# Optimization of Xylose Recovery in Oil Palm Empty Fruit Bunches for Xylitol Production
###### Abstract  
The hardest obstacle to make use of lignocellulosic biomass by using green technology is the existence of lignin. It can hinder enzyme reactions with cellulose or hemicellulose as a substrate. Oil palm empty fruit bunches (OPEFBs) consist of hemicellulose with xylan as the main component. Xylitol production via fermentation could use this xylan since it can be converted into xylose. Several pretreatment processes were explored to increase sugar recovery from lignocellulosic biomass. Considering that hemicellulose is more susceptible to heat than cellulose, the hydrothermal process was applied to OPEFB before it was hydrolyzed enzymatically. The purpose of this study was to investigate the effect of temperature, solid loading, and pretreatment time on the OPEFB hydrothermal process. The xylose concentration in OPEFB hydrolysate was analyzed using high-performance liquid chromatography (HPLC). The results indicated that temperature was more important than pretreatment time and solid loading for OPEFB sugar recovery. The optimum temperature, solid loading, and pretreatment time for maximum xylose recovery from pretreated OPEFB were 165 \({}^{\circ}\)C, 7%, and 60 min, respectively, giving a xylose recovery of 0.061 g/g of pretreated OPEFB (35% of OPEFB xylan was recovered). OPEFB; xylitol; hydrothermal process; hemicellulose; enzymatic hydrolysis; temperature; solid loading +
Footnote †: journal: Applied Sciences  
11 January 202014 February 20202014 February 202014 February 202014 February 20202  
There is a structural barrier in order to obtain hemicellulose from OPEFB since hemicellulose is covered by lignin and is attached to cellulose fibril. Lignin cover will hinder enzymes from reaching the hemicellulose since it is an amorphous polymer [10]. This makes it important to remove lignin from OPEFB via the pretreatment process. There were some efforts to pretreat a variety of lignocellulosic biomass, including pretreatment using dilute acid as reported on OPEFB [11], corn cob [12], the chip of _Eucalyptus globulus_[13], sunflower stalk, and tobacco stalk [14]. Others used a dilute base for OPEFB [15], bamboo [16], and wheat straw [17]. However, both acid and base pretreatment produce toxic compounds, furfural, carboxylic acid, soluble sugar, and soluble and insoluble lignin that will inhibit xylitol fermentation using wild strain yeast [18; 19]. Instead, a genetically modified strain was developed to overcome the detoxification process [20]. Another method is the organosolv pretreatment, which partially hydrolyzes the lignin bond with cellulose and hemicellulose, but most of the hemicellulose is solubilized [21; 22]. The milder process involves using a hydrothermal process such as that used on OPEFB [23], prairie cordgrass [24], hardwood [25], and seeds of _Euterep olerca_[26]. Hemicellulose is more susceptible to heat than cellulose; therefore, it needs a careful choice of pretreatment method and condition that will not depolymerize most of the hemicellulose and make it easier to be utilized by the enzyme [18]. Acid pretreatment produces an inhibitory substance, as seen in xylitol fermentation using wild strain yeast, which will need detoxification to reduce toxicity [27; 28; 29]. Alkaline pretreatment is better than acid pretreatment in sugar production, but it requires pH adjustment for the following fermentation process to produce xylitol [23]. The hydrothermal process is a mild process, but the process conditions such as temperature need to be set in order to obtain optimum xylose recovery. Temperature affects the success of the hydrothermal process, whereby a higher temperature worsens the effect on hemicellulose. In addition, solid loading (SL) and process time also affect the degree of hemicellulose recovery. If SL is too low, available water will increase the chances of OPEFB hydrolysis. Conversely, if SL is too high, water does not have enough opportunity to interact with OPEFB.