A novel economic concept, circular economy, has the ambitious purpose of expanding product lifespan, promoting recycling and re-using and closing the product lifecycle ^[1]. It focuses on economic and environmental sustainability, arguing that agri-food by-products are not waste, but resources to be valorized ^[1]. Sustainable development aims to meet the current needs of the population without compromising future generations in this process, where three interconnected dimensions have a major role: economical, environmental and social ^[2]. Sustainable practices can be applied to the sector of olive oil on an industrial scale. Olive tree (Olea europaea) cultivation is particularly widespread and increasingly relevant for the economy of countries such as Spain, Italy, Greece, and Portugal ^[3][4]. Extra-virgin olive oil is an important constituent of the Mediterranean diet, being known for its nutritional properties and health effects, especially against cardiovascular diseases ^[1][5]. These properties are due to the presence of high levels of fatty acids, particularly monounsaturated acids, as well as other valuable components such as phenolics, phytosterols, tocopherols, and squalene ^[5]. In fact, extra virgin olive oil contains 98% to 99% triglycerides and 1% to 2% minor components ^[6]. Unsaturated acids are up to 85% of olive oil composition, particularly oleic acid, whose percentage may range between 70 and 85% ^[6]. Unsaturated fatty acids have a particular impact as components of the lipid film on the skin surface, maintaining hydration, elasticity, as well as barrier integrity ^[4]. Regarding minor compounds, the most notorious representatives of this group are phenolic compounds. Lipophilic compounds with cosmetic interest such as α-tocopherol, and pigments such as carotenoids are also present in this minor group ^[6]. The unsaponifiable fraction of olive oil also contains squalene, a natural component of human sebum, which provides softness to the skin ^[4]. The emollient and protective properties of olive oil combined with great skin compatibility make it a popular ingredient in the cosmetic sector ^[7]. Because of its properties, olive oil can act as an active agent by itself or as an excipient of several formulations.

The olive oil industry generates high amounts of waste, particularly during the agricultural phase and oil production stage ^[8]. These by-products are mostly leaves, olive pomace (OP), olive stones and olive mill wastewater (OMWW) ^[4]. Olive oil extraction can be accomplished through discontinuous (pressing) or continuous processes (two or three-phase centrifugation) resulting in different wastes ^[9][10]. In the two-phase process, unlike the three-phase system, no water is used, thus being considered a more sustainable process ^[9]. A semi-solid residue, OP, is also produced, with different moisture content according to the method applied ^[10]. The water percentage can reach 70% when the two-phase process is used ^[10]. OP is a significant source of phenolic compounds since it retains most of the phenolic content of the olive fruit ^[11]. In fact, due to their chemical nature, only 1–2% of the phenolic content is found in olive oil ^[9][11]. On the other hand, this richness along with a low pH and high organic load makes this by-product phytotoxic and non-biodegradable ^[4][12]. Consequently, the disposal of these wastes encompasses an ecological concern because of their hazardous effects on soil and water ^[12]. As the production of olive oil is increasing, these difficulties grow to be more challenging.

2. Olive Oil By-Products

2.1. Olive Pomace

OP consists of olive husk and pulp, crushed olive stone and water with a moisture content of more than 60%. It is the main residue of olive oil production, representing 35–40% of the total weight of the processed olive ^[3][4][13]. Along with OMWW, these by-products are considered the most harmful to the environment due to their high phytotoxicity ^[3]. Additionally, OP contains considerable amounts of cellulose (30%) and pectic polysaccharides (39%), a rich lipid fraction, especially in oleic acid (75% lipid content), as well as squalene and a rich mineral composition ^[4][11]. OP is also characterized by containing antioxidants, such as tocopherols and several phenolic compounds ^[11]. This by-product composition is largely affected by the agronomic and technological conditions of olive oil production, including olive variety, culture cultivation, geographical origin, and extraction process ^[4].

2.2. Other Olive by Products: Leaves and OMWW

Olive leaves are the first by-product generated in olive oil production, in the pruning or harvesting phases and also in the olive pre-treatment cleaning step, representing 10% of the overall weight of processing olives ^[13][14]. A large proportion of olive leaves is underexploited or used for incineration, despite their chemical composition ^[14]. In fact, different compounds can be found in this matrix, which may vary according to several factors such as the selected extraction method, cultivation procedures, or climatic conditions, and include high percentages of fiber and polysaccharides, significant protein content and a wide variety of phenolic compounds ^[9][13][15]. Olive leaf extracts have been extensively reviewed and exhibit beneficial health effects, namely antioxidant, anti-inflammatory, hypoglycemic, anti-hypertensive, antimicrobial, anticancer, gastroprotective, and anticholesterolemic effects ^[15]. Olive mill oil wastewater (OMWW) is the main liquid effluent from the olive oil production process, with a red-to-black color, acidic pH and high conductivity ^[16][17]. Besides water, it contains sugars, organic acids, and mineral nutrients (especially potassium) and it is known to concentrate high amounts of phenolic compounds, due to their hydrosoluble nature ^[16]. However, these phenolic substances present phytotoxicity and biotoxicity which limits OMWW use for agriculture purposes and contributes significantly to environmental pollution, especially in the olive oil production areas ^[10]. 

3. Phenolic Compounds: Hydroxytyrosol (HT) as a Cosmetic Active

Some olive bioactive components with well-known antioxidant activity are phenolic compounds ^[4]. Olive phenolics are greatly heterogeneous and include, mainly, phenolic alcohols (tyrosol and HT) phenolic acids (caffeic and gallic acids and verbascoside), flavonoids (luteolin, apigenin and rutin), secoiridoids (oleuropein and ligstroside) and lignans (pinoresinol) ^[13]. Several studies revealed that phenolic compounds possess several health benefits, such as antibacterial, antihyperlipidemic, anti-tumor, antioxidant, cardioprotective, neuroprotective, anti-hepatotoxic, or anti-diabetic properties ^[13][18]. Phenolic compounds are released during olive oil processing and, due to their polar character, are found in great quantities in the remains, particularly in OP and OMWW ^[11][19]. HT, a phenolic alcohol, is the major phenolic compound present in OP, thus this by-product constitutes a major source of this bioactive molecule ^[11]. Among leaves, there is also a significant phenolic content with the most common compound being oleuropein ^[8][19]. HT is a product of oleuropein hydrolysis and has shown significant antioxidant properties ^[11][19]. In fact, the antioxidant capacity of HT is higher than that of other phenolic compounds with similar structures and other natural antioxidants such as vitamin C, or synthetic antioxidants such as butylated hydroxytoluene (BHT) ^[20][21].

3.1. Antioxidant, Anti-Aging and Photoprotector

Intrinsic aging is associated with the natural aging process due to physiological factors such as cellular senescence, which is triggered by oxidative stress caused by reactive oxygen species (ROS) in the dermal cells. Extrinsic aging is the result of environmental factors primarily ultraviolet (UV) radiation but also pollution or tobacco smoke. Photoaging also causes ROS production as well as transcription factors activation that mediates an inflammatory response. Additionally, proteins such as those of the extracellular matrix, intercellular lipids of the epidermal barrier and DNA are highly susceptible to damage by ROS. UV irradiation potently induces the transcription of matrix metalloproteases (MMPs) that degrade fundamental structural proteins such as collagen and elastin, leading to premature skin aging ^[22]. The synthesis of collagen and elastin also decreases naturally with age and, at the same time, the MMPs that degrade these proteins are up-regulated in fibroblasts and keratinocytes, an imbalance that leads to collagen and elastin deficiency, translated in visible signs, such as loss of skin cohesiveness, elasticity and wrinkles ^[22]. The cosmetic industry is searching for natural compounds and extracts with antioxidant potential to use as ingredients for their formulations and develop novel products that could delay the signs of aging. These anti-aging ingredients can act through one or more different mechanisms, such as providing a moisturizing effect, promoting cell renewal or repair, exhibiting anti-oxidant action, inhibiting MMPs and preventing photoaging ^[22].

3.2. Anti-Inflammatory

Maiuri et al. ^[23] presented a study in which HT at 200 μM was capable of inhibiting the protein expression of two mediator enzymes of the inflammatory response, nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), in J774 murine macrophages, stimulated with lipopolysaccharide (LPS). There is also evidence that its use in topic formulations or nutraceuticals could benefit inflammatory pathologies such as atopic dermatitis or psoriasis ^[24][25]. In vivo anti-inflammatory results could be observed after treatment with a nutraceutical, Alyvium^® (Solvitae Medica, Madrid, Spain) which contains 500 mg of an olive polyphenolic extract, in patients with mild to moderate psoriasis. In a three-month period, patients experienced improved cutaneous manifestations as well as a 25% affected area and severity index reduction ^[24].

3.3. Antimicrobial

Regarding antimicrobial activity, studies have not been consensual. In 1999, Bisignano and colleagues reported the antimicrobial activity of HT showing that low concentrations effectively inhibit the growth of several bacterial strains in the American Type Culture Collection (ATCC) ^[26]. However, Medina-Martinez et al. ^[27] had significantly different results demonstrating that culture media and bacterial strains can affect the antimicrobial results of HT.

3.4. Depigmentant

HT also influences tyrosinase activity, an enzyme involved in melanin production, reducing its activity ^[28]. HT presented similar results to a known depigmenting agent, kojic acid, against mushroom tyrosinase, with an IC₅₀ value of 13 µmol/L versus 14.8 µmol/L. Codina and Monjo patented a method to produce a dermocosmetic containing an olive extract with 40% HT to inhibit melanin synthesis, acting as a depigmenting agent, with in vivo skin compatibility ^[29].

4. Applications of Olive Oil By-Products in the Cosmetic Industry

4.1. Antioxidant and Anti-Aging

The anti-aging market is continually growing and, considering the characteristics of olive oil by-products, their potential use in cosmetic formulations is extremely relevant. Nunes et al. ^[3] developed oil-in-water (O/W) creams incorporating olive leaves extracts (5%) with promising results: no skin or eye irritation, in vivo skin compatibility and acceptability, antioxidant and antimicrobial activity, inhibition of elastase and photoprotection ability. Safety analysis methods included MTT and skin irritation assays using Human Corneal Epithelium and Reconstructed Human Epidermis (RHE) models SkinEthic^™ HCE and EpiSkin^™ (Lyon, France), respectively, and repeated applications of the samples under an occlusive patch for an in vivo irritation study with 51 volunteers.

4.2. Photoprotector

Antioxidants are typically used in sunscreens to complement UV filter protection, as they can reduce the damage induced by ROS generated by solar irradiation ^[30]. Recovery of phenolic compounds from OMWW was applied in different sunscreen formulations by Galanakis et al. ^[30]. An amount of 200 mg olive phenols powder was emulsified with 25 g base cream and the encapsulation of olive phenols in silica particles and/or the liposome effect on water resistance was tested. The higher water resistance (73%) was observed after the adsorption of olive phenols in silica particles and their emulsification in a base cream. Solutions with olive phenols extracted from OMWW with concentrations between 2.5 and 15 mg/L of olive phenols were combined with physical and chemical filters to evaluate the potential UV booster effect through in vitro sun protection factor (SPF) testing. The results confirmed OMWW photoprotector activity since the SPF value of the UV filter solutions increased linearly for all the tested phenolic concentrations ^[30].

56. Olive Pomace as a Potential Cosmetic Ingredient

56.1. Formulation’s Design

The development of a new cosmetic product must consider not only technical and regulatory requirements but also market placement and consumer demand ^[31]. In fact, natural products and sustainability have been, in recent years, a focus in the cosmetic industry driven by more eco-conscious consumers. However, formulating cosmetics with plant-based actives can represent technical challenges as well as quality, safety, and efficacy concerns ^[31]. Stability, skin bioavailability and effective concentrations of antioxidants are important when including them in topical formulations ^[22].

56.1.1. Extracts versus Isolated Compounds

Many natural ingredients may have more than one predominantly active compound with various biochemical actions, and this complexity can be very appealing, rather than the use of a purified component. On the other hand, complex extracts may cause difficulties in formulation, as well as problems with formula stability ^[32]. OP extracts offer a variety of interesting cosmetic ingredients and synergistic effects that are also to be considered since the interaction within phenolics or between phenolics and other antioxidants could potentiate their activity ^[33].

56.1.2. Olive Pomace Powders

Besides phenolics, other compounds can be incorporated into cosmetic formulations due to their chemical proprieties by improving sensory characteristics, such as texture or viscosity, or promoting hydration and skin protection ^[4]. Monosaturated fatty acids provide structural stability to cell membranes, squalene has emollient qualities as well as an effect as a skin barrier against solar rays and antioxidant proprieties, and minerals are part of the natural moisturizing factor, and essential to the stratum corneum hydration plasticity and homeostasis ^[4]. Fractionation approaches could also be an advantage to obtain different value-added products from OP. Ribeiro et al. ^[34] used centrifugation to separate the liquid from the solid fraction, followed by freeze-drying. The dry fraction was then sieved after a previous milling process, thus obtaining the pulp fraction and the stones fraction. Chemical composition analysis showed that fiber is the most abundant component in the pulp fraction also exhibiting a significant amount of protein (8–9% dry weight (dw)) and fat (15–21% dw). Antioxidant activity was evaluated using three different methods (DPPH^●, ABTS^●+ and ORAC) and presented higher values in the liquid fraction in all methods, related to the higher recovery of phenolic compounds such as HT. Minerals were also mostly present in the liquid fraction. Nevertheless, the pulp fraction possesses phenolic compounds mainly bound to fiber.

56.1.3. Encapsulation Strategies

Despite their benefits, phenolic compounds show low stability in environmental conditions such as exposure to light, oxygen, temperature and enzymatic activities ^[35]. Encapsulation strategies are being constantly developed in order to face limitations in the incorporation of certain active substances, such as bioavailability or stability issues and can also improve active the compound’s physicochemical characteristics or provide controlled release ^[36][37]. Encapsulation can also enhance bioactive skin penetration in addition to solving potential organoleptic constraints ^[36]. It is well-recognized that encapsulation can minimize phenolic instability and provide better formulations with an improvement in product shelf-life ^[30]. Kesente et al. ^[38] encapsulated olive leaf extract in biodegradable polylactic acid nanoparticles. These nanoparticles were characterized considering particle size, polydispersity index, zeta potential, in vitro phenolics release, and encapsulation efficiency. The loaded nanoparticles were incorporated in a cosmetic formulation and the stability of the formulation was studied for a three-month period using freeze cycles as well as storage at 5, 25 and 40 °C. The nanoparticles showed an adequate particle size and homogenous nanoparticle population with an encapsulation efficiency of 49.2%. The cosmetic formulation also demonstrated increased stability compared to the pure extract with respect to viscosity, pH and organoleptic characteristics.