## 1 Introduction  
Xplitol is a sugar alcohol with five carbon atoms and similar sweetening properties to sucrose. Xylitol is widely known for its various health benefits, including a great capacity to decrease the risk of dental caries. In addition, xylitol stands out for its high efficiency in the treatment of illnesses, such as diabetes, osteoporosis, iron deficiency, anaemia, muscle inflammation, or problems in respiratory system and colon [1]. The use of xylitol has increasingly gained interest for food and medicine manufacturing in the past several years [2]. Traditionally, it has been produced using a chemical synthesis process, reducing pure xylose at very high temperatures and pressures. However, due to the complexity of this chemical process, the need for pure xylose as feedstock and the associated high-energy requirements [3], novel biological processes, where xylitol is produced by xylose fermentation with microorganisms, have emerged. These alternative processes can be more environmentally friendly and lower energy consumption, making biological production of xylitol economically viable [4, 5]. To reach these objectives, the use of xylose sources coming from lignocellulosic biomass residues, originating in different industrial sectors (in many cases abundant and of low economic value), can be an important contribution. In this context, an interesting lignocellulosic residue for biological production of xylitol could be exhausted olive pomace (EOP) because it contains a significant amount of carbon sources, cellulose, and hemicellulose [6]. EOP is generated in the the olive oil sector together with other residues, such as olive stones, olive leaves, and olive pomace. EOP is generated in specific extraction industries, as a result of extraction with hexane of the residual oil contained in olive pomace, which in turn, is produced in olive mills [7; 8]. Olive pomace extraction industries in Spain are mainly located in the south of the country and can generate more than 1,182,000 tons per annum [8]. However, even though EOP can be employed in home heaters or small industrial boilers, due to strict environmental standards for emissions, it is now used less, and there is a certain surplus of this residue [9]. EOP contains cellulose, hemicellulose, lignin, and extractives (rich in value-added compounds, such as phenolic and antioxidants compounds) as its main components. Due to its high content in extractives (about 42%), an initial water extraction step is desirable in its conversion process, aimed at recovering these valuable compounds. Moreover, a pre-treatment stage is necessary, in order to disturb its structure and fractionate its main components [6]. In the pre-treatment stage, different types of technologies, such as autohydrolysis, dilute acid, alkalis, microwave, extrusion, organosolv, ionic liquid, deep eutectic solvents, steam explosion, or biologic treatment, have been tested with a vast variety lignocellulosic material [10; 11; 12]. Between them, dilute acid pretreatment is highlighted, as it is able to achieve high solubilization of hemicellulose fractions (>85%) and generate a pretreated solid composed mainly of cellulose, which is more easily accessible for enzymes in the subsequent enzymatic hydrolysis step [11; 13]. Nevertheless, fermentation inhibitory compounds, such as acetic and formic acids, furfural, 5-hydroxymethylfurfural (HMF), and phenolic compounds, might be generated during acid pre-treatment [11; 14]. Therefore, in order to minimize the generation of these undesirable compounds, the choice of adequate acid pretreatment conditions, such as temperature, time, and acid concentration, is essential [11]. Even so, in many cases the production of such compounds is unavoidable if good fractionation results are desired. Therefore, the treatment of acid hydrolysates with detoxification methods may be a suitable way of reducing the presence of inhibitor compounds and, thus, favouring its utilization as a sugar source.