**1. Introduction**

The current model of industrial development, based on an intensive usage of nonrenewable fossil resources, results in a number of negative effects, including health risks and global warming. Therefore, a gradual transition into sustainable alternatives based on cleaner and renewable raw materials is imperative [1]. In this sense, the biorefinery concept, based on the selective separation of the major feedstock components into "fractions" made up of compounds with similar properties, provides a framework for the integral conversion of biomass into a wide scope of fuels, chemicals, and materials. Lignocellulose biorefineries are expected to play a key role in the sustainable development of the industrial sector in the near future, and offer a sustainable path towards a bio-based economy through an efficient conversion of feedstocks from agriculture or forestry, including agroindustrial residues [2]. It can be noted that extracting value from wastes is compatible with the "circular economy" paradigm, which can be complemented with the utilization of green processing technologies.

### *Agronomy* **2020**, *10*, 760

Hazelnut (*Corylus avellana* L.) is a fruit made up of a shell with a kernel inside, which is used as a food. *Corylus avellana* originated from the Mediterranean region, and is one of the world's major commercial nut crops. Turkey is the main producer of *Corylus avellana*, covering approximately 70 percent of the world's production, followed by Italy, China, Spain, and USA. According to Food and Agriculture Organization (FAO) [3], more than one million tons of hazelnuts (with shell) were produced worldwide in 2017. Hazelnut shells (HS) account for more than 50% of the total nut weight, becoming an important byproduct in the hazelnut industry. HS are currently employed as low-value fuel [4,5]. However, HS is an abundant and low-cost raw material with a large potential as a raw material for biorefineries, where it can yield a number of value added products [6]. Some target products reported are xylooligosaccharides [7], phenolic antioxidants [4,5], bioethanol [8], materials for composites [9], levulinic acid, char [10], and furfural [11].

Nevertheless, the immense complexity of biomass together with the polymeric nature of its structural components makes fractionation almost inevitable. In this sense, decreasing the recalcitrance of the lignocellulosic matrix through cost effective and environmentally friendly technologies is a crucial objective [1]. Pretreatment is an essential processing step that entails the deconstruction of the lignocellulosic biomass by the disarray of lignin and the exposure of polysaccharides (cellulose and hemicelluloses) to hydrolytic reactions [12].

A number of biorefinery pretreatments are proposed in literature [2,13]. Some of them are used for obtaining products with industrial interest from HS as feedstock: hydrothermal [5,7,14], acid [15], alkaline [15,16] or the double step acid/alkaline pretreatments [17], oxidative (i.e., ozonolysis) [18], organosolv [19], or the combined effect of delignification pretreatments before acid hydrolysis [8].

Hydrothermal processing (also called autohydrolysis) is a pretreatment in which biomass is subjected to reaction with compressed hot water, enabling an extensive solubilization of hemicelluloses [20]. The reaction is catalyzed by hydronium ions resulting from water autoionization and in situ generated organic acids (mainly acetic acid coming from the hydrolysis of acetyl groups in hemicelluloses). As a result, hemicelluloses are depolymerized into low molecular weight polysaccharides, oligosaccharides (OS), monosaccharides, and minor amounts of other products. Additionally, the hydrothermal pretreatment increases the surface area and decreases the crystallinity of cellulose, resulting in an improved susceptibility toward the enzymatic hydrolysis. Additionally, lignin can be solubilized in subsequent processing stages.

In HS, hemicelluloses account for 24.6–30 wt% of the raw material. The major hemicellulose constituent is heteroxylan, made up of a backbone of linked xylose units substituted with acetyl and uronic groups, and bound phenolic compounds [5]. Consequently, optimal conditions of hydrothermal processing can result in relevant proportions of hemicellulose-derived xylooligosaccharides (XOS), which can be obtained at a reasonable yield and purity [21].

The implementation of an additional refining stage is necessary when high purity XOS are required, i.e., food-grade OS [22]. This point is important when OS are produced by hydrothermal pretreatment, because a variety of byproducts (monosaccharides, acetic acid, sugar degradation products, extractives, or acid soluble lignin) can be present in the reaction media. Membrane technology is an interesting alternative for XOS purification—the size-dependent separation achieved leads to concentrated solutions of purified XOS, while low molecular weight contaminants are removed in permeate. Some examples of XOS refining by membrane processing were reported for different agricultural residues, i.e., rice husks [23], peanut shells [24], and almond shells [25,26].

XOS are potential prebiotics, defined as nondigestible food ingredients that allow specific changes in both the composition and activity of the gastrointestinal microbiota, as stimulation of the growth of probiotic bifidobacteria and lactic acid bacteria, conferring benefits in the human health [27]. Some beneficial effects described in literature including the maintenance of the human health, the prevention of diseases, and the decreased risk of chronic diseases [22].

XOS containing esterified phenolic compounds are natural antioxidants, with potential applications in food, cosmetic, pharmacy, and nutraceutical industries. This type of XOS is considered as emerging prebiotics [7], and antioxidant activity [28] is derived from the presence of bound phenolics [27]. Therefore, this type of XOS could contribute to satisfying the increasing demand for ingredients of functional foods, whose demand is expected to reach more than 440 billion USD in 2022 [27].

In this work, hydrothermal pretreatment of waste HS was proposed as an initial step of a multistage process allowing the complete utilization of HS. This step aimed at the solubilization of hemicelluloses, leaving a treated solid made up of cellulose and lignin that could be valorized by further processing. HS were processed at different temperatures, and the solid and liquid phases were assayed for composition and yields to allow the formulation of material balances. The liquid phase from hydrothermal processing performed under optimal reaction was refined by membrane processing to yield a final product meeting the purity degree required for commercial food grade OS. The purified OS were assayed for monomeric constituents and structural features. Additionally, the total phenolic content and the in vitro antioxidant properties were considered to assess the potential of OS as ingredients for functional foods.

### **2. Materials and Methods**
