## 1 Introduction  
Oligosaccharides have attained a growing commercial interest in the last decades due to their application as food ingredients and putative prebiotic compounds, which might exert beneficial effects on the host health [1; 2; 3; 4; 5; 6; 7; 8; 9]. Some of these polymers, specifically xylo-oligosaccharides (XOS), are considered non-digesible fibers and can be produced from biodegradable and low-cost agricultural biomass, especially lignocellulosic material such as wheat straw, by chemical, auto-hydrolytic, enzymatic processes or a combination thereof [2; 4; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Xylo-oligosaccharide products are sugar oligomers with 2-7 xylose units, and their monomer (xylose) is connected by \(\beta\)-(1-4)-linkages, which can contain different side groups (e.g., \(\alpha\)-d-glucopyranosyl uronic acid or its 4-\(O\)-methyl derivative, acetyl groups, or ara-binofuranosyl residues), forming branched structures [23]. Associations in the scientific literature were made between consumption of XOS and potential prebiotic effects since these compounds pass through the upper gastrointestinal tract without being digested. They are metabolized by lactic acid bacteria (bifido bacteria and lactobacilli) in the lower intestine, which facilitates their conversion into short-chain fatty acids (SCFAs) like acetate, propionate and butyrate. Lately, research has highlighted the implications of the gut microbiota on human health, with SCFAs being correlated with reducing luminal pH and having protective action against acid-sensitive enteropathogens [24; 25; 26; 27; 28]. In recent years, XOS have been incorporated in various food applications (cookies, dairy products, beverages, fruit juices and chewing gums) [29; 30; 31; 32], animal feed [33] and nutraceuticals [16; 34], showing better physicochemical properties such as thermal stability (up to 100 \({}^{\circ}\)C) and acidity (pH 2.5 to 8) compared to the well-known inulin and fructooligosaccharides [5; 12; 35]. In Japan, XOS are approved as food ingredients by the Japanese Ministry of Health, Labour and Welfare, as Foods for Specified Health Uses (FOSHU), while in China they have been on the market since 2000 as food supplements [36]. In the European Union (EU), XOS obtained from corncobs have been authorized to be used as a novel food ingredient in various food categories (bread, breakfast cereals, biscuits, yoghurt, soy drink, fruit spreads and chocolate confectionary) at maximum levels varying from 3.5 g/kg to 30 g/kg, by the European Commission, as proved by the Commission Implementing Regulation (EU) 2018/1648 of 29 October 2018, based on the scientific opinion on the safety of XOS by the European Food Safety Authority [37; 38]. In the EU, foodstuffs produced with new technologies, derived from new sources, and made from new substances, as well as traditional foods consumed in non-EU countries that were not consumed to a significant degree within the EU before 15 May 1997, are considered \"novel foods\" in Regulation (EU) 2015/2283; therefore, they require premarket authorization by the European Commission after their safety is assessed by the European Food Safety Authority [39; 40]. Over the last years, various processes (enzymatic, chemical and autohydrolysis) have been exploited to produce XOS and fermentable sugars from lignocellulosic biomasses such as wheat by-products (straw, chaff and bran) [2; 3; 10; 13; 14; 15; 17; 18; 19; 20; 41; 42; 43], corn cobs [44], almond shells [20], olive stones [20], rice husks [44], barley straw [44], sugarcane straw [45] and poplar wood chips [46]. The state of the art in the manufacture and application of XOS up to 2019 was reviewed recently [10]. The use of agricultural biomass has a positive impact on the environment, by contributing to the valorization of the residues generated annually; however, there are some challenges arising for XOS production, since they present a spoilage risk during storage, are seasonal and have variable chemical composition [47; 48].