## 2 Phenolic Compounds Extraction from Renewable Resources within a Biorefinery Scheme
### Techniques for Phenolic Compounds Extraction from Agri-Food By-Products  
In this context, literature collects many research studies that have evaluated the extraction processes to obtain phenolic compounds and characterised their bioactivities (namely, antioxidant and antibacterial features) [34]. This antioxidant activity is related to a broad spectrum of health effects, including anti-allergenic, cardioprotective, antiatherogenic, antimutagenic, and anti-inflammatory [35]. Organic solvents (such as ethanol) are usually employed for phenolic compound extraction, achieving attractive yields. For instance, 32% of the extract was recovered from eucalyptus leaves (T = 50 \({}^{\circ}\)C; time = 225 min and ethanol-water = 56%), containing 92 mg/g of total phenolic compounds and 53 mg/g of total flavonoid compounds [31]. Currently, significant efforts have been devoted to developing suitable methods of biomolecule extraction from agri-food wastes focusing on alternatives to the conventional extraction processes. These traditional solvents (such as methanol, acetone, or hexane) present several disadvantages such as toxicity, volatility, and flammability, which limit their application. Therefore, emerging technologies (such as microwave-assisted, ultrasound-assisted, pulsed electric field, and ohmic heating) have been combined with alternative solvents to enhance the extraction of biomolecules from renewable resources. Recently, the use of smart solvents as deep eutectic solvents (DES) for the recovery of bioactive compounds from agri-food residues has been reviewed by [PERSON] and coworkers [36]. These avant-garde solvents have attracted great expectations due to their physicochemical features, low toxicity, and biodegradability. Moreover, DES combined with innovative techniques allows successful isolation of target biomolecules compared to conventional techniques of extraction. Therefore, techniques for phenolic compound extraction can be classified into: (i) conventional methods such as maceration, liquid-liquid, and solid-liquid extraction, which have been the most used for many years since they have the advantage of being relatively simple and effective with a broad tailoring window in the extraction of phenolic compounds at a low cost, (ii) non-conventional methods, developed to increase the yield of polyphenol extraction. These methods focus on using fewer organic solvents or alternative ones and improve the selectivity of phenolic compounds [37; 38; 39; 40].  
Some of the most used non-conventional methods (not an exhaustive list) are ultrasound-assisted extraction (UAE). This method uses ultrasound energy to increase the mass transfer through the cavitation mechanism. As a result, fragmentation, erosion, and pore formation occur in the cellular matrix, thus favouring interactions between the analytes and the solvent [37; 41; 42; 43]. The microwave-assisted extraction (MAE) is based on electromagnetic radiation to quickly rupture the cell membrane due to the increased temperature caused by the radiation [41; 44]. Finally, supercritical fluid extraction (SFE) is based on the use of fluids that can exceed their specific critical pressure, such as CO\({}_{2}\). This characteristic allows these fluids to have properties of both liquids and gases, such as lower surface tension and high diffusivity. These properties allow supercritical fluids (SF) to spread easily and penetrate faster within the solid matrix resulting in an efficient and highly selectivity extraction [38; 45; 46; 47; 48]. A summary of the advantages, drawbacks and their application to extract phenolic compounds from by-products is shown in Table 1.  
While these non-conventional methods have shown excellent results, the use of non-organic solvents is still required most of the time. However, they are required in lower volumes usually. The growing interest (and need) to improve these methods and reduce the use of organic solvents furthermore has led to investigating the use of safer and or natural alternatives, such as the following.