**2. Nature and occurrence of phenolic compounds: from food to waste**

Phenolic compounds (PC) are secondary metabolites with a high array of unique bioactive properties, which makes them vastly appreciated for their beneficial effects on human health and well-being. PC constitute a wide family of bioactive compounds comprising more than 8000 different structures already identified [3]. Besides the great structural heterogeneity, some common features allow classifying PC into two different families: flavonoids (flavones, flavonols, flavanols, flavanones, isoflavones, and anthocyanins) and non-flavonoids (phenolic acids and derivatives, stilbenes and lignans) as summarized in **Figure 1**. In nature, these structures could also occur esterified, acylated and/or glycosylated. Indeed, the extreme diversity of these compounds joined with their extensively

**Figure 1.** *Polyphenols classification and basic structures.*

probed bioactivities makes them one of the most largely studied family of plant metabolites.

PC are found almost in all families of plants and are concentrated in leaf tissue, the epidermis, bark layers, flowers and fruits. The PC distribution could vary within the plant tissues. Indeed, differences in PC composition from seeds, pulp and peels have been extensively studied [4]. Overall, anthocyanin pigments are mainly accumulated in peels while flavan-3-ols appear in higher concentration in seeds [5]. Furthermore, PC occurrence in Plant Kingdom depend on biotic and abiotic factors such as genetic variations, environmental conditions and agronomic practices among others [6]. Besides these variations, the PC content could vary during processing from technological and industrial processes to homemade practices such as culinary treatments or vegetables storage [7, 8]. All these features must be considered before analyzing the real intake of PC and related health outcomes observed. Indeed, the amount of a bioactive compound required to deliver the health-related effects is the key to design a new functional food. Besides the great diversity of PC and the wide range of foods containing them, the major daily intake comes from cereal grain accounting for over 50% of their total intake. Among phytochemicals identified in cereal grain the most important are the phenolic acids and condensed tannins (flavan-3-ols polymers) [9]. In parallel, flavonoids are the main bioactive compounds found in fruits [10] and herbs and spices are rich in hidroxybenzoic acids [11].

PC are not only present in foods but also Agro-Food wastes such as fruit pomace, wood material or even waste water [12]. In this context, the valorization of Agro-Food by-products have emerged over the last years especially because of their easy obtention in large volumes with reduced costs [13]. Apart from their functionality as a source of energy, Agro-Food by-products should also be considered as value-added residues due to their chemical heterogeneity, structure and subsequent applications in the food sector as functional food ingredients or nutraceuticals contributing not only for a sustainable and circular economy but also for the implementation of zero waste politics [14].

Agro-Food wastes have been traditionally used as organic fertilizer, livestock feeds, or as a source for biofuel production [13]. The evolution in Green-chemistry with cutting-edge technology to properly obtain bioactive compounds from Agro-Food wastes opens a new perspective to produce value-added products [15]. Much evidence has been highlighted to use by-products of fruit industry or wineries, among others. Edible parts of fruits and vegetables usually contain lower amounts of bioactive compounds than skin, twings or peels non-edible portions. Indeed, wastes obtained after pressing the juice are a valuable source of PC. In parallel, winery and overall beverages industry generates large volumes of pomace (a mixture of pulp, skin, seeds, and stem) with higher amounts of PC when compared with edible fruits and vegetables [16]. Among fruit-derived by-products or wastes, apple pomace contains high amounts of flavonols, flavanols, phenolic acids, dihydrochalcones and anthocyanins [17]. Furthermore, significant amounts of a well-known antidiabetic agent named phlorizin is widely found in apple-derived by-products [18]. Peels from fruit such as banana peel, rich in phenolic acids, flavonols, flavanols, and catecholamines accounting three times the edible part of the fruit, have been reported as providing strong antioxidant and anti-microbial activities, in addition to exhibit other health benefits like reducing cholesterol and blood sugar, neuroprotective effect and anti-angiogenic activity were also described [13]. Citrus peels have been also studied to recover high-value bioactive compounds like flavones (apigeninglucoside and diosmetin-glucoside) and flavanones (eriocitrin and hesperidin) from lemon peel and phenolic acids (hydroxybenzoic and caffeic acids) and flavanones (hesperidin and narirutin) from orange peel and pulp [19]. Potato peel has been

reported as containing phenolic acids and beetroots peel and pulp contain flavonoids, phenolic acids and betalains, which exhibited good antioxidant activity and hepatoprotective effects [20]. One of the most recently studied food material was onion and garlic skin/peel, which generates huge volume of wastes especially rich in quercetin derivatives as well as other aglycone flavonoids exhibiting bioactivities like antioxidant, antimicrobial, antispasmodic, and antidiabetic activity [21]. Berries by-products such as branches obtained from elderberry processing are a valuable source of anthocyanins [22]. Furthermore, seeds/kernel which is a major waste after processing holds promise as a potential therapeutic source with numerous PC being isolated such as flavonols and gallotannins [23].

Some of the mentioned food by-products or Agro-Food wastes have been already proposed as additive in the formulation of bakery and dairy products to enhance their contents in bioactive compounds [24, 25]. In addition, the presence of natural compounds, pigments and volatile compounds can enhance the sensory properties and overall quality of the final product but some research is needed to deep in the technological effects of these new ingredients with highly probed bioactivities.
