Quercus suber: A Promising Sustainable Raw Material for Cosmetic Application

Abstract

There is a drive within the cosmetic industry towards the development of more sustainable products, supported by consumer awareness of the environmental footprint. The cosmetic industry is rising to meet consumer demand by following practices, such as the use of by-products from agro-industrial waste. Quercus suber is a tree prevalent in the Mediterranean basin. The extraction of cork is considered sustainable, as this process does not harm the tree, and the amount of cork produced increases with the number of extractions. Beyond this, the cork industry produces by-products that are used to sustain the industry itself, such as cork powder, which is reused for generating energy. Additionally, cork and cork by-products contain bioactive compounds mainly with antioxidant activity that can be of use to the cosmetic industry, such as for antiaging, anti-acne, anti-inflammatory, and depigmenting cosmetic products. We provide the reader with an overview of the putative cosmetic applications of cork and its by-products as well as of their bioactive compounds. It is noteworthy that only a few cork-based cosmetic products have reached the market, namely antiaging and exfoliant products. Clearly, the use of cork upcycled cosmetic ingredients will evolve in the future considering the wide array of biological activities already reported.

1. Introduction

For several years now, planet Earth has been facing a significant increase in human activity, reaching worrying levels regarding the consumption of natural resources and other environmental issues, such as climate change, pollution, destruction of forests, and consequently, a decrease in biodiversity [1,2]. Sustainable development is defined by the United Nations as meeting the needs of the present without compromising the ability of future generations to meet their own needs, which is directly related to the preservation of natural resources [1,3]. Many industries started to adopt this concept following the rising interest of consumers in eco-friendly natural products, and the cosmetic industry was no exception [4,5,6]. Sustainability has been a challenge for this industry since formulating products with new eco-friendly ingredients can lead to stability, aesthetics, and effectiveness issues [7].

With the growing environmental concerns about climate change and sustainability, consumers expect cosmetic products to contain ingredients of natural origin from sustainable and renewable resources [8]. Thus, cosmetic industries have created innovative green products and have increasingly focused on strategies to reduce their environmental footprint using a life cycle assessment (LCA) [9]. The LCA is an approach that takes into account the environmental impact, namely resources and emissions at all stages of the production of a product [9]. The stages analyzed go from the sourcing of the raw materials to a post-consumer phase, where recycling and waste disposal take place [9]. Also, the reuse of by-products that are considered waste for the agronomic industry has gained increasing attention from the cosmetic industry, adopting the concept of circular beauty [10]. The use of these upcycled raw materials are one of the great pillars to reduce the environmental impact [9,10].

2. Quercus suber and the Cork Industry

Quercus L. is a genus that belongs to the Fagaceae family and includes several species of trees, around 450 different species scattered throughout the world [11,12]. This genus can be divided into two subgenera: one composed of species that live in temperate regions generally in the Northern Hemisphere, named Quercus, and another group that includes trees typical from subtropical regions in Asia, mostly in the east and southeast, called Cyclobalanopsis [13].

Quercus suber, the cork oak, is a slow-growing evergreen tree that can live up to 200 years or more and is native to Mediterranean countries, such as European-like Portugal, Spain, France, Italy and North-African-like Tunisia, Algeria, and Morocco [14,15]. Q. suber has a unique bark, thick and porous and with fissures, that protects the tree cells from the aggressions of the outside environment, such as forest fires [16]. This bark, known as cork, is exploited by humans without endangering the tree vitality since it has the ability to regenerate as it is being harvested through the years [17]. Additionally, cork oak has the capacity to prevent soil erosion and desertification, to regulate the hydrological cycle, and to reduce CO₂ emissions, protecting the biodiversity around it. For those reasons, this tree is part of an agroforestry system called “montado” in Portugal and “dehesa” in Spain that brings together forests, livestock, and agriculture [17,18,19,20].

Cork oak was highly used for its wood to build ships or manufacture tools, although nowadays, the primary use of this tree is the extraction of cork, the common name given to the cork oak outer bark that has outstanding properties, making it an important raw material for numerous applications [12,16]. Cork is a light material, is impermeable and compressible, is a good acoustic insulator, has low thermal conductivity, and has a high capacity to absorb energy and to resist impact and friction [17]. Thus, the main applications of cork are insulation, flooring, cork agglomerates and natural cork stoppers [21,22]. As several studies have shown, cork is also a source of bioactive compounds [13,23,24,25,26,27,28,29]. Cork extracts revealed the presence of substances with biological activity that could be incorporated in pharmaceutical and cosmetic formulations [4,13,27,28,29,30,31,32,33,34,35,36]. Therefore, we can consider that cork is a renewable and sustainable resource, currently gaining more interest as a raw material [7,9]. Nowadays, the cork industry is important in the economy of countries, such as Portugal and Spain. Portugal detains about 55% of the world production of cork [22]. The whole process begins with the first cork harvest. When the tree reaches between 25 and 30 years, the process of extraction of cork starts [12,16,17,22]. The first harvested cork is called “virgin cork”, has poor quality, and cracks easily; thus, it is not used in most applications [16,37]. Then it takes 9 to 12 years to extract subsequent layers of cork from the same tree; thus, the bark obtains the adequate thickness [16,37]. The second extracted cork is called “first reproduction cork” and has better quality; however, it is used essentially to produce cork agglomerates [16,37]. Only in the third extraction, the “second reproduction cork”, cork is used for its major purposes, such as to produce cork wine stoppers [16,22,37]. To avoid any damage to the tree, cork stripping is only done manually when the tree has perfect physiological conditions to remove the cork, being a seasonal activity in the transition from spring to summer [17]. On the other hand, as can be seen, this process is fully sustainable since the tree remains intact, and throughout each extraction it produces more and more cork [19].

Subsequently, the cork industry produces a lot of waste to manufacture the final products. However, this industry is sustainably developed since, for example, the cork powder is used as a fuel in generators to produce heat and energy in the factories that process cork [17,19,25,38]. Cork waste is often reused, and it is increasingly valued both for the production of new materials and for incorporation in pharmaceutical and cosmetic products since its chemical composition is being highly studied, revealing interest in certain bioactive compounds that it presents [19,25]. Therefore, this industry presents high ecological, economic, and social value; thus, cork oak forests and cork waste need to be correctly managed to continue to be sustainable [17].

3. Cork By-Products and Applications

One of the main problems inherent in the cork industry is related to the amount of waste that is generated by the processes of cork production and transformation, reaching values close to 50,000 tons per year [4]. Therefore, it is important to overcome this situation to reduce waste through its valorisation on many industrial applications. Figure 1 show the main by-products of the cork industry.

In the production of cork disks and stoppers, some of these remnants are appropriate to obtain agglomerates through the application of high pressure and temperature in autoclaves [24]. Agglomerates, as the final product, keep some characteristics from the natural cork, such as elevated resistance and low thermal, acoustic and vibrational conductivity [39]. As expanded agglomerates result from exposure to superheated steam without using synthetic adhesives, this material is considered an environmentally sustainable by-product [19,39].

Accompanying industrial transformation processes, cork powder emerges from the granulometric separation once these particles are not viable to produce agglomerates [25]. This by-product is considered the main waste, representing about 25% of the raw materials that gather particles with dimensions lower than 0.25 mm [40]. Cork powder has a high heating value which is commonly used as a combustion on boilers for energy production [24,41]. Furthermore, cork powder can also be used as a filling agent to improve the quality of cork stoppers, incorporated in agglomerates and briquettes, on linoleum production, in agriculture, in the fabrication of explosives, and as a source of relevant chemicals [4,41,42]. Recent studies point to the pertinent ability of cork waste to prepare biomass materials [43]. Activated carbon can be prepared from the chemical or physical activation of cork where its adsorption properties are improved [44]. This transformation has proven to be effective to control the atmospheric levels of CO₂, storing carbon for long periods and reducing their release to the atmosphere [45]. In addition, cork-based activated carbon has been shown to be able to remove some pharmaceuticals from water, such as paracetamol, isoprofen, or iopamidol, and is a relatively fast adsorber of methylene blue [46,47]. This biosorbent has begun to gain some notoriety, constituting a sustainable alternative for contaminants removal, including heavy metals, such as Cu (II), Zn (II), Cr (VI), and Ni (II), due to the low costs, great efficacy, and environmental protection legislation [41,47,48].

Black condensate is another by-product that comes from the production of black agglomerate in the insulation corkboard industry. These corkboards result from the treatment of cork particles under elevated pressure and temperature conditions (250–500 °C) which originates a dark liquid that works as an adhesive and vapours that condensate in autoclave pipes, and black condensate that is removed as a pasty solid [24,49]. It is currently used to produce energy from its combustion, although its hydrophobic character can be used as a potential protective coating [49].

In one of the initial stages of cork stopper production, cork planks are boiled in water to increase its elasticity and to remove impurities, where cooking wastewaters are obtained. Usually, industries reuse these waters for several cycles resulting in a dark effluent known as cork-boiling wastewaters, with a high content of water-soluble compounds [50]. Although the composition of these waters is dependent on the type of cork and the number of boiling cycles, the main components that are present include phenolic compounds, tannins, and 2,4,6-trichloroanisol without suberin [51]. This by-product needs a previous treatment before disposal because it exceeds the legal limits of contaminants imposed for residual wastewaters [50]. Therefore, several methodologies, essentially based on chemical, physical, and even biological processes, have been tested over the years to decrease the level of contaminants [50]. For cork industry applications, gamma radiation treatment values this effluent by increasing the antioxidant potential of phenolic compounds whose recovery can be beneficial to other industries [51].

4. Bioactive Compounds on Cork and Cork By-Products

The composition of cork includes a variety of compounds from different chemical families, namely terpenes, sterols, saccharides, suberin, lignin, and other phenolic compounds, whose concentrations are dependent on several factors, such as climate, region, age, or the part of the tree [23,52]. Cork extractives are constituted by compounds with low molecular weight that are not connected to structural elements [33]. Aliphatic extractives, also known as waxes, are originated using nonpolar solvents, such as hexane or dichloromethane, while phenolic extractives are obtained from polar solvents, such as water or ethanol [53].

Suberin is the major component found on cork cell walls (30–50%), responsible for the low permeability and elasticity of this material, working as a protective barrier from the environment [54]. Cork suberin is a lipophilic polyester macromolecule, where monomers, such as long-chain fatty components, glycerol, hydroxyfatty, and phenolic acids are connected by ester groups [54,55,56]. The analysis of its monomeric fractions is possible thanks to depolymerization methods, concluding that suberin is constituted by an aromatic and an aliphatic domain. Among them, long-chain ω-hydroxyfatty acids and α,ω-dicarboxylic acids are the main aliphatic components, while ferulic acid is the principal aromatic component [57]. Despite being normally discarded due to its poor quality, virgin cork usually has more suberin than reproduction cork. In addition, cork powder is another rich source of suberin that can be valued in new applications [58]. For example, the aliphatic components from suberin are scarce in nature and can favour their industrial interest for the synthesis of polymeric materials [56,59]. Suberin extracts also showed antimutagenic properties and skin-firming properties [36] and acted in a desmutagenic manner [60].

Lignin is another hydrophobic polymer present on cork which works as the mechanical support of cell walls, believed to be the principal aromatic fraction of cork [61,62]. Studies indicate that lignin appears in the three layers from cork cell walls, although at different concentrations [63]. Cork powder often contains greater amounts of lignin than the original cork [64]. Additionally, lignin contains UV-absorbing properties which make it interesting to incorporate in sunscreens [65].

The minor components, cellulose and hemicellulose, are hydrophilic polysaccharides that confer structural rigidity to cork cells [66]. Even so, the bark of Q. suber L. is essentially composed of the monomeric units of glucose, xylose, and arabinose, contrary to what happens with other species [23].

Waxes include lipophilic, aliphatic, and aromatic compounds that along with suberin contribute to cork impermeability [67]. Triterpenes are the most abundant compounds found on waxes in addition to still having n-alkanes, n-alkanols and fatty acids [53]. Cerin (1), friedelin (2), betulin (3), betulinic acid (4), and sterols are examples of triterpenes that can be used as bioactive components (Figure 2) [68,69]. Dichloromethane cork extracts have been found to have a high amount of friedelin (2), betulin (3), betulinic acid (4), and β-amyrin (5), as well as sterols, such as sitost-4-en-3-one (6) (Figure 2) [28]. However, a higher number of sterols can be obtained using supercritical CO₂ as a solvent. Cork extraction using a dichloromethane/methanol mixture demonstrated that the most abundant triterpenes on cork are cerin (1) and friedelin (2), while betulinic acid (4) and friedelin (2) are the main components from cork powder and black condensate, respectively [59,67].

Phenolic compounds are important secondary metabolites that present a wide range of biological activities [70]. In fact, many epidemiological studies point to the health benefits of fruit and vegetable intake, owing to the presence of many antioxidant phytochemicals [71]. They represent the second most abundant group of organic compounds in the plant kingdom produced as a response to the influence of biotic and abiotic factors and whose main functions include support, hormonal regulation, seed gemination, protection against pathogens, with herbivores and UV radiation also being involved in flavour, smell and colour [72].

These compounds have at least one hydroxyl group attached to the aromatic ring with great structural diversity ranging from simple to complex structures obtained through the shikimate pathway [73]. Phenolics, also known as phenylpropanoids, can be classified into essentially two large groups named as flavonoids and nonflavonoids. Chemically, flavonoids have a fifteen-carbon backbone (C6-C3-C6) with two phenyl rings, A and B, attached through a three-carbon chain that normally arises as a heterocyclic pyran ring [74]. On the other hand, nonflavonoids encompass phenolic compounds, usually with a relatively simpler structure than flavonoids, such as phenolic acids, coumarins, stilbenes, hydrolysable, and condensed tannins and lignans [75].

The antioxidant, anti-inflammatory, antimicrobial, and anticancer properties of phenolic compounds make them very attractive for pharmaceutical, cosmetic, or food applications [76]. Hence, the valuation of this raw material and its by-products increasingly involves the identification of their phenolic composition [27].

Cork phenolics are obtained by polar solvent extraction, and even though its composition is variable within trees and geographic location, it essentially includes phenolic acids and aldehydes, coumarins, flavonoids, and tannins [31]. Methanol/water mixtures are the most frequently employed methods to extract cork phenolics, often followed by an organic solvent [32,76]. The cork extracts can also be prepared through sequential extraction with increasing polarity solvents [77]. In 2015, Bouras and co-workers reported a microwave-assisted extraction method, using different proportions of water, methanol, and ethanol, demonstrating that the use of these alcohols promotes a significant improvement of polyphenol recovery. Among them, p-coumaric (7), syringic (8), and sinapic (9) acids are the major compounds on bark extracts (Figure 3) [78]. In the same year, a new method for the extraction of phenolic compounds from cork granulates using a mixture of water with propylene glycol was reported [76,79].

Hydrolysable tannins and low molecular weight phenolic compounds are suggested as the potential bioactive compounds from Quercus suber bark [27]. The most common phenolic acids in cork are ellagic (10), protocatechuic (11), gallic (12), vanillic (13), ferulic (14), and caffeic (15) acids, whereas phenolic aldehydes include vanillin (16), protocatechuic aldehyde (17), and coniferyl aldehyde (18) (Figure 3) [80,81].

Tannins can be monomeric or polymeric and even condensed or hydrolysable, which are related to bitterness and astringency of wines as a result of binding to salivary proteins [82]. Several studies indicate that these phenolics, namely castalagin (19), grandinin (20), vescalagin (21), and roburin (22) are capable of migrating from cork stoppers to wine solutions after bottling and may interfere with its organoleptic properties, such as taste, colour, or bitterness or participate on wine oxidation (Figure 4) [83,84,85].

The first reported HPLC analysis of cork extract prepared with a methanol/water mixture showed that the most abundant phenolic compounds are the phenolic acids, ellagic (10), and protocatechuic (11) [32]. Aldehydes, such as protocatechuic aldehyde (17), coniferyl aldehyde (18), and vanillin (16) and coumarins, such as scopoletin (23) and aesculetin (24), also appear, although in much smaller amounts (Figure 5) [24,32].

Santos et al. prepared cork extracts by two distinct extraction routes using a mixture of methanol 20% followed by diethyl ether fractionation and sequential extraction with methanol and water and analysed the differences in the phenolics extracted according to the solvent [80]. Thus, even though the aqueous extract had the higher phenol and p-hydroxybenzoic acid (25) (Figure 3) contents, the amount of ellagic acid (10) was vestigial, while some compounds, such as coumaric (7), vanillic (13), or ferulic (14) acids did not appear [4].

The cork hydroglycolic extract prepared by Batista and co-workers was shown to be mainly constituted by ellagic acid (10) and ellagitannins, such as castalagin (19), vescalagin (21), and roburin (22), as well as protocatechuic acid (11) and gallic acid (12) [79].

In 2013, Santos and co-workers extended the identification of cork phenolics to extracts prepared from cork powder and black condensate using methanol/water mixture (50%). Ellagic acid (10), gallic acid (12), protocatechuic acid (11), quinic acid (26) (Figure 6), and aesculetin (24) were present in all extracts. However, ferulic acid (14) only appears on cork powder extract, while coumaric acid (7), vanillin (16), coniferyl aldehyde (18), and p-hydroxyphenyllactic acid (27) (Figure 6) emerged on black condensate extract [24]. The major compounds on cork extract were ellagic (10) and gallic (12) acids, while on cork powder extract they were gallic acid (12) and aesculetin (24), and finally on black condensate they were gallic acid (12), coniferyl aldehyde (18), and aesculetin (24).

As mentioned before, cork-cooking wastewaters are also rich in phenolic acids, mostly ellagic (10) followed by gallic (12), protocatechuic (11), and ferulic (14) [26,86].

Biological Activity of Cork Extracts and Cork By-Products

Cork and its by-products constitute a source of bioactive compounds with antioxidant activity that can be of use to the cosmetic industry [87]. In the scientific literature, some studies have already described the extraction of compounds from cork, cork acorns, and cork by-products, such as cork powder, black condensate, and cork-cooking wastewater. These extracts showed the presence of phenolic acids, such as ellagic (10), protocatechuic (11), gallic (12), vanillic (13), ferulic (14), and caffeic (15) and ellagitannins with antioxidant and protective DNA activity as well as collagenase and elastase inhibitory activity that can be interesting for antiaging cosmetics (

Table 1. Chemical composition and biological activity of cork extracts and cork by-products.

Extraction Solvent and Source Material	Composition	Quantification		Biological Activity	References
methanol/water (80:20); diethylether   granulated cork from Spain	Ellagic acid (10)	228.4	µg of compound/g of dry cork	--------	[32]
	Protocatechuic acid (11)	48.8
	Vanillic acid (13)	27.4
	Gallic acid (12)	18.3
	Scopoletin (23)	12.7
	Vanillin (16)	16.1
	Coniferaldehyde (18)	11.2
	Protocatechuic aldehyde (17)	8.1
	Caffeic acid (15)	12.1
	Ferulic acid (14)	10.7
	Aesculetin (24)	7.5
	Sinapaldehyde	4.5
supercritical CO2   granulated cork	Friedelin (2)	30.6	mg of compound/extract	--------	[28]
	Sitost-4-en-3-one (6)	22.5
	β-Sitosterol	6.59
	Betulinic acid (4)	4.93
	Betulin (3)	3.13
dichloromethane   granulated cork	Friedelin (2)	30.2	mg of compound/extract	--------	[28]
	Sitost-4-en-3-one (6)	4.1
	Betulinic acid (4)	10.5
	Betulin (3)	3.9
	Protocatechuic aldehyde (17)
	Vanillin (16)
	Protocatechuic acid (11)
	Gallic acid (12)
	Conyferaldehyde (18)
	Caffeic acid (15)
	Ferulic acid (14)
	Ellagic acid (10)
	Ellagic acid-pentose
	Ellagic acid-deoxyhexose
wine solution (12% ethanol, 5.0 g/L tartaric acid, pH = 3.2); ethyl acetate   granulated cork	Ellagic acid-hexose	--------		---------	[31,83,84]
	Valoneic acid dilactone
	HHDP-glucose
	Valoneic acid
	Dehydrated tergallic-C-glucoside HHDP-galloyl-glucose
	Trigalloy-glucose
	Di-HHDP-glucose
	HHDP-digalloyl-glucose
	Tetragalloyl-glucose
	Castalagin (19)
	Vescalagin (21)
	Di-HHDP-galloyl-glucose
	Trigalloyl-HHDP-glucose
	Pentagalloyl-glucose
	Mongolicain A and B
water; water/ethanol (50:50)   granulated cork	Castalagin (19)	46.9	mg of compound/g extract	Antioxidant activity (DPPH (EC50) = 5.32 ± 0.45 µg of extract/mL; ORAC = 2.11 ± 0.24 mgTeq/gextract)	[29]
	Ellagic acid (10)	26.7
	Vescalagin (21)	22.4
	Gallic acid (12)	2.9
dichloromethane; methanol/water   cork powder	Betulinic acid (4)	11719	mg of compound/kg of cork powder	--------	[67]
	Cerin (1)	2060
	Friedelin (2)	2009
	Ellagic acid (10)	1347
	Betulin (3)	875
	β-Sitosterol	254
	Ursolic acid	104
	Lupeol	60
subcritical water   granulated cork	Gallic acid (12)	4.9 ± 0.9	mg of compound/g extract)	Antioxidant activity (EC50 = 0.25 mg extract/mg DPPH)	[76]
	Ferulic acid (14)	0.6 ± 0.1
	Caffeic acid (15)	0.5 ± 0.1
	Ellagic acid (10)	6800–8200		Antioxidant activity (ORAC = 22,603 ± 2097 ymolET/L; HORAC = 15,712 ± 1419 µmolEAC/L; HOSC = 22,678 ± 3225 pmolET/L; DPPH = 1.68 (IC50) mL/L; O2− = 11.08 (IC50) mL/L) Antiaging activity: inhibition of MMP-1, MMP-3, MMP-9 activity; inhibition of ROS formation in keratinocytes and fibroblasts. Depigmenting activity: inhibition of tyrosinase activity; inhibition of melanin production in melanocytes. Anti-inflammatory activity: inhibition of NO production; reduction of IL-6, TNF-α, CCL5 levels; reduction of the activation of NF-kB. Inhibition of lipid accumulation in keratinocytes (inhibition of SREBP-1 gene expression)
	Roburin (22) and Grandinin (20)	500–3200
water/propylene glycol (40:60)   granulated cork	Castalagin (19)	1800–2100	µg of compound/g of dry cork		[79]
	Vescalagin (21)	800–1900
	Protocatechuic acid (11)	100–130
	Gallic acid (12)	60–100
methanol/water (80:20); diethyl ether   granulated cork from Portugal	Ellagic acid (10)	2031.5	mg of compound/kg of dry cork	Antioxidant activity (DPPH (IC50) = 2.79 ± 0.15 µg of extract/mL)	[80]
	Caffeic acid (15)	57.6
	Salicylic acid	32.7
	Gallic acid (12)	30.6
	Eriodictyol	27.4
	Protocatechuic acid (11)	17.5
	Vanillin (16)	14.3
	Aesculetin (24)	4.9
	Naringenin	2.6
	Vanillic acid (13)	Trace
	p-coumaric acid (7)	Trace
	Ferulic acid (14)	Trace
methanol   granulated cork from Portugal	Ellagic acid (10)	1576.9	mg of compound/kg of dry cork	Antioxidant activity (DPPH (IC50) = 3.58 ± 0.20 µg of extract/mL)	[80]
	Aesculetin (24)	106.7
	Protocatechuic acid (11)	59.0
	Gallic acid (12)	48.1
	Vanillin (16)	Trace
	Vanillic acid (13)	Trace
	Quinic acid (26)	Trace
water   granulated cork from Portugal	Ellagic acid (10)	526.5	mg of compound/kg of dry cork	Antioxidant activity (DPPH (IC50) = 5.84 ± 0.29 µg of extract/mL)	[80]
	Gallic acid (12)	241.6
	Protocatechuic acid (11)	118.3
	Caffeic acid (15)	12.9
	p-hydroxybenzoic acid (25)	1.0
	p-hydroxyphenyllactic acid (27)	Trace
	Ellagic acid (10)	1246.46 ± 0.18	of dry cork		[24]
	Ellagic acid-pentoside	770.16 ± 0.15
	Gallic acid (12)	736.48 ± 1.63
	Aesculetin (24)	391.59 ± 1.10
methanol/water (50:50)   granulated cork	Quinic acid (26)	372.86 ± 1.94	mg of compound/kg	Antioxidant activity (DPPH (IC50) = 4.77 ± 0.02 µg of extract/mL)
	Methyl gallate	251.43 ± 0.06
	Brevifolin-carboxylic acid	102.03 ± 0.08
	Protocatechuic acid (11)	79.26 ± 0.10
	Ferulic acid (14)	Trace
	Coniferyl aldehyde (18)	Trace
	p-hydroxyphenyllactic acid (27)	Trace
	Valoneic acid dilactone	168.01 ± 0.70
	Caffeic acid isoprenyl ester	127.98 ± 0.28
	Isorhamnetin-rhamnoside	Trace
	Eriodictyol	Trace
	Isorhamnetin	Trace
methanol/water (50:50) cork powder   (by-product)	Ellagic acid (10)	527.59 ± 1.70	mg of compound/kg of dry cork powder	Antioxidant activity (DPPH (IC50) = 3.33 ± 0.02 µg of extract/mL)	[24]
	Gallic acid (12)	263.04 ± 0.52
	Aesculetin (24)	176.80 ± 0.60
	Quinic acid (26)	137.02 ± 0.50
	Methyl gallate	96.93 ± 0.56
	Ellagic acid-pentoside	46.18 ± 0.15
	Valoneic acid dilactone	46.05 ± 0.11
	Protocatechuic acid (11)	16.44 ± 0.01
	Ferulic acid (14)	14.77 ± 0.02
	Coniferyl aldehyde (18)	Trace
	Caffeic acid isoprenyl ester	82.47 ± 0.29
	Brevifolin-carboxylic acid	53.72 ± 0.15
	Isorhamnetin-rhamnoside	Trace
	Isorhamnetin	Trace
methanol/water (50:50)black condensate   (by-product)	Coniferyl aldehyde (18)	194.34 ± 0.56	mg of compound/kg of dry black condensate	Antioxidant activity (DPPH (IC50) = 1.57 ± 0.01 µg of extract/mL)	[24]
	Aesculetin (24)	125.28 ± 0.65
	Gallic acid (12)	118.46 ± 0.61
	Quinic acid (26)	117.17 ± 0.30
	Ellagic acid (10)	52.52 ± 0.18
	p-hydroxyphenyllactic acid (27)	49.36 ± 0.12
	p-coumaric acid (7)	35.76 ± 0.22
	Vanillin (16)	32.47 ± 0.25
	Caffeic acid (15)	17.68 ± 0.05
	Protocatechuic acid (11)	9.97 ± 0.03
	Ferulic acid (14)	Trace
	Eriodictyol	Trace
water/ethanol   granulated cork	Castalagin (19)	47.2	mg of compound/g of extract	Antioxidant activity (DPPH (EC50) = 7.9 ± 0.02 µg of extract/mL; ORAC = 1533 ± 147 mgTeq/gextract; FRAP = 1963 ± 126 mgTeq/gextract; TEAC = 802 ± 8 mgTeq/gextract)	[90]
	Vescalagin (21)	22.8
	Ellagic acid (10)	26.5
	β-O-ethylvescalagin	24.4
	Gallic acid (12)	0.6

HHDP—hexahydroxydiphenyl. --------: unreported biological activity.

) [4,79,88,89].

In addition to the in vitro studies that already exist and prove the biological activity of these extracts, there is also an in vivo study that proves the tensor and smoothing effect that cork extracts have on human skin [36]. Additionally, due to the presence of these phenolic compounds in cork and its by-products, there is the possibility of incorporating these ingredients in sunscreens, as it has been proven that these compounds, namely lignin [65], can absorb UV radiation.

Another known activity of phenolic compounds and flavonoids is the inhibition of tyrosinase in melanocytes by in vitro and in vivo studies [4]; hence, its interest in the development of depigmenting cosmetics is predictable. Thus, correlating the existence of these compounds in cork, namely ellagic (10) and gallic (12) acids, protocatechuic aldehyde (17) and ellagitannins, with their depigmenting activity, one of the possible cosmetic applications of cork is in depigmenting products for the treatment of skin blemishes [4,79,91].

Polyphenols can also inhibit the accumulation of lipids in keratinocytes and inhibit the expression of the SREBP-1 gene, which makes them promising in combating acne [92]. A cork hydroglycolic extract has also been studied for this activity, with favourable results [79]. In addition, cork powder has bioabsorbent properties, removing pollutants and oily substances. For this fact, it is being studied for protection of the environment and for the treatment of acne, absorbing the accumulated sebum on the skin [4,41,79]. In addition, for skin problems, such as acne, anti-inflammatory and antimicrobial cosmetics can be used. Compounds present in cork, such as suberin, friedelin (2), polysaccharide, and phenol (gallic acid (12) and ellagitannins) extracts have also demonstrated anti-inflammatory activity by the NO inhibitory activity in the presence of a pro-inflammatory stimulus and inhibitory activity of NF-kB transcription factor activation of a cork hydroglycolic extract [4,79]. On the other hand, cork extracts present bioactive compounds, namely protocatechuic (11) and gallic (12) acid, as well as ellagitannins, that have antimicrobial properties [80,93,94]. The antimicrobial activity of cork was also proven by preliminary studies, mainly against Staphylococcus aureus, with a MIC value of 6 mg/mL [4,95].

5. Current Cosmetic Applications of Cork

Cork and its by-products have been increasingly sought as a source of new ingredients for pharmaceutical and cosmetic use (Figure 7). In terms of commercial applications, cork is the main ingredient in a brand of anti-aging cosmetics which claims that the suberin present in the cork extract has a lifting effect on the skin [4,96]. There is only one cork cosmetic ingredient with established in vitro [79] and in vivo [36] activity on human skin described in the “CosIng” database, namely Quercus suber bark extract. This ingredient may be used alone as a unique ingredient or combined with other ingredients, such as plant extracts. The “CosIng” database is a cosmetic ingredients database from the European Commission, and it provides information on cosmetic ingredients, including their regulatory status according to the Regulation (EC) No 1223/2009 [97]. Several cork-based ingredients are available from different suppliers. The ACTISCRUB™ Cork by Lipotec is a cosmetic raw material constituted by a Q. suber bark extract and has exfoliant and peeling activity [98]. The Suberlift™ by Ashland Specialty Chemical is another cork raw material containing Q. suber bark extract with tensor and firming effects on human skin [99]. One last example, the DIAM Oléoactif^® by Hallstar, is a raw material constituted by a mixture of Cocos nucifera oil, oak root extract, and Q. suber bark extract with soothing, antiaging, anti-redness, and anti-inflammatory effects [100].

Furthermore, cork was also studied to be used in skin exfoliants. A granulated cork was studied and considered suitable as a mild exfoliant due to its morphology and properties. Cork particles were also tested as stabilizers of a Pickering emulsion for topical application with antioxidant and anti-elastase activity [4,34].

6. Conclusions

Cork obtained from Q. suber bark represents a natural and sustainable resource for various applications since its extraction does not harm the tree. The cork industry is also considered sustainable since most of the cork by-products that are formed during the process are reused to self-sustain the factory. However, the main problem inherent to this industry is the amount of waste generated throughout the process. For this reason, a new perspective on the potentialities of cork by-products aroused to take full advantage of its properties. Cork and its by-products are currently being studied for pharmaceutical and cosmetic application. Cork extract composition encompasses several compounds from different chemical families, thus attracting attention due to the different biological activities, translating into a variety of possible applications.

The growing concern for the environmental footprint is also reflected on the skin care market where consumers started to demand more natural and sustainable products. Cork extracts have a considerable number of bioactive compounds, especially phenolics. Cork phenolics, such as caffeic, gallic, ellagic, ferulic, vanillic, and protocatechuic acids, show remarkable antioxidant activity, owing to their radical-scavenging properties. Currently, cork and some cork extracts are already used in antiaging cosmetics and as mild exfoliants and are registered as cosmetic ingredients in official databases.

Even taking into account all the studies already carried out regarding cork and its by-products and their current applications, there is still room for innovation in this area. Although the results of in vitro studies on cork extracts are promising, they are still very preliminary, and it is necessary to conduct more studies to prove their effectiveness and safety for skin applications, namely studies to support skin depigmentation and anti-acne and antimicrobial claims as well as toxicological studies, according to the European Cosmetics Regulation No 1223/2009, “The SCCS Notes of Guidance for the Testing of Cosmetic Ingredients and their Safety Evaluation” and “Technical Document on Cosmetic Claims” [101,102,103]. In addition, more specific studies are needed regarding the analysis of bioactive compounds in cork by-products, such as cork powder, “black condensate”, and cork-cooking wastewater. Clearly, cork is an important resource for the cosmetic industry, but further studies are needed to unveil and confirm the full spectrum of its potentialities.