Wood Waste from Fruit Trees: Biomolecules and Their Applications in Agri-Food Industry

  Citation: Aliaño-González, M.J.; Gabaston, J.; Ortiz-Somovilla, V.; Cantos-Villar, E. Wood Waste from Fruit Trees: Biomolecules and Their Applications in Agri-Food Industry. Biomolecules 2022,12, 238. https:// doi.org/10.3390/biom12020238 Academic Editors: Carmen Ancìn-Azpilicueta, Irene Esparza Catalán and Nerea Jiménez-Moreno Received: 8 January 2022 Accepted: 27 January 2022 Published: 1 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biomolecules Review Wood Waste from Fruit Trees: Biomolecules and Their Applications in Agri-Food Industry Maria Jose Aliaño-González 1,2 , Julien Gabaston 3, Victor Ortiz-Somovilla 4and Emma Cantos-Villar 1,* 1IFAPA Rancho de la Merced, Consejería de Agricultura, Ganadería, Pesca y Desarrollo Sostenible, Junta de Andalucía, 11471 Jerez de la Frontera, Spain; [email protected] 2Departamento de Química Analítica, Facultad de Ciencias, Universidad de Cádiz, 11510 Cadiz, Spain 3 Departamento de Nutrición y Bromatología, Toxicología y Medicina Legal, Área de Nutrición y Bromatología, Facultad de Farmacia, Universidad de Sevilla, 41012 Seville, Spain; [email protected] 4IFAPA Alameda del Obispo, Consejería de Agricultura, Ganadería, Pesca y Desarrollo Sostenible, Junta de Andalucía, Avenida Menéndez Pidal, 14004 Córdoba, Spain; victor[email protected] *Correspondence: [email protected]; Tel.: +34-671560353 Abstract: In the European Union (EU), a total of 11,301,345 hectares are dedicated to the cultivation of fruit trees, mainly olive orchards, grapevines, nut trees (almond, walnut, chestnut, hazelnut, and pistachio), apple and pear trees, stone fruit trees (peach, nectarine, apricot, cherry, and plum), and citrus fruit trees (orange, clementine, satsuma, mandarin, lemon, grapefruit, and pomelo). Pruning these trees, together with plantation removal to a lesser extent, produces a huge amount of wood waste. A theoretical calculation of the wood waste in the European Union estimates approximately 2 and 25 million tons from wood plantation removal and pruning, respectively, per year. This wood waste is usually destroyed by in-field burning or crushing into the soil, which result in no direct economic benefits. However, wood from tree pruning, which is enriched in high added-value molecules, offers a wide spectrum of possibilities for its valorization. This review focuses on the contribution of wood waste to both sustainability and the circular economy, considering its use not only as biomass but also as a potential source of bioactive compounds. The main bioactive compounds found in wood are polyphenols, terpenes, polysaccharides, organic compounds, fatty acids, and alkaloids. Polyphenols are the most ubiquitous compounds in wood. Large amounts of hydroxytyrosol (up to 25 g/kg dw ), resveratrol (up to 66 g/kg dw), protocatechuic acid (up to 16.4 g/kg), and proanthocyanins ( 8.5 g/kg dw ) have been found in the wood from olive trees, grapevines, almond trees and plum trees, respectively. The bioactivity of these compounds has been demonstrated at lower concentrations, mainly in vitro studies. Bioactive compounds present antioxidant, antimicrobial, antifungal, biostimulant, anti-inflammatory, cardioprotective, and anticarcinogenic properties, among others. Therefore, wood extracts might have several applications in agriculture, medicine, and the food, pharmaceutical, nutraceutical, and cosmetics industries. For example, olive tree wood extract reduced thrombin-induced platelet aggregation in vitro ; grapevine tree wood extract acts a preservative in wine, replacing SO 2 ; chestnut tree wood extract has antifungal properties on postharvest pathogens in vitro ; and stone tree wood extracts are used for aging both wines and brandies. Moreover, the use of wood waste contributes to the move towards both a more sustainable development and a circular economy. Keywords: wood waste; olive trees; grapevine; nut trees; stone fruit trees; pome trees; citrus fruit trees; bioactive compounds; phenolic compounds; circular economy 1. The Abundance and Importance of Fruit Trees in Europe In the EU, a total of 11,301,345 hectares (ha) are dedicated to the cultivation of fruit trees (Figure 1A). Olive orchards cover the largest area, with more than 5 million hectares (45% of the total area), followed by vineyards, with more than 3 million ha (28%). Almond trees and other nuts are the third most abundant (11%), with more than 1,200,000 ha. Biomolecules 2022,12, 238. https://doi.org/10.3390/biom12020238 https://www.mdpi.com/journal/biomolecules Biomolecules 2022,12, 238 2 of 46 Meanwhile, 611,540 ha (5%) are dedicated to apple and pears trees 609,710 ha (5%) are dedicated to stone fruits (peaches, nectarines, apricots, cherries, and plums), with citrus fruits covering 502,366 ha (4%). The last group of fruit crops includes figs, avocados, kiwis, other tropical fruits, and bananas, which cover 141,000 ha (around 1.3%). Figure 1. ( A ) Fruit tree area in the EU and ( B ) distribution of fruit trees areas in EU countries ( 1B ) [ 1 ]. Regarding the distribution of the fruit growing area in the EU member countries, Spain stands out with 43% of the total surface, followed by Italy with 21%, Greece with 10%, France with 8%, and Portugal with 6% (Figure 1B). Olives and grapes are the two major perennial crop systems traditionally grown in the Mediterranean Basin. Regarding olive production , Spain is by far the largest producer, responsible for over 50% of EU production, followed by Italy, Greece, and Portugal (Figure 2A) . Other leading olive-producing countries by annual metric tons are France and Croatia. Grapes are an important cultural, economic, and ecological feature of the Mediterranean Basin but also a cosmopolitan crop, with the largest acreage and the highest economic value among fruit crops globally. The EU members with the highest grapevine surfaces are Spain, France, and Italy (Figure 2A). The production of edible nuts in Europe has gradually increased since 2013. The mostproduced edible nuts in Europe are hazelnuts and almonds, while walnuts, chestnuts, and pistachio nuts are produced to a lesser extent. Spain, the Netherlands, and Italy are major producers of processed edible nuts. In terms of primary production (growing, harvesting, and drying), the largest European producers are Spain (led by almonds), Italy (hazelnuts), Portugal (chestnuts), and France (walnuts) (Figure 2B). Biomolecules 2022,12, 238 3 of 46 Thousands of varieties of apples are grown worldwide, many of which have been created and selected to grow in varied climates. This has enabled commercial apple production to take place in almost all member states. Just over a quarter (26.6%) of the EU-27 0 s harvested apple production came from Poland in 2019. The other principal appleproducing member states were Italy (19.9%) and France (15.1%) (Figure 2B). Figure 2. ( A ) Distribution of olive trees and grapevines and, ( B ) nut trees, apple and pear trees, stone fruit trees, citrus fruit trees, and other trees by European countries (ha). By contrast, the production of oranges and peaches is much more restricted by climatic conditions, and about 93% of these crops produced in the EU-27 come from Spain, Italy, and Greece (Figure 2B). The information presented above means that Spain can be considered an important fruit producer. Moreover, apart from the fruits, other aspects may be considered. The total area of fruit tree cultivation in the EU is 11.33 Mha, thus making the EU an important producer of wood waste from fruit trees, due to their annual pruning. A theoretical calculation of the potential pruning availability of the EU estimates approximately 25 million tons (MT) per year of wood dry weight [ 2 ]. More specifically, 8 MT of wood waste is produced in Spain every year. When considering a fruit tree plantation managed according to sustainability criteria, a wide range of ecosystem services can be detected. Indeed, apart from fruit production, these services may include climate change mitigation by reducing greenhouse gas (GHG) emissions, fossil fuel consumption and increasing carbon sequestration, soil nutrient recovery and cycling, improving water use efficiency and water regulation, preventing run-off and controlling erosion, the biological control of pests and diseases, biodiversity, pollination, and many others [2]. Biomolecules 2022,12, 238 4 of 46 The following section focuses on the contribution of wood waste to both sustainability and the circular economy, considering its use not only as biomass but also as a potential source of bioactive compounds. 2. The Contribution of Wood Waste to a Circular Economy and Sustainable Bioeconomy According to the United Nations, the global population is expected to increase from 7.7 billion (2019) to 9.7 billion in 2050 [ 3 ]. This prospect raises several concerns about the global consumption of biomass, fossil fuels, metals, and minerals, which is likely to double, and annual waste production, following the current trend, will increase by 70% in the next 40 years [ 4 ]. Undoubtedly, these premises challenge the move toward both a more sustainable development and a circular economy. The circular economy is a model of production and consumption that involves sharing, leasing, reusing, repairing, refurbishing, and recycling existing materials and products as much as possible, thereby extending the life cycle of products. Within an envisioned circular bio-based economy, a key component is the valorization of biomass wastes and residues into valuable products. In practice, it implies reducing waste to a minimum. When a product reaches the end of its life, its materials are kept within the economy wherever possible. These can be productively used again and again, thereby creating further value [ 5 ]. The EU is set to transition from a linear to a circular economy, turning waste into a resource in order to increase resource efficiency and close the loop in a circular economy [6]. Wood comes mainly from forest biomass [ 7 ]. The main wood-based products are: (i) cork and wood, (ii) pulp and waste paper, (iii) cork and wood manufacturers (excluding furniture), and (iv) paper, paperboard, and by-products [ 8 ]. However, in other cases, wood is not produced biomass but, rather, residual biomass, which is the case of wood from the pruning of fruit trees. Fruit tree pruning is an important process in the management of many orchards. Its main purpose is to facilitate crown shaping for optimal fruit production and efficient harvesting. A properly formed crown is able to produce and bear a large fruit yield [ 9 ]. Pruning is performed at least once a year to promote continual and optimal production through the control of tree physiology. In fact, by cutting tree shoots, the number of fruits per plant is reduced, thus allowing a better distribution of nutrients throughout trees, an effective canopy light exposition, and an optimal bearing potential of re-growth, therefore achieving quantitative and qualitative improvements in fruit crops [10]. Great differences in wood production can be found between fruit trees and even in the same fruit tree. Wood waste production depends on factors, such as the type of crop, variety and age, the form of the tree, density, the type of pruning, the climate and soil conditions, and other agronomic operations, such as irrigation [ 11 , 12 ]. On average, annual pruning from crops in good climatic and agronomic conditions can usually produce from 0.5 to 2.0 tons/ha of wood (dry weight, dw). Considering the current fruit tree area in Europe (11.33 Mha), around 25 MT of wood dw are currently produced each year. This wood must be removed for both disease control and to facilitate future tending activities. Consequently, it is usually destroyed by in-field burning or by crushing onto the soil, which results in no direct economic benefits [ 12 ]. Collecting the wood from pruning poses a series of logistical difficulties due to several factors: (i) its dispersion in the territory, (ii) the size and layout of the plantations, and (iii) the production of biomass per hectare is low in comparison to forestry wood [13]. However, this waste has been proposed as an excellent biomass source. Biomass from pruning, being a residual material, does not create any additional demand for land and can deliver substantial GHG emission savings compared to fossil fuels, reducing both the pressure on fossil fuel reserves and energy dependence. Pruning residues, when removed from the cultivated field, can be directed to energy conversion to obtain renewable forms of energy (heat and power), thus offering a supplemental “provisioning” service. This option is of relevance, considering that replacing the use of fossil fuels with renewables is a Biomolecules 2022,12, 238 5 of 46 relevant international goal, within the frame of the former Kyoto protocol (from 2005 to 2020) and the Paris agreement thereafter (from 2020 to 2030 and 2050). With the use of agroforest waste as biomass, fossil fuels are being displaced by renewable energy sources, and, consequently, the concentration of carbon dioxide is gradually reduced from the atmosphere, due to the “carbon neutrality” of biomass feedstock, thus mitigating climate change [2,14]. Moreover, wood from tree pruning offers a wide spectrum of possibilities for their reuse and valorization, since they are still enriched in high added-value molecules, as foreseen by the circular economy [ 15 ]. Plants, in particular, produce secondary metabolites, which are not directly involved in the basic functions of growth, development, and reproduction of the organism but are essential for long-term survival and play multiple roles, including defense against predators or the attraction of pollinators. The above metabolites are endowed with numerous biological activities, making them also extremely important for human health and well-being. Moreover, due to their chemical and biological properties, applications for these metabolites have also been found in many other fields, serving as pigments, cosmetics, antioxidants, antifeedants, and so on [16]. Wood from plantation removal is usually produced when vines and olive or fruit trees are cleared out at the end of the lifetime of a plantation. In some cases, the termination of a plantation is driven by changes in the food market (in order to grow a new fruit or grape variety), by agricultural policies (for modernization and reconversion of plantations), or by other particular reasons (plague/disease, farmer, or exploitation manager). Considering the ha of different fruit trees and their half-life time, between 5–20 T dw/ha are discarded from trees, which may reach more than 2 MT yearly. As for pruning, the wood from plantation removals is mostly under-utilized in Europe [ 17 ], although the traditional use of firewood from the aerial part of the tree may be usual in some areas. In such cases, the stumps, roots, and thin branches are not used. In many cases, the whole tree is just uprooted, piled with others, and burned in the open air. In summary, wood is a valuable waste product that is available in huge amounts every year, and its valorization deserves to be studied. 3. The Main Fruit Trees in Europe: Bioactive Compounds from Wood and Applications Orchards could also be ‘exploited’ as a starting material through extracting secondary metabolites with high intrinsic values, due to their wide range of biological activities. For instance, several studies have demonstrated the potential uses of the secondary metabolites derived from agricultural wood extracts, including the antioxidant compounds present in vineyard wood and the radical scavenging properties of extracts derived from olive tree wood, as detailed below. In general, the following schema of wood compounds and applications can be established (Figure 3). 3.1. Olive Trees Olive trees (Olea europaea) are small trees between 8 and 15 m tall with life expectancies of up to 1000 years, the oldest one dating from 2000 years ago [ 18 ]. Throughout history, the fruit of these trees has been appreciated and consumed as table olives and incorporated into bread, soups, salads, etc. In addition, oil obtained from these olives is one of the most consumed agricultural products around the world. In fact, the total harvested production of olives just for oil in the European Union in 2019 was 9.8 million tons, of which 5.6 million tons were collected in Spain [ 19 ]. This positions olive trees as one of the trees with the largest productive areas in the European Community, with Spain being the country with the highest productive area in the EU, as described in the introduction section. Biomolecules 2022,12, 238 6 of 46 Figure 3. Bioactive compounds present in olive, grapevine, nut, stone fruit, pome fruit, and citrus fruit wood waste, main bioactive properties, and applications reviewed in the current work. Biomolecules 2022,12, 238 7 of 46 The production of olive wood discarded during olive pruning could vary from 3.5 to 10.5 kg/tree each year, which results in between 1.31 and 3.02 tons of discarded wood/ha [ 12 , 20 ]. In Spain, this is around 7 million tons of wood per year that is not leveraged. For the above reasons, many authors have investigated olive wood composition to propose extra uses for it. 3.1.1. Bioactive Compounds from Olive Wood Waste Olive trees contain a significant amount of carbohydrates, such as cellulose, hemicellulose, and lignin [ 21 ]. In fact, the olive tree has 32% more cellulose than other materials currently used in biorefinery, such as birchwood (26%), peanut shells (25%), or waste tea (17%) [ 22 – 24 ]. Within the carbohydrates group, different sugars have been identified in olive wood, in particular fructose, galactose, and glucose [ 25 – 27 ], explaining why it has been employed as an energy source in a multitude of situations. In addition, many interesting compounds have been isolated from olive tree wood, including lignans, fatty acids, nitrogen compounds, and iridoid [ 15 , 28 – 30 ]. These compounds are of great interest, as they exhibit properties related to plant and even human health. However, the quantification of these compounds in olive tree wood and, consequently, the evaluation of these properties has not been performed yet. Ortega-García et al. [ 26 ] extracted different phenolic compounds from Olea europaea L. cv. Picual wood using solid–liquid extraction. The extracts were analyzed by HPLC, and the results showed the presence of hydroxytyrosol (1500–2120 mg/kg dw), tyrosol (1720– 3640 mg/kg dw ), and oleuropein (22,300–58,020 mg/kg dw). These compounds have a crucial role in the nutritional and organoleptic properties of olives and olive oil. In addition, they present an important action in defending plants against pathogens and herbivores [ 31 , 32 ]. Finally, these three compounds have exhibited significant antioxidant properties, even preventing oil oxidation during storage and inhibiting the growth of Gram-positive microorganisms [ 33 ]. Pérez-Bonilla et al. [ 34 ] also evaluated the presence of different phenolic compounds from olive tree wood and analyzed their DPPH values (%) . A total of six phenolic compounds were found: hydroxytyrosol (76.7%), tyrosol (3.7%), cycloolivil (19.7%), 7-deoxyloganic acid (6.8%), oleuropein (20.4%), and ligustroside (7.4%). Luján et al. [ 35 , 36 ] evaluated the presence of other phenolic compounds in wood from the Arbequina, Empletre, and Picual olive tree varieties using ultrasound- and microwaveassisted extraction coupled to analytical techniques, such as gas or liquid chromatography and mass spectrometry. The authors reported the following concentrations of phenolic compounds: verbascoside (1338–1681 mg/kg dw), apigenin-7-glucoside ( 10–16 mg/kg dw ), and luteolin-7-glucoside (167–202 mg/kg dw). Faraone et al. [ 20 ] studied the presence of phenolic compounds in olive tree wood using GC-MS. The results exhibited a total phenolic compound content of 156.040 mg/kg dw and antioxidant activity of 188.840 mg Trolox equivalent (TE)/kg dw. In addition, 22 organic compounds (alcohols, aldehydes, and ketones), 13 phenolic acids, 2 benzenoid aromatics, 1 pyranone, and 3 amines were identified in olive trees. Allopurinol was the most concentrated organic compound. Lastly, Salido et al. [ 37 ] also analyzed the composition of wood extracts from 10 Spanish olive cultivars and found the following 16 phenolic compounds: (-)-olivil, (-)-olivil 4-O- β -D- glucopyranoside, 2”-hydroxyoleuropein, isojaspolyoside A, jaspolyanoside, jaspolyanoside isomer, jaspolyoside, jaspolyoside isomer, ligustroside, oleoside 11-methyl ester, (+)-1- hydroxypinoresinol 1-O- β -D-glucopyranoside, 7-deoxyloganic acid, eriodictyol, hydroxytyrosol, and tyrosol. The most abundant compounds were (-)-olivil and ligustroside (Table 1). However, as far as we know, these properties have not been evaluated in extracts obtained from olive tree wood. Biomolecules 2022,12, 238 8 of 46 Table 1. Main bioactive compounds from olive tree wood and their applications. * n.d.: not detected. ** ↑ : Increase. Chemical structures by families are shown in Figure S1. Family Compounds (Concentration or Relative Area) Properties and Applications References Olive wood Polyphenol Benzenoid aromatic 20-formyl-2,3,4,40-trimethoxy-1,10-biphenyl (n.d.–0.72%) 4-(hydroxyacetyl)-1,10-biphenyl (n.d.–2.1%) Crucial role in nutritional and organoleptic properties Defense against pathogens and herbivores ↑Antioxidant activity (188.840 mg TE/kg dw) ↑Antimicrobial properties (IC50: 80%) ↑Enzyme inhibition related to Alzheimer’s pathology ↑Reduction of platelet aggregation in vitro DPPH: 116.5 mg TE/g CUPRAC: 192.91 mg TE/g FRAP: 105.23 mg TE/g [20,26,34–37] Flavanone eriodictyol (3000.0–71,030.0 mg/kg dw) Flavone apigenin-7-glucoside (10.0–15.9) luteolin-7-glucoside (167.0–202.0 mg/kg dw) Phenolic acids 3,4-dihydroxybenzeneacetic acid (n.d.–10.0%) 3,4-dihydroxy-benzeneacetic acid (5.1–5.6%) 3,5-dimethoxy-4-hydroxyphenylacetic acid (n.d.–6.7%) 3-formylbenzoic acid (0.9–1.6) 3-hydroxy-4-methoxybenzoic acid (n.d.–1.22%) 3-hydroxy-butanoic acid (n.d.–0.1%) 4-methoxy-40,50-methylenedioxybiphenyl-2-carboxylic acid (n.d.–1.0%) 4-methoxy-40,50-methylenedioxybiphenyl-2-carboxylic acid (n.d.–0.9%) 7-deoxyloganic acid (n.d.–1880.0 mg/kg dw) docosanoic acid (0.1–0.2%) eicosanoic acid (0.3–0.5%) hexadecanoic acid (0.7–1.2%) octadecanoic acid (0.1–2.6%) oleic acid (1.0–2.0%) Phenylethanoid hydroxytyrosol (1500.0–2920.0) tyrosol (1720.0–3640.0) Phenylpropanoids verbascoside (1338.0–1681.0) Biomolecules 2022,12, 238 9 of 46 Table 1. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Olive wood Alcohols, Aldehydes, and Ketones 1-(2,4,6-trihydroxy-3-methylphenyl)-1-butanone (1.6–1.8) 1-(2,4,6-trihydroxyphenyl)-2-pentanone (n.d.–1.7) 2,6-dimethoxyphenol (0.1–0.7) 2-methoxy-4-vinylphenol (0.4–0.82) 2-methoxyphenol (0.1–0.5) 2-methyl-6-methylene-7-octen-2-ol (n.d.–3.4) 2-methyl-6-methylene-7-octen-2-ol (n.d.–3.8) 3,4-dihydroxyphenyl-2-propanone (n.d.–3.9) 3-methyl-1-(2,4,6-trihydroxy-3-methylphenyl)-1-butanone (n.d.–2.9) 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol (5.5–9.4) 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol (n.d.–17.4) 4-(4-hydroxyphenyl)-2-butanone (n.d.–1.9) 4,40-methylenebis[2,6-dimethoxyphenol] (n.d.–0.2) 4-ethoxy-2,5-dimethoxybenzaldehyde (n.d.–5.7) 4-ethoxy-2,5-dimethoxybenzaldehyde (0.5–0.6) 4-hydroxy-2-methylacetophenone (n.d.–0.2) 4-hydroxy-3,5-dimethoxybenzaldehyde (n.d.–0.8) 4-hydroxybenzeneethanol (1.8–3.7) allopurinol (8.1–11.4) benzaldehyde (0.1–0.7) catechol (n.d.–0.5) ↑Antioxidant activity (Relative Antioxidant Capacity Index: 0.53) DPPH (%): 188.84–26.40 mg TE/g FRAP: 408.80–400.31 mg TE/g [20] Amines 1-methyl-N-vanillyl-2-phenethanamine (n.d.–10.3%) 2,6-dimethyl-4-pyrimidinamine (n.d.–7.8%) N-formyltyramine (n.d.–1.6%) Crucial role in nutritional and organoleptic properties [20] Secoiridoids (−)-olivil (12,020.0–51,810.0 mg/kg dw) (−)-olivil 4-O-β-D-glucopyranoside (5300.0–15,980.0 mg/kg dw) 2”-hydroxyoleuropein (11,920.0–29,610.0 mg/kg dw) isojaspolyoside A (n.d–12,480.0 mg/kg dw) jaspolyanoside (n.d.–13,030.0 mg/kg dw) jaspolyanoside isomer (n.d.–12,130.0 mg/kg dw) jaspolyoside (n.d.–11,220.0 mg/kg dw) jaspolyoside isomer (n.d.–13,400.0 mg/kg dw) ligustroside (13,280.0–21,450.0 mg/kg dw) oleoside 11-methyl ester (n.d.–3760.0 mg/kg dw) oleuropein (22,300.0–58,020.0) Defense against pathogens and herbivores ↑Antimicrobial properties (IC50: 80%) ↑Enzyme inhibition related to Alzheimer’s pathology ↑Reduction of platelet aggregation in vitro [20,37] Biomolecules 2022,12, 238 16 of 46 vanillic acid, and syringic acid. To a lesser extent, the almond wood biomass also contains ferulic acid and p-coumaric acid, as well as the cinnamaldehydes sinapinaldehyde and coniferyl aldehyde. Focusing on polysaccharide content, almond wood waste contains 22.3% glucan (glucose polymer) and 11.1% xylan (xylose polymer), making a total of 33.4% (dried biomass) fermentable sugars. Furthermore, almond pruning residues were also composed of lignin with a value of 20.0% of biomass [84]. Application of Almond Wood Waste in the Agri-Food Industry Few data on the bioactivities of almond pruned wood were available in the literature, but some studies investigated the properties of phenolic acids present in woody biomass and isolated from other almond by-products, such as the seeds or aerial parts (stems and leaves). In this sense, the 4-hydroxybenzoic, protocatechuic, and vanillic acids isolated from almond seeds showed an antibacterial effect against two Gram-positive bacteria ( S. aureus and Bacillus subtilis) and five Gram-negative bacteria (P. aeruginosa,S. typhimurium, Proteus vulgaris ,E. coli, and K. pneumoniae) [ 85 ]. The protocatechuic acid isolated from almond skin also inhibited Helicobacter pylori (MIC range, 128 to 512 mg/L), which is a Gramnegative bacterium known to be a gastric pathogen of humans (responsible for chronic gastritis, peptic ulcers, intestinal metaplasia, and cancer) [ 86 ]. Moreover, protocatechuic acid and p-hydroxybenzoic acid, followed by vanillic acid and syringic acids, exhibited antioxidant capacities through a DPPH assay with values from 4.5 to 12.9 µ mol TE/g [ 87 ]. Vanillic acid was also involved in the inhibition of the β -glucosidase enzyme, which is involved in various metabolic processes and is part of promising therapeutics in the treatment of related diseases [ 88 ]. Overall, almond wood waste, rich in polyphenols, especially phenolic acids, might be valorized for medical applications, in cosmetic products, or as food additives through their antibacterial and antioxidant properties. 3.3.2. Walnut Wood Waste Bioactive Compounds from Walnut Wood Waste In wood from pruned walnut trees, polyphenols were the main compounds, especially the phenolic acids gallic acid, ellagic acid, and ferulic acid, as well as the flavonol quercetin- 3- β -D-glucoside [ 89 ]. In a water extract of pruned wood, gallic acid reached 98.0 mg/kg, ferulic acid reached 48.2 mg/kg, and quercetin-3- β -D-glucoside reached 38.3 mg/kg. In an ethanolic extract, the major compounds were quercetin-3- β -D-glucoside, with 150.2 mg/kg, and ellagic acid, with 101.4 mg/kg [ 89 ]. In a methanolic extract of walnut sawdust, the amount of ellagic acid reached 393.0 mg/kg (Table 3) [90]. Wood from pruned walnut trees also contained other phenolic acids, such as chlorogenic, neochlorogenic, p-coumaric, protocatechuic, caffeic, vanillic, syringic, and salicylic acids [ 84 , 89 , 91 ]. Other flavonoids were also characterized, such as kaempferol, quercetin, epicatechin, and procyanidin B1 and B2 [ 89 , 91 ]. In addition, this wood was composed of phenolic aldehydes (vanillin, 4-hydroxybenzaldehyde and 3,4-dihydroxybenzaldehyde) and cinnamaldehydes (coniferyl aldehyde, sinapinaldehyde, and 4-hydroxycinnamylaldehyde) (Table 3) [84,91]. Apart from polyphenols, walnut pruned wood contained polysaccharides, with 26.5% glucan, 10.6% xylan, and 20.0% lignin content [ 84 ]. Several organic compounds were also identified by GC-MS, such as elemicin, which is a phenylpropene, the terpenoid curzerene, trans-benzylidenacetone, and the esters butyl palmitate and isobutyl stearate (Table 3) [ 91 ]. Application of Walnut Wood Waste in the Agri-Food Industry Examining the antioxidant activities of pruned wood, Fernández-Agullóet al. [ 89 ] tested aqueous and hydro-ethanolic extracts through DPPH, FRAP, and ABTS assays. The extracts demonstrated a radical scavenging activity (EC 50 values of 95–180 mg/L), reducing power (0.9–1.7 µ mol ascorbic acid eq/mg), and lipid peroxidation inhibition (EC 50 values of 757–1232 mg/L). Biomolecules 2022,12, 238 17 of 46 Another study investigated the antifungal property of wood trunk of walnut against Trametes versicolor, a white-rot fungus involved in poplar wood disease. Samples of poplar wood were impregnated with walnut wood extract, followed by exposition to T. versicolor for 14 weeks. The lowest wood weight loss was observed in the samples treated with a 1.5% concentration of extractive solution, obtaining value of 30.4%, while the highest weight loss was measured in the untreated control (36.9%). Therefore, the authors concluded that walnut wood was able to slow down the effect of the fungus attack [92]. Furthermore, a recent study investigated the evolution of the phenolic composition and sensory characteristics of a red wine stored for 30 days in contact with toasted chips from walnut, a non-conventional wood species in enology. The wine kept in contact with walnut chips showed significantly higher values for the oligomeric fraction of proanthocyanidins, whereas, from a sensory point of view, a tendency for lower scores for most sensory descriptors was obtained [93]. Apart from studies developed on walnut extracts, the major compounds of walnut wood were also tested through their isolation from wood or other walnut by-products. Regarding immunomodulatory function, a study highlighted that gallic acid and ellagic acid decreased lymphocyte T-cell activation/growth and the secretion of interleukins. These data suggest that supplementation of gallic and ellagic acids to walnut-containing foods might ameliorate walnut-induced allergic reaction [ 94 ]. Related to inflammatory diseases, gallic acid exhibited inhibitory activity on the nitric oxide production of macrophages. Nitric oxide is one of the most important reactive nitrogen species and constitutes an important target in the inflammatory response/anti-inflammatory treatment [ 95 ]. Gomes et al. [96] argued also that ellagic acid glycoside and quercetin derivatives from walnut may exert an antibacterial effect against S. aureus and may be proposed as promising agents for the reduction of the occurrence of dairy food industry contaminations. 3.3.3. Chestnut Wood Waste Bioactive Compounds from Chestnut Wood Waste Among all nut trees, chestnut wood is certainly the most studied, and wood trunk extracts are already commercially available (Farmatan ® , Tanex ® , and Silvafeed ® ). This success and this recognition are due to its composition of polyphenols and, more precisely, in hydrolysable tannins. Among hydrolysable tannins, two main classes are distinguished, ellagitannins, which contain a hexahydroxydiphenoyl unit and give rise to ellagic acid by hydrolysis, and gallic acid derivatives [ 97 ]. Owing to recent technological advances, especially in mass spectrometry, the deciphering of the chemical composition of chestnut wood is more developed. In recent studies, several ellagitannin derivatives were identified, such as ellagic acid, ellagic acid pentoside, vescalin, vescalagin, castalin, castalagin, punicalagin, pedunculagin, roburin A-E, (mono-, ditri-, tetra-, and penta-)galloyl glucose, valoneic acid dilactone, and castavaloninic acid [ 97 , 98 ]. Gallic acid was also characterized. In addition to these compounds, other ellagitannins and gallic acid derivatives were putatively identified with the proposed structures supported by molecular formula information, HR-MS fragmentation, and UV data [ 97 ]. From a quantitative point of view, the phenolic fraction of chestnut wood extracts could vary between 2–4% of dw, depending on both the solvent and temperature employed for the extraction [99] (Table 3). Biomolecules 2022,12, 238 18 of 46 Table 3. Main bioactive compounds from nut tree wood and their applications. * ↑: Increase; ↓: Decrease. Chemical structures by families are shown in Figure S1. Family Compounds (Concentration or Relative Area) Properties and Applications References Almond wood Polyphenol Phenolic acid protocatechuic acid (16.4 g/kg) 4-hydroxybenzoic acid (12.1 g/kg) salicylic acid (11.4 g/kg) vanillic acid (7.9 g/kg) syringic acid (3.6 g/kg) ferulic acid (1.4 g/kg) p-coumaric acid (1.4 g/kg) ↑in vivo learning and memory performance ↑antibacterial activity ↑antioxidant capacity: DPPH = 4.5–12.9 µmol TE/g ↓β-glucosidase enzyme [84–88] Organic compound Cinnamaldehyde sinapinaldehyde (0.7 g/kg) coniferyl aldehyde (1.4 g/kg) Polysaccharide - glucan (223 g/kg) xylan (111 g/kg) Walnut wood Polyphenol Phenolic acid gallic acid (19.8–98.0 mg/kg) ellagic acid (101.4–393.0 mg/kg) ferulic acid (48.2 mg/kg) chlorogenic acid (1.8–4.9 mg/kg) neochlorogenic acid (1.3–4.5 mg/kg) p-coumaric acid (1.9–3.3 mg/kg) protocatechuic acid caffeic acid vanillic acid syringic acid salicylic acid ↑antioxidant capacity: DPPH = 95–180 mg/L FRAP = 0.9–1.7 µmol AAE/mg BCB = 757–1232 mg/L ↑antifungal activity ↑antibacterial activity wine aging: ↑proanthocyanidins ↓wine sensory descriptors ↑anti-inflammatory capacity: ↓lymphocyte T-cell activation/growth ↓secretion of interleukin-4,-10,-γ ↓nitric oxide [84,89,91–96] Flavonol quercetin-3-β-D-glucoside (38.3–150.2 mg/kg) quercetin (0.2 mg/kg) kaempferol (0.1 mg/kg) Flavanols epicatechin (0.1–0.6 mg/kg) procyanidin B1-B2 (0.1 mg/kg) Aldehydes Vanillin 4-hydroxybenzaldehyde 3,4-dihydroxybenzaldehyde Cinnamaldehyde coniferyl aldehyde sinapinaldehyde 4-hydroxycinnamylaldehyde Biomolecules 2022,12, 238 19 of 46 Table 3. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Terpene Sesquiterpene Curzerene Fatty acid Ester butyl palmitate isobutyl stearate Phenylpropanoïde Phenylpropene Elemicin Polysaccharide - glucan (265 g/kg) xylan (106 g/kg) Chestnut Wood Polyphenol Ellagitannin ellagic acid (461.4 g/kg) vescalin (68.5 g/kg) vescalagin (13.0 g/kg) castalin (77.2 g/kg) castalagin (11.8 g/kg) 1-O-trihydroxibenzene castalagin punicalagin pedunculagin roburin A–E galloyl glucose valoneic acid dilactone castavaloninic acid ↑antioxidant capacity: DPPH scavenging = 1234 mg TE/g) FRAP = 2263 mg TE/g ↑oxidative stability of meat by adding extract in pig diet ↑antioxidant status and cholesterol metabolism by adding extract in broiler diet ↑modulation of the gut microbiota ↑antifungal activity ↑antibacterial activity ↑wine aging with high value phenolic compounds ↑anticarcinoma activity towards breast cancer [100–108] Gallotannin gallic acid (4.5 g/kg) Terpene Triterpene bartogenic acid bartogenic acid 28-O-β-D-glucopyranosyl ester chestnoside A–B Monoterpene dehydrovomifoliol (2.72 mg/kg) 3-oxoretro-α-ionol (2.39 mg/kg) Organic compound Lactone β-methyl-γ-octalactone (30.2 mg/kg) Aliphatic aldehyde nonanal (3.3 mg/kg) 3-octen-1-ol (0.6 mg/kg) Biomolecules 2022,12, 238 20 of 46 Table 3. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Aromatic aldehyde syringaldehyde (61.4 mg/kg) vanillin (28.1 mg/kg) sinapinaldehyde (19.4 mg/kg) propiovanillone (12.2 mg/kg) butyrovanillone (6.5 mg/kg) trans-isoeugenol (7.03 mg/kg) methyl syringate (5.8 mg/kg) Furane furfural (0.6 mg/kg) 5-methyl-furfural (0.16 mg/kg) 5-hydromethylfurfural (0.39 mg/kg) Biomolecules 2022,12, 238 21 of 46 Apart from polyphenols, some terpenes were also identified in chestnut wood, such as the triterpenoids bartogenic acid, bartogenic acid 28-O- β -D-glucopyranosyl ester, chestnoside A, and chestnoside B [ 100 ]. Using subcritical water extraction, Alarcon et al. [ 101 ] reported that chestnut wood not only contained flavoring compounds, such as terpenoids and norisoprenoids (dehydrovomifoliol), but also lactones ( β -methyl- γ -octalactones), aliphatic aldehydes (nonanal), and benzenic (vanillin derivatives) compounds. Application of Chestnut Wood Waste in the Agri-Food Industry Over the past five years, the biological activity of chestnut wood remains an attractive and highly studied topic, owing to the presence of hydrolysable tannins and other compounds. Regarding the antioxidant abilities of chestnut wood extracts, Faraone et al. [ 102 ] showed a high reducing ability of radical DPPH (1234 mg TE/g) and ferric reducing antioxidant power (2263 mg TE/g). Additionally, Tomažin et al. [ 103 ] studied the oxidative stability of meat by adding chestnut wood extract to the diet of pigs. Among various indicators, the main effects observed were an increase in the water holding capacity and a positive color evolution of dry-cured bellies, due to the antioxidant supplementation. Similarly, Liu et al. [104] investigated the use of chestnut wood extract as an alternative feed additive in the broiler diet and demonstrated increases in antioxidant status, cholesterol metabolism, and growth performance without affecting normal meat quality. Molino et al. [ 105 ] also exhibited that chestnut wood positively modulated the gut microbiota using an in vitro digestion-fermentation assay. Furthermore, another study showed the antifungal activity of chestnut wood extract against different filamentous fungi of agronomical and food interest, such as telluric phytopathogens (Fusarium oxysporum,Fusarium solani,Rhizoctonia solani, and Sclerotium rolfsii) and post-harvest pathogens (Botrytis cinerea,Penicillium digitatum, and Penicillium italicum) [ 106 ]. Hydrolysable tannins from chestnut wood extract also showed antibacterial activities by inhibiting the growth of S. aureus,Streptococcus uberis,P. aeruginosa,Streptococcus agalactiae,E. coli, and K. pneumoniae (concentration > 630 mg/mL) [107]. Chestnut wood barrels have also been used in the aging of wine brandies. In fact, a transfer of compounds occurs between the wood and beverage, influencing the physicochemical and sensory properties of the wine. Wine aged in chestnut barrels is distinguished by a higher phenolic content and higher levels of some low molecular weight compounds, such as gallic acid, vanillic acid, vanillin, and volatile phenols [108]. Apart from polyphenols, other compounds from chestnut have been also studied. In fact, the cytotoxicity of triterpenoids isolated from C. sativa wood was investigated against two cancer cell lines (PC3 and MCF-7). The results showed that the terpene chestnoside B exhibited a stronger cytotoxicity (IC 50 of 12.3 mM) towards MCF-7 cells than positive controls, highlighting the potential of triterpenoids from chestnut wood to prevent breast cancer [100]. Even though chestnut wood is commercialized nowadays, especially in the field of enology, and may not be considered agricultural waste to such an extent, other applications should be promoted. Chestnut wood could be valorized for pharmaceutical purposes or as a food additive, due to its antioxidant, anti-carcinoma, antifungal, and antibacterial properties. 3.4. Apple and Pear Trees According to European statistics, the pome fruits are the fourth most common species cultivated around the European Union, covering over 600,000 ha, with around 9 MT being produced every year. Among the pome genus, apples and pears are the main fruits consumed. Apples represent 80% of the total pome production, and pears the remaining 20% (Figure 5). Biomolecules 2022,12, 238 22 of 46 Figure 5. Proportion of pome production in Europe. At pruning, between 2–5 tons/ha are produced from apple and pears trees each year. Up to 6.8 and 5.6 kg wood/ha have been described in the Czech Republic [ 109 ]. If we add the fallen wood from climatic events or fruit harvesting processes, there is a huge amount of wood produced as a by-product that is not given extra value. 3.4.1. Bioactive Compounds from Apple and Pear Wood Waste Some authors have evaluated the availability of nutrients in pome tree wood. Apple tree wood showed a higher macronutrient content, with a concentration of 6810 and 13,900 mg/kg of N and Ca available, respectively. Meanwhile, pear tree wood showed a higher micronutrient content, with 144 and 118 mg/kg of Fe and B, respectively [ 110 ]. The amount of carbohydrates has been also evaluated in apple wood, detecting the presence of starch, sorbitol, sucrose, and glucose or fructose, with concentrations from 3 up to 58 mg/g dw [111] . In addition, interesting values of organic and fatty acids, alcohols, lignin, and cellulose have been detected in apple tree wood [ 112 ]. However, to the best of the authors’ knowledge, these compounds have not been isolated or identified in wood from pear trees yet. Few researchers have focused their studies on the bioactive compounds in apple tree wood. Withouck et al. [ 113 ] analyzed the composition of wood from Malus domestica ‘King Jonagold’ apple trees. The results exhibited a total phenolic compound concentration of 9600 mg/kg dw and a total flavonoid content of 2570 mg/kg. After ultrasound-assisted extraction, the following families were identified: flavanols (( − )-epicatechin gallate, ( − )-epicatechin, (+)-catechin, procyanidin B1 and B2), phenolic acids (gallic acid, caffeic acid, vanillic acid, p-coumaric acid, and ferulic acid), flavonols (rutin, quercetin, and kaempferol-3-glucoside], flavanones (naringin and naringenin), and dihydrochalcone (phloridzin and phloretin). The total phenolic compound concentration reached up to 46,600 mg/kg dw. In addition, the obtained extracts exhibited a significant antioxidant activity, with a DPPH assay of 28.4 mg TE/g dw, and a significant antimicrobial property, with a 68% reduction in the growth of Enterococcus faecalis. Finally, Moreira et al. [ 114 ] used microwave-assisted extraction (MAE) for the analysis of Malus domestica apple tree wood. The authors reported a total phenolic and total flavonoid concentration of 2670 mg/kg dw and 1000 mg/kg dw, respectively. The authors analyzed the extracts obtained by HPLC-PDA and identified a total of 18 phenolic compounds in the apple wood: (+)-catechin, kaempferol, kaempferol-3-O-glucoside, kaempferol-3-O-rutinoside, myricetin, naringenin, naringin, phloretin, phlorizin, quercetin, rutin, 4-hydroxyphenilacetic acid, chlorogenic acid, cinnamic acid, ferulic acid, gallic acid, protocatechuic acid, sinapic acid, and resveratrol. Phlorizin was the most abundant phenolic compound, with concentrations ranging from 6890 to 8770 mg/kg dw. Biomolecules 2022,12, 238 23 of 46 3.4.2. Applications of Pome and Pear Wood Waste in the Agri-Food Industry Apple and pear wood waste has been employed principally as firewood to obtain heating or electric energy [ 115 ] and as biochar [ 116 ], since it presents the highest net calorific value (around 15.7 MJ/kg) in comparison with other woods [ 104 , 109 ]. Moreover, this wood has also been burned to produce biocarbon or vinegar [ 117 – 119 ]. The direct application of these woods in the field is its degradation to contribute certain nutrients to the soil [120]. Moreover, apple tree wood has been used in the production of particleboards, with structural applications, and for furniture production [ 121 ]. Another study relied on the lignocellulosic content of apple and pear wood waste, among others, to demonstrate its worth as a substrate for growing the oyster mushroom, P. ostreatus, one of the most edible mushrooms because of its nutritional quality, ease of cultivation, and economic potential. P. ostreatus is a saprotroph species with the ability to degrade cellulose, playing an important ecological role in the disintegration of organic waste. The authors concluded that pruning residues from apple trees and pear trees can be successfully used as raw materials to obtain substrates for P. ostreatus cultivation [ 120 ]. Pear wood waste, containing a considerable amount of cellulose, may be useful to produce paper, plastics, photographic film, magnetic tapes, protective coatings, and electrical parts [122]. In addition, antioxidant activity has been attributed to apple wood extracts. The antioxidant activity of acetone/water extracts was evaluated by the DPPH method, reaching values of 80% (% reduction/g dry weight), which significantly proves its high antioxidant activity [ 113 ]. The same authors also researched the antimicrobial effect of apple wood extracts. Bark extracts obtained by acetone/water mixture presented the highest phenolic (22.84 mg GAE/g DW) and antioxidant activity (1.07 mM FeSO 4 .7H 2 O/g dw for the FRAP assay), while the extract also inhibited the growth of the Gram-positive bacteria E. faecalis and S. aureus by 100% [ 113 ]. Moreover, phlorizin, the most abundant phenolic compound in wood extracts, is able to lower glucose plasma concentrations independent of insulin [ 123 ]. Therefore, the authors suggest that these antioxidant extracts can be used in the food industry, pharmaceutics, and cosmetics. A summary of the bioactive compounds isolated from apple wood and their applications can be found in Table 4. 3.5. Stone Fruit Trees In Europe, the stone fruits are the fifth most cultivated common fruit in terms of surface area (0.6 million hectares of land) and the fourth most produced (7.3 MT). Examining the proportions of each stone fruit, peaches (Prunus persica L.) are the most produced, accounting for 42%, followed by plums (Prunus domestica L.) and nectarines (Prunus persica var. nucipersica), which account for 20% and 16% of the European production, respectively. Finally, cherries (Prunus avium) and apricots (Prunus armeniaca) also represent an important part of stone fruit production, accounting for 13% and 9%, respectively (Figure 6) [124]. Similar to other common fruit species, stone fruit production is accompanied by the generation of wood residues, mainly by annual pruning and, to a lesser extent, by the renewal of the orchard. Several studies of peach groves mentioned consensual values of 2.5, 2.7, and 2.9 tons of pruned wood/ha per year [ 80 , 125 , 126 ]. Similar values were reported for nectarine orchards [ 80 ]. On the contrary, heterogenous data on plum trees are reported by Pari et al. [ 126 ], who estimated 0.8 tons/ha per year of pruned wood, by Bilandzija et al. [80] , who estimated 2.1 tons/ha per year, and by Burg et al. [ 109 ], who estimated 4.6 tons/ha per year [ 80 , 109 , 126 ]. In cherry orchards, previous reports showed that annual pruning results in between 1.8 and 2 tons/ha per year of wood residues, while in apricot groves the amounts range between 1.2 and 1.9 tons/ha per year. Globally, stone fruit trees appear to produce around 2 tons/ha per year of wood waste. The following sections deal with the chemical compounds and bioactivities relevant to the agri-food domain of peach, plum, cherry, and apricot woody biomasses. Unfortunately, due to the absence of literature data, nectarine wood valorization is not discussed. Biomolecules 2022,12, 238 24 of 46 Table 4. Main bioactive compounds (mg/kg dry wood) from apple tree wood and their applications. * ↑ : Increase. Chemical structures by families are shown in Figure S1. Family Compounds (Concentration (mg/kg dw)) Properties and Applications References Apple wood Polyphenols Dihydrochalcone phloridzin (6890.0–8770.0) phloretin (167.0–195.0) Flavanol (+)-catechin (103.0–301.0) (−)-epicatechin (−)-epicatechin gallate procyanidin B1 procyanidin B2 ↑antioxidant activity DPPH: 28.4 mg Trolox eq/g FRAP: 36.1 mg ascorbic acid eq/g ↑antimicrobial activity: 68% growth reduction of Enterococcus faecalis and Staphylococcus aureus [113,114] Flavonol kaempferol (123.0–219.0) kaempferol-3-O-glucoside (1500.0–1960.0) kaempferol-3-O-rutinoside (76.9–161.0) quercetin (214.0–229.0) rutin (68.3–98.4) myricetin (1930.0–2590.0) Flavanones naringin (n.d.–182.0) naringenin (n.d.–31.0) Phenolic acid 4-hydroxyphenilacetic acid (395.0–677.0) caffeic acid chlorogenic acid (n.d.–185.0) cinnamic acid (403.0–815.0) p-coumaric acid ferulic acid (40.9–89.0) gallic acid (n.d.–43.1) protocatechuic acid (183.0–269.0) sinapic acid (63.0–113.0) vanillic acid Stilbenes resveratrol (157.0–320.0) Biomolecules 2022,12, 238 25 of 46 Figure 6. Proportion of stone fruit production in Europe. 3.5.1. Peach Wood Waste Bioactive Compounds from Peach Wood Waste The chemical composition of pruned wood from peach orchards was studied by Nakagawa et al. [ 127 ], who attempted to isolate the active compounds from an ethanol extract. The following polyphenols were identified: two flavanols, catechin and 4 0 -O- methylcatechin; the flavonol quercetin 3-O- β -D-glucopyranoside; the chalcone designated as 4,2 0 ,4 0 -trihydroxy-6 0 -methoxychalcone 4 0 -O- β -D-glucopyranoside; the phenolic acid ferulic acid; and a phenolic compound called phenyl O- β -D-glucopyranoyl-(1 → 6)- β -D- glucopyranoside. In addition, two pentacyclic triterpenoids named oleanolic acid and ursolic acid were also characterized. Apart from secondary metabolites, peach wood was composed of 42.4% crude fiber, such as cellulose or lignin, 3.9% crude protein, and 1.8% ethereal extract, which contains fats and fatty acids [120]. Application of Peach Wood Waste in the Agri-Food Industry From the biological investigation, wood waste from peach tree pruning and its major compounds were studied with a view towards understanding their antioxidant, anti-lipase, and anti-dementia activities. The free radical scavenging capacities of water and ethanolic extracts were tested using the ORAC assay and showed values of 0.2 and 3.0 mg TE/mg , respectively. The flavonol quercetin 3-O- β -D-glucopyranoside was shown to be the main antioxidant compound. The anti-lipase activity, which is linked to the fight against obesity, exhibited IC 50 values of 54.9 µ g/mL, with oleanolic acid as the main contributor. The neuroprotective activity of ethanolic extract was evaluated through its acetylcholinesterase inhibition capacity, and IC 50 values of 83.4 µ g/mL were found. In this case, 4 0 -O-methylcatechin, chalcone derivative and ferulic acid were stated as the main active compounds [127]. Aiming to discover a new natural antihypertensive agent, Kim et al. [ 128 ] tested the vasorelaxant effect of pruned wood extract from peach trees and studied its associated mechanisms. Their findings demonstrated that ethanolic extract caused a concentrationdependent vasorelaxation of rat aortic rings. Peach wood waste could thus be valorized as a functional food for hypertension treatment and could be implemented in nutraceutical or pharmaceutical areas [128]. Another study into the lignocellulosic content of peach tree shoots demonstrated that peach wood could also be used in agriculture to promote the development of P. ostreatus [120] . 3.5.2. Plum Wood Waste Bioactive Compounds from Plum Wood Waste A chemical study of pruning residues from plum trees was performed recently, highlighting the presence of polyphenols. Among them, two flavanols (catechin and epicatechin), Biomolecules 2022,12, 238 32 of 46 while González et al. [151] estimate that this Iberian country produces 5 MT of orange wood waste. In this sense, as orange production represents 58% of the citrus production, the pruning from citrus fruit in Spain can be estimated at between 4 and 9 MT per year. Moreover, as Spain accounts for half of the European citrus production, the amount of pruning residues in Europe could be estimated at between 8 and 18 MT per year. Figure 7. Proportion of citrus fruit production in Europe. 3.6.1. Orange Wood Waste Bioactive Compounds from Orange Wood Waste Regarding orange tree residues, wood extracts belong to different chemical families. A recent study into pruning biomass from sweet orange (C. sinensis) showed the presence of polyphenols and alkaloids [ 149 ]. Using LC-MS, the authors showed the main compounds to be the cinnamic acid derivatives caffeic acid, the flavan-3-ol catechin, and an unknown glycosylated compound with a relative abundance at 69.7%, 14.4%, and 15.1%, respectively. Using the Folin–Ciocalteu index, the authors estimated a total polyphenol content of between 35 and 50 GAE/g, depending on the extraction process [ 149 ]. Focusing on the composition of wood biomass from bitter orange (C. aurantium) trees, a recent study screened a total of 295 metabolites and found 39 compounds using HPLC-Q-TOF-MS [ 152 ]. These included three amino acids (asparagine, glutamine, and pipecolic acid), two ribonucleosides ((hydroxy-)adenosine), ten flavanones (derivatives of eriodictyol, hesperitin, hesperidin, and naringenin), fifteen flavones (derivatives of luteolin, diosmetin, acacetin, apigenin, 3 0 ,4 0 ,5 0 -trimethoxyflavone, cirsimaritin, nobiletin, and tangeretin), two coumarins (xanthotoxol and scopoletin), one alkaloid (M-acetylnorsynephrine-rhamnoside), and six new compounds (citrus A, B, C, H, I, J) as flavonoids, alkaloids, and terpenoids [ 152 ] (Table 6). Wood waste from orange trees has also been subjected to hydrodistillation to produce essential oils. Essential oils from pruned tree shoots of sweet orange trees (C. sinensis) were analyzed by GC-MS and exhibited 24 volatile compounds. The main compounds were the monoterpene sabinene, followed by limonene, δ -3-carene, terpinen-4-ol, trans-ocimene, and β -myrcene [ 153 ]. Another work, focusing on essential oil from the wood of bitter orange (C. aurantium), showed that the major compound was the monoterpene limonene, followed by dimethyl anthranilate, β -fenchol, dodecane, 4-carvomenthenol, G -terpinene, cis-4-thujanol, and thymol [154]. Application of Orange Wood Waste in the Agri-Food Industry As described for other wood waste, the most common practice for the disposal of these citrus pruning residues is their controlled combustion in the field, allowing ashes Biomolecules 2022,12, 238 33 of 46 to be incorporated into the soil as an amendment [ 155 ]. When open burning is forbidden because of possible environmental risks (zones near natural reserves), the wood waste is ground and used as green manuring. However, wood tree biomass is known to be rich in secondary metabolites, which can be valorized in different industrial sectors, especially in the agri-food industry [156,157]. Regarding extracts of sweet orange wood, they showed antioxidant capacity through three different tests. DPPH, FRAP, and BCB ( β -carotene bleaching assay) allowed for the measurement of their radical scavenging activity, reducing power, and lipid peroxidation inhibition, respectively. Wood extract obtained from a water autoclave extraction had high DPPH scavenging activity (35.4 mg TE/g) and reducing power (87.3 mg TE/g), while an ethanolic extract exhibited better lipid peroxidation inhibition, with an antioxidant activity (AA) value of 70.3% [ 149 ]. Despite orange wood extracts showing good antioxidant properties, as far as we know, no studies are available in the literature regarding their in vitro or in vivo bioactivities or their potential applications. In this sense, further investigation is required and represents an interesting challenge, due to the large amount available and the complexity of orange wood chemical profiles. The possible application of essential oil derived from orange tree pruning has also been investigated. Eldahishan et al. [ 153 ] studied the antibacterial and antifungal activities of C. sinensis tree shoot essential oil with a concentration of 12.5 µ L against Gram (+) bacteria (S. aureus,S. pyogenes, and E. faecalis), Gram (-) bacteria (K. pneumoniae,E. coli, and S. typhimurium), and fungi (Trichophyton mentagrophytes,Aspergillus fumigatus, and C. albicans). The oil showed a better antibacterial effect against S. aureus (responsible for skin and soft tissue infections, i.e., abscesses, furuncles, and cellulitis), S. pyogenes (responsible for strep throat), and S. typhimurium (causes a protracted bacteremic illness referred to as typhoid fever), as well as antifungal properties against A. fumigatus (responsible for severe pulmonary allergic disease) [ 153 ]. Furthermore, Okla et al. [ 154 ] reported the antibacterial effect of essential oil extracted from bitter orange wood against Agrobacterium tumefaciens, Dickeya solani, and Erwinia amylovora. Overall, essential oils from the hydrodistillation of orange pruning residues showed antibacterial and antifungal properties, allowing it to be valorized for medical purposes and for use in cosmetic products and food additives. 3.6.2. Mandarin Wood Waste Bioactive Compounds from Mandarin Wood Waste Mandarins (C. reticulata) are one the smallest citrus fruits produced in Europe, and research has been performed into the chemical composition of the woody residues generated every year. A study of mandarin tree wood was performed with an acetonic extraction. Six compounds were described: two acridone alkaloids (citruscridone and citrusinine-I), a polyphenol cinnamic acid (valencic acid), two triterpenes limonoid (limonexic acid and limonin), and a coumarate ester (p-hydroxyphenyl-ethyl-p-coumarate) [ 158 ] (Table 6). Mandarin wood is thus composed of a mixture of different chemical compounds that could involve various molecular targets and several signaling pathways. Biomolecules 2022,12, 238 34 of 46 Table 6. Chemical composition, quantitation, major bioactivities, and applications of wood biomasses from citrus trees. * ↑ : Increase. Chemical structures by families are shown in Figure S1. Family Compounds (Concentration or Relative Area) Properties and Applications References Orange wood (C. sinensis) Polyphenol Phenolic acid caffeic acid (69.7%) ↑antioxidant capacity: DPPH scavenging = 35.4 mg Trolox eq/g) FRAP = 87.3 mg Trolox eq /g BCB = 70.3% antioxidant activity ↑antibacterial activity (S. aureus,S. pyogenes, and S. typhimurium) ↑antifungal activity (A. fumigatu) [149,152,159] Flavanol catechin (14.4%) Lignan Unknown—m/z384 (1.0%) Alkaloid - Unknown—m/z323 (7.4%) Essential oil Monoterpene sabinene (33.0%) limonene (18.7%) δ-3-carene (9.4%) terpinen-4-ol (6.2%) trans-ocimene (6.1%) β-myrcene (4.4%) (C. aurantium) Polyphenol Flavanone Eriodictyol eriodictyol-O-glucoside hesperitin hesperitin-O-rhamnoside hesperitin-O-glucoside hesperidin neohesperidin naringenin-O-arabinoside-O-rhamnoside-O-arabinoside 40-methoxy-flavanone-O-rhamnoside Biomolecules 2022,12, 238 35 of 46 Table 6. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Orange wood Flavone luteolin-O-glucoside luteolin-O-glucoside-O-rhamnoside-O-glucoside diosmetin-O-glucoside diosmetin-O-glucoside-O-rhamnoside acacetin-O-glucuronic acid-O-rhamnoside apigenin apigenin-O-glucoside-O-rhamnoside apigenin-O-glucuronic acid-O-arabinoise-O-rhamnoside 30,40,50-trimethoxyflavone 30,40,50-trimethoxyflavone-O-rhamnoside cirsimaritin-O-arabinose cirsimaritin-O-arabinose-O-rhamnoside-O-rhamnoside cirsimaritin-O-rhamnoside-O-arabinose-O-arabinose nobiletin tangeretin Coumarin Xanthotoxol Scopoletin Alkaloid Proto-alkaloid m-acetylnorsynephrine-rhamnoside Amino acid Asparagine glutamine pipecolic acid Ribonucleoside hydroxy-adenosine adenosine Essential oil Monoterpene limonene (38.1%) β-fenchol (6.8%) 4-carvomenthenol (4.2%) G-terpinene (3.6%) cis-4-thujanol (3.49%) thymol (3.3%) linalool (2.9%) 6,7-dihydrogeraniol (2.2%) ↑antibacterial activity (A. tumefaciens,D. solani, and E. amylovora) [154] Sesquiterpene valencene (3.3%) Biomolecules 2022,12, 238 36 of 46 Table 6. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Alkane dodecane (5.3%) undecane (2.1%) Other dimethyl anthranilate (8.1%) Mandarin wood Polyphenol Phenolic acid valencic acid (0.21 mg/g) ↑antibacterial activity (S. aureus, methicillin-resistant strain of S. aureus,E. coli, and P. aeruginosa) ↑antifungal activity (C. albicans,C. neoformans, and M. gypseum) [158] Alkaloid Acridone citruscridone (0.15 mg/g) citrusinine-I (0.08 mg/g) Terpene Limonoid limonexic acid (0.39 mg/g) limonin (0.44 mg/g) Lemon wood Essential oil Monoterpenes limonene (55.2%) (E)-β-ocimene (1.6%) β-myrcene (1.5%) β-pinene (1.5%) ↑expression levels of apoptotic protein caspase-8 ↓expression levels of anti-apoptotic protein BcL-2 and proliferative marker Ki-67. ↑cytotoxic effect [159] Sesquiterpenes trans-caryophyllene (2,2%) Oxygenated geranial (7.9%) neral (6.1%) eryl acetate (5.6%) geranyl acetate (5.1%) citronellal (1.7%) nerol (1.4%) trans-geraniol (1.1%) Biomolecules 2022,12, 238 37 of 46 Table 6. Cont. Family Compounds (Concentration or Relative Area) Properties and Applications References Pomelo wood Polyphenol Flavanone naringenin (5.2 g/kg) eriocitrin narirutin naringin melitidin [161] Flavone rhiofolin (0.1 g/kg) vicenin-2 apigenin-6/8-C-glucoside-O-arabinoside apigenin-6/8-C-glucoside-O-rhamnoside apigenin-8-C-glucoside luteolin-7-O-rutinoside Coumarins meranzin hydrate meranzin isomeranzin marmin bergapten imperatorin isoimperatorin (0.03 g/kg) Terpene Limonoid limonin isoobacunoic acid nomilin obacunone Grapefruit wood Terpene Limonoid limonin (0.2 g/kg) [160] Biomolecules 2022,12, 238 38 of 46 Application of Mandarin Wood Waste in the Agri-Food Industry Wood extracts from C. reticulata and its main compounds were evaluated for their antibacterial and antifungal activities [ 158 ]. An acetonic extract showed better antimicrobial activity against four bacteria (S. aureus, a methicillin-resistant strain of S. aureus,E. coli, and P. aeruginosa) and antifungal activity against two yeasts (C. albicans and Cryptococcus neoformans) and a dermatophyte (Microsporum gypseum). Among the purified compounds of the acetonic extract, the alkaloid citruscridone and the polyphenol valencic acid inhibited the growth of M. gypseum, with MIC values of 200 and 128 mg/L and minimum fungicidal concentration values of 200 mg/L for both compounds. The other compounds (citrusinine-I, limonexic acid, p-hydroxyphenyl-ethyl-p-coumarate, and limonin) showed no activities up to a dose of 200 mg/L [ 158 ]. Overall, mandarin wood waste, rich in polyphenols (flavonoids, phenolic acids, and coumarins), alkaloids (acridone and acridine) and terpenes, could be valorized for medical purposes, in cosmetic products, or as a food additive, thanks to its antibacterial and antifungal properties. 3.6.3. Lemon Wood Waste Bioactive Compounds from Lemon Wood Waste Lemon (C. limon) trees generate a significant amount of wood waste in orchards. However, few studies have focused on the chemical characterization and potential biological activities of the wood or tree shoots of lemon trees. The only available study deals with the valorization of the woody biomass of lemon through essential oils. The essential oils of C. limon were isolated from freshly collected tree shoots by hydro-distillation. Using a GC-MS apparatus, 35 compounds were identified that accounted for 99.22% of the essential oils from the pruned wood of lemon trees. Among them, the main compounds were the monoterpene limonene, geranial, neral, neryl acetate, and geranyl acetate [159]. Application of Lemon Wood Waste in the Agri-Food Industry Although the bioactivities of essential oil from wood biomass have not been explored, a lemon leaf oil with a similar composition (i.e., limonene as the main compound, followed by geranial and neral) was tested for its cytotoxic activity against breast carcinoma cells (MCF-7). The lemon essential oil exerted a significant increase in the expression levels of the apoptotic protein caspase-8, as well as a significant decrease in the expression levels of both the anti-apoptotic protein BcL-2 and the proliferative marker Ki-67. In addition, the compound geranial showed a high affinity against caspase-8 and caspase-9, demonstrating the promising cytotoxic effect of lemon oil through enhancing apoptosis [ 159 ]. Overall, although further studies are needed, it appears that the wood biomass from lemon tree pruning could be valorized for medicinal purposes, owing to its anticancer properties. 3.6.4. Grapefruit and Pomelo Wood Waste Bioactive Compounds from Grapefruit and Pomelo Wood Waste Although grapefruit (C. paradisi) and pomelo (C. maxima) represent less than 1% of citrus production in Europe, some studies have investigated the biomolecules present in their woody waste. A study into grapefruit pruning explored the presence of one of the main triterpenoid limonoids, known as limonin. It was found that increases in limonin levels of up to 0.2 g/kg can be found in grapefruit wood, depending on the season [ 160 ]. The chemical profile of pomelo wood residues was also determined by rapid resolution liquid chromatography coupled to a quadrupole time-of-flight mass spectrometry (RRLC-MS/QTOF). The analysis of the methanolic extract showed the presence of 22 compounds , including several polyphenols, such as flavones (vicenin-2, rhiofolin, derivatives of apigenin, and luteolin) and flavanones (eriocitrin, narirutin, naringin, melitidin, and naringenin). The extract also contained several coumarins (meranzin hydrate, meranzin, isomeranzin, marmin, bergapten, imperatorin, and isoimperatorin) and terpene limonoids (limonin, isoobacunoic acid, nomilin, and obacunone) [161] (Table 6). Biomolecules 2022,12, 238 39 of 46 Application of Grapefruit and Pomelo Waste in the Agri-Food Industry To date, the application of grapefruit and pomelo wood waste has not been investigated. A summary of the bioactive compounds isolated from citrus fruit wood can be found in Table 6. 4. Challenges and Perspectives Millions of tons of wood waste are generated yearly. The most frequent use of this waste is its incorporation into the soil. In the recent years, these residues have been used as biomass for energy production, thus reducing the use of fossil fuels, which is a relevant international goal, decreasing greenhouse gas emissions and contributing to mitigating climate change. Moreover, wood waste is a rich source of secondary metabolites that are not directly involved in the basic functions of growth, development, and reproduction of the organism, but possess many bioactive properties. These compounds have been widely studied in food and plants, and, to a lesser extent, in wood residues. The last ten years have seen a 7-fold increase in the study of wood waste valorization (Scopus ® ). Olive tree wood extracts have shown many health-related properties, which is a reason why they have been proposed for use in pharmacy and medicine, specifically to treat Alzheimer’s disease and diabetes. Grapevine cane extracts have potent antioxidant and antimicrobial properties, thereby being of use in the food industry as an alternative to SO 2 in winemaking. Chestnut wood extracts possess antifungal activities that could be beneficial in agriculture. Citrus wood extracts, mainly orange and mandarin, have a composition and flavor that are of use in the cosmetics industry. However, as described in the current review, the technology readiness levels (TRL) of these applications are low. There is still a lack of pilot scale studies demonstrating the viability and profitability of wood waste uses. Therefore, considering all the bioactive properties of both wood extracts and the compounds described in them, along with the beneficial effects on sustainability and the contribution to the circular economy, the use of wood extracts deserves to be more widely studied. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/biom12020238/s1, Figure S1: Chemical structure of bioactive compound families investigated during this research. Own elaboration using Fisher Scientific website: https://www.fishersci. es/es/es/search/chemical/substructure.html (accessed on 5 January 2022). Author Contributions: Conceptualization, V.O.-S. and E.C.-V.; investigation, M.J.A.-G. and J.G.; resources, M.J.A.-G., J.G., and E.C.-V.; writing—original draft preparation, M.J.A.-G., J.G., V.O.-S. and E.C.-V.; writing—review and editing, M.J.A.-G. and E.C.-V.; visualization, V.O.-S.; supervision, E.C.-V. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Ministerio de Economía, Industria y Competitividad, and INIA (RTA2015-00005-C02-01). M. J. Aliaño-González is supported by the Sistema Nacional de Garantía Juvenil, the Agencia Estatal de Investigación, and the Fondo Social Europeo (FSE) with the Youth Employment Initiative: “The FSE invests in your future”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. FAOSTAT. Available online: https://www.fao.org/faostat/en/#home (accessed on 5 January 2022). 2. Libutti, A.; Cammerino, A.R.B.; Monteleone, M. Management of Residues from Fruit Tree Pruning: A Trade-Off between Soil Quality and Energy Use. Agronomy 2021,11, 236. [CrossRef] 3. World Population Prospects—Population Division—United Nations. Available online: https://population.un.org/wpp/ (accessed on 5 January 2022). Biomolecules 2022,12, 238 40 of 46 4. Kaza, S.; Yao, L.C.; Bhada-Tata, P.; Van Woerden, F. What a Waste 2.0: A Global Snapshot of Solid Waste Management to 2050, 1st ed.; World Bank: Washington, DC, USA, 2018; ISBN 978-1-4648-1329-0. 5. Circular Economy: Definition, Importance and Benefits | News | European Parliament. Available online: https://www.europarl. europa.eu/news/en/headlines/economy/20151201STO05603/circular-economy-definition-importance-and-benefits (accessed on 5 January 2022). 6. Malinauskaite, J.; Jouhara, H.; Spencer, N. Waste Prevention and Technologies in the Context of the EU Waste Framework Directive: Lost in Translation? Eur. Energy Environ. Law Rev. 2017,26, 66–80. 7. Verkerk, P.J.; Martinez de Arano, I.; Palahí, M. The Bio-Economy as an Opportunity to Tackle Wildfires in Mediterranean Forest Ecosystems. Forest Policy Econ. 2018,86, 1–3. [CrossRef] 8. Forestry in the EU and the World. Available online: https://ec.europa.eu/eurostat/web/products-statistical-books/-/KS-31-11- 137 (accessed on 5 January 2022). 9. Cichy, W.; Witczak, M.; Walkowiak, M. Fuel Properties of Woody Biomass from Pruning Operations in Fruit Orchards. Biol. Res. 2017,12, 6458–6470. [CrossRef] 10. Velázquez-Martí, B.; Fernández-González, E.; López-Cortés, I.; Callejon-Ferre, Á.J. Prediction and Evaluation of Biomass Obtained from Citrus Trees Pruning. J. Food Agric. Environ. 2013,11, 1485–1491. 11. García-Galindo, D.; Gómez-Palmero, M.; Pueyo, E.; Germer, S.; Pari, L.; Afano, V.; Dyjakon, A.; Sagarna, J.; Rivera, S.; Poutrin, C. Agricultural Pruning as Biomass Resource: Generation, Potentials and Current Fates. An Approach to Its State in Europe. In Proceedings of the European Biomass Conference and Exhibition, Amsterdam, The Netherlands, 6–9 June 2016; pp. 1579–1595. 12. Velázquez-Martí, B.; Fernández-González, E.; López-Cortés, I.; Salazar-Hernández, D.M. Quantification of the Residual Biomass Obtained from Pruning of Trees in Mediterranean Olive Groves. Biomass Bioenergy 2011,35, 3208–3217. [CrossRef] 13. CIRCE; CERTH. Biomass from Agricultural Pruning and Plantation Removals; European Union’s Horizon 2020 Research and Innovation Programme; CIRCE: Zaragoza, Spain; CERTH: Athens, Greece, 2018; p. 50. 14. Law, B.E.; Harmon, M.E. Forest Sector Carbon Management, Measurement and Verification, and Discussion of Policy Related to Climate Change. Carbon Manag. 2011,2, 73–84. [CrossRef] 15. Chiocchio, I.; Mandrone, M.; Tomasi, P.; Marincich, L.; Poli, F. Plant Secondary Metabolites: An Opportunity for Circular Economy. Molecules 2021,26, 495. [CrossRef] 16. Kabera, J.N.; Semana, E.; Mussa, A.R.; He, X. Plant Secondary Metabolites: Biosynthesis, Classification, Function and Pharmacological Properties. J. Pharm. Pharmacol. 2014,2, 377–392. 17. uPruning; CIRCE Sector Analysis and Strategic Plan at National and EU Level. uP_running. 2018. Available online: https://www.up-running.eu/wp-content/uploads/2016/10/D2.2_Sector-Analysis-and-Strategic-Plan-at-national-and-EU- levels_compressed.pdf (accessed on 5 January 2022). 18. Olive Wood—Characteristics, Types and Uses. Available online: http://www.woodassistant.com/wood-database/olive-wood/ (accessed on 3 January 2022). 19. Olive Oil. Available online: https://ec.europa.eu/info/food-farming-fisheries/plants-and-plant-products/plant-products/ olive-oil_en (accessed on 3 January 2022). 20. Faraone, I.; Russo, D.; Bruno, M.R.; Todaro, L.; D’Auria, M.; Milella, L. Focus on Olea europaea L. Pruning by-Products: Extraction Techniques, Biological Activity, and Phytochemical Profile. Biofuel. Bioprod. Biorefin. 2021,15, 1835–1849. [CrossRef] 21. Zabed, H.; Sahu, J.N.; Suely, A.; Boyce, A.N.; Faruq, G. Bioethanol Production from Renewable Sources: Current Perspectives and Technological Progress. Renew. Sust. Energy Rev. 2017,71, 475–501. [CrossRef] 22. Dhyani, V.; Bhaskar, T. A Comprehensive Review on the Pyrolysis of Lignocellulosic Biomass. Renew. Energy 2018 ,129, 695–716. [CrossRef] 23. Gueye, M.; Richardson, Y.; Kafack, F.T.; Blin, J. High Efficiency Activated Carbons from African Biomass Residues for the Removal of Chromium(VI) from Wastewater. J. Environ. Chem. Eng. 2014,2, 273–281. [CrossRef] 24. Gurten, I.I.; Ozmak, M.; Yagmur, E.; Aktas, Z. Preparation and Characterisation of Activated Carbon from Waste Tea Using K2CO3.Biomass Bioenergy 2012,37, 73–81. [CrossRef] 25. Ballesteros, I.; Ballesteros, M.; Cara, C.; Sáez, F.; Castro, E.; Manzanares, P.; Negro, M.J.; Oliva, J.M. Effect of Water Extraction on Sugars Recovery from Steam Exploded Olive Tree Pruning. Bioresour. Technol. 2011,102, 6611–6616. [CrossRef] 26. Ortega-García, F.; Peragón, J. HPLC Analysis of Oleuropein, Hydroxytyrosol, and Tyrosol in Stems and Roots of Olea europaea L. Cv. Picual during Ripening. J. Sci. Food Agric. 2010,90, 2295–2300. [CrossRef] 27. Romero-García, J.M.; Niño, L.; Martínez-Patiño, C.; Álvarez, C.; Castro, E.; Negro, M.J. Biorefinery Based on Olive Biomass. State of the Art and Future Trends. Bioresour. Technol. 2014,159, 421–432. [CrossRef] 28. Castejón, M.L.; Montoya, T.; Alarcón-de-la-Lastra, C.; Sánchez-Hidalgo, M. Potential Protective Role Exerted by Secoiridoids from Olea europaea L. in Cancer, Cardiovascular, Neurodegenerative, Aging-Related, and Immunoinflammatory Diseases. Antioxidants 2020,9, 149. [CrossRef] 29. Cecchi, L.; Bellumori, M.; Cipriani, C.; Mocali, A.; Innocenti, M.; Mulinacci, N.; Giovannelli, L. A Two-Phase Olive Mill by- Product (Pâté) as a Convenient Source of Phenolic Compounds: Content, Stability, and Antiaging Properties in Cultured Human Fibroblasts. J. Funct. Food. 2018,40, 751–759. [CrossRef] Biomolecules 2022,12, 238 41 of 46 30. Jimenez-Lopez, C.; Fraga-Corral, M.; Carpena, M.; García-Oliveira, P.; Echave, J.; Pereira, A.G.; Lourenço-Lopes, C.; Prieto, M.A. ; Simal-Gandara, J. Agriculture Waste Valorisation as a Source of Antioxidant Phenolic Compounds within a Circular and Sustainable Bioeconomy. Food Funct. 2020,11, 4853–4877. [CrossRef] 31. Konno, K.; Hirayama, C.; Yasui, H.; Nakamura, M. Enzymatic Activation of Oleuropein: A Protein Crosslinker Used as a Chemical Defense in the Privet Tree. Proc. Natl. Acad. Sci. USA 1999,96, 9159–9164. [CrossRef] 32. Uccella, N. Olive Biophenols: Biomolecular Characterization, Distribution and Phytoalexin Histochemical Localization in the Drupes. Trends Food Sci. Technol. 2000,11, 315–327. [CrossRef] 33. Robards, K.; Prenzler, P.D.; Tucker, G.; Swatsitang, P.; Glover, W. Phenolic Compounds and Their Role in Oxidative Processes in Fruits. Food Chem. 1999,66, 401–436. [CrossRef] 34. Pérez-Bonilla, M.; Salido, S.; van Beek, T.A.; Linares-Palomino, P.J.; Altarejos, J.; Nogueras, M.; Sánchez, A. Isolation and Identification of Radical Scavengers in Olive Tree (Olea europaea) Wood. J. Chromatogr. A 2006 ,1112, 311–318. [CrossRef] [PubMed] 35. Japón-Luján, R.; Luque de Castro, M.D. Small Branches of Olive Tree: A Source of Biophenols Complementary to Olive Leaves. J. Agric. Food Chem. 2007,55, 4584–4588. [CrossRef] [PubMed] 36. Luján, R.J.; Capote, F.P.; de Castro, M.D.L. Temporal Metabolomic Analysis of O-Glucoside Phenolic Compounds and Their Aglycone Forms in Olive Tree and Derived Materials. Phytochem. Anal. 2009,20, 221–230. [CrossRef] [PubMed] 37. Salido, S.; Pérez-Bonilla, M.; Adams, R.P.; Altarejos, J. Phenolic Components and Antioxidant Activity of Wood Extracts from 10 Main Spanish Olive Cultivars. J. Agric. Food Chem. 2015,63, 6493–6500. [CrossRef] [PubMed] 38. Amirante, R.; Clodoveo, M.L.; Distaso, E.; Ruggiero, F.; Tamburrano, P. A Tri-Generation Plant Fuelled with Olive Tree Pruning Residues in Apulia: An Energetic and Economic Analysis. Renew. Energy 2016,89, 411–421. [CrossRef] 39. Vera, D.; Jurado, F.; de Mena, B.; Hernández, J.C. A Distributed Generation Hybrid System for Electric Energy Boosting Fueled with Olive Industry Wastes. Energies 2019,12, 500. [CrossRef] 40. Martín-Lara, M.A.; Blázquez, G.; Zamora, M.C.; Calero, M. Kinetic Modelling of Torrefaction of Olive Tree Pruning. Appl. Therm. Eng. 2017,113, 1410–1418. [CrossRef] 41. Ate¸s, S.; Gür, M.; Özkan, O.E.; Akça, M.; Olgun, Ç.; Güder, A. Chemical Contents and Antifungal Activity of Some Durable Wood Extractives vs. Pleurotus ostreatus.BioResources 2015,10, 2433–2443. [CrossRef] 42. Tarfaya, B.; Belahcene, M.; Makhloufi, N.; Bensafi, H.; Boulanouar, A.; Mebarki, L.; Mohamed, O.; Algeria, B. Biological Activities of Olea europea Sylvestris Tar, Growing Wild in South West of Algeria. Int. J. Curr. Microbiol. App. Sci. 2014,3, 771–777. 43. Özkan, O.E.; Zengin, G.; Akça, M.; Balo˘glu, M.C.; Olgun, Ç.; Altuner, E.M.; Ate¸s, S.; Aktümsek, A.; Vurdu, H. DNA Protection, Antioxidant, Antibacterial and Enzyme Inhibition Activities of Heartwood and Sapwood Extracts from Juniper and Olive Woods. RSC Adv. 2015,5, 72950–72958. [CrossRef] 44. Zbidi, H.; Salido, S.; Altarejos, J.; Perez-Bonilla, M.; Bartegi, A.; Rosado, J.A.; Salido, G.M. Olive Tree Wood Phenolic Compounds with Human Platelet Antiaggregant Properties. Blood Cells Mol. Dis. 2009,42, 279–285. [CrossRef] 45. OIV Advanced Search on Database. Available online: https://www.oiv.int/en/statistiques/recherche (accessed on 3 January 2022). 46. Vineyards in the EU—Statistics. Available online: https://ec.europa.eu/eurostat/statistics-explained/index.php?title= Vineyards_in_the_EU_-_statistics (accessed on 3 January 2022). 47. Arvanitoyannis, I.S.; Ladas, D.; Mavromatis, A. Potential Uses and Applications of Treated Wine Waste: A Review. Int. J. Food Sci. Technol. 2006,41, 475–487. [CrossRef] 48. Çetin, E.S.; Altinöz, D.; Tarçan, E.; Göktürk Baydar, N. Chemical Composition of Grape Canes. Ind. Crop. Prod. 2011 ,34, 994–998. [CrossRef] 49. Ferreyra, S.; Torres-Palazzolo, C.; Bottini, R.; Camargo, A.; Fontana, A. Assessment of In-Vitro Bioaccessibility and Antioxidant Capacity of Phenolic Compounds Extracts Recovered from Grapevine Bunch Stem and Cane by-Products. Food Chem. 2021 , 348, 129063. [CrossRef] 50. Barros, A.; Gironés-Vilaplana, A.; Teixeira, A.; Collado-González, J.; Moreno, D.A.; Gil-Izquierdo, A.; Rosa, E.; Domínguez-Perles, R. Evaluation of Grape (Vitis vinifera L.) Stems from Portuguese Varieties as a Resource of (Poly)Phenolic Compounds: A Comparative Study. Food Res. Int. 2014,65, 375–384. [CrossRef] 51. Teixeira, A.; Baenas, N.; Dominguez-Perles, R.; Barros, A.; Rosa, E.; Moreno, D.A.; Garcia-Viguera, C. Natural Bioactive Compounds from Winery By-Products as Health Promoters: A Review. Int. J. Mol. Sci. 2014,15, 15638–15678. [CrossRef] 52. Aliaño-González, M.J.; Richard, T.; Cantos-Villar, E. Grapevine Cane Extracts: Raw Plant Material, Extraction Methods, Quantification, and Applications. Biomolecules 2020,10, 1195. [CrossRef] 53. Gabaston, J.; Leborgne, C.; Waffo-Teguo, P.; Valls, J.; Palos Pinto, A.; Richard, T.; Cluzet, S.; Mérillon, J.-M. Wood and Roots of Major Grapevine Cultivars and Rootstocks: A Comparative Analysis of Stilbenes by UHPLC-DAD-MS/MS and NMR. Phytochem. Anal. 2019,30, 320–331. [CrossRef] [PubMed] 54. Gabaston, J.; Leborgne, C.; Valls, J.; Renouf, E.; Richard, T.; Waffo-Teguo, P.; Mérillon, J.-M. Subcritical Water Extraction of Stilbenes from Grapevine By-Products: A New Green Chemistry Approach. Ind. Crop. Prod. 2018,126, 272–279. [CrossRef] 55. Ioannidou, O.; Zabaniotou, A. Agricultural Residues as Precursors for Activated Carbon Production—A Review. Renew. Sust. Energ. Rev. 2007,11, 1966–2005. [CrossRef] 56. Zacharof, M.-P. Grape Winery Waste as Feedstock for Bioconversions: Applying the Biorefinery Concept. Waste Biomass Valori. 2017,8, 1011–1025. [CrossRef]