) and pH in the pretreatment liquids <sup>a</sup> .

<sup>a</sup> Mean values from triplicates; ND, not detected. Standard deviations shown in parentheses. Same codification of experimental conditions as in Table 1.

For monosaccharides derived from oligosaccharides/disaccharides, the highest concentrations were 15.0 ± 0.3 g/L for Xylo-OS (xylose derived from oligosaccharides/disaccharides) and 2.7 ± 0.1 g/L for Gluco-OS (glucose derived from oligosaccharides/disaccharides) (Table 3). The maximum concentrations of the others (Arabino-OS, Galacto-OS, and Manno-OS) were around 1 g/L or lower. For A-HTP, Xylo-OS, Gluco-OS, and Manno-OS reached the highest concentrations at 190 ◦C, whereas the highest concentrations of Arabino-OS and Galacto-OS were instead found at 175 ◦C. This indicates that oligosaccharides containing arabinose and galactose units were more heat-labile. This is supported by data from the SA-HTP series showing higher concentrations of Xylo-OS, Gluco-OS, and Manno-OS for SA-190 than for SA-160, whereas Arabino-OS and Galacto-OS do not follow that pattern. In most cases, the concentrations of monosaccharides derived from oligosaccharides/disaccharides were higher than the concentrations of the corresponding monosaccharides in the same pretreatment liquid (Table 3).

The pH of the A-HTP liquids decreased with pretreatment temperature, from 5.0 for 160 ◦C to 3.5 for 205 ◦C (Table 3). As further discussed below, the decrease of pH with increasing pretreatment temperature results from the formation of carboxylic acids. For the SA-HTP series, the pH was only slightly (0.2–0.5 pH units) lower than for the corresponding pretreatment liquids in the A-HTP series.
