## 4 Discussion
180 \({}^{\circ}\)C, which had the highest XOS to total sugar ratio, and 190 \({}^{\circ}\)C conditions were selected for evaluating XOS production from SSB with several variations of the reaction time (Figure 3). The highest XOS content (15.4%) in this study was obtained at 180 \({}^{\circ}\)C for 20 min and 190 \({}^{\circ}\)C for 15 min. This XOS content can be converted to conversion ratio based on the initial amount of xylan in SSB (64.1%), which may be considered as a noteworthy value compared with the results of previous autohydrolysis studies: 60.9% from miscanthus [32], 61% from the brewery's spent grain [40], 51.7% from rice husks, and 51.8% from corn cob [41]. Glucan was presented as a major carbohydrate in SSB, even though it contained twice as much as the XGM in this study. Glucose has been recognized as a platform chemical from biomass and can be easily produced by enzymatic hydrolysis if the cell wall structure is appropriately degraded by the pretreatment [42]. The glucose to total sugar ratio, which was calculated in the same manner as the case of XOS, gradually increased with an increase in the reaction temperature and time (Figures 2 and 4). Meanwhile, less than 4% of glucose was released into the liquid hydrolysate even at the highest reaction temperature (Tables 2 and 3). This means that the amount of remaining glucan had not changed much by changing the autohydrolysis conditions, but the cell wall structure might be degraded toward a suitable form for cellulase activity at high reaction temperatures and long reaction times. The highest glucose to total sugar ratio (94.2%) was observed at 200 \({}^{\circ}\)C for 20 min, and an indication of glucan degradation could not be found under the autohydrolysis conditions in this study. The ideal autohydrolysis condition for XOS production could be suggested by the variations in reaction temperature and time (Figure 5). A XOS to total sugar ratio of 68.7% was obtained at 190 \({}^{\circ}\)C for 5 min, and a short reaction time can be an advantage for large-scale plants [43]. However, the result of this condition is concerned that the XOS might consist of a significant amount of high DP of XOS. According to this assumption, the liquid hydrolysate under this condition may require further processing to reduce the DP by xylanase. Additionally, this condition showed a relatively low glucose to total sugar ratio (83.5%), another main product of autohydrolysis, than other conditions. In this term, 180 \({}^{\circ}\)C for 20 min can be an alternative to 190 \({}^{\circ}\)C for 5 min because the XOS to total sugar ratio in this condition (67.6%) is slightly lower than that at 190 \({}^{\circ}\)C for 5 min. Meanwhile, the higher XGM monomer content (1.4%) in this condition means that the average DP might be lower than that at 190 \({}^{\circ}\)C for 5 min. If xylanase treatment followed by autohydrolysis induces the formation of XOS with a narrow DP from 2 to 4, the XOS DP variation in the liquid hydrolysate should be considered as an important factor in determining the amount of xylanase consumed. In this regard, 180 \({}^{\circ}\)C for 20 min might be more favorable for producing high-value XOS than at 190 \({}^{\circ}\)C for 5 min. On the other hand, 190 \({}^{\circ}\)C for 15 min can be chosen to obtain a high purity of glucose, 91.4% of glucose to total sugar ratio, even though this condition showed a moderate level of XOS to total sugar ratio (64.4%). This trade-off relationship between XOS and glucose might be controlled by inducing an additional process between the autohydrolysis and enzymatic hydrolysis steps. For instance, mechanical refining can be employed for solid residues, and it improves glucose conversion by collapsing the cell wall structure, even though the solid residue is produced under low reaction temperature conditions [44].