## 4 Discussion  
Numerous studies have attempted to improve the release performance of XOS from biomasses that directly connected the productivity of XOS [16; 17; 18; 19]. However, control of impurities in the liquid  
Figure 4: Glucose content and conversion ratio after enzymatic hydrolysis: (**a**) 180 \({}^{\circ}\)C; (**b**) 190 \({}^{\circ}\)C.  
Figure 5: Mass balance of the XOS and glucose production from sweet sorghum bagasse. fraction after treatment is considered a major factor affecting the market price of XOS as well as its yield [2; 32]. Due to the fact that the purification process of XOS is costly, it can dramatically increase if a high amount of impurities are simultaneously mixed with XOS [28]. Another reason for the importance of controlling the impurities is related to the XOS activity [26]. In other words, the concept of impurity can be expanded to monomeric sugars, such as xylose [33]. XOS has several beneficial activities for the human body as a prebiotic, while xylose does not have any function like XOS [34]. Thus, preventing the excess generation of xylose is an essential factor for increasing the productivity of XOS. The chemical composition of SSB showed good potential for XOS production (Table 1). It had a high amount of xylan, and a low amount of ash and other hemicellulosic sugars. In particular, a very tiny amount of galactan and mannan in SSB has been reported in previous studies [35; 36]. Therefore, the XGM monomer in the liquid hydrolysate can be considered as xylose alone, and the XGM oligomer implied XOS in this study. The acid-hydrolysis pathway of xylan is shown by the variation of the XGM content with an increase in the reaction temperature (Table 2). The xylan seemed to be fully degraded and dissolved in liquid hydrolysate at 190 \({}^{\circ}\)C. Then, the XGM content sharply dropped at 200 \({}^{\circ}\)C because it converted to HMF and furfural, which are degradation products. The pH level gradually decreased by providing more heat energy due to the cleavage of the acetyl group from the hemicellululose chain, which resulted in more acidic circumstances at higher reaction temperature conditions [37]. This means that xylan degradation might be accelerated by positive feedback from heat energy and acids during autohydrolysis. This phenomenon was revealed by the variation in the XGM monomer content, which increased more in the higher reaction temperature conditions (Figure 1). Thus, autohydrolysis proved that the xylan structure in SSB can be sufficiently decomposed without the addition of chemical catalysts. As mentioned above, the quality of XOS might be controlled by reducing the impurities, including the monomeric sugars. In this term, the ratio between the XOS and the sum of glucose, arabinose, and XGM monomer in the liquid hydrolysate was calculated to evaluate the quality of XOS, and it was named XOS to total sugar ratio in this study. In other words, the XOS to total sugar ratio implies the proportion of XOS compared to the total sugars in the liquid hydrolysate. When the autohydrolysis conditions had a designated reaction time (20 min), the 170 \({}^{\circ}\)C condition showed a better XOS to total sugar ratio (64.2%) than that at 190 \({}^{\circ}\)C (57.5%) (Figure 1). The liquid hydrolysate at 170 \({}^{\circ}\)C might have fewer impurities than that at 190 \({}^{\circ}\)C, but the quantity of XOS and monomers at 190 \({}^{\circ}\)C (13.9% and 4.5%) was higher than that at 170 \({}^{\circ}\)C (11.6% and 0.5%). Considering the acid-hydrolysis pathway of xylan during autohydrolysis, the high amount of XGM monomer implies that the liquid hydrolysate at 190 \({}^{\circ}\)C could have a higher amount of low DP of XOS than that at 170 \({}^{\circ}\)C [38]. According to previous research, the low DP of XOS presented performed better as a prebiotic than high DP (above 4) of XOS [39].