## 2 Phenolic Compounds Extraction from Renewable Resources within a Biorefinery Scheme
### Co-Production of Phenolic Compounds within a Biorefinery Scheme
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **Advantages** & **Limitations** & **By-Products** & **Compounds** & **Refs** \\ \hline Ultrasonic & Lower temperature & Certain phenolic compounds & Acerola pomace & Phenolic compounds & [32] \\ extraction & & are unstable & & & \\  & Less use of solvents & Formation of OH radicals & & Anthocyanins & \\  & & Unavoidable use of organic solvents & Pomegranate peel & Phenolic compounds & [33] \\  & & Challenging to scale & & & \\  & & Mango peel & Phenolic compounds & [34] \\  & & Grape pomace & Phenolic compounds & [35; 36; 37] \\  & & & Anthocyanins & \\  & & Strawberry & Phenolic compounds & [38] \\  & & by-product & & \\  & & Orange peel & Phenolic compounds & [39] \\  & & Citrus pomace & Phenolic compounds & [40] \\  & & & Hesperidin & \\  & & Olive pomace & Phenolic compounds & [41] \\ \hline Microwave-assisted extraction & Quick process & Affinity of the solvent to polyphenol-radiation & Grape and blueberry pomace & [42] \\  & It can be done in absence of light & Risk due to & Grape pomace & Phenolic compounds & [43; 44] \\  & Low energy input & Damage to the analyte & & & \\  & & since radiation causes & & \\  & & a rapid increase & & \\  & & in temperature & & \\  & It can be coupled & Different plants and their parts can have & Kiwberry leaves & Phenolic compounds & [45] \\  & such as UAE & different levels & & \\  & & of interaction & & & \\  & & with microwaves & & & \\  & & Acai pomace & Phenolic compounds & [46] \\  & & & Spent coffee & Caffe acid & [47] \\  & & & grounds & \\  & & & Sunflower cake & Chlorogenic acid & [48] \\ \hline Supercritical & Easy penetration & Limited to compound & Cacao pod husk & Phenolic compounds & [49] \\ fluid Extraction & to the matrix & with low or & & & \\  & & medium polarity & & & \\  & Selective extraction & High initial & Grape marc & Phenolic compounds & [43; 70] \\  & of bioactives & investment & pomace and skins & & \\  & Rapid & Particle size of the raw & & & \\  & and simple & material to assure & & & \\  & & penetration but not & & & \\  & & clogging the filter & & & \\  & & (0.04–0.08 mm & & \\  & & optimum) & & & \\  & No need for organic & & & \\  & solvent, or in & & & \\  & very low quantities & & & \\  & CO\({}_{2}\) is the most & & & \\  & common solvent with is & & & \\  & a Generally Recognised & & & \\  & As Safe (GRAS) and & & & \\  & inexpensive solvent & & & \\  & Use of low temperatures, & & & \\  & which is ideal for heat & & & \\  & sensible bioactives & & & \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Green methods for polyphenol extraction from agri-food by-products and wastes. Furthermore, olive agro-industrial wastes have been extensively evaluated as sources of bioactive compounds. Olive tree (_Olea europaea_) pruning and olive mill leaves were submitted to different extraction processes to obtain total phenolic and flavonoid contents with antioxidant activity being the most abundant benzaldehyde, followed by p-vinylguaiacol and o-guaiacol [32]. _Pinus_ by-products as a source of bioactive phenolic compounds using green and sustainable valorisation techniques were recently reviewed by [PERSON] and collaborators [72]. They included emerging technologies of extraction (ultrasounds, supercritical fluids, pressurised liquids, and electric fields) to obtain bioactive compounds within a biorefinery concept [73]. Avocado waste can also be processed to obtain bioenergy and bioproducts using a single or integrated process [74]. Avocado industrial residues were also evaluated for the production of oligosaccharides and polyphenolics, obtaining 3.48 g gallic acid equivalents/100 g and 10.80 g frolox equivalents/100 g of residue, using autohydrolysis treatment at 150 \({}^{\circ}\)C [75].