Virgin olive oils are obtained from the fruit of the olive tree (Olea europaea L.) solely by mechanical or other physical means under conditions that do not cause alterations in the oil, especially thermal conditions, and that have not been subjected to any other treatment other than washing, decantation, centrifugation or filtration [1]. Olive oil is a natural emollient that has been used for millennia for the treatment of the skin. This oil has a high percentage of pure oleic acid, which, together with two other essential fatty acids, linoleic and linolenic acid, also present in olive oil, play a fundamental role in maintaining the barrier function of the skin, facilitating the penetration of active substances into the deeper layers, reducing transepidermal water loss (TEWL) and improving the protective function [2]. Other components of olive oil of importance in skin care are antioxidants, especially phenols and polyphenols (tyrosol, hydroxytyrosol (HT), caffeic acid, and oleuropein, among others), in addition to tocopherols. Antioxidants are anti-aging actives used in skincare ^[3][4][3,4]. These compounds can be found both in the oil and in the by-products obtained during its production. The olive oil industry produces large amounts of waste, such as olive pomace, olive mill wastewater and leaves. Wet olive pomace, or “alpeorujo”, is the main by-product obtained by the two-phase centrifugation system during the extraction of virgin olive oil. It contains the solid residue of the olive (consisting of the stone, the skin and the mesocarp) together with the vegetation waters (natural water in the olive plus the water added during the extraction process). Alpeorujo is a semi-solid residue with a high moisture content (55–65%), and it includes a large amount of organic matter in its composition, among which many phenolic compounds can be found [5]. Therefore, it is a very attractive by-product for obtaining compounds with high added value for cosmetics ^[6][7][8][6,7,8]. Olive leaves are waste from the pruning of the olive tree and represent 10% of its total weight [9]. They are usually used as animal feed ^[10][11][10,11]; however, they may be used in other applications, such as cosmetics, pharmaceuticals and food. The concentration of phenolic compounds in olive leaves changes with the quality, origin and variety of the olive tree [12]. Furthermore, many of the compounds found in olive leaves show a wide range of biological activity in addition to their antioxidant capacity, such as antifungal, antibacterial [13], antiviral [14] and therapeutic ^[15][16][15,16].

2. Olive By-Products for Skin Care

The compounds derived from the olive tree, which are used in cosmetics because of their properties, can be categorized as hydrophilic and lipophilic agents. Hydrophilic compounds with important cosmetic uses are mainly polyphenols. The group of lipophilic compounds includes fatty acids, vitamins and squalene. All of them are extensively used in cosmetics and can be found in the olive, in olive leaves and branches, in the olive stone and in by-products that include olive mill wastewater (OMWW), alpeorujo, olive pomace and cake.

2.1. Hydrophilic Compounds: Polyphenols

Polyphenols stand out among the hydrophilic components due to their antioxidant capacity. Antioxidants are commonly used in cosmetics and dermatology to delay skin aging. Aging is a complex process that involves intrinsic and extrinsic factors. The most relevant extrinsic factors are ultraviolet radiation (UV), infrared (IR), visible light, blue light, alcohol consumption and cigarette smoke; they all have a cumulative effect on the skin. It becomes leathery, drier, showing signs of hyperpigmentation and deep wrinkles ^{[17][18][19][20][21][22][23][24]}[30,31,32,33,34,35,36,37]. These factors are involved in the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which cause lipid peroxidation, damage to desoxiribonucleic acid (DNA), degradation of the extracellular matrix, protein damage and glycation, also generating an inflammatory response of the skin, which triggers the immune response and causes additional oxidative stress that increases the damage ^[25][26][27][38,39,40]. The cosmetic and pharmaceutical industries are investigating the mechanisms involved in these processes and are looking for new anti-aging active ingredients ^[17][28][30,41]. Polyphenols are powerful antioxidants with anti-free radical activity; they can prevent or reduce the damage caused by ROS and RNS ^[6][29][30][6,42,43] and produce significant improvements in the elasticity, thickness and moisture of the skin when used topically ^{[19][26][31][32][33]}[32,39,44,45,46], thus resulting in good anti-aging agents ^{[4][34][35][36][37]}[4,47,48,49,50].

2.1.1. Polyphenols as Anti-Aging Agents

Rodrígues et al. [6] highlighted the antioxidant capacity associated with possible anti-aging action and protection from UVA and UVB radiation. Rodrígues et al. ^[30][43] mentioned the bioactives recovered from OMWW, which are particularly rich in polyphenols, HT, oleuropein, caffeic and ferulic acid, which could be used as complements to sunscreens, anti-aging and antibacterial agents. Tamasi et al. ^[38][61] analyzed extra virgin olive oil (EVOO) and the by-products from the olive industry, olive fruit and pomace, extracted the antioxidant components and identified and quantified tyrosol, HT and oleuropein. They performed cytotoxicity studies on the extracts and found that pomace is a promising source of bioactives and antioxidants with no cytotoxic effects.

2.1.2. Polyphenols in UV Protection

Additionally, it is important to highlight that olive derivatives rich in polyphenols have interesting properties in relation to photoprotection and antimicrobial activity. Olive polyphenols may contribute to photoprotection, and they increase the sun protection factor (SPF) when combined with UV filters, extending the UV protection to areas of the spectrum in which the UV filters do not have good absorption. Rodrígues et al. ^[30][43] also reviewed several studies in which natural antioxidants, among which olive antioxidants are found, contribute to improving the stability of UV filters and act as boosters of others.

2.1.3. Polyphenols as Antimicrobial Agents

Regarding antimicrobial activity, Ribeiro et al. ^[35][48] underlined a growing interest in finding natural components with antimicrobial activity that can replace synthetic antimicrobial preservatives. The phenolic compounds found in olive wastes may fit this purpose ^[35][36][48,49]. EU and Mercosur legislation, among others, establish requirements regarding the microbiological quality of cosmetic products and present positive lists of raw materials that can be considered preservatives, understanding by such “substances which are exclusively or mainly intended to inhibit the development of micro-organisms in the cosmetic product” ^[39][40][64,67].

2.2. Lipophilic Compounds

Olive oil and its derivatives are included in many skin and hair care cosmetic products, where they play a variety of roles. They contribute to the replacement of natural lipids, to retaining water in the stratum corneum of the skin and to improving cell renewal, adding elasticity and suppleness to the skin. They facilitate combing and provide shine to the hair. They form the oily phase of emulsions and can act as over-greasing agents in detergents for the skin and hair. Additionally, olive oil can contribute to the improvement of skin conditions, such as atopic dermatitis, psoriasis and eczema, moisturizing the dry, flaky skin, which becomes supple and flexible ^[6][41][6,72]. Several lipophilic compounds in the olive tree are also present in the skin. Their properties range from being emollient, moisturizing, and protective to being an antioxidant; they show healing activity and repair the function of the lipid barrier ^[41][42][72,73], and also act as lubricants, resulting in soft, elastic and lubricated skin, providing a feeling of well-being. Their activity results from their capacity to remain on the surface of the skin for long periods of time ^[43][44][45][74,75,76]. They can also be found in other cosmetic formulations, such as emulsions, oils, suspensions and gels ^[46][77].

2.2.1. Fatty Acids

Oleic acid (C18:1), the most abundant fatty acid from the olive tree, holds a particular interest in topical formulations and may be used as a facilitator for the entry of certain active ingredients through the skin barrier ^[41][47][48][72,78,79]. Essential polyunsaturated fatty acids (EFAs) linoleic (C18:2) and linolenic (C18:3) are important components of the cell membrane that cannot be synthesized by our body and whose topical application may contribute to the improvement of skin conditions, such as psoriasis, topical dermatitis and eczema. They also have anti-irritant and anti-inflammatory effects, protecting the skin against UV-induced damage ^[41][49][50][22,23,72].

2.2.2. Vitamin E

Vitamin E occurs in eight isomeric forms, α-tocopherol being the one with the highest concentration in olive oil ^[34][51][47,80]. Vitamin E is a very well-known non-enzymatic antioxidant ^[52][81]. It is widely used in topical formulations due to its free radical scavenging power, prevention of lipid oxidation of fatty acids and protection of cell membranes ^[53][69], participating, as well, in signaling pathways regarding inflammation, apoptosis and cell differentiation ^[51][80]. Vitamin E may be isolated from the oil or lipid waste, such as cake, and also from olive leaves ^[54][82] and from OMWW, associated with the remaining fat ^[55][19]. It is a lipophilic antioxidant, which could be used to complement hydrophilic antioxidants, the polyphenols, in formulations with both hydrophilic and lipophilic components, such as emulsions.

2.2.3. Carotenoids

Carotenoids are natural pigments with multiple conjugated double bonds, high antioxidant capacity and low water solubility, thus hindering their incorporation into cosmetic products. They may be extracted from pomace and the fatty remains of OMWW ^[55][19]. β-carotene is found in the leaves of Olea europea and can be extracted with conventional techniques. When applied topically, it prevents UVA damage on the dermis, reduces oxidative stress, prevents the loss of antioxidant enzymes and the apoptosis of fibroblasts and shows anti-inflammatory effects ^[52][81].

2.2.4. Squalene

Squalene is a hydrocarbon with several double bonds from the family of triterpenes ^[56][57][25,84]. It is one of the most abundant lipids on the skin’s surface ^[42][58][73,85]. The European Cosmetic Ingredient database ^[59][29] describes it as an animal (fish) or plant-derived raw material used in cosmetics, such as hair and skin conditioner, emollient and solvent. It was first discovered as a component of liver oil from some varieties of sharks and was used in cosmetics for a long time. Vegetable sources are now mostly used to obtain squalene in order to protect biodiversity. Lozano-Grande et al. ^[60][86] highlighted Olea europea as the main source of squalene. It may be used topically as an antioxidant, antibacterial, or antifungal. It has also been pointed out that other sources of squalene related to the olive industry are olives, leaves, pomace, olive oil and alpeorujo ^[61][62][21,87].

3. Extraction Technologies of Bioactive Compounds from Olive By-Products

3.1. Solid–Liquid Extraction

Solid–liquid extraction involves the removal of soluble compounds present in a vegetable matrix using organic solvents or mixtures of them. It results in a very convenient type of extraction since the solvent provides a physical medium to which the target molecules are transferred ^[63][89]. It is important to select the extraction solvent properly, as well as the experimental conditions (time, temperature, solid:liquid ratio, stirring, particle size), in order to achieve a quantitative extraction of the target compounds ^[64][90]. Hydroalcoholic mixtures are the most employed to recover phenolic compounds from olive by-products, especially when the extracts are intended to be used for cosmetic or food products since ethanol is considered a GRAS (generally recognized as safe) solvent; therefore, it does not implicate any harm to consumers. Many authors have employed different solvents to extract polyphenols from olive leaves, such as water, ethanol, acetone, ethyl acetate and its aqueous solutions ^[65][66][67][91,92,93]. Recently, the potential use of olive leaf extracts from the “Negrinha do Freixo” and “Cornicabra” varieties, prepared by maceration in ethanol/water, was evaluated. The extracts, obtained in optimized conditions (6 h, 50% ethanol, 1:20 w/v), showed great potential as an anti-aging agent, exhibiting antioxidant activity as well as the ability to inhibit the enzymes elastase, collagenase and tyrosinase. They also exhibited antimicrobial activity against Escherichia coli, Staphylococcus aureus and Bacillus cereus ^[53][69]. Solid–liquid extraction with water has also been proposed as a green approach to recover phenolic compounds from olive pomace ^[68][94].

3.2. Ultrasound-Assisted Extraction (UAE)

UAE is an extraction technology that has proved effective for extracting a wide range of compounds from different matrixes. It uses mechanical waves that propagate longitudinally through a fluid with a frequency between 20 and 2000 kHz ^[69][98]. The waves induced in the solvent alternate high- and low-pressure cycles, called compressions and rarefactions, respectively, which promote the displacement and eviction of molecules from their original location ^[69][70][98,99]. The negative pressure generated during rarefaction exceeds the attraction forces between the liquid molecules, separating them by creating cavitation bubbles. This cavitation effect breaks down the vegetable matrix, leading to higher penetration of the solvent in the cell structure, higher mass transfer rates and shorter extraction times ^[71][72][88,100]. Extractions can be performed in an ultrasonic bath or using an ultrasonic probe immersed in the solvent ^[73][101]. In terms of operational costs, UAE offers advantages against other “green” technologies without sacrificing extraction efficiency. This was the conclusion after comparing UAE, MAE and PLE for recovering polyphenols from olive pomace ^[74][102]. UAE has also been used to extract polyphenols from olive pomace with water as a solvent ^[75][103]. The effect of three factors (power, time and the sample/solvent ratio) on the total phenol content and antioxidant activity of the extracts was studied by the application of a Box–Behnken design, proposing the optimal conditions as 2 g/100 mL of water, 250 W and 75 min at 30 °C. UAE improved extraction efficiency, obtaining extracts with high total phenol content (TPC) levels and strong antioxidant activity.

3.3. Microwave-Assisted Extraction (MAE)

Microwaves are radiations from the electromagnetic spectrum whose frequency oscillates between 300 MHz (radio radiation) and 300 GHz (infrared radiation) ^[76][77][107,108]. This technology implies the use of microwave energy to heat the extraction solvent in contact with the solid matrix. The heating produces an increase in the intracellular pressure that facilitates the rupture of the cell wall and the release of the compounds of interest into the solution ^[78][109]. The effect of microwaves on exposed materials is strictly related to the conversion of electromagnetic energy into heat ^[76][107]. Thus, the MAE refers to the use of the heat generated by microwaves to accelerate the processes of extraction with solvent. These extractions are characterized by a low energy cost and short process times, which are some of the main advantages ^[79][110]. Prolonged extraction times could negatively affect thermolabile compounds, such as polyphenols, due to localized heating, which is why this is an important parameter that must be considered for preserving the integrity of the compounds being extracted ^[80][111]. Other factors that may affect the efficiency of the extraction are the power used, type of solvent, sample:solvent ratio, particle size and the number of cycles. Therefore, it is important to find the optimal conditions when trying to scale up to the industrial level ^[81][112].

3.4. Supercritical Fluid Extraction (SFE)

SFE involves the use of fluids at pressure and temperature levels above their critical points ^[82][116]. Some of the characteristics of fluids in a supercritical state are the relatively low viscosity and high diffusion coefficient, compared with conventional organic solvents, which enhances their performance as extraction solvents, as they present mass transfer properties similar to those of a gas and solvation characteristics similar to those of a liquid ^[83][117]. Moreover, the density and diffusivity of supercritical fluids can be adjusted by modifying the temperature and pressure conditions, adapting their selectivity against different molecules. Since supercritical solvents operate at high pressures, the separation carried out between the solute and solvent is very simple and efficient by simply decompressing the system ^[84][118]. This also allows the obtention of solvent-free extracts ^[83][117]. The most widely used solvent in the food and pharmaceutical industries is supercritical carbon dioxide (sc-CO₂). Due to its relatively low critical temperature and pressure values (31.1 °C, 7.39 MPa), moderate extraction conditions can be applied, protecting target molecules from thermal degradation. CO₂ is considered a nontoxic, non-flammable and GRAS substance, which allows safe working conditions. CO₂’s low polarity makes it suitable for extracting nonpolar compounds, such as lipids and carotenoids. Polar modifiers or co-solvents can be added in order to efficiently extract other types of molecules, such as flavonoids or simple phenols.

3.5. Pressurized Liquid Extraction (PLE)

The basic principle underlying PLE is that the solvent is used at temperatures higher than its boiling point and at a high enough pressure to keep it in the liquid state during the extraction process. These conditions result in faster extractions, during which high yields are obtained with small volumes of solvent. The high temperatures increase the solubility of the analytes in the solvent while the viscosity and the superficial tension of the solvent decrease, allowing greater penetration of the solvent in the matrix ^[83][117]. The extractions can be performed in two ways: static or dynamic; the static one is the most widely used ^[85][127]. A particular case of extraction with pressurized liquids happens when the solvent used is water, which is known as extraction with subcritical water. At the temperatures used (between 100 and 374 °C, the subcritical temperature for water), the hydrogen bonds weaken, which changes the dielectric constant of water and, therefore, its polarity. Depending on the working temperature, itwe can selectively extract different compounds: the more polar ones at a lower temperature and the less polar ones at a higher temperature. In this way, the selectivity of subcritical water allows the manipulation of the composition of the extracts by changing the working conditions. Other factors that affect the extraction are time, pressure, the addition of an organic solvent or a surfactant and the water flow rate ^[83][117].

3.6. Deep Eutectic Solvents (DES)

One of the newest and least studied technologies in relation to the recovery of bioactive compounds from olive waste is the use of deep eutectic solvents (DES). DES is defined as a multimolecular solvent that contains a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The HBD and the HBA interact through hydrogen bonds in order to generate a stable liquid that is quite viscous at room temperature, with a lower fusion point than those of the individual components ^[86][132]. One of the advantages of DES is the ease of its preparation and the fact that it can be generated from a variety of substances, typically, choline chloride ([Ch]Cl) as HBA, and sugars, organic acids and urea as HBD. This allows for the creation of “tailor-made” solvents, depending on each particular application. They also present other benefits, such as low toxicity, low volatility and low cost ^[87][133]. One practical disadvantage that may arise when working with DES is the high viscosity; however, this can be solved by the addition of a small amount of water ^[88][134].

4. Applications of Olive Extracts in the Cosmetic Industry

There is abundant information regarding studies about extracts from olive by-products and the cosmetic benefits of their components. Most of the extracts reported in the bibliographic references were obtained by solid–liquid extraction; for example, Oliveira et al. ^[53][69] obtained phenolic extracts by solid–liquid extraction from a mixture of pulverized olive leaves from two Portuguese cultivars (Negrinha do Freixo and Cornicabra), using different ethanol:water ratios at room temperature. The phenolic extracts exhibited high antioxidant capacity, and they inhibited enzymes in the skin associated with aging, such as elastase, collagenase and tyrosinase. The authors concluded that the extracts possessed anti-aging potential. Kishikawa et al. (2015) ^[89][138] reported that ethanolic extracts from olive leaves inhibit the growth of Staphylococcus aureus and reduce melanin synthesis in melanoma B16 cells. Galanakis et al. (2018) ^[90][62] obtained powdered extracts by concentrating OMWW for 15 h (26–30 °C) in an electric vacuum filter. The residual water was mixed with ethanol 96% (1:1 w/w), and the insoluble solute was removed. The supernatant liquid was mixed with maltodextrin and then spray-dried. These authors studied the potential of the extract obtained as UV filter boosters. They found that the UV absorption of the synthetic filters increased as a function of the concentration of olive phenols, while the relationship between the increase in SPF and the concentration of olive phenols was linear. Lecci et al. ^[29][42] reported the results of their analysis of extracts obtained by ultrafiltration of the OMWW that came from six different Italian cultivars. All the extracts showed high contents of HT and tyrosol, and three of them showed high amounts of verbascoside. The antioxidant capacity was dependent on the cultivar and related to the polyphenol content. The cytotoxic effect was evaluated, and a determination of the concentration of polyphenols in the extracts that could be used safely was conducted by means of tests in cultures of human epidermal keratinocytes, adult (HEKa).  The use of some olive derivatives or olive-derived extracts in cosmetic formulations could present several inconveniences or challenges regarding the technical or sensory aspects. The main problem is related to their stability, given that polyphenols are highly susceptible to degradation. Another concern could be the fact that lipophilic compounds could result in undesirable absorption when applied to the skin. On the other hand, compounds that act as UV filters are hydrophilic, which is inadequate for their use in water-resistant formulations. The smell of some active compounds may be unpleasant for some consumers. This is the case with vitamin E, squalene and some fatty acids ^[91][92][141,142], which can also show stability problems. To overcome all these issues, encapsulation could be an alternative, thus preserving the bioactive products against oxidation, changes in environmental conditions and possible interactions with other active products in the formulation, as well as masking their smell. Microencapsulated olive oil may result in an attractive raw material for the cosmetic industry ^[93][143]. A few examples of encapsulated olive-derived extracts with cosmetic purposes are reported in the literature. Panagiotopoulou et al. ^[94][57] used microencapsulation as a means to favor a product’s stability and to protect labile bioactives. These authors worked with an aqueous extract from olive leaves with high oleuropein and HT contents, obtained enzymatically and encapsulated by spray-drying. The microparticles were incorporated into a cosmetic cream, which was evaluated in terms of its rheology, thermal stability, microbiological and sensory characteristics.