## 2 Phenolic Compounds Extraction from Renewable Resources within a Biorefinery Scheme  
Environmental concerns over fossil fuel dependence and increasing demand for energy and chemicals require an urgent change in the economic production model [27]. Consequently, European Union policies highly encourage the search for renewable resources for sustainable development that contributes to a transition from a linear economy to a circular economy [28]. In this framework, residues are considered sources of new raw materials able to be reincorporated into the productive system, allowing the achievement of equilibrium between the economy, environment, and society [29].  
In this context, sectors such as the agriculture and agri-food industries are considered the highest producers of wastes and/or by-products worldwide. These residual biomasses are usually under-valorised and discarded, causing environmental damage. Nevertheless, it is known that wastes generated in these industries (such as citrus processing, winemaking, olive oil production, and seaweed hydrocolloid industry) are important sources of a wide range of bioactive compounds such as phenolic compounds. The extraction of phenolic compounds within a biorefinery scheme is the desired formula to obtain a broad spectrum of marketable products and to attain an integral valorisation of these residues (Figure 2). For the feasibility of biorefieries based on agri-food residues, one first stage of extraction of these biomolecules with pharmaceutical and/or food applications is a priority, followed by the manufacturing of biofuels, materials of chemicals. In addition, this cascading biomass processing has to fulfill green chemistry principles to satisfy the increasing demand for environmentally friendly and safe processes [30]. Moreover, this first step of biorefinery aims to meet requirements such as a cost-effective, versatile and straightforward process for extracting a wide range of compounds [31]. Therefore, the extraction of phenolic compounds from wastes implies a double benefit: reducing costs and environmental problems related to waste management and, on the other hand, improving the feasibility of biorefieries and increasing the value-added compounds that can be obtained [32].  
Figure 2: Polyphenolic compound extraction from residual biomass within a biorefinery scheme.  
Generally, lignocellulosic-based wastes can include agri-food and forestry residues, dedicated crops, and short rotation crops. Lignocellulosic biomasses are composed of non-structural components (protein, ashes, extractives) and structural components (mainly cellulose, hemicellulose, and lignin). Residues from food and agricultural industries usually include peels, seeds, hulls, and shells rich in non-structural components (such as phenolic compounds). Traditionally, biomass processing includes physical (extrusion, milling), physicochemical (steam explosion, AFEX), and chemical (e.g., alkali, autohydrolysis, organosolv, acid) treatments. Nevertheless, harsh conditions are necessary for breaking down the recalcitrant lignocellulosic structure to improve the enzymatic access to cellulose. This approach can hinder the integral valorisation of lignocellulosic biomass since more susceptible fractions (such as hemicellulose and extractives) can be degraded. In this context, milder conditions are employed first to extract thermolabile biomolecules, such as some phenolic compounds present in non-structural components of residual biomass [33].