## 1 Introduction  
_What straw (WS) is an agricultural residue from wheat grass (Triticum aestivum L.) that presents many interesting characteristics that facilitate its biotechnological upgrade in a bio-refinery framework_ _[_1_]__. Specifically, it is an herbaceous crop, soft material that can be transported in relatively high-density form and typically has a low water content (i.e., about 6-8%)_ _[_2_]_ _that enables its easy storage [3]. Moreover, it is a very abundant material and it does not present an excessive commercial value (about 97 USD/ton) [4]. The average yield of WS is 1 kg per 1.3-1.4 kg of grain, which leads to a considerable surplus amount [5] even considering the fractions to be used for soil improvement and livestock use. Based on the data from FAO [6], world annual WS production was about 564 million tons in 2018. Despite the production showed a slightly decrease in 2018, these amounts are significant enough to consider WS as a complementary source in the production of fibers and antioxidants. The main constituents of WS cell walls are cellulose (33.7-40%), hemicelluloses (21-26%), and lignin (11-22.9%) [7]. The cell walls also contain small amounts of p-hydroxy-cinnamic acids such as ferulic and p-coumaric acids, glycoproteins, pectins, wax, and ash [7]. Currently, WS is used for low value applications such as mulch [8], animal-feed [9], bedding [10], energy [11] and pulp production [12]. Despite this, it is considered to have high potential for the production of second-generation bioethanol in Europe [13] and different studies were conducted in the past regarding the bound phenolic compounds of WS and their antioxidant properties [14; 15; 16; 17; 18]. Plant cell walls are three-dimensional structures formed with a polysaccharide network of which cellulose, hemicelluloses and pectin are the most important components. It is well established that they contain hydroxyl cinnamic acids covalently linked to polysaccharides through ester linkages [19]. Phenolic compounds in plants are present in the free, esterified or bound forms. Phenolic acids may form both ester and ether linkages owing to their bifunctional nature through reactions involving their carboxylic and hydroxyl groups, respectively, which allows phenolic acids to form cross-links with cell wall macromolecules [20]. The antioxidant phenolics and cellulose can be isolated from WS during the same lignocellulosic fractionation process, such as an acid/alkaline hydrolysis process [7]. The literature review indicates the use of acid and alkali hydrolysis in wheat, rice, rye, barley and maize straws [21]; wheat straw, wheat bran, maize bran, rice bran and sugar cane bagasse [22]; wheat bran [23] and corn fiber [24]; barley bran and corn cobs [25]; corn bran [26]; corn cob [27]; triticale bran and straw [28]; maize bran [29]; rice bran [30; 31]; barley bran, corn cobs, corn leaves and oat fiber [25; 32]; barley husk [33]. From the environmental point of view, it is better to avoid the use of chemicals which are used in conventional fractionation process such as the sulfuric acid in the acid hydrolysis step. Even though a powerful agent for cellulose hydrolysis, it is toxic, corrosive and hazardous and the concentrated acid must be recovered after hydrolysis to make the process economically feasible [34]. For these reasons, auto-hydrolysis instead of acid hydrolysis process is gaining more attention nowadays and was already tested to recover phenolics from grape stalks [35; 36]; corn cob [37; 38]; diamond shells and chestnut burs [37], barley husks [39]; olive tree branches [40] and corn stalks [41]. Hence, in the present study, a potential environmentally friendly auto-hydrolytic process was applied for the recovery of both antioxidants and cellulose from wheat straw in a three-step process consisting in a high pressure/temperature auto-hydrolysis step, followed by an alkaline hydrolysis and a final bleaching step.