# Single-Stage Fractionation of Vine Shoots Using Microwave Heating
## 1 Introduction  
Agro-biorefineries represent a sustainable approach to manufacture bio-based products from lignocellulosic agricultural residues, operating in clean and efficient processes enabling the separation of their main constituents. The worldwide area under vines was estimated at 7.4 million ha in 2018, with 13% of the world vineyard corresponding to Spain [1]. Vine shoots are agricultural wastes generated in vineyards at an estimated annual rate of 2-4 tons/ha [2], which are often abandoned in the field or burned, causing a number of ecological and environmental problems [3, 4]. Owing to their large availability and compositional features, vine shoots are a potential resource for manufacturing a scope of bioproducts, such as lactic acid, xylitol [5], oligosaccharidesandics (OS) [6], ethanol [7, 8], biogas [9], glucose and lignin [10]. The major hindrance for using lignocellulosic biomass as an industrial raw material lies on its complex and heterogeneous structure, defined by the presence of interpenetrated polymers (hemicellulosees, cellulose and lignin, named \"structural components\") with different chemical and physicochemical properties. For example, lignin is an aromatic polymer that can be dissolved by selected organosolvents, hemicelluloses are ramified polysaccharides of limited polymerization degree susceptible to hydrolysis reactions, and cellulose is a polysaccharide composed of anhydroogluces units that presents a limited susceptibility to hydrolysis. The vine shoots composition (referred to the structural components) is as follows: cellulose, 33-36 wt% [5, 8]; hemicelluloses, 19-23 wt%; and lignin, 27.1-26.6 wt% [4, 8]. The fractionation of lignocellulosic involves the breakdown of the complex interlinks between the structural components, improving the accessibility of the fractionationagents to cellulose and hemicelluloseoses [11]. Usually, this goal is achieved by consecutive treatments, which allow an integral benefit of the feedstock with minimal or no waste generation [12; 13]. A number of conversion techniques have been considered for this purpose [14; 15; 16], and several have been applied to vine shoots, including hydrothermal processing [6; 8], acid hydrolysis [5], delignification with alkaline solutions or organosolvents [17] and enzymatic methods [4]. Organic solvents (pure or in mixtures with water, eventually in the presence of a catalyst), are suitable agents for lignocellulosic fractionation: in typical treatments, lignin and hemicelluloseases are partially dissolved and transferred to the liquid phase, whereas cellulose remains in solid phase. Although ethanol, glycerol and acetone are the most used solvents, 1-butanol represents an interesting alternative, owing to its partial water miscibility [18]. This characteristic allows the one-step fractionation of lignocellulosic into three streams containing products derived from the structural components: lignin is dissolved in the organic phase, hemicellulose-derived products appear in the aqueous one, and cellulose is kept in solid phase. Recently, studies have been reported on biomass fractionation using partially immiscible solvents [19; 20; 21]. 1-butanol is considered as a green solvent that can be produced from biomass [22]. Efficient fractionation processes must provide an extensive lignin solubilization, yielding products that keep the structure and properties of the original polymer [23], and providing solids with enhanced cellulose contents. Additionally, the chemical treatment should be able to reduce the crystallinity of cellulose, increasing the available surface area, and enhancing the cellulose susceptibility to hydrolysis [24]. The enzymatic hydrolysis of cellulosic solids from fractionation leads to glucose solutions suitable as fermentation media for obtaining bioproducts such as ethanol or lactic acid [5; 25]. The partial hydrolysis of hemicelluloseosamine occurring in the aqueous phase leads to the formation of polysaccharide fragments (low molecular polysaccharides and/or OS) as main products [12].