# Optimization of Xylose Recovery in Oil Palm Empty Fruit Bunches for Xylitol Production
## 5 Discussion  
Among the three variables studied, temperature gave the most prominent effect of xylose recovery both in the solid and the liquid pretreated OPEFB (Figure 1a,b). In agreement with the characteristics of hemicellulose, which is more susceptible to heat than other lignocellulosic components [33], an optimum value of xylose recovery was observed within the range of studied temperature. Hemicellulose tends  
Figure 5: Contour plot for maximum total xylose recovery on hydrolysate of a) solid pretreated OPEFB and b) liquor pretreated OPEFB. to solubilize more than cellulose in a higher pretreatment temperature [24; 26; 34; 35]. The soluble hemicellulose can be hydrolyzed further into furfural and become an inhibitor to enzymes [36]. The optimum temperature found in this study was in the same range of the optimum temperature obtained for the hydrothermal pretreatment of prairie cordgrass [24]. Other studies stated that the pretreatment of sugarcane bagasse, sugarcane straw, wood, and OPEFB at temperatures higher than 165 \({}^{\circ}\)C resulted in dissolved hemicellulose [37; 38; 39; 40]. However, our current study differs from previously reported results. The xylose recovery in liquid pretreated OPEFB was lower than in the solid pretreated OPEFB, as can be seen in Figures 2-4. During the hydrothermal process, some of the water was absorbed into the oil palm empty fruit bunches, leaving only a portion of the water, with a maximum of 50% of the initial volume. This led to the lower concentration of hemicellulose in the remaining liquid. A previous experiment using wheat straw in a continuous hydrothermal pretreatment process resulted in a hemicellulose extraction of 70% at 195 \({}^{\circ}\)C for 6-12 min [41]. Another experiment using catalpa wood resulted in a different result, which showed an extraction of 33.9% and 38.8% hemicellulose at 160 \({}^{\circ}\)C and 170 \({}^{\circ}\)C, respectively [42]. These results showed that there was a connection between lignocellulosic type and hemicellulose on the extraction recovery and conditions, as reported earlier [43]. Solid loading (SL) effects on xylose recovery for pretreated solid and liquid were not statistically significant (\(p\)-values of 0.957 and 0.93, respectively) up to SL of 15%, as can be seen in Figure 1a,b. This is important because a higher SL (>15%) lowers the xylose recovery, as reported earlier [44; 45]. Hydrothermal pretreatment uses water as the agent to break the bond between lignin and hemicellulose. A high SL means that there is less water in the system that should be in contact with lignocellulosic biomass, which may lower xylose recovery [46]. Another study stated that a high SL pretreatment (20% SL) required a combination pretreatment using maleic acid and a saccharification process [46]. Another difficulty in using high solid loading was obtaining a pumpable slurry for the subsequent enzymatic hydrolysis [47]. An increase in the pretreatment time resulted in a slight rise in xylose recovery in solid and liquid pretreated OPEFB hydrolysate, as can be seen in Figures 2 and 3. A further increase in xylose recovery was reported to be less important as time prolonged further. Extending the hydrothermal pretreatment time of OPEFB at 127.9 \({}^{\circ}\)C from 60 to 90 min was reported not to give a considerable increase in xylose recovery [23]. A similar report on OPEFB hydrothermal pretreatment showed that a longer pretreatment time resulted in a lower sugar recovery [44]. Furthermore, extending the hydrothermal pretreatment time to 180 min at 170 \({}^{\circ}\)C lowered the xylose recovery of beechwood [25]. A longer pretreatment time leads to the formation of furfural, soluble lignin, and insoluble lignin, which was reported to inhibit the enzymatic hydrolysis process [25; 48]. Compared to previous research using grapevine followed by acid hydrolysis, where 60% of xylan was recovered in the liquid pretreated biomass [25].