## 3 Results and Discussion  
Table 1 shows the pretreatment hydrolysates' pH, the percentage of compounds that were released, and the characteristics of the solid residues for the pretreatment experiments carried out at maximum temperature during 30 min, for the pin wood chips. The results for the reaction times of 0 and 15 min at 170 \({}^{\circ}\)C maximum temperature, as well as, those for the corresponding wood chips with conventional size and replications are available in Appendix A (Table A1). The percentages of glucose, combined XMG, products of carbohydrate degradation (furfural and HMF), formic and acetic acids and also the combined soluble lignin and all other degradation products (pseudo-lignin) absorbing at 205 nm (SLY) that were released into the hydrolysates, concerning the initial biomass weight, were calculated following the HPLC determination of the corresponding compounds' concentrations in the pretreatment hydrolysates. As it can be perceived in Table 1, the formation of acidic components (acetic and formic acids) during the pretreatments led, in general, to a decrease of the hydrolysates pH (initial liquor pH vs. final pH). The increase in the severity of the pretreatment conditions (namely acid charge and maximum temperature) has led to higher extraction of hemicelluloses, as expected. The increase of temperature from 170 \({}^{\circ}\)C to 190 \({}^{\circ}\)C, even during a short period of time (30 min), had an important impact on carbohydrates extraction; for mild chemical reaction conditions (e.g., 2s_1 ac) the XMG removal increased from 14.5% to 18.5% on wood, which practically is the XMG content in the wood. Interestingly, very high percentages of carbohydrates can also be released from the wood by delignification (without acid addition), as revealed by the experiments 15s_0 ac (at 170 \({}^{\circ}\)C and 190 \({}^{\circ}\)C), highlighting the positive role of delignification, associated with the decrease in pH due to the acetic acid release from the xylan, on the hemicellulose extraction. As we can see in Table 1, the lignin content in the solid residue can be higher than that of the raw material. This can be due to the conjugation effect of two factors: the increase of lignin content because of hemicelluloses extraction and/or the formation of pseudo-lignin. In fact, when the SLY and the amount of residual lignin present the solid residue (as Klason lignin, determined directly through the aforementioned NREL procedure, or estimated using the kappa number; please see Figure A1 for the relationship between the lignin content determined through the two methodologies) are counted together, taking in account the SRY, the result is higher than the initial lignin content of the wood (23.1%), particularly for harsher hydrolysis pretreatment conditions (e.g., 15s_5 ac and 2s_2.5 ac). This increase is in good agreement with the degradation of carbohydrates in aromatic compounds and its precipitation on the solid residue, as was reported [21; 26; 27]. To improve the confidence in the reported values, the global mass balance was also done (column named \"total\" in Table 1) and the values are close to 100%. With the purpose of evaluating the efficiency of the enzymatic hydrolysis stage, the lignin content in each solid residue was determined (Table 1) and the calculations of the total carbohydrate enzymatic yield were based on the lignin-free material. Using the previously mentioned NREL procedure, the carbohydrates composition of the solid residue was also evaluated for several samples. Hemicelluloses are practically absent in those carried out under the more acidic conditions (such as 0s_2.5 ac, 5s_2.5 ac and 15s_5 ac), in accordance with their high concentration in the corresponding hydrolysates (Table 1).