## 1 Introduction  
Due to an emerging interest in a healthy lifestyle, probiotics have attracted attention [1]. Probiotics have several beneficial effects on the human body, especially as it reduces the risk of colon-related diseases and dysfunctions [2]. Prebiotics have also attracted great interest because they facilitate the growth of probiotics and inhibit the growth of pathogenic microorganisms in the human intestine [3]. Xylooligosaccharides (XOS) are one of the fascinating products of biomass that have unique properties as prebiotics [4]. XOS do not only reduce the risk of colon cancer, but also improve beneficial biological activities, including reducing dental caries, boosting the immune system, and restraining the growth of pathogens [5]. Food and food additives have been recognized as conventional methods for XOS ingestion, but the application area of XOS is expanding the pharmaceutical, chemical, and nutraceutical industries [6]. The global market size of XOS is expected to grow by 4.1% annually and reach 135.7 million dollars by 2026 [7]. XOS is typically defined as two to ten xylose combined by \(\beta\)-1,4 linkages with arabinose, acetyl groups, and uronic acid substitution [8]. Chemical (acid pretreatment and autohydrolysis) and biological (enzymatic hydrolysis by xylanase) approaches have been developed for hydrolyzing and extracting the xylan chain in a biomass [9]. Concentrated acid pretreatment using inorganic acids has been reported to result in fast isolation of the xylan fraction, but this method is not environmentally friendly and corrodes the equipment [10]. Meanwhile, the biological method can control the generation of byproducts, as shown in the results of acid pretreatment, but it has drawbacks such as long reaction time, relatively low XOS yield, and high cost of xylanase [11,12]. Autohydrolysis is considered an eco-friendly pretreatment method because it does not use any chemicals for the reaction [13]. When a biomass is heated in water, the uronic and acetyl groups from the hemicellulose fraction can be released in the form of an acid followed by acid-hydrolysis of the carbohydrate [14]. This acidic circumstance of autohydrolysis from the side groups of hemicellulose does not lead to an extremely low pH condition compared to the mineral acids of typical acid pretreatments [15]. Thus, autohydrolysis can be a less corrosive and low-cost approach for XOS production than other chemical methods [13]. Several types of xylan-rich biomasses are used for producing XOS, such as corn stover, wheat straw, sugarcane straw, and sugarcane bagasse [16; 17; 18; 19]. However, the straws from agricultural residues usually contain a high amount of minerals, including amorphous silica, which can cause chemical reactions to occur in an undesired manner [20]. The production of numerous XOS has been focused on the utilization of sugarcane bagasse, while interest for sweet sorghum is still lower than that of other biomasses [21]. Sweet sorghum can be cultivated in most regions of the world, from temperate to tropical [22]. It typically grows over 3 m tall, ensuring a high energy density per cultivating area up to 20.2 tons per ha [23]. Additionally, sweet sorghum bagasse (SSB) contains an abundant amount of xylan, which is an advantage for XOS production [24]. The physiological activities of XOS have been considered to be strongly related to the linkage-type, substituted side groups, and the degree of polymerization (DP) [25]. Regarding the DP, XOS has a low DP from xylobiose (DP = 2) to xylotetraose (DP = 4), and performs better as a prebiotic than XOS with a high DP [26; 27]. On the other hand, xylose and furfural, which are dehydrated products from xylose, can be excessively produced when high temperature or/and low pH conditions are induced to focus on the cleavage of the xylan structure [28]. Therefore, suitable autohydrolysis conditions for improving the XOS yield and purity should be investigated.