# Recovery of Polyphenols from Agri-Food By-Products: The Olive Oil and Winery Industries Cases
## 1 Introduction  
Agri-food industries bring about a large amount of waste. It is estimated that around 90 million tons of food waste are generated in the European Union each year, which constitutes a serious problem for the environment [1]. However, it is well known that agri-food waste and by-products can be sources of valuable compounds, such as polyphenols, with relevant functional biological activities, such as antioxidant, anticancer, antihypertensive, or anti-cholesterol [2; 3; 4]. Bioconomy covers the production of renewable biological resources and the conversion of these resources and wastes into value-added products, maintaining the value of products and materials for as long as possible while minimizing the use of resources and the generation of wastes [5]. Biomass volume or mass and its added value are inversely related. Thus, in the bottom part of the pyramid of biomass (see Figure 1) the most common application can be found, i.e., the use of biomass as an energy source, which has the lowest priority since it provides the lowest value (15-40 \(\upc\)/ton). Conversely, in the top part, the biomass used in pharmacy and cosmetics could result in products that have associated thehighest values (e.g., 10,000-20,000 \(\copyright\)/kg). Food and feed find the second and third place respectively, and chemicals are in the fourth position [5]. Phenolic compounds are products of high added value, with potential applications in the pharmaceutical, cosmetic, and food industries, and their recovery from waste can be highly interesting [6]. Studies indicate that consuming polyphenols reduces the risk of degenerative diseases, by inhibiting the oxidation of low-density lipoproteins [7]. Polyphenols may also offer protection against the development of cancer, cardiovascular diseases, diabetes, or osteoporosis [6]. The antioxidant capacity of the polyphenols correlates with their chemical structure; in general, they mainly prevent the formation of free radicals connected involved in the processes of autoxidation, by donating hydrogen atoms or electrons [4]. Phenolic acids and polyphenols are secondary metabolites of plants, generally related to the defense against ultraviolet radiation, pathogens, and environmental stress [7; 8]. Polyphenols are characterized by the presence of more than one phenol group per molecule, as well as other possible functional groups. However, the term polyphenols are quite often used in a broader sense, encompassing phenolic acids. Most of the phenolic metabolites may be classified into four large groups: phenolic acids, flavonoids, stilbenes, and lignans, as shown in Figure 2[4]. Figure 1: Pyramid of biomass value (adapted from [5]). Figure 2: Chemical structures of phenolic acids and polyphenols representative of the main families. Phenolic acids have a carboxylic acid functional group and can be divided into hydroxybenzoic acids and hydroxycinnamic acids, based on their C1-C6 and C3-C6 structures, respectively. The latter group includes compounds such as caffe, ferulic, \(p\)-coumaric, chlorogenic, or synaptic acids, while hydroxybenzoic acids include, among others, \(p\)-hydroxybenzoic, protocatechuic, syringic, vanillic, and gallic acids, and related tannins hydrolyzable tannins [8]. Flavonoids consist of two aromatic rings that are linked by three carbon atoms that form an oxygenated heterocyclic ring. Depending on the type of heterocyclic involved, they can be classified into different subclasses: flavonols, flavones, isoflav, flavanones, anthocyanin, and flavanols [4]. Stilbenes are a group of compounds derived from phenylpropanoids, characterized by the main chain of 1,2-diphenylethylene (C6-C2-C6). They are present in the human diet in low quantities and the main representative is resveratrol, detected in more than 70 species of plants, including grapes, berries, and peanuts [4]. Lignans are produced by oxidative dimerization of two units of phenylpropane.