Deep Eutectic Solvents as a Potential Alternative Extraction Technique for the Isolation of Phenolic Compounds from Economically Important European Tree Species

Abstract

The by-products of the wood-processing industry are still predominantly used for energy generation, despite being a rich source of high-value phenolic compounds. This review focuses on the valorization of bark from economically crucial European tree species. Based on an extensive literature survey, three deciduous species (Fagus sylvatica, Quercus robur/petraea, Carpinus betulus) and three coniferous species (Pinus sylvestris, Picea abies, Abies alba) were selected on the basis of their distribution in the European Union, their industrial relevance, and the composition and bioactivity of their extractive phenolic fractions. Conventional and nonconventional extraction techniques are briefly compared, with particular emphasis on deep eutectic solvents (DESs) as emerging green media for the selective isolation of phenolics from bark and other lignocellulosic residues. DESs are typically renewable, nontoxic, biodegradable, and nonflammable, and their tunable composition allows them to be tailored to specific target compounds. The literature data demonstrate that DES-based extractions can provide phenolic-rich extracts with high antioxidant and antimicrobial activities and, in some cases, can outperform conventional solvents. Finally, the potential applications of bark-derived phenolic extracts in the pharmaceutical, agricultural, food, polymer processing, and cultural heritage sectors are outlined. The review also identifies knowledge gaps in DES selection, extract purification, and solvent recovery, highlighting future research needs for integrating DESs into sustainable wood-biomass biorefineries.

1. Introduction

Forests cover approximately 3.870 million hectares of the Earth’s surface, of which more than a quarter is in Europe, which represents approximately 1.040 million hectares [1]. The European Union (EU) and its member states collectively hold around 5% of the total forest area in the world, equivalent to approximately 158 million hectares, which covers 37.7% of the territory of the EU. Between 1990 and 2010, the forested area in the EU expanded by around 11 million hectares [2]. A similar trend can be observed in Slovakia, where the forest area has increased by approximately 1% since 1990, currently representing 41.3% of the total area of the country [3]. Within the EU, coniferous forests dominate (42%), followed by deciduous forests (40%) and mixed forests (18%) [4].

In contrast, deciduous forests prevail in Slovakia, covering 64.3% of the total forest area [3]. The European landscape hosts a rich diversity of deciduous and coniferous tree species [1]. Among the most common deciduous species are Fagus sylvatica, Quercus robur/petraea, Carpinus betulus, Betula pendula, Fraxinus excelsior, Alnus incana/glutinosa, Populus nigra/alba, Robinia pseudoacacia, and even subtropical species such as Eucalyptus globulus, which is widespread in the northern part of the Iberian Peninsula. The dominant conifers include Pinus sylvestris, Picea abies, and Abies alba. Pinus pinaster, on the other hand, thrives in the Mediterranean region, as well as along the Pyrenean and Apennine peninsulas and on the eastern coast of France. Larix decidua occurs less frequently. All these tree species are widely used in various industries, generating substantial amounts of residual biomass, including bark, leaves, and needles [3,4]. Despite the predominance of deciduous forests in Slovakia, coniferous wood processing plays an important role in the national economy, particularly in the pulp and paper and construction industries. Conifers offer the advantage of faster growth and higher biomass accumulation compared to slow-growing hardwood species, making coniferous forests a valuable renewable resource in Europe [1,2,3,4,5].

European forests represent a significant source of renewable biomass, with approximately half of renewable energy derived from wood. About 42% of harvested wood is used in energy production, 24% in sawmills, 17% in the paper industry, and 12% in the production of wood-based panels [2] (Figure 1). Industrial wood processing generates a substantial quantity (15–50 wt%) of by-products, including bark, leaves, needles, and cones, which are used primarily for energy purposes [2,3,4,5].

Numerous studies have shown that these by-products, especially bark, contain valuable bioactive compounds, mainly of phenolic nature. While the bark of deciduous trees is typically rich in phenolic substances, the bark of conifers is characterized by a higher content of resins and terpenes [6]. Phenolic compounds exhibit a wide range of biological activities, including antioxidant, antiviral [7], antibacterial, antifungal [8], cardioprotective and neuroprotective [9,10], anti-inflammatory [11,12], and anticancer effects [12,13]. As secondary plant metabolites, they have become a significant focus of scientific research. Due to these properties, phenolic compounds are widely used in the pharmaceutical, cosmetic and food industries, for example, to extend the shelf life of food [14]. They are also investigated as polymer additives for their antioxidant potential [15] and as agrochemical agents for the protection of plants, particularly against harmful insects. Their biological activity arises mainly from their structure, which consists of at least one benzene ring substituted with one or more hydroxyl (-OH) groups. The hydrogen atoms in these groups readily participate in radical reactions, making phenolics efficient radical scavengers [16,17].

2. Selected Trees in the Forests of the European Union

2.1. Pedunculate and Sessile Oaks (Quercus robur/petraea)

The pedunculate oak (Quercus robur) and the sessile oak (Quercus petraea) are among the most widespread deciduous tree species in Europe [18]. Oak bark typically constitutes 10–20% of the total stem volume, while in the crown section, the proportion of bark can reach 20–35% of the total volume. The bark of Quercus robur consists of standard lignocellulosic components, approximately 25 wt% cellulose, 9 wt% hemicelluloses, and 38 wt% lignin (

Table 1. The chemical composition of wood and bark [6,29].

Chemical Composition of Wood and Bark (wt%)	Coniferous Trees		Deciduous Trees
	Wood	Bark	Wood	Bark
Lignin	25–30	40–55	18–25	40–50
α-Cellulose	45–58	Not detected	42–48	Not detected
Polysaccharides	66–72	30–48	74–80	32–45
Hemicelluloses	13–18	Not detected	18–27	Not detected
Extractive compounds	2–9	2–25	2–5	5–10
Ash	0.2–0.6	1–2.5	0.2–0.6	1.5–11

). It also contains approximately 16 wt% extractives [19], mainly composed of polyphenolic compounds, including tannins and phenolic acids [18]. The identified phenolic molecules in oak bark (Table 2) include ellagic acid, gallic acid, vanillic acid, catechin [20,21], vanillin, syringic acid and ferulic acid, as well as aldehydes such as syringaldehyde, coniferylaldehyde and sinapaldehyde. Coumarins such as aesculetin and scopoletin have also been detected [20]. Phenolic compounds are also present in oak wood, where, in addition to the substances mentioned above (ellagic and gallic acids, vanillin, syringaldehyde and coniferylaldehyde), other compounds have been identified, such as eugenol, ellagitannins (vescalagin, grandinin) and tannin roburin, a characteristic phenolic compound of oak species [22,23,24]. In addition to bark and wood, oak leaves have also been investigated for the extraction of phenolic compounds. In addition to phytochemicals, glycosidic derivatives of quercetin, which are commonly found in red fruits and vegetables, have been identified in these extracts [25]. The most abundant extractive phenolic compounds in oak bark are ellagic acid, gallic acid, and catechin [21]. Catechin, a flavan-3-ol compound, represents approximately 1 wt% of all extractives in oak bark. This biomolecule exhibits multiple beneficial properties, including antioxidant activity, free radical scavenging activity, and anti-inflammatory, antibacterial, and especially anticancer effects [26,27,28]. The biological activities of oak bark extract rich in gallic and ellagic acids have also been evaluated. When tested using the DPPH (2,2-diphenyl-1-picrylhydrazyl) method, Quercus robur bark extract exhibited high antioxidant activity, comparable to that of the pure ellagic acid standard (IC50 = 30 µg·mL⁻¹). Antibacterial activity assays conducted against Pseudomonas aeruginosa, Bacillus cereus, Listeria monocytogenes, Escherichia coli, Micrococcus flavus, and Staphylococcus aureus demonstrated strong antibacterial effects, particularly against Pseudomonas aeruginosa, Escherichia coli, and Micrococcus flavus. Due to the presence of catechin, known for its anticancer potential, the bark extract of Quercus robur was also tested against several cancer cell lines, including breast carcinoma (MCF-7), cervical cancer (HeLa), leukemia (Jurkat), and colon cancer (HT-29) [26,27,28,29]. Among the oak extracts tested (Quercus acutissima, Quercus macrocarpa, and Quercus robur), the pedunculate oak extract showed the highest apoptotic activity against the examined cancer cell lines [21]. These findings highlight the potential of pedunculate oak bark as a valuable source of bioactive compounds with added value and promising biological properties. Although the identified phenolic compounds (ellagic acid, gallic acid, and catechin) are also widely distributed in other plant species, oak bark represents an accessible and sustainable source of extractives with significant pharmaceutical potential [21,22,23,24,25].

2.2. European Beech (Fagus sylvatica)

The European beech (Fagus sylvatica), which belongs to the same botanical order (Fagales) as oak and hornbeam, is the second most widespread deciduous tree species in Europe after oak. Beech wood is extensively utilized in furniture and flooring production, as well as in the automotive and paper industries, and also serves as a source of fuelwood. The bark of the beech represents approximately 5–7% of the total volume of the trunk. It is commonly used for energy purposes, mainly for the generation of heat or electricity through combustion [30], but it is also used in animal husbandry as bedding material. However, beech bark is rich in bioactive phenolic molecules. Using HPLC-MS/MS, a total of 37 phenolic compounds were identified, including catechin [31,32], epicatechin, quercetin glucoside (quercetin-O-hexoside), four pentosyl and three hexosyl glycosides of taxifolin, B-type (6) and C-type (6) procyanidins, di-O-glycosides of syringic and coumaric acids, and coniferyl alcohol and sinapyl alcohol glycosides [31], as well as taxifolin and syringin [32]. Extracts obtained from Fagus sylvatica leaves were also analyzed, revealing the presence of phenylpropanoids such as cis-coniferyl and cis-syringin, derivatives of hydroxycinnamic acids (ferulic, caffeic and p-coumaric acids), flavanols, flavan-3-ols and proanthocyanidins. Among flavonoids, compounds such as apigenin, kaempferol, naringenin, quercetin derivatives and catechin and epicatechin were identified in leaf extracts (Table 2) [33,34].

A comparison of the phenolic profiles of oak and beech reveals substantial chemical similarity, likely attributable to their shared taxonomic classification within the Fagales order. Given this chemical similarity, it can be assumed that beech exhibits biological activities comparable to those observed in oak extracts. Similarly to oak, beech extracts also contain catechin as one of the predominant flavonoids. The extracts of Fagus sylvatica were tested for antibacterial activity against Staphylococcus aureus (Gram-positive) and three Gram-negative bacteria: Pseudomonas aeruginosa, Salmonella typhimurium, and Escherichia coli. The results showed that Staphylococcus aureus was the most sensitive strain to the beech bark extract. This finding is consistent with another study that evaluated the antibacterial activity in Fagus sylvatica leaf extracts, which showed the most potent inhibitory effects against the Gram-positive bacteria Staphylococcus aureus and Staphylococcus epidermidis [35,36].

Furthermore, beech bark extract was investigated for its anticancer and antiproliferative properties. This experiment was carried out on two cancer cell lines: melanoma (A375) and lung carcinoma (A549). Antitumor activity was observed only in the melanoma A375 line, showing the highest inhibitory effect at the maximum tested concentration of 2.5 mg/mL. No significant cytotoxic effects were detected on lung carcinoma cells. In contrast, a mild stimulatory effect was observed at lower concentrations, which decreased as the dose increased. In particular, the extract demonstrated selectivity, showing no cytotoxicity toward noncancerous cell lines [37].

Table 2. Selected phenolic compounds present in selected European woody species [22,23,33,34,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59].

Tree Species	Phenolic Acids	Flavonoids *	Stilbenes *	Tannins
Hardwood Bark
Beech	ellagic acid gallic acid vanillic acid	catechin and quercetin	-	roburin
Oak	ellagic acid gallic acid vanillic acid coumaric acid and syringic acid	catechin, epicatechin, taxifolin and quercetin	-	-
Hornbeam	chlorogenic acid and ellagic acid	myricetin, luteolin, quercetin and apigenin	-	-
Softwood bark
Pine	-	kaempferol, quercetin and taxifolin	pinosylvin	-
Spruce	-	quercetin and myricetin	resveratrol piceid piceatannol	-
Fir	gallic acid vanillic acid coumaric acid and ferulic acid	catechin, taxifolin, epicatechin and quercetin	-	-

* Glycosides and derivatives of phenolic compounds are also present.

Table 2. Selected phenolic compounds present in selected European woody species [22,23,33,34,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59].

Tree Species	Phenolic Acids	Flavonoids *	Stilbenes *	Tannins
Hardwood Bark
Beech	ellagic acid gallic acid vanillic acid	catechin and quercetin	-	roburin
Oak	ellagic acid gallic acid vanillic acid coumaric acid and syringic acid	catechin, epicatechin, taxifolin and quercetin	-	-
Hornbeam	chlorogenic acid and ellagic acid	myricetin, luteolin, quercetin and apigenin	-	-
Softwood bark
Pine	-	kaempferol, quercetin and taxifolin	pinosylvin	-
Spruce	-	quercetin and myricetin	resveratrol piceid piceatannol	-
Fir	gallic acid vanillic acid coumaric acid and ferulic acid	catechin, taxifolin, epicatechin and quercetin	-	-

* Glycosides and derivatives of phenolic compounds are also present.

2.3. Hornbeam (Carpinus betulus)

The common hornbeam (Carpinus betulus) is a typical deciduous tree of the Northern Hemisphere that covers a substantial portion of the territory of the EU. Although hornbeam belongs to the same order (Fagales) as oak and beech, it is classified as a member of the Betulaceae family (the birch family). This taxonomic relationship is supported not only by botanical similarities but also by the chemical profiles of both genera, which include the characteristic triterpenol betulin [38,39]. According to recent studies, the antioxidant activity of hornbeam leaves is significantly higher compared to that of other European deciduous species [40]. Falling leaves of Carpinus betulus have also been shown to contain higher levels of total phenolic compounds than those of other deciduous or coniferous trees [41]. Furthermore, the phenolic extracts of Carpinus betulus are believed to possess promising anticancer potential. This assumption is based on the dominant phenolic biomolecules identified in its chemical profile, including chlorogenic acid, ellagic acid, ellagitannins, and flavonoid glycosides (myricetin, luteolin, quercetin, and apigenin) (Table 2). These flavonoid glycosides are known not only for their anticancer effects but also for their high biological activities, including antidiabetic, antibacterial, antiviral, antifungal, anti-inflammatory and hepatoprotective properties. These effects are associated with both O-glycosides and C-glycosides, although the latter, despite being less common, tend to exhibit stronger biological activities compared to oxygen-linked glycosides [42]. The antitumor activity of hornbeam extracts was tested in several cancer cell lines, including colon adenocarcinoma, prostate adenocarcinoma, and brain tumors. The results showed notable effects for the leaf extract of ethyl acetate in the U-373 cell line, as well as for bark extracts in LoVo cells. Furthermore, methanolic bark extracts exhibited significant cytotoxicity against LoVo and U-373 cells [23].

2.4. Scots Pine (Pinus sylvestris)

As with Quercus robur and Quercus petraea, which are among the most widespread deciduous species in Europe, Scots pine (Pinus sylvestris) represents the most characteristic coniferous tree of the Northern Hemisphere and the EU. Due to its abundance, it is widely used in the wood-processing industry for paper, furniture, and flooring and as a source of firewood. Coniferous species generally have a higher bark content compared to deciduous trees. The substantial amount of this so-called residual biomass, along with its richness in value-added compounds, makes Scots pine bark an attractive raw material for obtaining bioactive molecules (Table 1) [6,29].

The predominant phenolic compounds in Scots pine are stilbenes of the pinosylvin type, such as pinosylvin and pinosylvin monomethyl ether. Furthermore, Pinus sylvestris contains notable amounts of flavonoids, including kaempferol, quercetin, taxifolin, and their derivatives (Table 2). Similarly to extracts of oak (Quercus robur/petraea) and beech (Fagus sylvatica), Scots pine extracts were tested for antibacterial activity against Gram-positive (Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes) and Gram-negative bacteria (Escherichia coli, Salmonella infantis, Pseudomonas fluorescens). The results showed a similar trend to those observed in hardwood extracts, with more substantial antibacterial effects against Gram-positive bacteria [43,44]. The antibacterial efficiency of pine extracts was also demonstrated against ‘paper degrading’ bacteria such as Burkholderia multivorans, Alcaligenes xylosoxidans, and Bacillus coagulans [45]. Such properties may have potential applications in the sterilization and preservation of historical and cultural paper-based materials [43,44,45]. Flavonoids, including quercetin and taxifolin [45,46], exhibit remarkable anticancer properties. The effects of quercetin were examined in various cancer cell lines, including colorectal carcinoma (CT-26), prostate adenocarcinoma (LNCaP, PC3), acute lymphoblastic leukemia T cells (MOLT-4), ovarian cancer cells (CHO) and breast cancer cells (MCF-7). Quercetin-induced apoptosis was observed in all tested lines, with the most pronounced effects in CT-26, LNCaP, and MOLT-4 cells. In vivo studies in tumor-bearing mice (MCF-7, CT-26) revealed a significant reduction in tumor volume in groups treated with quercetin compared to controls [46,47]. Quercetin has also been investigated for its neuroprotective effects. Although its mechanism of action is not fully understood, evidence suggests that quercetin exerts protective effects against neurotoxic agents, neuronal damage, and oxidative stress, as well as neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases [46,47,48,49].

2.5. Norway Spruce (Picea abies)

The Norway spruce (Picea abies) is the second most widespread coniferous species in Europe, only surpassed in prevalence by Scots pine. Compared to pine, it grows faster and therefore produces more biomass. This, combined with its wide distribution across the EU, makes it a valuable source of raw material for various industries (construction, papermaking, furniture production and fuelwood). Like other tree species, its processing generates large amounts of by-products such as bark, needles, and branches [49,50].

Spruce bark contains high concentrations of stilbenes, lignans, flavonoids, and tannins (Table 2). The most common identified flavonoids are quercetin and myricetin, while dominant stilbenes include resveratrol and its glucosides, such as piceid, along with piceatannol [43,50]. Resveratrol is one of the most studied natural phenolic compounds due to its beneficial health effects, including antioxidant, anti-aging, anti-inflammatory, antiviral, anticancer, cardioprotective, and neuroprotective activities [9,51,52,53]. Studies on spruce extracts have confirmed that phenolic compounds exhibit greater inhibitory activity against Gram-positive bacteria [43]. The cardioprotective effects of resveratrol were demonstrated using ex vivo rat heart models subjected to 30 min of global ischemia followed by 120 min of reperfusion. Resveratrol significantly improved the recovery of cardiac mechanical function, reduced infarct size, and decreased myocardial injury markers (CK-MB), confirming its strong cardioprotective potential [52]. The anticancer effects of resveratrol have been evaluated in multiple cancer cell lines and in vivo animal models. This stilbene demonstrated efficacy in all stages of carcinogenesis (initiation, promotion, and progression) [54]. For example, at a concentration of 50 µM, it induced apoptosis in human neuroblastoma cells (SH-SY5Y, NGP, SK-N-AS), and at 100 µM, similar results were observed in colorectal carcinoma cells (DLD1, HT29). It also inhibited breast cancer progression in both estrogen-positive (MCF-7) and estrogen-negative (MDA-MB-231) cell lines. In vivo tests in nude mice inoculated with these cancer types showed that a 10 mg/kg dose of resveratrol administered over two days significantly reduced tumor progression. Piceatannol, a stilbene that differs from resveratrol by an additional hydroxyl group at the C4 position, exhibited even more vigorous anticancer activity. Tests in prostate, colon, melanoma, and leukemia cell lines confirmed its potent antitumor properties. Furthermore, since piceatannol is a metabolic derivative of resveratrol formed in the liver, it contributes to prolonging and improving the biological activity of resveratrol [54,55,56].

The typical chemical structures and skeletal classifications of these prominent bioactive phenolics commonly found in European tree barks are systematically illustrated in Figure 2.

2.6. Silver Fir (Abies Alba)

Silver fir (Abies alba) grows predominantly in mixed beech–fir and spruce–fir forests. In Slovakia, these forests represent approximately 24% and 21% of the total forest area. Among coniferous species, fir is the third most widespread member of the Pinaceae family [3]. Beyond botanical similarities, Abies alba shares a comparable phenolic profile with Norway spruce and Scots pine. Identified compounds include gallic, vanillic, p-coumaric and ferulic acids; flavonoids such as catechin, epicatechin, taxifolin, and quercetin (Table 2); and lignans including secoisolariciresinol, pinoresinol, lariciresinol and isolariciresinol [57,58,59].

Given these similarities, silver fir extracts are expected to exhibit analogous biological effects, including antibacterial, anticancer, and cardioprotective properties. Studies on the biological activities of individual phenolic constituents support these assumptions. Taxifolin, one of the most abundant flavonoids, has been explicitly evaluated for its antitumor potential in various cancer cell lines (colon, breast, prostate, lung, and skin) as well as in animal models. The results highlight the promise of taxifolin as a compound capable of inhibiting cancer cell growth and inducing apoptosis [60].

3. Extraction Techniques for Isolation of Phenolic Compounds

3.1. Conventional Extraction Techniques

Conventional extraction methods include maceration, digestion, decoction, percolation (solvent flow through the sample), infusion, serial fractional (continuous) extraction, Soxhlet extraction, and hydrodistillation and steam distillation. The main advantages of these techniques are their technical simplicity and the accessibility of the required equipment. However, they also present significant drawbacks, such as prolonged extraction times, high solvent consumption, and, in cases that require elevated temperatures (hydrodistillation, Soxhlet extraction, decoction, or percolation), the potential loss of volatile and thermolabile compounds [61,62,63,64].

Maceration and related techniques, such as decoction, percolation, infusion, digestion, and Soxhlet extraction, can be grouped under the category of maceration-based processes, differing mainly in temperature and extraction duration. The principle of maceration involves immersing a plant sample in an appropriate solvent (such as water, oil, alcohol, or organic solvents such as toluene, hexane, or benzene) [65].It is stored at room temperature in a sealed container for a specified period of time. The sample may be stirred periodically or occasionally during the process [61,62,63]. Before maceration, samples are typically ground or crushed to increase the surface area available for contact with the solvent. The ratio of sample to solvent is a crucial parameter that influences the extraction yield. Although maceration is among the slowest extraction techniques, lasting from several days to weeks, it remains one of the simplest [65,66]. The efficiency of the process can be improved by regular stirring, which improves diffusion and promotes the renewal of the solvent near the plant material. The extraction yield generally increases over time until the equilibrium is reached between the solid and liquid phases [64]. After extraction, the liquid phase is separated from the solid residue, which is subsequently pressed to recover the remaining extract. The two liquid fractions are then combined and filtered to remove impurities [61,62,65]. Maceration is widely used in the food industry, especially in winemaking and distillation [66,67,68,69]. This method also enables the valuation of wood industry by-products by extracting high-value compounds for industrial applications [65].

For example, polyphenols such as catechin have been isolated from Pinus pinaster sawdust, while robinetin and dihydrorobinetin were extracted from Robinia pseudoacacia L. wood. Todaro et al. employed maceration to extract flavonoids, polyphenols, and tannins from Populus nigra sawdust using n-hexane, followed by an ethanol–water mixture of 70:30 (v/v) ratio [70,71,72]. The resulting extracts exhibited satisfactory total phenolic content (TPC) [73,74,75] and strong antioxidant activity [72,76]. When maceration is carried out with gentle heating, the process is known as digestion. This technique is applied to extract poorly soluble or polyphenolic compounds from plant tissues. The process typically lasts 24 h using ethanol as solvent, which disrupts cell walls and improves the diffusion of extractable compounds [61,63,77].

3.2. Nonconventional Extraction Techniques

Although conventional extraction methods require less equipment investment, they have notable disadvantages, such as long extraction times, the need for large volumes of often toxic solvents, and the need for solvent regeneration [61]. These methods also require substantial energy for heating, and elevated temperatures can cause degradation of thermolabile compounds, thereby reducing the quality of the extract [78]. Additionally, heating processes require subsequent cooling, increasing water consumption. To overcome these limitations, new nonconventional extraction techniques have been developed to minimize the use of energy and solvents while maximizing extraction yield and quality [65]. Such methods include microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pressurized liquid extraction (PLE, also known as accelerated solvent extraction—ASE), supercritical CO₂ extraction (SFE), and extraction using deep eutectic solvents (DES-based extraction). These approaches are considered to be more efficient, effective, and environmentally friendly than conventional methods. However, for the isolation of volatile organic compounds (VOCs), particularly essential oils, traditional techniques such as hydrodistillation and steam distillation remain the most suitable [79].

3.3. Deep Eutectic Solvents as Bark Extraction Agents

3.3.1. The Position of DES Extraction Among Current Extraction Methodologies

To properly define the position and efficiency of DESs in the field of green chemistry, it is essential to contrast them with established extraction approaches.

Table 3. Comparison of traditional, non-traditional, and DES-based extraction methods for the isolation of phenolic compounds [61,62,63,64,65,66,67,68,69,70,73,74,75,76,77,78,79,80,81,82].

Extraction Category	Method	Main Advantages	Main Disadvantages
Traditional methods	Maceration	Simple protocol and low operation costs. No sophisticated equipment required. Suitable for thermolabile compounds (at room temperature).	Extremely long extraction times (hours to days). Low extraction yields. High consumption of hazardous organic solvents.
	Soxhlet extraction	High extraction efficiency due to continuous recycling of fresh solvent. No filtration required after extraction.	High risk of thermal degradation of sensitive phenolics. High energy consumption (continuous heating). Large volumes of toxic organic solvents needed.
Non-traditional methods	Ultrasound-assisted extraction (UAE)	Short extraction time. Enhanced mass transfer via acoustic cavitation. Reduced solvent consumption.	Difficult to scale up to industrial levels. Potential degradation of compounds due to localized high temperatures/pressures.
	Microwave-assisted extraction (MAE)	Very rapid heating and short extraction times. High extraction yields. Reduced environmental impact compared to Soxhlet.	High equipment costs. Restricted to polar or moderately polar solvents. Risk of thermal degradation for highly volatile/sensitive compounds.
DES-based extraction	DES extraction (often combined with UAE/MAE)	Highly tunable selectivity (‘designer solvents’). Negligible vapor pressure and high thermal stability. Biodegradable, nontoxic, and nonflammable (for most biomass-derived DESs). High extraction capacity for both hydrophilic and hydrophobic phenolic compounds.	High viscosity limits mass transfer (requires dilution with water or heating). Difficult and energy-intensive recovery of both target compounds and the solvent. Lack of comprehensive Life Cycle Assessment (LCA) data.

presents a critical comparison between traditional extraction techniques (maceration, Soxhlet extraction), non-traditional advanced methods (UAE, MAE), and DES-based extraction. While traditional methods suffer from high organic solvent consumption and long extraction times, advanced methods offer faster kinetics but often rely on high-cost equipment. DES-based extraction emerges as a highly promising sustainable alternative with tailorable selectivity, though high fluid viscosity and challenging solvent recovery remain key technical hurdles to overcome for industrial deployment [73,74,75,76,77,78,79,80,81,82].

3.3.2. The Concept of DESs and Their Classification

The term eutectic comes from the Greek word etektos, which means ‘easily melted.’ Extraction using DESs is possible because the melting point of a mixture of two or more components is lower than the melting points of each compound. Consequently, the DES remains in a liquid state under conditions where its components are solid [80,81]. DESs are generally composed of ionic compounds (e.g., salts) and molecular substances held together by hydrogen bonds [82]. They usually consist of a hydrogen bond acceptor (HBA), such as a quaternary ammonium salt, and a hydrogen bond donor (HBD), such as urea, carboxylic acids, or amines [82,83] (Table 4).

Based on their constituent components, DESs can be classified into four main types according to the general formula Cat⁺X⁻zY, where Cat⁺ represents an ammonium, phosphonium or sulfonium ion; X is a Lewis base (typically a halide anion); and Y denotes a Lewis or Brønsted acid [84,85]. Type I DESs are prepared from metal halides combined with organic salts, most often quaternary ammonium salts. Type II DESs utilize hydrated metal salts, thereby reducing moisture sensitivity and enhancing industrial applicability. Type III DESs, mainly based on choline chloride as the HBA and on various HBDs, including amides, alcohols, carboxylic acids, sugars, polyols, or amino acids, are the most studied and widely applied [86,87]. DESs that are prepared by combining metal halides as HBAs and HBDs with amino (R-NH₂) or hydroxyl (R-OH) groups are classified as type IV DESs [88]. In contrast to the I-IV types of DESs, type V DESs are composed of a combination of nonionic compounds such as carboxylic acids, alcohols, or saccharides, which can function simultaneously as HBDs and HBAs. Types III and V are also known as natural deep eutectic solvents (NADESs) due to their low toxicity and biodegradability [89,90,91]. In addition, DESs can be synthesized from pharmaceutically active compounds such as ibuprofen, lidocaine, or phenylacetic acid, producing the so-called therapeutic DESs, a fifth group of nonionic DESs [91,92,93].

Table 4. Common HBAs, HBDs, and additives used in the preparation of various DES systems [94].

HBD with Various Functional Groups
Carboxylic Acid-Based DESs
Monocarboxylic acids
Formic acid	Acetic acid	Butyric acid	Lactic acid	Glycolic acid	Propionic acid Levulinic acid
Di/Tricarboxylic acids
Oxalic acid	Malonic acid	Succinic acid	Malic acid	Citric acid	Maleic acid Glutaric acid
Amine/amide-based DESs
urea, formamide, acetamide, ethanolamine, diethanolamine, imidazole, methyl diethanolamine
Polyalcohol/saccharide-based DESs
glycerol ethylene glycol propylene glycol 1,4-butanediol glucose fructose xylitol sucrose
DESs based on phenolic compounds (derived from lignin)
resorcinol catechol vanillin p-hydroxybenzaldehyde p-hydroxybenzyl alcohol p-hydroxybenzoic acid p-coumaric acid gallic acid salicylic acid
HBAs with various functional groups
choline chloride betaine guanidine hydrochloride proline choline dihydrogen citrate
ethyl ammonium chloride acetylcholine chloride choline dihydrogen phosphate benzyl triethyl ammonium chloride benzyl trimethyl ammonium chloride
Catalysts/co-solvents
Inorganic and organic acids
sulfuric acid phosphotungstic acid phosphomolybdic acid silicotungstic acid p-toluenesulfonic acid
Metal salts or hydrates
AlCl3 × 6H2O FeCl3 × 6H2O CrCl3 × 6H2O FeCl2 × 4H2O ZnCl2 CuCl2
Non-polar solvents n-butanol
Others water and alkaline hydrogen peroxide

Table 4. Common HBAs, HBDs, and additives used in the preparation of various DES systems [94].

HBD with Various Functional Groups
Carboxylic Acid-Based DESs
Monocarboxylic acids
Formic acid	Acetic acid	Butyric acid	Lactic acid	Glycolic acid	Propionic acid Levulinic acid
Di/Tricarboxylic acids
Oxalic acid	Malonic acid	Succinic acid	Malic acid	Citric acid	Maleic acid Glutaric acid
Amine/amide-based DESs
urea, formamide, acetamide, ethanolamine, diethanolamine, imidazole, methyl diethanolamine
Polyalcohol/saccharide-based DESs
glycerol ethylene glycol propylene glycol 1,4-butanediol glucose fructose xylitol sucrose
DESs based on phenolic compounds (derived from lignin)
resorcinol catechol vanillin p-hydroxybenzaldehyde p-hydroxybenzyl alcohol p-hydroxybenzoic acid p-coumaric acid gallic acid salicylic acid
HBAs with various functional groups
choline chloride betaine guanidine hydrochloride proline choline dihydrogen citrate
ethyl ammonium chloride acetylcholine chloride choline dihydrogen phosphate benzyl triethyl ammonium chloride benzyl trimethyl ammonium chloride
Catalysts/co-solvents
Inorganic and organic acids
sulfuric acid phosphotungstic acid phosphomolybdic acid silicotungstic acid p-toluenesulfonic acid
Metal salts or hydrates
AlCl3 × 6H2O FeCl3 × 6H2O CrCl3 × 6H2O FeCl2 × 4H2O ZnCl2 CuCl2
Non-polar solvents n-butanol
Others water and alkaline hydrogen peroxide

Natural deep eutectic solvents (NADESs) can be classified into five main groups:

• Ionic liquids composed of acids and bases;
• Neutral NADESs prepared solely from carbohydrates or from carbohydrates combined with other polyalcohols;
• Neutral NADESs with bases, formed from carbohydrates or polyalcohols and organic bases;
• Neutral NADESs with acids, prepared from carbohydrates or polyalcohols and organic acids;
• Amino acid-based NADESs, formed from amino acids and organic acids or carbohydrates [93,94].

3.3.3. Methods of DES Preparation

In general, two primary methods are employed for the preparation of DESs. The first method is heating, which involves continuously stirring and heating the mixture of selected components, typically in a water bath, until a homogeneous liquid is formed [95,96]. The heating temperature typically ranges between 50 and 100 °C, since higher temperatures may cause degradation of the DES components through esterification reactions [97]. The second method is the lyophilization of the aqueous DES solution. Each element is first dissolved in water, and the resulting solutions are mixed to form a homogeneous liquid. The aqueous mixture is then lyophilized to remove all the water from the DES system [98,99]. A similar technique to lyophilization is vacuum evaporation, where a rotary vacuum evaporator is used instead of a lyophilizer [87].

3.3.4. Physical–Chemical Properties of DESs

DESs have attracted significant research attention due to their advantageous physicochemical properties, including low volatility, nonflammability, low vapor pressure, and chemical and thermal stability. Additionally, DESs exhibit high chemical versatility, allowing their composition to be tailored and optimized for specific applications. This flexibility arises from the wide variety of chemical compounds capable of forming deep eutectic mixtures [93].

Most biomass-derived DESs are generally considered nonflammable and biodegradable; however, their exact toxicological and environmental profile is strictly dependent on their specific chemical composition and individual components [90,91,92,93,94]. The renewability of DESs cannot be generalized, as it strongly depends on the sources of the HBD and HBA used. While NADESs utilizing organic acids or sugars offer high biorenewability, the overall environmental footprint must always be assessed individually based on preparation and application conditions. In summary, terms such as ‘biodegradable’, ‘non-toxic’, or ‘green’ should not be interpreted as universal traits inherent to all deep eutectic systems. The ecological profile of a DES is highly tunable and sensitive, dictated not only by the nature of its individual constituents but also by its thermal stability, operating conditions, and potential for degradation during the extraction process [87,88,89,90,91,92,93].

As mentioned above, DESs are composed of two or more pure substances. These mixtures can be represented by a solid–liquid phase diagram, which allows the determination of the melting temperature as a function of the molar composition. For binary mixtures, the eutectic point corresponds to the intersection of the melting curves of the two components. According to the definition proposed by Martins et al., the term deep eutectic solvent should refer exclusively to mixtures whose melting temperature is lower than the ideal eutectic temperature. Otherwise, such systems cannot be distinguished from ordinary eutectic mixtures [95,96,97,98,99]. It has also been observed that a lower molecular weight of the HBD results in a more pronounced decrease in the freezing temperature of the resulting DES [95,96].

The main physicochemical properties of DESs include density, viscosity, ionic conductivity, and polarity. Density is one of the fundamental properties of liquids, and most DESs exhibit higher densities than water (1 g·cm⁻³), typically ranging from 1.3 to 1.6 g·cm⁻³ at 25 °C. However, some hydrophobic DESs show densities lower than those of water. The density of a DES decreases linearly with increasing temperature and also depends on the chosen hydrogen bond donor [100,101,102,103,104,105,106].

Another crucial property is viscosity. Most DESs reported to date exhibit high viscosities (η > 100 mPa·s) at room temperature due to the extensive hydrogen bonding network between their components. Examples of highly viscous DESs include carbohydrate-based systems, such as choline chloride:sorbitol (1:1), with a viscosity of 12,730 mPa·s at 30 °C, and choline chloride:glucose (1:1), with a viscosity of 34.400 mPa·s at 50 °C [107]. To mitigate this issue without relying on elevated temperatures, which risk the thermal degradation of heat-sensitive flavonoids and stilbenes, three primary strategies are currently being deployed. First, a secondary ecofriendly co-solvent, commonly water (typically regulated between 10 and 30 wt%), can be precisely added to act as a macroscopic viscosity modifier. Second, the fundamental screen-and-design approach favors the synthesis of naturally low-viscosity DESs (at room temperature) by utilizing specific HBDs such as short-chain organic acids (e.g., lactic, levulinic, or formic acid) instead of bulky, multi-hydroxyl carbohydrates like glucose or sucrose. Third, from a mechanical engineering perspective, the integration of advanced process intensification technologies, such as acoustic cavitation via UAE or hydrodynamic cavitation, mechanically disrupts the fluid boundary layers around the bark particles, artificially accelerating mass transfer rates even within highly viscous deep eutectic mediums [101,102,103,104,105,106,107].

On the contrary, specific DESs exhibit significantly lower viscosities; for example, choline chloride: ethylene glycol (1:2) has a viscosity of 37 mPa·s at 25 °C, while hydrophobic menthol: acetic acid (1:3) has a viscosity of 7.61 mPa·s at the same temperature. Reported viscosity values for identical DES systems often vary substantially. For example, the viscosity of choline chloride:urea (1:2) at 30 °C ranged between 152 and 527.28 mPa·s, while that of choline chloride:oxalic acid (1:1) at 40 °C was 202–2142 mPa·s. These discrepancies can be attributed to differences in preparation methods, reagent purity, or measurement techniques [107,108]. Since viscosity directly influences ionic conductivity, most DESs exhibit low conductivities (κ < 2 mS·cm⁻¹). Increasing the temperature reduces the viscosity and consequently enhances the conductivity. Ionic conductivity is significantly affected by the choice of the HBD and HBA, their molar ratio, and the water content [109,110].

3.3.5. Techno-Economic Aspects and Engineering Challenges of Industrializing DES Extraction

When considering the transition of DES-based extraction from batch laboratory configurations to continuous industrial scale-up, several critical engineering and economic parameters emerge as major hurdles. The primary challenge is dictated by fluid hydraulics and mass transfer restrictions, which are inherently tied to the viscosity of the solvent [100,101,102,103]. While low-viscosity formulations (ƞ < 100 mPa.s) are technologically highly attractive for standard pumping and continuous-flow architectures, many high-performance DESs display significantly higher viscosities. Processing these highly viscous fluids at an industrial scale substantially increases pumping energy requirements, causes severe pressure drops across continuous pipes, and compromises turbulent mixing, which is essential for rapid mass transfer from the bark matrix. Furthermore, the lack of standardized pilot-plant data for continuous counter-current or semi-continuous extraction loops with DESs limits reliable thermodynamic and kinetic modeling. To foster sustainable future development, research must pivot toward (1) screening and designing low-viscosity eutectic mixtures without compromising extraction yields, (2) optimizing the precise addition of water as a viscosity regulator without triggering premature solvent dissociation, and (3) designing specialized continuous-flow reactors (such as Taylor–Couette or oscillatory flow reactors) specifically tailored for handling non-Newtonian or highly viscous green fluids (DESs) [104,105,106,107].

Although DESs are widely celebrated as ‘green solvents’ due to their negligible vapor pressure, nonflammability, and low volatility, a truly objective evaluation requires addressing their potential ‘non-green’ aspects. The common assumption that DESs are inherently benign simply because they are often composed of individual components that are safe or food grade has been challenged by recent toxicological studies [84,85]. The formation of the hydrogen bonding network can result in synergistic effects, yielding a DES complex that exhibits significantly higher cytotoxicity and ecotoxicity than its separate constituents. Furthermore, the sustainability of the DES preparation process itself must be scrutinized; the standard synthesis typically requires prolonged heating and stirring at elevated temperatures, contributing to a non-negligible energy footprint. Another critical drawback is the massive consumption of water or volatile organic solvents often required during downstream processing to reduce DES viscosity or to precipitate the target phenolic compounds [95,96,97,98]. Finally, there is currently a severe lack of comprehensive LCA data for DES-based processes. It is premature to call DESs a universally ecological solution. First, comprehensive life cycle analyses (LCAs) are needed to accurately quantify the environmental impacts associated with component production, operational energy consumption, and subsequent recycling or disposal [103,104,105].

To ensure the economic viability and sustainability of shifting DES-based extractions from laboratory to industrial scale, the development of efficient strategies for downstream product isolation and solvent recovery is crucial. Due to the negligible vapor pressure of DESs, conventional distillation techniques are inapplicable. Currently, three main technical approaches are being explored, each presenting unique engineering challenges (

Table 5. Overview of recovery strategies for phenolic compounds and DESs, along with their main technical difficulties [105,106,107,108,109,110,111,112,113].

Methods of Recovery	Mechanism/Principle	Main Technical Difficulties and Challenges
Antisolvent precipitation	Water is added to disrupt the DES hydrogen bonding network, decreasing the solubility of target phenolics and forcing them to precipitate.	High energy consumption required to evaporate water during subsequent DES recycling. Restricted mainly to hydrophobic or poorly water-soluble phenolic compounds.
Macroporous resin adsorption	Phenolics are selectively adsorbed onto solid polymeric resins (e.g., XAD series), while the DES passes through. Phenolics are then recovered using a volatile eluent (ethanol).	High viscosity of the DES impairs mass transfer and flow rate, leading to column clogging. Requires initial dilution, leading to partial DES dissociation. Gradual loss of resin adsorption capacity over repeated cycles.
Membrane filtration (ultrafiltration or nanofiltration)	Pressure-driven separation occurs, where target molecules and DES components are separated based on molecular weight cut-off (MWCO) and size exclusion	Severe membrane fouling due to the complex nature of bark extracts. High operating pressures required to process highly viscous DES fluids. Limited chemical compatibility of some commercial membranes with aggressive DES components (e.g., organic acids).
Supercritical CO2 (scCO2) extraction	Supercritical CO2 is used as a green solvent to selectively back-extract phenolic compounds directly from the DES phase.	High investment costs for high-pressure industrial equipment. Low solubility of highly polar, multi-hydroxyl phenolic compounds in pure scCO2 (requires co-solvents).

). The first approach is antisolvent precipitation (typically using water), which disrupts the hydrogen bonding network of the DES and reduces the solubility of hydrophobic phenolics, causing them to precipitate. However, the subsequent recovery of the DES requires evaporating large volumes of water, a process that is extremely energy-intensive and compromises the green character of the method [93,94,95,96,97]. The second strategy involves adsorption on macroporous resins, where phenolic compounds are selectively retained on the resin and later eluted with a volatile solvent like ethanol, while the DES passes through [99,100,101,102,103]. The primary technical difficulty here is the high viscosity of DESs, which slows down the mass transfer and can cause severe column clogging unless the mixture is diluted. Lastly, membrane separation techniques (ultrafiltration and nanofiltration) offer a pressure-driven alternative for separating target molecules based on molecular weight. The main hurdles for membrane deployment are membrane fouling caused by the complex biomass matrix and the high operating pressures required to overcome the hydrodynamic resistance of viscous deep eutectic fluids [105,106,107,108,109,110,111,112,113].

From a techno-economic perspective, describing DES frameworks as universally ‘cheap’ or ‘cost-effective’ requires a rigorous comparison with conventional industrial solvents such as ethanol or methanol. At an industrial scale, the initial bulk purchase cost of standard HBAs like choline chloride is relatively low; however, high-purity HBDs and the energy required for large-scale, controlled DES synthesis can initially elevate operating expenditures [94,95,96,97,98,99,100]. Nevertheless, the economic viability of switching to DESs in a biorefinery context is justified by several unique thermodynamic advantages. First, due to their negligible vapor pressure, solvent losses through evaporation are virtually zero, eliminating the expensive volatile organic compound recovery systems mandated for ethanol or methanol. Second, the high thermal and chemical stability of DESs allows for multiple recycling loops (often up to 4–6 cycles without a significant drop in capacity), amortizing the initial solvent cost. Lastly, the superior selectivity of tailored DESs for specific bark phenolics reduces the mass and complexity of downstream purification steps, which traditionally represent up to 60–80% of total production costs in industrial extraction plants. Therefore, while raw DES mixtures may exhibit higher initial procurement costs per liter than crude methanol, their process-cycle economics and reduced environmental compliance costs present a highly competitive techno-economic profile [102,103,104,105,106,107,108,109,110,111,112,113].

4. Isolation of Phenolic Compounds from Forest Biomass Using DESs/NADESs

The applicability of DESs (including NADESs) for the valorization of plant residues, such as needles and bark, has been confirmed by several studies [81,114,115,116,117]. For example, the extraction of polyphenolic compounds from maritime pine (Pinus pinaster) was performed using a DES composed of levulinic acid and formic acid (70:30). The extraction was carried out at 30 °C under ultrasound-assisted conditions (efficiency 80%; amplitude 35 kHz). The resulting extracts exhibited potent antioxidant and antimicrobial activity [115]. A DES has also been used for the isolation of lignin from Pinus pinaster. A mixture of lactic acid, tartaric acid, and choline chloride was used in a molar ratio of 4:1: 1, with extraction carried out at 175 °C for 1 h. This process produced 95 wt% of total lignin with a purity of 89 wt%, demonstrating that DES-based extraction represents a promising ‘green’ approach to the valorization of wood waste (Figure 3) [118].

Furthermore, the extraction of phenolic biomolecules from spruce bark was carried out successfully using a DES based on choline chloride combined with lactic acid and various diols (1,3-propanediol, 1,3-butanediol, 1,4-butanediol, and 1,5-pentanediol). The extraction process was carried out at 60 °C for 2 h, and the antioxidant activity of the extracts was assessed spectrophotometrically using the DPPH radical scavenging assay. The total phenolic content ranged from 233.6 to 596.2 mg GAE/100 g of dry spruce bark, while radical scavenging activity varied between 81.4 and 95% [114]. Phenolic compounds, which are abundant in the selected tree species listed above, have also been successfully isolated from various other plant matrices using DESs.

Table 6. Phenolic compounds isolated from various natural raw materials by selected types of DESs [119,120,121,122,123,124,125,126].

DES Composition	Molar Ratio and Water Content	Target Phenolic Compounds	Source Material (Literature)	Prospective European Tree Species	Extraction Conditions and Yield	Ref.
PART A: Prospective applications based on food industry by-products (proof of concept models)
Choline chloride + Lactic acid	1:2 20 wt%	Quercetin glycosides	Walnut tree leaves (Juglans regia L.)	Beech Oak Hornbeam Pine	* HAE (50 °C, SESR of 0.15 g/5 mL, 600 rpm for 60 min) 5.7–9.9 mg/g	[119]
Choline chloride + Lactic acid	1:2 20 wt%	Chlorogenic acid	Mulberry leaves (Morus alba L.)	Hornbeam	* MAE (60 °C, power 600 W, SESR of 0.05g/mL for 20 min) 4.507 mg/g	[120]
Choline chloride + Glycerol	1:3 10 wt%	Total polyphenols and flavonoids	Lemon peels (Citrus limon)	Beech Oak Hornbeam, Pine Spruce Fir	* UAE (80 °C, sonification power 140 W, frequency 37 kHz for 90 min, SESR of 100 mL/g 53.76 mg GAE/g, 19.42 mg RE/g	[121]
Choline chloride + Lactic acid	1:2 -	Apigenin	Virgin olive oil (Olea europaea)	Hornbeam	* SLE (40 °C, with agitation for 1 h, being vortexed for 1 min every 15 min, SESR of 1 g/g 0.120 mg/kg	[122]
PART B: Validated applications on bark matrices (directly related to this review)
Levulinic acid + Formic acid	28:12 (v/v) 60 (v/v)	Polyphenols	Bark (Pinus Pinaster)	All investigated species	USE (20 kHz, 130 W) 1.5 g of biomass/10 mL solvents, 30 °C, 40 min) 315.50 mg GAE/g	[123]
Choline chloride + Glycolic acid	1:2 10 wt%	Total phenolic compounds	White birch bark (Betula papyrifera)	All investigated species	UAE (56 °C, 16 min, 423 W, 1:51.49 g/mL 29.17 mg/g	[124]
L-proline + Sorbitol	1:2 10 wt%	Flavonoids	Bark (Cercis chinensis)	All investigated species	UAE (42 °C, 40 min and 8 wt% of water) 87.6 mg/g	[125]
Levulinic acid + Formic acid	70:30 (v/v)	Polyphenols	Maritime pine (Pinus pinaster Aiton)	All investigated species	UAE (30 °C, 40 min, with ultrasound amplitude of 80% at 37 kHz) ~60 mg/g	[115]

* HAE = heat-assisted extraction; UAE = ultrasound-assisted extraction; SLE = solid–liquid extraction; MAE = microwave-assisted extraction; wt% = weight percentage.

summarizes some types of DESs used for the extraction of phenolic compounds from plants sourced primarily from the food industry. These are mainly citrus or onion peels, as well as pomace from wine or oil production, such as olive oil production. Similarly to tree bark, these plant by-products represent a significant portion of the biological waste generated during the processing of food, beverages, or spices. However, as can be seen in Table 6, these food by-products are successfully used to isolate phenolic compounds such as catechin, quercetin, apigenin, myricetin, epicatechin, kaempferol, and phenolic acid (gallic acid, ellagic acid and coumaric acid), which are also present in industrially crucial European tree species. The success of obtaining phenolic compounds from food waste using DESs could indicate that the DESs mentioned in Table 6 could be suitable solvents for the recovery of bark as a by-product of the wood-processing industry [119,120,121,122,123,124,125,126].

5. Use of Phenolic Compounds from Bark Extracts

Phenolic compounds, as mentioned previously, are integral components of all plants, where they act as a form of a ‘defense system’. Plants naturally synthesize these secondary metabolites in response to environmental stimuli. Consequently, the content of extractive substances, particularly phenolic compounds, varies according to the habitat of the plant, climatic factors (such as sunlight exposure), and the degree of biotic stress caused by pests, including insects, molds, fungi, viruses, and bacteria. Through the presence of these chemical constituents, plants exhibit improved resistance to external stressors and improved adaptability to their surrounding environment [126,127]. Due to their diverse biological activities, phenolic compounds provide crucial protection to plants, demonstrating antioxidant, antibacterial, antifungal, antiviral, and other bioactive effects [128,129]. In addition to these biological functions, they have been shown to exhibit anticancer, neuroprotective and antidiabetic properties, as well as UV-blocking capabilities [130,131,132]. These multifaceted characteristics make phenolic compounds highly attractive for industrial applications. Beyond their importance in the pharmaceutical, cosmetic and food industries, they also represent a sustainable and renewable alternative to fossil-based chemicals for use in polymer production, agriculture, and the conservation or restoration of materials from cultural heritage [127,128,129,130].

5.1. Polymer Industry

Plastics are widely used in almost every industrial sector due to their low cost, weight, and excellent mechanical properties [133]. Plastic production has increased dramatically in the last 70 years, from approximately 2 million tons in 1950 to more than 460 million tons in 2021. In Europe, 57.2 million tons of plastic was produced in 2021, with 39.1% used in packaging, 21.3% in construction, 8.6% in the automotive industry, 6.5% in electronics, and 3.1% in agriculture. The remaining share was allocated to consumer goods, furniture, sports, and health. However, plastics currently pose a significant environmental challenge [134,135]. Therefore, developed countries are looking for strategies to reduce the environmental impact of plastic waste, such as recycling or energy recovery. Another promising approach is the development of biodegradable and compostable plastics, as well as biobased additives, which have recently attracted considerable attention from researchers [136,137,138].

Biodegradable and renewable phenolic compounds obtained from plants represent valuable substances with high industrial potential due to their unique structures and their broad occurrence in plant matrices (bark, leaves, needles, etc.) [139]. One of the most widespread biopolymers in plants is lignin, which is composed of phenolic monomeric units. Recent research has focused on its depolymerization to develop new renewable polymers and composites [140]. Other significant groups of phenolic compounds in plants, as secondary metabolites, include flavonoids, tannins, stilbenes, phenolic acids, lignans, and coumarins. These biomolecules are also extensively studied as additives for conventional polymers [132] in the preparation of phenolic resins as alternative phenol substitutes in phenol–formaldehyde resins, as well as polyurethane resins [141,142,143,144].

Quercetin, a natural antioxidant belonging to the flavonoid family, was tested as a thermooxidative stabilizer in polyethylene (PE). Its antioxidant properties were compared to the conventional stabilizer Irganox 1010, commonly used in industrial practice. The results indicated that quercetin is significantly more effective than Irganox 1010. However, some drawbacks were observed, including high melting temperature, lower solubility in PE, and yellow discoloration of the final product [145]. Condensed tannins, as well as flavonoids, extracted from the bark of Pinus radiata and Acacia mearnsii were tested as stabilizers for linear low-density polyethylene (LLDPE) films. Tannins from Acacia tannins exhibited higher stabilization effects than those of pine bark. Stabilization was enhanced when used in conjunction with copolymers such as ethylene-vinyl alcohol (EVOH/EVA) or maleic anhydride-modified polyethylene (MAPE), resulting in improved UV stability of PE films. Despite being less effective than conventional stabilizers, condensed tannins have shown potential as green additives for the polymer industry [133].

Condensed tannins from coniferous bark, particularly spruce, also show potential as phenol substitutes in phenol–formaldehyde (PF) resins [142]. Sain et al. [146] investigated the use of natural alternatives such as tannins and lignin, combined with glyoxal, to produce biobased PF resins. Their results demonstrated that these natural resins exhibited properties comparable or even superior to those of traditional PF resins used in fiberboard production. However, the properties of the cured resins varied depending on the curing temperature [146]. Condensed tannins were also successfully applied in biobased polyurethane (PU) foams, in combination with furfural, furfuryl alcohol, and glyoxal, forming urethane and methylene bridges between components [147]. Biobased PU foams and tannin-based resins can improve the mechanical and thermal properties of polymers used in the automotive, construction, or woodworking industries [145].

5.2. Agricultural and Food Industry

In addition to their use as additives in polymers for various structural materials or as substitutes for phenol and formaldehyde in polyfluorophenol resins, these natural compounds have also been successfully applied in the development of packaging materials that come into contact with food. In addition to commonly available polymeric films such as polyethylene (PE) or polypropylene (PP), edible packaging materials (coatings) are also used to protect food, particularly fruits and vegetables. In general, these coatings are prepared from polysaccharides such as starch, alginate, or chitosan. The relatively low antioxidant activity of chitosan is usually enhanced by adding natural extracts with a high content of phenolic compounds [148,149,150]. For example, an edible chitosan coating enriched with green tea extract was applied to strawberries, significantly contributing to their extended shelf life and freshness. Similarly, edible coatings based on chitosan containing blueberry leaf extract also had a positive effect on the storage and freshness of the fruit. To extend the shelf life after harvest of tomatoes, 30% v/v ethanol and methanol extracts from fresh broccoli residues were used. The results showed that the application of broccoli extract to cherry tomatoes reduced weight loss and maintained the firmness, color, phenolic content, and antioxidant capacity of the fruit during 36 days of storage at 21 °C compared to untreated fruits [151,152].

In addition to edible films for protecting fruits and vegetables, phenolic extracts have also been used as antioxidants in animal-derived foods, such as meat products. Tea extracts rich in catechins were successfully applied to inhibit lipid oxidation in minced meat, poultry, and fish. Likewise, grape pomace extract proved to be a suitable antioxidant for meat products and was incorporated into minced poultry meat. This extract, used at concentrations of 1 wt% and 2 wt%, effectively inhibited the development of thiobarbituric acid, with samples treated with grape pomace extract exhibiting values of thiobarbituric acid ten times lower compared to untreated samples [153,154].

In addition to synthetic preservatives used to maintain the freshness, color, and taste of foods, synthetic sprays are also applied to plants such as cereals, fruits, and vegetables to protect against fungal diseases, viral diseases, or pest infestations [155,156,157]. However, these synthetic crop protection agents are often toxic to the environment and animals, particularly beneficial insects such as bees [158]. As an alternative protection method against fungal diseases caused mainly by fungi of the genera Fusarium, Aspergillus and Penicillium, which produce most known mycotoxins, 21 phenolic compounds in plant extracts were tested for their ability to inhibit fungal growth. The results of this study showed that at a concentration of 1000 µg/mL, ten phenolic compounds (thymol, carvacrol, isoeugenol, eugenol, 2-ethylphenol, 4-ethylphenol, salicylaldehyde, 2-methoxy-4-methylphenol, 4-ethylguaiacol and salicylic acid) exhibited the most potent inhibitory effects on fungal development, with thymol and carvacrol showing the highest inhibitory efficacy [155].

In addition to the antifungal properties of phenolic biomolecules, the insecticidal effects of these phytochemicals have also been studied. In recent years, the protection of crops has required the development of environmentally friendly pest control agents to ensure food safety, sustainability, and environmental protection [158]. Natural extracts with insecticidal effects have been shown to be successful in their original natural form or as precursors for their synthetic derivatives. Evidence of the insecticidal effects of phenolic biomolecules is provided in the study by Gautam et al. [156], who examined purified phenolic extracts of Alocacia nilotica (catechin, chlorogenic acid, and umbelliferone) for their inhibitory activity against the stages of development of the eggs, pupae, and adults of the tobacco cutworm Spodoptera litura. These biomolecules effectively inhibited larval growth and development, resulting in a reduced population of adult individuals due to the phenolic compounds used. These studies collectively indicate that phenolic compounds derived from natural sources could represent a suitable and sustainable alternative to conventional synthetic pesticides [157,158,159].

Another potential application of phenolic compounds extracted from plants is the reduction in cattle methane production. The antimetabolic effects of phenolic compounds, such as tannins and flavonoids, have been investigated. Essential oils derived from white thyme and oregano have been shown to reduce methanogenesis in the rumen of cattle without negatively affecting the digestibility of the feed. Condensed and hydrolyzable tannins have been found to represent promising solutions for reducing CH₄ emissions [159]. Tannins may limit methane synthesis during digestion either directly or indirectly by inhibiting methanogens or protozoa [160]. However, it is essential to note that inhibition of methane by tannins can vary considerably depending on the source and type of tannin, its molecular weight, and the methanogenic colonies present in the digestive system of the animals [159].

5.3. Pharmaceutical and Cosmetic Industries

The biological properties of phenolic biomolecules have been used for thousands of years in traditional folk medicine. These compounds are also widely used in the production of natural cosmetics. Currently, phenolic compounds extracted from various parts of plants have been shown to exhibit antibiotic, antiviral, antifungal, anticancer, neuroprotective, cardioprotective, and other health benefits [161]. As a result of all of these positive effects, phenolic biomolecules represent suitable and nontoxic alternatives to synthetic drugs, which often have adverse side effects. In addition to their therapeutic benefits, phenolic compounds such as spruce bark lignin or stilbenes have shown favorable effects in blocking UV radiation, as confirmed by studies in which these compounds were incorporated into sunscreen formulations [130,131].

The antibiotic properties of phenolic phytocompounds have been confirmed in several studies. For example, an ethanol extract of nutmeg (Myristica fragrans), containing 3′,4′,7-trihydroxyflavone, is effective against Gram-negative bacteria, such as Providencia stuartii and Escherichia coli. Furthermore, flavonoids and other phenolic plant compounds exhibit effects against Propionibacterium acnes, the leading bacterial cause of acne, a common skin condition, particularly among adolescents. An extract of Impatiens balsamina L. containing kaempferol effectively inhibited the growth of P. acnes and, in combination with clindamycin and quercetin, showed positive synergistic effects in suppressing bacterial growth. Similar antibacterial effects against P. acnes were also observed for flavones extracted from the roots of Scutellaria baicalensis Georgi in both in vitro and in vivo models [162,163].

In addition to the antibacterial properties mentioned above, flavonoids can also promote apoptosis in various types of cancer cells. For example, recent studies have revealed that quercetin exhibits promising anticancer effects against prostate and breast cancer. The effects of quercetin were also investigated in nine cancer cell lines, including prostate adenocarcinoma, colon carcinoma, acute lymphoblastic leukemia, and ovarian cancer cells. The results of this investigation showed that quercetin could induce apoptosis in all cell lines tested compared to control groups [47,164].

Tylkowski et al. investigated the biological activity of extracts from Norway spruce needles (Picea abies). They found that the spruce needle extract was rich in the flavonoid luteolin, which exhibited cytotoxic effects against cancer cells of oral squamous cell carcinoma. Resveratrol, mainly present in red fruits but also found in coniferous trees, has been identified as a potentially effective compound in the treatment of the early stages of Alzheimer’s disease [10,165,166,167].

5.4. Protection of Cultural Heritage Objects

Cultural and historical relics encompass a diverse range of items, including buildings, artifacts, books, manuscripts, works of art, garments, tools, and similar objects. Historical objects are subject to various degradation processes caused by biological factors such as micro- and macroorganisms (biodeterioration), as well as by physical and chemical agents that induce material changes in cultural artifacts. The most significant threats to the preservation of historical and modern works are bacteria, fungi, yeast, molds, mosses, and insects. Fungi are among the leading causes of deterioration of paintings, textiles, wood, paper, parchment, and even ceramics or stone. In fact, biological deteriorating agents, such as mold and fungi, are ubiquitous in historical and modern objects housed in libraries, museums, and archives. This is often due to the poor physical condition of the buildings of these institutions, which promotes the continuous colonization of objects of cultural heritage by microorganisms [168,169,170].

Various types of plant extracts, including essential oils from basil, clove, tea tree, pine and eucalyptus, have been investigated for their protective properties against deterioration of historical objects. These extracts were applied to a mosaic located in the archeological park of Ostia Antica in Rome. As a reference biocide, Preventol RI50 (quaternary ammonium salt) and two commercially available biobased products, Essenzio and Biotersus, were used. The study demonstrated moderate biocidal effects of individual essential oils compared to conventional Preventol RI50. However, when mixtures of specific essential oils were applied, a synergistic enhancement of biocidal properties was observed in the treatment of the Roman mosaic [171].

The antimicrobial activity of Origanum vulgare essential oils was also studied against seven different species of sac fungi of the genus Aspergillus, which contaminate various materials used in artwork, such as paper, silk, and even stone. These extracts were also compared with commercially used biocides. The study concluded that Origanum vulgare extracts exhibited more potent antifungal activity than the commercial biocidal agent [172].

Palla et al. [170] also investigated extracts from O. vulgare and Thymus vulgaris, which were applied to 18th-century wooden sculptures affected by the fungus Aspergillus flavus and the woodboring beetle Anobium punctatum. The outcome of this study showed that volatile compounds present in essential oils reduced the viability of fungal colonies and simultaneously acted as repellents against woodboring insects [170]. However, more research is essential to investigate the biocidal properties of plant-derived extractive compounds as alternative, sustainable, and nontoxic agents for protecting historical, cultural and natural heritage relics.

6. Conclusions

The main objective of this review was to highlight the potential of deep eutectic solvents as green alternatives to conventional organic solvents for the valorization of wood-processing by-products, particularly bark from economically crucial European tree species. Bark derived from selected deciduous and coniferous species, such as Fagus sylvatica, Quercus robur/petraea, Carpinus betulus, Pinus sylvestris, Picea abies, and Abies alba, is rich in phenolic compounds with pronounced antimicrobial, antioxidant, neuroprotective, and cardioprotective activities, as well as UV-blocking properties. These characteristics make bark phenolics attractive for high-value applications in the pharmaceutical, cosmetic, food, and polymer processing industries, including their use as natural antioxidants, UV stabilizers, and renewable components in biobased adhesives.

A critical comparison of conventional and nonconventional extraction techniques reveals that DESs constitute a promising solvent class. Their tunable composition allows them to be tailored to specific phenolic targets and, in many cases, results in higher total phenolic content and improved antioxidant activities compared to traditional solvents such as water, methanol, or ethanol. However, data from the literature also demonstrate that DES performance is highly system-dependent. Some DES formulations are efficient delignification or extraction media, while others perform worse than optimized aqueous extractions for the same bark matrix. This underlines that DESs cannot be treated as universally ‘better’ solvents, but rather as designer media that require rational selection for each feedstock and target compound group.

Future research should therefore focus on the systematic design of DESs/NADESs for bark phenolic extraction, considering not only green chemistry principles but also the molecular interactions between solvent components and target phenolics, viscosity and mass transfer limitations, and the influence of water content. The future success of DES deployment relies on establishing integrated, economically viable processes that seamlessly combine efficient biomass extraction with downstream target compound isolation and subsequent solvent recovery. In this context, the use of NADESs or therapeutic DESs that can function simultaneously as extraction media and final formulations offers an attractive route to minimize downstream processing.

In general, the available evidence indicates that DESs have great potential to become key tools for the selective recovery of phenolic compounds from bark and other lignocellulosic residues. When combined with appropriate recovery and recycling strategies, DES-based extraction can contribute to more sustainable, integrated wood-biomass biorefineries and to the substitution of fossil-based chemicals in several industrial sectors.