The Potential of Coffee and Olive by Products as Ingredient in Cosmetics Formulations and Their Extraction Techniques

Abstract

This review concentrates on the bioactive potential of two significant agri-food by-products: coffee by-products (coffee pulp and husk, spent coffee grounds, and silverskin) and olive by-products (olive mill wastewater, pomace, stones, and leaves). These residues are produced in substantial quantities, and despite their considerable application potential, they remain predominantly underutilized, thereby contributing to environmental burdens and economic losses. Their richness in bioactive compounds is unequivocal. Specifically, coffee by-products are abundant in caffeine and chlorogenic acids, whereas olive by-products serve as excellent sources of oleuropein, hydroxytyrosol, and tyrosol. Such compounds possess health-promoting properties and are promising active ingredients for cosmetic formulations, owing to their antioxidant, anti-aging, UV protective, antimicrobial, emollient, and moisturizing effects. This review not only compiles the bioactive compounds present in these by-products and explores their potential applications but also examines the extraction methods employed for their recovery. Both conventional techniques (solvent extraction) and green extraction technologies (ultrasound-assisted extraction, microwave-assisted extraction, and supercritical fluid extraction) are discussed. These innovative and environmentally friendly approaches enhance extraction efficiency and are aligned with sustainability objectives. In this context, the importance of incorporating natural ingredients into cosmetic products is emphasized, both to meet regulatory and environmental standards and to satisfy the increasing consumer demand for safer, more effective, and environmentally sustainable formulations.

1. Introduction

Cosmetic and hygiene products have been an integral part of human culture and self-care practices for many years. However, in recent decades, substantial improvements have been realized owing to advancements in cosmetic formulation and technology, alongside continuous growth in the global market. The cosmetic industry targets specific demographics and now constitutes a significant sector of the global economy, with anticipated further development in the coming years [1,2]. In recent times, consumer concerns have markedly increased regarding skin aging, dermatological imperfections, and conditions such as acne and skin cancer [3]. This heightened awareness has driven research and innovation within the industry, leading to the development of new products that address consumer demands for aesthetically pleasing and healthy skin [4]. Concurrently, consumers are increasingly adopting eco-conscious lifestyles and displaying greater concern for the ingredients contained in cosmetic products, seeking items with natural components and lower environmental impact. Terms such as “natural,” “absence of chemical toxic substances,” or “product tolerability” significantly enhance a product’s appeal. Consequently, it is imperative to consider consumer sensitivity toward environmentally friendly and biodegradable products, while also ensuring product efficacy [3,5]. Consumer preferences, combined with rising interest from organizations and the scientific community regarding environmental sustainability, have spurred innovation in the cosmetic industry, which has historically been associated with considerable ecological impacts. Indeed, the industry is actively pursuing more environmentally sustainable raw materials for product formulation [6]. Within the European regulatory framework, the European Commission (EC) has demonstrated an interest in promoting regulations that involve nations and citizens in the transition towards a sustainable future [7].

This review article offers a comprehensive evaluation of potential solutions for this notably significant issue by incorporating recent data on natural products, specifically coffee and olive by-products, as well as their biological composition, extraction techniques, and applications in cosmetic formulations—an aspect that is seldom examined.

2. Methodology of Search

A comprehensive literature review was undertaken utilizing PubMed and Google Scholar to identify scholarly articles, reviews, and monographs relevant to the subject matter. Various search term combinations were employed, with the key words such as: cosmetic industry, cosmetic products; circular economy; natural ingredients, plant extracts; coffee by-products, coffee silverskin, coffee pulp, parchment, spent coffee grounds, coffee husk, coffee by-products bioactive compounds, polyphenols, caffeine, chlorogenic acids, cosmetic applications, photoprotection, antiaging, antioxidant activity; extraction methods, green methods, conventional methods; olive by-products, olive pomace, olive mill wastewater, olive stones; olive by-products bioactive compounds, hydroxytyrosol, oleuropein, tyrosol. Priority was given to publications from 2020 onward to incorporate recent advancements; however, earlier studies of notable scientific significance were also evaluated. Some relevant articles were identified through references cited in the selected publications.

3. Circular Economy

The European Commission’s new agenda for sustainable growth, known as the European Green Deal, encompasses a series of proposals aimed at addressing climate change and environmental degradation—issues of significant global concern. In March 2020, a revised Circular Economy Action Plan (CEAP) was introduced, with the objective of alleviating environmental pressures and enhancing sustainability [8]. The circular economy model is designed to transform products typically destined for waste by applying the principles of reduce, reuse, and recycle (3Rs) to their material components, thereby establishing a closed-loop system and assigning value to discarded materials [9]. This strategy emphasizes the sustainable management of resources, wherein waste is reintegrated into the production cycle, becoming a resource for subsequent manufacturing processes. Such an approach prolongs the lifespan of products, maximizes their value, and minimizes waste [10]. Empirical evidence indicates that this model is efficacious not only in decreasing the volume of final waste but also in reducing the dependence on virgin natural resources, thereby contributing to a more sustainable future [7] (Figure 1). Waste upcycling yields economic advantages, including job creation, cost savings, and mitigation of supply related risks such as price volatility, resource scarcity, and import dependency. Furthermore, it benefits the environment by conserving natural resources for future generations and decreasing landscape and habitat disruption, as the development of more efficient and sustainable products reduces energy and resource consumption [11].

The concept of a circular economy, exemplified by the utilization or extraction of by-products from waste materials, has been implemented within the cosmetic industry prior to its widespread adoption [2]. Extracts of natural origin, obtained from waste generated during food and plant processing, have been extensively studied to extract bioactive compounds such as vitamins, enzymes, essential oils, antioxidants, among others [3]. These constituents are highly valuable in the cosmetic sector, enhancing the functional properties of products through the provision of antioxidant, antimicrobial, and antiaging effects, among other benefits. This approach not only adheres to the principles of a circular economy but also promotes the development of greener, value-added cosmetic products [12,13]. For example, antioxidants that prevent product degradation and diminish oxidative damage to cellular components can be derived from plants, fruits, and grains. Polyphenols, flavonoids, and flavonols are noteworthy active compounds sourced from nature that offer multiple health benefits [14]. In fact, a broad range of natural ingredients can be incorporated into cosmetics, serving as replacements for synthetic substances. Additionally, cosmetics formulated with natural ingredients are generally preferred by consumers and support more sustainable practices [3].

4. Natural Products in Cosmetics

The utilization of natural ingredients in cosmetics originated in ancient civilizations across diverse cultures. Primarily derived from plants, minerals, and animals, these ingredients have been extensively employed. For example, Egyptians (5000 BC–30 BC) utilized oils, creams, and perfumes made from myrrh, lavender, rose, and almond oil. Kohl and henna were utilized for cosmetic purposes and hair coloring [15,16]. In Mesopotamia, now referred to as the Middle East (3000 BC), herbs, oils, and wood were used to produce various types of incense, while pigments served as color cosmetics, notably for eye makeup. The Greeks and Romans (800 BC–500 AD) employed chalk, lead-based face powders, and other ingredients such as soft coral, narcissus bulbs, honey, and orris root for skincare and dermatological applications. Over time, cosmetic formulations evolved, leading to the replacement of plant and mineral-based products with synthetic and chemical counterparts. Currently, however, the cosmetics industry is increasingly returning to natural and sustainable products, illustrating a cyclical pattern [17].

Plant extracts, which are a rich source of biologically active compounds, are increasingly employed due to their safety profile, compatibility with all skin types, minimal side effects, and greater availability, offering advantages over synthetic ingredients [18]. These extracts possess numerous valuable properties, including medicinal benefits, that can aid in the management of certain skin disorders, particularly inflammatory conditions such as acne, psoriasis, or atopic dermatitis. When incorporated into cosmetic formulations, plant extracts can provide antioxidant, antibacterial, astringent, moisturizing, regenerating, cleansing, smoothing, and/or lightening effects [19,20]. They are obtained from various plant components, including fruits, leaves, roots, bark, stems, branches, seeds, flowers, or even the entire plant. Factors such as cultivation practices, plant health, harvesting methods, and post-harvest processing techniques (including fragmentation, drying, and extraction methods) can critically influence the composition and bioactivity of the extracts [21]. Standard approaches to ensure the consistency of bioactive content involve monitoring the raw material using markers (e.g., hydroxytyrosol for olive by-products); this monitoring can be conducted employing NIR or fluorescence spectroscopy combined with chemometrics to identify variations, as documented in [22,23].

Importantly, agri-food by-products generated during food processing, such as peels, seeds, pomace, or husks, offer a sustainable and cost-effective alternative source of valuable plant extracts. The valorization of these by-products not only contributes to waste reduction and environmental sustainability but also enables the recovery of high-value bioactive compounds with high potential to be incorporated in cosmetic products [21].

Aging is characterized as a progressive and continual decline in cellular and organismal functions over the lifespan, representing a complex biological process influenced by both extrinsic and intrinsic factors [24]. Manifestations of skin aging include hyperpigmentation, diminished elasticity and increased laxity, the appearance of fine lines and wrinkles, telangiectasia, uneven skin texture, enlarged pores, periorbital puffiness, and keratosis [24,25]. The exploration of active substances to inhibit or mitigate these signs has been a longstanding area of research. In recent times, plant extracts have gained considerable prominence as sources of such active compounds, given their significant potential to delay the skin aging process. Consequently, plant extracts are increasingly incorporated into anti-aging formulations due to their capacity to protect the skin from oxidative stress, enhance immune responses, and absorb UV radiation, among other beneficial effects [25]. For example, residues from the coffee industry, such as defective coffee beans, silverskin, and spent coffee grounds, are produced in substantial quantities and contain high-value bioactive compounds, notably phenolic constituents such as chlorogenic acids and caffeine. The application of these residues has demonstrated safety, stability, and efficacy in improving skin qualities, alongside favorable consumer acceptance, thereby supporting the valorization of coffee by-product extracts as novel natural ingredients [26].

Consequently, olive by-products (e.g., olive mill wastewater, olive pomace, and leaves) contribute to the generation of significant waste quantities, thereby raising environmental concerns due to their elevated content of phytotoxic phenolics, high organic load, and low pH. Nonetheless, research has demonstrated that extracts derived from these by-products possess valuable properties, and their incorporation into cosmetic formulations offers advantageous benefits. Simultaneously, the utilization of these by-products presents an environmentally sustainable solution that alleviates their phytotoxic effects [27].

5. Coffee By-Products

5.1. Bioactive Compounds of Coffee By-Products

Coffee stands as one of the most widely consumed beverages globally, resulting in substantial quantities of coffee by-products annually. The bioactive compounds present in this plant hold potential applications in skincare products [28]. According to Ribeiro et al. [29], there are 124 species within the genus Coffea (Coffea sp.), of which only two are of significant commercial importance: Coffea canephora (commonly known as robusta) and Coffea arabica. In C. arabica beans, up to 17% of the matrix comprises a complex lipid fraction that yields coffee oil, whereas in robusta, this fraction can constitute up to 10%. The concentration of phenolic compounds in green coffee beans varies by species, with Arabica containing lower levels (4–8%) and Robusta exhibiting higher levels (7–14%). Despite robusta possessing higher caffeine content, Arabica coffee is generally regarded as of superior quality due to its sensoritonal properties [30]. The leading producers—Brazil, Vietnam, Colombia, Indonesia, Ethiopia, India, and Honduras—account for approximately 80% of global production [31].

There are two primary methods for processing coffee cherries: dry processing and wet processing, each producing distinct by-products. The dry method, also referred to as the natural process, involves drying the coffee cherry immediately after harvesting, followed by mechanical de-husking to remove the outer layers and extract the green coffee beans [32]. Conversely, the wet processing, also known as the washing method, encompasses a series of steps—including pulping, fermentation, washing, and drying—that systematically remove the fruit layers, necessitating significant water consumption [33]. Husks are the by-product of the dry method, accounting for approximately 45% of the coffee cherry’s weight, comprising the fruit’s outer layers such as skin, pulp, mucilage, and parchment. From the wet processing, the principal by-products are coffee pulp and parchment [34] (Figure 2). Although exceptions exist, Robusta coffee is generally processed using the dry method, whereas the wet method is predominantly employed for Arabica coffee.

During the roasting process, the beans undergo expansion and the removal of coffee silverskin. Following grinding and brewing, the residual coffee grounds are ultimately collected [35]. In on-farm processing, the coffee beans possess a moisture content ranging from 10% to 12%, thereby ensuring the preservation of quality during transportation [36].

In addition to the aforementioned coffee by-products, defective coffee beans and coffee sieving residues may also contain valuable compounds of high interest. It is noteworthy that various coffee by-products exhibit distinct chemical compositions, which are influenced by the coffee species and the processing methods employed [37]. Nonetheless, the literature indicates that coffee by-products are generally abundant in carbohydrates, proteins, vitamins, nutrients, and a range of bioactive compounds [38]. In a study conducted by Machado et al. [37], the nutritional profiles of different coffee by-products were analyzed. Except for parchment, all samples demonstrated significant protein content, with silverskin exhibiting the highest concentration at 16.3% dry weight (dw), followed by sieving residue at 14.6% dw, and defective coffee beans at 13.3% dw. Coffee silverskin also displayed notably high levels of dietary fiber (56–62%), particularly insoluble fiber predominantly composed of cellulose (18%) and hemicellulose (13%), as well as total minerals (8%) and protein (19%). Other pertinent bioactive compounds identified include chlorogenic acids (1–6%), caffeine (0.8–1.25%), and melanoidins (17–23%), among other antioxidant compounds [39]. Coffee pulp (CP) and husks (CH), aside from their carbohydrate, mineral, and protein content, also contain significant quantities of caffeine, chlorogenic acid, and tannins. The presence of these compounds restricts their application as animal feed or fertilizer due to concerns regarding animal health, seed germination, and plant growth [40,41,42]. However, these by-products possess valuable applications, such as in biogas and bioethanol pro-duction, mushroom cultivation, and the extraction of bioactive compounds. Utilizing a simple and low-cost enzymatic extraction method, chlorogenic acid was identified as the predominant compound (36.1%) obtained from CP and CH, with caffeic acid closely following at 33% [40].

Spent coffee grounds (SCGs), akin to other mentioned by-products, have also been explored for their potential applications owing to their high concentrations of organic constituents such as carbohydrates (60.3–82.0%), lipids (6.0–38.6%), proteins (11.5–18.0%), and additional compounds including caffeine (0.02–0.4%), chlorogenic acid (1.8–11.5%), caffeic acid, cafestol, and kahweol [30,43]. Consequently, SCGs may be transformed into high-value products, encompassing biofuels, bioactive compounds, and biomaterials. When compared to other lignocellulosic residues, SCGs are distinctly abundant in hemicellulose (30–40 wt%) and lignin (25–30 wt%), while their cellulose content remains relatively modest (8.6–13.3 wt%) [30]. Cellulose and hemicellulose can undergo hydrolysis into mannose and galactose, serving as substrates for microbial fermentation and the synthesis of other valuable chemicals [30].

The extraction methodology employed is of paramount importance to ensure the proper processing of coffee by-products as well as their storage conditions. Inadequate storage conditions may result in microbial contamination, the development of mycotoxins, and the degradation of bioactive compounds. Such issues could restrict their application in cosmetic formulations unless proper processing, safety protocols, and quality control measures are implemented [44]. A study examining spoilage mechanisms during storage revealed that anaerobic storage of spent coffee grounds (SCG) for up to 14 days facilitated significant bacterial proliferation, predominantly Bacilli, lactic acid bacteria, and acetic acid bacteria [44]. Conversely, extensive fungal colonization, including contamination by Aspergillus flavus—a species known for its capacity to synthesize aflatoxins—was observed under aerobic storage conditions. Post-processing, these by-products exhibit minimal microbial contamination, thereby supporting their potential valorization for cosmetic applications. Nonetheless, during storage and processing at recycling facilities, they remain highly susceptible to biochemical degradation and microbial proliferation. Therefore, establishing comprehensive storage and processing guidelines is essential [45]. Another study investigated chemical and microbial contamination in coffee silverskin (CS) extracts, with some samples testing positive for Ochratoxin A [46]. Mycotoxins are expected to undergo substantial degradation during the roasting process [47]. Effective mitigation of Ochratoxin A during coffee transportation and storage necessitates maintaining low temperatures (3–7 °C) and water activity (aw) ≤ 0.60 to prevent bean rehydration [48].

5.2. Extraction Methods for Coffe By-Products

The chemical profile of extracts derived from coffee by-products is substantially dependent on the extraction methodology employed, as variations in specific parameters, such as the choice of solvent, result in the recovery of differing quantities and types of bioactive compounds [49]. Indeed, the solvent exerts a predominant influence on the extraction process, given that solvents with varying polarities interact distinctively with the chemical structures of the constituents, thereby leading to variations in the biological activities associated with each by-product and solvent combination [40].

Conventional methods, such as solid–liquid extraction, remain widely employed for the recovery of bioactive compounds from coffee by-products owing to their simplicity and minimal operational costs. Nonetheless, these techniques are considered less environmentally sustainable, as they frequently necessitate substantial quantities of organic solvents [50]. Soxhlet extraction, a representative of solid–liquid extraction predominantly utilized at laboratory scale, facilitates efficient and selective extraction of target compounds; however, it demands large solvent volumes and is time-consuming [29] (see Figure 3). Mechanical pressing, a traditional solvent-free method, does not generate potentially toxic residues but generally yields lower extraction efficiencies and diminished oil purity [51].

To enhance the recovery yield of compounds, parameters such as temperature, duration, solvent type, and liquid-solid ratio can be optimized according to the specific by-product. Typically, solvents such as alcohols and acetone, with varying water ratios, are employed for the extraction of phenolic compounds [52]. In the study conducted by Gokhan Zengin et al. [40], various solvents—including H₂O, MeOH, MeOH:H₂O (50:50), and EtOH:H₂O (70:30)—were utilized to obtain extracts of CS and SCG. As anticipated, the phenolic compound content in both by-products was affected by the choice of solvent. The SCG extracts had higher levels of bioactive compounds, with the highest antioxidant activity observed using EtOH:H₂O, followed by MeOH:H₂O, MeOH, and H₂O, in that order. The fact that water was the least effective solvent can be explained by the non-polar nature of plant cell walls, which restricts the extraction of certain compounds. In a study conducted by Ramón-Gonçalves et al. [53], ethanol-water mixtures with low ethanol ratios, ranging from 15 to 20% (v), proved to be a simple and cost-effective method for extracting polyphenols from SCGs, such as chlorogenic acid (9.69 mg/g), p-coumaric acid (0.155 to 0.348 mg/g), trans-ferulic acid (0.110 to 0.177 mg/g), rutin (0.086 mg/g), naringin (0.096 to 0.423 mg/g), and resveratrol (0.066 to 0.093 mg/g).

The presence of residual solvents can influence skin safety and the claims of natural products, as methanol, acetone, or hexane may be toxic and can alter the molecular organization of skin lipids and proteins. This alteration could compromise the skin’s protective barrier function and induce skin irritation [54]. Regulatory frameworks highlight that solvents can interfere with the classification or commercialization of an extract as natural. Consequently, green solvents such as water, ethanol, and CO₂ are regarded as the most acceptable options in terms of safety, product quality, and consumer perception [55,56].

Conventional methods are increasingly being replaced by greener and more efficient alternatives, which offer shorter processing times, reduced use of solvents, and improved extraction yields [49]. Nonetheless, some of these alternatives may necessitate substantial investment for scaling up into industrial practices, thus presenting a barrier for certain industries. Microwave-assisted extraction (MAE) is less time-consuming and requires smaller volumes of solvent; however, it faces limitations in terms of extraction uniformity and equipment costs. MAE has achieved an extraction yield of 84 ± 2.8 mg/g for chlorogenic acid and approximately 72.5 mg/g of caffeine [57]. Supercritical fluid extraction (SFE), particularly with carbon dioxide, facilitates the extraction of oil with high purity, representing an efficient and environmentally friendly method, notwithstanding the high costs associated with the equipment [29]. SFE has yielded extraction amounts of 6.45 mg/g for caffeine and 0.55 mg/g for chlorogenic acid [56].

Finally, in the study conducted by Ibtissam Bouhzam et al [58], three uncomplicated and straightforward extraction techniques were compared at a laboratory scale for the extraction of chlorogenic acid and caffeine. This investigation demonstrated a comparison among Water Ultrasound-Assisted Extraction (UAE) (Figure 4), Water and Vortex Extraction, and Supramolecular Solvent Extraction. All three procedures lasted 1 min, and ultrasound-assisted water extraction at ambient temperature was identified as the most effective, producing the highest concentrations of chlorogenic acid (1.15 mg/g) and caffeine (0.972 mg/g). Conversely, the utilization of supramolecular solvent resulted in a lower recovery of chlorogenic acid in the supra-phase, likely owing to its stronger affinity for the aqueous lower phase (Table 1). The extraction conditions of the various methods are summarized in Table 2.

Table 1. Comparison between conventional and green extraction methods with pros, cons, and typical yields for coffee by-products extraction.

Category	Method	Pros	Cons	Yield
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	Simple; Economic; Accuracy/reproducibility [49]	Not environmentally friendly; Time consuming; Poor processing adaptability [49]	(ethanol-water solvent) Chlorogenic acid (9.69 mg/g) p-coumaric acid (0.155 to 0.348 mg/g) rutin (0.086 mg/g) [53]
Green	Microwave-assisted (MAE)	Less time consuming; Efficient and environmentally friendly; [29,57]	Expensive equipment; Limitations concerning extraction uniformity [29,57]	Chlorogenic acid (84 ± 2.8 mg/g) Caffeine (approx. 72.5 mg/g) [57]
	Supercritical fluid Extraction (SFE)			Chlorogenic acid (0.55 mg/g) Caffeine (19.49 mg/g) [57]
	Water Ultrasound-Assisted Extraction (UAE)			Chlorogenic acid (1.15 mg/g) Caffeine (0.972 mg/g) [58]
	Water and Vortex Extraction			Chlorogenic acid (0.827 mg/g) Caffeine (0.766 mg/g) [58]
	Supramolecular Solvent Extraction			Chlorogenic acid (0.116 mg/g) Caffeine (0.885 mg/g) [58]

Table 1. Comparison between conventional and green extraction methods with pros, cons, and typical yields for coffee by-products extraction.

Category	Method	Pros	Cons	Yield
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	Simple; Economic; Accuracy/reproducibility [49]	Not environmentally friendly; Time consuming; Poor processing adaptability [49]	(ethanol-water solvent) Chlorogenic acid (9.69 mg/g) p-coumaric acid (0.155 to 0.348 mg/g) rutin (0.086 mg/g) [53]
Green	Microwave-assisted (MAE)	Less time consuming; Efficient and environmentally friendly; [29,57]	Expensive equipment; Limitations concerning extraction uniformity [29,57]	Chlorogenic acid (84 ± 2.8 mg/g) Caffeine (approx. 72.5 mg/g) [57]
	Supercritical fluid Extraction (SFE)			Chlorogenic acid (0.55 mg/g) Caffeine (19.49 mg/g) [57]
	Water Ultrasound-Assisted Extraction (UAE)			Chlorogenic acid (1.15 mg/g) Caffeine (0.972 mg/g) [58]
	Water and Vortex Extraction			Chlorogenic acid (0.827 mg/g) Caffeine (0.766 mg/g) [58]
	Supramolecular Solvent Extraction			Chlorogenic acid (0.116 mg/g) Caffeine (0.885 mg/g) [58]

Table 2. Comparison of extraction conditions for coffee by-products.

Category	Method	Temperature or Power	Time	Solvent Ratio	Ref.
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	60 °C	30 min	EtOH:H2O (50:50)	[59]
Green	Microwave-assisted (MAE)	500 W	2 min	H2O 15 mL/g	[60]
	Supercritical fluid Extraction (SFE)	Room temperature	5 min	EtOH: H2O 4:1 v/w	[61]
	Water Ultrasound-Assisted Extraction (UAE)	30–50 °C 100–300 W	5–45 min	EtOH 5:1–30:1	[52]
	Water and Vortex Extraction	50/60 Hz, 195 W	1 min	0.7 g of SCG to 4 mL of ultrapure water.	[58]
	Supramolecular Solvent Extraction	-	1 min	0.7 g of SCG to ethanol (1.2 mL), 1-hexanol (0.96 mL), and water until a total volume of 4 mL	[58]

Table 2. Comparison of extraction conditions for coffee by-products.

Category	Method	Temperature or Power	Time	Solvent Ratio	Ref.
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	60 °C	30 min	EtOH:H2O (50:50)	[59]
Green	Microwave-assisted (MAE)	500 W	2 min	H2O 15 mL/g	[60]
	Supercritical fluid Extraction (SFE)	Room temperature	5 min	EtOH: H2O 4:1 v/w	[61]
	Water Ultrasound-Assisted Extraction (UAE)	30–50 °C 100–300 W	5–45 min	EtOH 5:1–30:1	[52]
	Water and Vortex Extraction	50/60 Hz, 195 W	1 min	0.7 g of SCG to 4 mL of ultrapure water.	[58]
	Supramolecular Solvent Extraction	-	1 min	0.7 g of SCG to ethanol (1.2 mL), 1-hexanol (0.96 mL), and water until a total volume of 4 mL	[58]

Extraction of bioactive compounds from plant by-products for cosmetic applications has yielded promising results in laboratory settings. However, studies focusing on process optimization and industrial implementation remain relatively limited. Further research is required to address scalability, process economics, solvent recovery, and integration into existing production lines. While most extraction methods currently produce low quantities of bioactive compounds from coffee by-products, these materials have been demonstrated to be valuable ingredients for cosmetic formulations. Derivatization of these bioactive compounds is feasible, enabling the creation of new ingredients with high commercial value for industrial use. A recent study demonstrated the modification of spent coffee oils through cost-effective processes suitable for scaling-up. This approach also aligns with the increasing consumer preference for eco-friendly and naturally derived cosmetics [62].

Figure 4. Water Ultrasound-Assisted Extraction (UAE) of SCG oil. Adapted from Miladi et al. [63].

Figure 4. Water Ultrasound-Assisted Extraction (UAE) of SCG oil. Adapted from Miladi et al. [63].

5.3. Applications in Cosmetic Formulations

As previously mentioned, coffee by-products possess potential applications within the cosmetic industry, owing to their content of caffeine and phenolic compounds, such as tannins, flavonoids, and chlorogenic acids, notably 5-caffeoylquinic acid. These constituents are primarily responsible for antioxidant, anti-inflammatory, and antibacterial activities [64].

One of the many external factors that contribute to skin aging is UV exposure. UVA rays penetrate deeply into the epidermis and dermis, causing collagen and elastin fiber degradation through oxidative stress and activation of matrix metalloproteinases [65]. A study conducted by Cho Y-H. et al. [66] investigated the anti-wrinkle effects of chlorogenic acid and other compounds extracted from Coffea arabica on UV-B-stimulated mouse fibroblasts, demonstrating the anti-wrinkle potential of chlorogenic acid. Therefore, extracts of coffee by-products can be used to protect against UV light, which damages the skin and accelerates skin aging. Applying these extracts as sunscreen additives would be effective, since the presence of double bonds or aromatic rings in the molecular structure of phenolic compounds provides them with UV absorption properties in the range of 200–400 nm [66]. Polyphenols have protective and healing effects on the skin, and according to the literature, caffeic acid reduces dermatitis and pigmentation resulting from previous UVB radiation exposure [67,68].

Overall, phenolics have been reported as relatively photostable, maintaining their activity under UV exposure and demonstrating no significant phototoxicity in cellular models. Consequently, phenolic compounds are promising candidates for products exposed to sunlight; however, their photostability varies considerably depending on the specific compound and is often enhanced through encapsulation, combination with UV filters, or synergistic antioxidants to ensure long-term safety and efficacy [69,70].

As previously noted, coffee by-products are a source of caffeine and chlorogenic acids. Caffeine (C₈H₁₀N₄O₂) possesses notable applications owing to its ability to penetrate the skin barrier, thereby enhancing the condition of skin and hair through their biological effects. Its utilization is prevalent due to its antioxidant, photoprotective, and lipolytic properties [26,71]. Additionally, caffeine may promote hair growth by boosting scalp microcirculation, which enhances the delivery of nutrients and oxygen to hair follicles through skin cell and follicular proliferation, increases metabolic activity, and, owing to its antioxidant properties and capacity to stimulate IGF-1 expression, triggers and sustains the anagen phase of hair growth [72,73]. Due to its antioxidant properties, this compound is capable of neutralizing free radicals, thereby diminishing oxidative stress and decelerating the processes associated with skin aging. Consequently, the inclusion of these extracts in cosmetic formulations can improve skin protection and vitality [26]. Furthermore, caffeine has been shown to enhance microcirculation within blood vessels, which may reduce the visibility of cellulite. A study conducted by Rodrigues et al. [74] involved the isolation of caffeine from CS, which was subsequently incorporated into nanostructured lipid carriers (NLCs) to develop a formulation for cellulite treatment. In vivo assessments using pig ear skin demonstrated that caffeine in CS extracts exhibited improved nanoparticle penetration. Caffeine is extensively utilized in topical skincare formulations and is generally regarded as safe and well-tolerated, with no substantial reports of irritation [75]. Nevertheless, in vitro evidence suggests that excessive caffeine exposure may diminish collagen synthesis in human cultured skin fibroblasts and may also exert inhibitory effects on DNA biosynthesis [76]. Chlorogenic acid (C₁₆H₁₈O₉) is the primary phenolic compound found in extracts of coffee by-products, holding significance for topical formulations owing to its antioxidant properties observed in both in vitro and in vivo studies. Additionally, this compound is recognized as a potential therapeutic agent for combating photoaging and exerting anti-inflammatory effects. The therapeutic efficacy of chlorogenic acid was assessed in Wistar rats, demonstrating an acceleration in wound healing processes. Furthermore, this compound is regarded as a photostable molecule effective against UVA and UVB radiation, exhibiting notable photoprotective activity. The antimicrobial properties of chlorogenic acid are particularly valuable for formulation purposes, as it can serve as a preservative, thereby replacing synthetic alternatives [64,77].

Previous studies have detailed potential applications of bioactive compounds derived from coffee in cosmetic products. Grigolon et al. [78] conducted a chemical profiling of a coffee seed (CS) extract, which identified the presence of cafestol and kahweol fatty acid esters, along with acylglycerols, β-sitosterol, and caffeine. The authors conducted in vitro experiments on keratinocytes, which demonstrated heightened expression of genes associated with oxidative stress response and skin barrier function. In a study involving volunteers, after 28 days, the extract was observed to increase skin hydration. Additionally, the extract facilitated skin regeneration following irritation, evidenced by reductions in redness and transepidermal water loss. A research study conducted by Rodrigues et al. [79], involving twenty volunteers who utilized a base cream containing a CS extract, revealed results in skin hydration and firmness comparable to those achieved with an equivalent base formulation containing 1.5% hyaluronic acid. In a study conducted by Rodrigues et al. [80], a formulation containing 2.5% CS extract demonstrated no cytotoxic effects in vitro, thereby indicating its safety for topical application when evaluated on reconstituted human epidermis. As previously noted, the constituents of coffee’s lipid fraction provide beneficial attributes for the development of products with photoprotective properties.

The antioxidant activity of coffee by-products, ranging from strong to weak, depends on the specific by-product and the processing method employed [28,81]. Phenolic compounds such as chlorogenic acids and tannins, along with caffeine present in coffee by-products, could serve as active agents against oxidative stress and damage, owing to their redox properties, which enable them to scavenge and neutralize free radicals [82,83]. Consequently, coffee by-products possess numerous potential applications in topical cosmetic formulations [28] (

Table 3. Bioactive compounds from coffee by-products and their potential applications in cosmetic formulations.

Bioactive Compounds	Properties	Cosmetic	References
		Formulations
Chlorogenic acid	Photoprotective; Antioxidant; Anti-wrinkle; Anti-aging; Anti-inflammatory; Antimicrobial	Anti-aging products; Products with therapeutic effects; Suncreams	[64,66,77]
Caffeine	Antioxidant; Anti-aging; Photoprotective; Lipolytic activity; Microcirculation enhancer	Products for cellulite treatment, hair growth, oxidative damage, photoprotection and skin protection and vitality	[26,71,72,73,74,82]
Cafestol; Kahweol; Acylglycerols; β-sitosterol	Antioxidant; Moisturizer; Regenerator; Photoprotective	Skin hydration and firmness creams; Products for skin regeneration;	[78,79,80]

). Several commercial cosmetic products incorporate extracts of coffee by-products; for instance, the VICHY brand offers an under-eye serum targeting dark circles, with caffeine as a key ingredient, attributable to its antioxidant and anti-inflammatory proper-ties [84].

The presence of multiple compounds can influence each other’s activity. Although there is a lack of recent primary studies specifically investigating the combined effects of bioactive compounds from by-products in cosmetic applications, evidence shows complexation between caffeine and chlorogenic acids in aqueous solutions [85]. In a study conducted by Xu et al., [86] the results demonstrated that the combination of chlorogenic acid and caffeine had anti-obesity effects and regulated lipid metabolism in high-fat diet-induced obese mice. Studies have reported that caffeine interacts with many biomolecules, including nucleic acids, polyphenols, and various drug molecules, thereby affecting their physiological properties (e.g., solubility, bioavailability, pharmacology) [87].

6. Olive By-Products

6.1. Bioactive Compounds of Olive By-Products

Olive trees are regarded as among the earliest cultivated plants in human history, with over thirty species documented. Only Olea europaea L. is consumable [88], a fruit tree originating from Asia Minor and Syria [89]. Currently, the principal producers of olives and olive oil are Spain, Italy, and Greece. In recent decades, the consumption of olive oil has risen, attributed to its organoleptic qualities and health advantages [90]. Olive oil is notably rich in monounsaturated fatty acids, as well as phenolics, phytosterols, tocopherols, and squalene. In the composition of extra virgin olive oil, triglycerides constitute 98% to 99%, while minor components account for 1% to 2%, with phenolic compounds representing the most significant constituents of this minor fraction. Olive oil predominantly comprises unsaturated fatty acids, constituting up to 85%, chiefly oleic acid, which varies from 70% to 85% [89,91].

The waste and by-products generated from the olive processing industry have experienced an increase. Consequently, environmental issues have emerged due to their elevated content of phytotoxic compounds (e.g., phenolics), despite these compounds also offering health benefits to humans. Therefore, it is imperative to develop strategies for their recovery and reuse in other industries, in alignment with circular economy policies and sustainability objectives [92]. Given that these bioactive compounds possess notable technological and pharmaceutical properties, they can be utilized as active ingredients in cosmetic applications following extraction and/or purification [93]. The olive fruit is structurally composed of three primary parts: the skin, or epicarp, which accounts for approximately 1–3% of the total fruit weight and is rich in chlorophylls, carotenoids, and anthocyanins crucial for the fruit’s coloration; the pulp, or mesocarp, which comprises the majority of the fruit, representing 70–80% of its weight and serving as the reservoir of nutritional and bioactive constituents; and the stone, or woody endocarp, containing the seed and the woody shell, which constitutes 18–22% of the olive’s weight [88].

Olive oil production is principally a mechanical process, which can be classified into three distinct types: the traditional pressing method, characterized by low efficiency and high labor costs; and two continuous methods, specifically the two-phase and three-phase centrifugal separation techniques, which are the most widely employed. The two-phase centrifugal separation has gained favor in recent years, as the three-phase method necessitates large volumes of water during the centrifugation and decantation stages [94]. The three-phase extraction process yields not only olive oil but also substantial quantities of olive mill wastewater (OMWW) and olive pomace. Conversely, the two-phase process omits the need for additional water, thereby considerably reducing the volume of OMWW produced. This process also results in a semi-solid residue known as olive pomace [95,96]. Overall, the principal by-products derived from olive processing include leaves and pruning biomass, OMWW, olive pomace, and olive stones [93] (see Figure 5). Among these, olive pomace represents the most polluting agricultural by-product in the Mediterranean region [97].

While the content of bioactive compounds is influenced by factors such as the type of by-product, olive variety or cultivar, harvesting period, processing conditions, and extraction techniques [95], olive by-products are generally abundant in phenolic compounds. These include phenolic alcohols (tyrosol and hydroxytyrosol), phenolic acids (caffeic acid, gallic acid, and verbascoside), flavonoids (luteolin, apigenin, and rutin), secoiridoids (oleuropein and ligstroside), and lignans (pinoresinol) [90,93]. Madureira et al. [98] identified hydroxytyrosol as the predominant phenolic compound in olive pomace (25 mg/g of extract), followed by hydroxytyrosol 1-β-glucoside (9.8 mg/g of extract) and tyrosol (5.9 mg/g of extract). The composition of olive pomace varies, comprising cellulose (30%), pectic polysaccharides (39%), hemicellulosic polymers rich in xylans and glucuronoxylans (14%), xyloglucans (15%), and mannans (2%). Additionally, squalene has been reported in olive pomace [90].

Olive leaves are regarded as a by-product of olive cultivation, arriving in substantial quantities at the olive mill. They contain valuable compounds, including flavones such as luteolin-7-glucoside (0.145–0.165 mg/g), apigenin-7-glucoside (0.197–0.217 mg/g), and luteolin (0.216–0.224 mg/g); flavonols like rutin (0.289–0.651 mg/g); substituted phenols such as tyrosol (3–14 mg/g), hydroxytyrosol (6–24 mg/g), vanillic acid (0.010–0.020 mg/g), and caffeic acid (0.012–0.018 mg/g); and secoiridoids including oleuropein (0.13–21.34 mg/g). Oleuropein, the predominant phenolic compound in olives, is a complex molecule derived from elenolic acid, glucose, and hydroxytyrosol [99,100].

Olive mill wastewater is an organic-rich effluent containing a significant concentration of phytotoxic phenolic compounds. Additionally, it comprises sugars and organic acids, along with mineral nutrients such as nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg), among others. In the study conducted by Cuffaro et al. [96], the polyphenolic profile of OMWW extract from two different olive cultivars, Leccino (CL) and Frantoio (CF), was reported. Two samples of CL and CF (CL1, CL2, and CF1, CF2) were collected and characterized, revealing that, although present in varying quantities, these samples were particularly rich in various phenolic compounds. CL1 and CF1 exhibited high concentrations of oleacein (314.628 ± 19.535 and 227.273 ± 3.974 µg/mg, respectively), as well as tyrosol and hydroxytyrosol (17.602 ± 0.792, 18.808 ± 0.447 µg/mg, and 55.875 ± 1.511, 71.919 ± 3.943 µg/mg, respectively). Notably, CL1 contained higher levels of oleocanthalic acid compared to CF1 (10.628 ± 0.484 µg/mg and 2.149 ± 0.091 µg/mg, respectively). In addition to these compounds, present in lower quantities yet remaining significant, were caffeic acid (2.791 ± 0.101 µg/mg and 1.453 ± 0.044 µg/mg for CL1 and CL2, respectively), and (2.234 ± 0.112 µg/mg and 3.144 ± 0.142 µg/mg for CF1 and CF2, respectively), as well as verbascoside (1.624 ± 0.035 µg/mg and 0.958 ± 0.018 µg/mg for CL1 and CL2, respectively), and (18.182 ± 0.437 µg/mg and 0.909 ± 0.035 µg/mg for CF1 and CF2, respectively).

The olive stones constitute a lignocellulosic material predominantly composed of hemicellulose (21.9%), cellulose (31.29%), and lignin (26.5%) [100]. Consequently, this by-product holds significant interest owing to its chemical and physical properties as well as its calorific value. Additionally, minor proportions of fat (5.53%), proteins (3.20%), and sugars (0.48%) have been documented [88]. This by-product is among the most abundant sources of secondary metabolites. According to Saad et al. [101], thirty-one metabolites were identified via methanolic extraction, encompassing phenolic alcohols, hydroxycinnamic acid derivatives, secoiridoids, flavonoids, terpenes, and fatty acids.

Therefore, the reutilization of olive waste as an innovative, renewable, and economical source of high-value bioactive compounds holds significant potential owing to its numerous applications as functional ingredients within the food, pharmaceutical, and cosmetics sectors, in accordance with the principles of a circular economy [96] (Figure 6). Given their high phytotoxic properties, their valorization additionally aids in mitigating environmental concerns [96].

6.2. Extraction Methods for Olive By-Products

According to the literature, the methods used to extract bioactive compounds from olive by-products are very similar to those applied to coffee by-products. The extraction techniques for olive by-products also include conventional technologies, such as solid–liquid extractions evolving organic solvents, distillation, heat reflux, and filtration processes to separate and obtain the bioactive compounds [102]. When the extracts are to be applied in the food or cosmetic industry, the solvents employed are mainly hydroalcoholic mixtures, since ethanol is recognized as safe for consumers [27]. The solid-liquid extraction method is straightforward, cost-effective, and exhibits high accuracy and reproducibility. However, it is not environmentally sustainable, is time-consuming, and demonstrates limited adaptability in processing. The extraction yield of hydroxytyrosol using a water-ethanol mixture as a solvent ranges from 10 to 15 mg per gram [103,104].

The procedures encompass the recovery of phenolics through the condensation of steps (thermal concentration, filtration, or lyophilization), followed by subsequent extraction stages utilizing methanol, ethanol, or hydro-alcoholic solutions [93].

The employment of innovative or environmentally friendly technologies has been on the rise, including high-hydrostatic pressure, ultrasound-assisted extraction (UAE), microwave-assisted water extraction (MAE), pulsed electric field (PEF), radiofrequency drying, high-voltage electrical discharge, supercritical fluid extraction (SFE), and pressurized liquid extraction (PLE) [93,105].

In a study conducted by Madureira et al. [106], ultrasound-assisted extraction (UAE) was utilized to recover hydroxytyrosol and tyrosol, yielding results of 36 ± 2 mg g⁻¹ and 14 ± 1 mg g⁻¹, respectively. In another investigation employing microwave-assisted extraction (MAE), it was possible to solubilize 5.87 mg of hydroxytyrosol per gram and 46.70 mg of mannitol per gram from exhausted olive pomace [107]. These methods, in comparison to conventional techniques, are characterized by reduced time requirements and greater environmental sustainability. Presently, UAE is acknowledged as a sustainable technique that facilitates energy-efficient processes and produces notable improvements in extraction yield and economic viability at an industrial scale. Recent innovations in ultrasound technology have been developed to meet current industrial demands, including the availability of continuous-flow devices suitable for large-scale operations. Consequently, the optimization of extraction parameters at the laboratory level remains a crucial step prior to the implementation at an industrial scale [108]. A research study aimed to establish an optimized, reproducible, and industrially scalable UAE protocol for olive oil by-products, specifically olive leaves and olive pomace, with emphasis on maintaining the chemical stability and integrity of key phenolic compounds. Among the evaluated variables, the ethanol concentration in the extraction solvent was identified as the most significant factor exerting the highest impact [109].

The optimization of operational parameters for each extraction method is essential to prevent the degradation of macromolecules and the oxidation of labile compounds [94].

The application of deep eutectic solvents (DES), multicomponent systems comprising hydrogen bond donors and acceptors that interact to form a stable liquid, has recently emerged as a sustainable technique for extracting bioactive compounds from olive by-products [110]. These solvents are easy to prepare, cost-effective, and environmentally benign. They can be synthesized from various substances, permitting the development of a “tailor-made” solvent tailored to specific applications [27,111]. This methodology yielded an extraction of 4.98 mg/g for hydroxytyrosol [112].

Natural Deep Eutectic Solvents (NaDES) constitute a subset of deep eutectic solvents composed of naturally occurring, biomolecular components such as amino acids, sugars, organic acids, and other bio-based molecules [100]. These solvents are engineered to be biocompatible and environmentally sustainable, thereby aligning with the principles of green chemistry [113]. When optimized for specific applications, deep eutectic solvents have demonstrated considerable efficiency; however, they are characterized by high viscosity, which may present a limitation [111,114]. In the study conducted by Cabrera et al. [115], the extraction of phenolic compounds utilizing NADES was investigated, and the findings indicated a hydroxytyrosol content of 1.24 mg/g dry basis.

Enzyme-Assisted Extraction (EAE) is a process that uses hydrolytic enzymes to break down cell walls, enabling the separation of bioactive compounds from plant raw materials. It is environmentally friendly and cost-effective, integrating well with existing industrial processes [116]. Cellulase and viscoenzyme are two enzymes commonly used in EAE. These enzymes can produce new derivatives from the extracts by cleaving bonds in macromolecules, such as cell walls. For example, cellulase hydrolyzes vegetable cell walls by breaking down cellulose into glucose. Meanwhile, viscoenzyme is a multicomponent carbohydrase capable of breaking glycosidic bonds in plant cell walls through hydrolysis, releasing intercellular components. Therefore, EAE has shown superior extraction yields compared to hydroalcoholic solvents, indicating its potential for large-scale use [117]. The downside of this method is that enzymes can be costly, and extraction efficiency may vary depending on the substrate and enzyme used [56]. Using cellulase and viscoenzyme, it is possible to obtain 0.512 mg/g of hydroxytyrosol during extraction [117] (Table 4).

Conventional solvent extractions generally provide the highest yields; however, they are not highly sustainable. Green technologies such as Ultrasound-Assisted Extraction (UAE) and Microwave-Assisted Extraction (MAE) often attain higher yields with satisfactory purity, while utilizing reduced amounts of solvent. The optimal balance for extracting polar bioactive compounds, such as polyphenols, may be achieved through MAE or UAE employing water/ethanol as solvents [118]. The extraction conditions of the various methods are summarized in Table 5.

Table 4. Comparison of conventional and green extraction methods, including their pros, cons, and typical yields for extracting olive by-products.

Category	Method	Pros	Cons	Yield
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	Simple; Economic; Accuracy/reproducibility [104]	Not environmentally friendly; Time consuming; Poor processing adaptability [104]	(ethanol-water solvent) Hydroxytyrosol (10–15 mg/g) [103]
Green	Microwave-assisted (MAE)	Less time consuming; Efficient and environmentally friendly; [107]	Expensive equipment; Limitations concerning extraction uniformity; [107]	Hydroxytyrosol (5.87 mg/g) Manitol (46.70 mg/g) [107]
	Deep eutectic solvents (DES)	Solvents easy to prepare; Efficient; Inexpensive, and environmentally friendly; [27,111]	Challenges in compound and solvent recovery; [119]	Hydroxytyrosol (4.98 mg/g) [112]
	Water Ultrasound-Assisted Extraction (UAE)	Less time consuming; Efficient and environmentally friendly; [108]	Expensive equipment; Limitations concerning extraction uniformity [108]	Hydroxytyrosol (36 ± 2 mg/g) Tyrosol (14 ± 1 mg/g) [106]
	Natural Deep Eutectic Solvents (NaDES)	Biocompatible and environmentally friendly; Efficient; [113]	High viscosity; [114]	Hydroxytyrosol (1.24 mg/g) [115]
	Enzyme-Assisted Extraction (EAE)	Environmentally friendly and cost-effective; [116]	Enzymes can be expensive; Extraction efficiency may vary with substrate and enzyme; [57]	(Using cellulase and viscoenzyme) Hydroxytyrosol (0.512 mg/g) [117]

Table 4. Comparison of conventional and green extraction methods, including their pros, cons, and typical yields for extracting olive by-products.

Category	Method	Pros	Cons	Yield
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	Simple; Economic; Accuracy/reproducibility [104]	Not environmentally friendly; Time consuming; Poor processing adaptability [104]	(ethanol-water solvent) Hydroxytyrosol (10–15 mg/g) [103]
Green	Microwave-assisted (MAE)	Less time consuming; Efficient and environmentally friendly; [107]	Expensive equipment; Limitations concerning extraction uniformity; [107]	Hydroxytyrosol (5.87 mg/g) Manitol (46.70 mg/g) [107]
	Deep eutectic solvents (DES)	Solvents easy to prepare; Efficient; Inexpensive, and environmentally friendly; [27,111]	Challenges in compound and solvent recovery; [119]	Hydroxytyrosol (4.98 mg/g) [112]
	Water Ultrasound-Assisted Extraction (UAE)	Less time consuming; Efficient and environmentally friendly; [108]	Expensive equipment; Limitations concerning extraction uniformity [108]	Hydroxytyrosol (36 ± 2 mg/g) Tyrosol (14 ± 1 mg/g) [106]
	Natural Deep Eutectic Solvents (NaDES)	Biocompatible and environmentally friendly; Efficient; [113]	High viscosity; [114]	Hydroxytyrosol (1.24 mg/g) [115]
	Enzyme-Assisted Extraction (EAE)	Environmentally friendly and cost-effective; [116]	Enzymes can be expensive; Extraction efficiency may vary with substrate and enzyme; [57]	(Using cellulase and viscoenzyme) Hydroxytyrosol (0.512 mg/g) [117]

Table 5. Comparison of extraction conditions for olive by-products.

Category	Method	Temperature or Power	Time	Solvent Ratio	Ref.
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	25 °C	180 min	EtOH: H2O 5:1 v/w	[120]
Green	Microwave-assisted (MAE)	250–350 W	2–3 min	Solvent free Sample Amount 5–10 g.	[121]
	Deep eutectic solvents (DES)	55 °C	5 min	ChCl:AA (1:2) ethanol (80:20 w/w)	[122]
	Water Ultrasound-Assisted Extraction (UAE)	500 W 30 °C	10 min	EtOH: H2O 50% v/v	[123]
	Natural Deep Eutectic Solvents (NaDES)	60 °C	10 min	Choline chloride and caffeic acid (1:2)	[124]
	Enzyme-Assisted Extraction (EAE)	50 °C	120 min	Cellulase or viscoenzyme 1.0%	[125]

Table 5. Comparison of extraction conditions for olive by-products.

Category	Method	Temperature or Power	Time	Solvent Ratio	Ref.
Conventional	Solid–Liquid Extraction (e.g., Soxhlet)	25 °C	180 min	EtOH: H2O 5:1 v/w	[120]
Green	Microwave-assisted (MAE)	250–350 W	2–3 min	Solvent free Sample Amount 5–10 g.	[121]
	Deep eutectic solvents (DES)	55 °C	5 min	ChCl:AA (1:2) ethanol (80:20 w/w)	[122]
	Water Ultrasound-Assisted Extraction (UAE)	500 W 30 °C	10 min	EtOH: H2O 50% v/v	[123]
	Natural Deep Eutectic Solvents (NaDES)	60 °C	10 min	Choline chloride and caffeic acid (1:2)	[124]
	Enzyme-Assisted Extraction (EAE)	50 °C	120 min	Cellulase or viscoenzyme 1.0%	[125]

6.3. Applications in Cosmetic Formulations

Olive trees can supply compounds for the cosmetic industry, including both hydrophilic and lipophilic agents. Hydrophilic compounds primarily consist of polyphenols, whereas lipophilic compounds encompass fatty acids, liposoluble vitamins, and squalene [27,125]. In the hydrophilic fraction, polyphenols are characterized by their multifunctionality, exhibiting antioxidant properties along with anti-aging, photoprotective, and antimicrobial activities [126,127]. The lipophilic compounds also possess various valuable properties, including antioxidant, emollient, and moisturizing effects. They facilitate healing by supporting the repair of the skin’s lipid barrier function; can be employed as lubricants to achieve soft, elastic, and well-lubricated skin; and have a remarkable ability to adhere to the skin surface, thereby providing prolonged activity [27,128,129].

The application of olive by-products has been investigated for skin treatments employing various topical delivery systems. For instance, creams are recognized for their efficacy in hydration and emollience. The utilization of natural ingredients in creams has witnessed an increasing trend [130]. In a study conducted by Nunes et al. [131], oil-in-water creams incorporating three distinct extracts derived from olive oil by-products were produced without the use of organic solvents and were compared to a control cream devoid of extracts. The authors concluded that these extracts might serve as UV-protection enhancers by promoting the absorption of specific synthetic UV filters. The results further suggest their potential use as preservative enhancers. Moreover, in an in vitro assessment conducted on human keratinocytes and corneal epithelium cells, these extracts did not induce skin or eye irritation. In vivo efficacy assessments also demonstrated that these formulations exhibit excellent compatibility and acceptability with human skin. Another investigation conducted by Wanitphakdeedecha et al. [130] evaluated the efficacy of a cream containing olive leaf extract in facial rejuvenation. A pilot study was carried out involving 36 participants with photoaged skin, who applied the olive leaf extract-containing cream. Observations revealed changes in the biophysical properties of the skin, including melanin and erythema indices, transepidermal water loss, skin hydration, skin pH, sebum levels, texture, and wrinkles. Notably, significant improvements in the reduction in wrinkles were identified after merely one month of treatment, while enhancements in skin barrier function, hydration, and texture were observed after two months. In conclusion, the cream containing olive leaf extract demonstrated benefits in skin rejuvenation in human subjects.

As previously stated, hydroxytyrosol (C₈H₁₀O₃), a phenolic compound and the predominant product derived from the hydrolysis of oleuropein (C₂₅H₃₂O₁₃), is abundantly present in olive by-products and constitutes one of the most potent naturally occurring antioxidants [131]. Oleuropein, obtained from olive leaves, exhibits antibacterial efficacy comparable to that of synthetic preservatives, as reported by Fuad Al-Rimawi et al. [132]. In this study, a microbiological analysis was conducted to evaluate two fungal species and three bacterial species; the results indicated that the product containing natural preservatives performed similarly to the formulation utilizing synthetic preservatives.

Hydroxytyrosol, which develops throughout olive maturation, oil preservation, and table olive processing, is the major contributor to the sensory diversity of olive products [133]. Hydroxytyrosol can be used in formulations that suppress melanin production, thereby reducing skin pigmentation, and acting as a whitening, lightening, and depigmenting compound. This compound has not only antioxidant, but also anticancer, anti-inflammatory, and antimicrobial activities [95,134,135,136]. Smeriglio et al. [137] examined the safety and efficacy of a cream containing hydroxytyrosol as an active ingredient, verifying its ability to enhance the epidermal barrier, mitigate inflammatory responses, and promote skin repair.

Nunes et al. [138] examined the nutritional, chemical, and antioxidant profiles of functional ingredients derived from various olive pomace samples, as well as their antimicrobial activity against S. aureus, E. coli, and C. albicans. Among the evaluated ingredients, the sample containing the highest concentration of hydroxytyrosol (220 mg/100 g) exhibited the most potent antibacterial activity, as evidenced by the lowest minimal inhibitory concentration (MIC) values against the strains tested. The antibacterial mechanisms of hydroxytyrosol are linked to its capacity to chelate transition metals, thereby diminishing the reactivity of iron and copper through the formation of inert metal–ligand complexes, which subsequently reduce their bioavailability for bacterial growth. Additionally, it may exert its effects by decreasing intracellular ATP concentrations, disrupting membrane potential, and/or reducing bacterial protein content [138,139].

Tyrosol (C₈H₁₀O₂), as a hydrophilic compound, presents certain limitations in its incorporation into oil-based cosmetic products. Nevertheless, it is feasible, under green chemistry conditions, to synthesize lipophilic esters of tyrosol, thereby enhancing its suitability for emulsions, creams, and other oil-based cosmetic formulations. These lipophilic derivatives exhibit superior antioxidant and surfactant properties and also possess anti-inflammatory effects [140]. Although tyrosol demonstrates valuable properties for inclusion in cosmetic formulations, unlike hydroxytyrosol, the literature lacks controlled clinical or targeted in vitro investigations regarding its dermal tolerability, particularly for sensitive or atopic skin. Therefore, the topical application of tyrosol should be supported by formal dermal safety assessments, such as reconstructed human epidermis assays and trials involving volunteers with sensitive or atopic skin, prior to broader cosmetic use [137,141].

Regarding lipophilic compounds, oleic acid (C₁₈H₃₄O₂) stands as the most abundant fatty acid in olive oil by-products and can be employed in cosmetic formulations as an enhancer of transdermal penetration for specific active compounds [128,142]. Vitamin E, specifically α-tocopherol, is the predominant form found in olive oil and serves as a potent non-enzymatic antioxidant. It is extensively incorporated into topical formulations due to its capacity to neutralize free radicals, prevent lipid peroxidation of fatty acids, and safeguard cellular membranes [143,144]. Vitamin E (C₂₉H₅₀O₂) also possesses the ability to modulate pathways involved in inflammation, apoptosis, and cell differentiation. Carotenoids, natural pigments with photoprotective properties, are capable of preventing UVA-induced dermal damage, reducing oxidative stress, preserving enzyme activity, and inhibiting fibroblast apoptosis, while also exerting anti-inflammatory effects [145]. Squalene (C₃₀H₅₀), a polyunsaturated triterpenic hydrocarbon characterized by six carbon double bonds [146], is found in olive pomace and offers notable advantages for skin. In addition to its antioxidant properties, squalene serves as a sink for highly lipophilic xenobiotics, aiding in their elimination from the organism. This compound has potential applications as an ingredient in cosmetic and dermo-protective creams due to its emollient properties [90,147] (

Table 6. Olive by-products’ bioactive compounds and their potential applications in cosmetic formulations.

Bioactive Compounds	Properties	Cosmetic Formulations	References
Hydroxytyrosol	Whitening, lightning and depigmenting Antioxidant Anti-aging Photoprotective Anti-inflammatory Antimicrobial	Formulations for reducing skin pigmentation; Creams to enhance the epidermal barrier, inflammatory responses and for skin repair;	[95,126,127,131,134,136]
Tyrosol	Antioxidant Anti-aging Photoprotective Anti-inflammatory Surfactant	Emulsions, creams and oil-based formulations.	[126,140]
Oleic acid Vitamin E Squalene	Antioxidant Emolient Moisturizer Anti-inflammatory	Formulations with healing and lubricant activity; Formulations to prevent oxidative stress; Dermo-protective creams	[27,125,128,129,142,147]
Carotenoids	Antioxidant Photoprotective Anti-inflammatory	Formulations to prevent UVA induced dermal damage and reduce oxidative stress.	[145]

). Several commercial cosmetic products incorporate extracts derived from olive by-products. For instance, OLIVEDA, a distinguished skincare brand, offers various product lines, including cleansing balms, hair serums, facial masks, and face creams containing olive by-product ingredients—most notably, olive leaf cell extracts rich in hydroxytyrosol—owing to their antioxidant and anti-aging properties [148].

Cosmetic formulations must undergo multiple physical-chemical and microbiological evaluations prior to market release. Safety assessment reports have been prepared for the utilization of olive-derived ingredients in various cosmetic formulations, including leave-on products [149]. Such documentation underscores the importance of comprehensive safety assessments, encompassing dermatological compatibility testing, to safeguard consumer welfare and ensure product efficacy. Various studies have conducted dermatological compatibility tests to identify potential adverse skin reactions, with most yielding favorable outcomes that support the safe integration of these innovative ingredients into cosmetic products [131]. These evaluations enable companies to achieve clear regulatory compliance and substantiate the stability and efficacy of their final products [56,150].

Phenolic compounds derived from coffee and olive by-products, such as hydroxytyrosol and chlorogenic acids, are prone to degradation under unfavorable conditions such as elevated temperatures, exposure to light, and alkaline pH levels. Such degradation can impair their antioxidant and antimicrobial properties. To mitigate this issue, cosmetic formulations commonly utilize stabilization strategies, including pH adjustment, incorporation of antioxidants or chelating agents, and encapsulation techniques such as liposomes, nanoemulsions, and biopolymeric carriers, which have demonstrated significant improvements in the shelf-life and bioactivity of these compounds [151,152].

Coffee and olive by-products possess valuable properties individually and can be utilized concurrently to achieve a synergistic effect. A recent study investigated the development of a hair care formulation employing lipid nanocarriers loaded with enriched coffee silverskin extract and olive pomace. The findings indicated that the synergistic combination of these two by-products elicited multiple beneficial effects, including the stimulation of hair growth, anti-dandruff activity, and improved nourishment of the hair [153].

7. Conclusions and Future Perspectives

Throughout this work, it became clear that the cosmetic industry can significantly benefit from valuing natural ingredients, especially those derived from agro-industrial by-products like coffee and olive residues. What is often seen as waste contains a wide range of compounds with real potential for skincare applications. Several studies confirm the role of by-product extracts as active ingredients since they possess antioxidant, antimicrobial, moisturizing, antiaging, and UV-protective properties, demonstrating their effectiveness both in vitro and in vivo by improving skin hydration, elasticity, barrier function, and appearance. Using these by-products in cosmetic products not only introduces new active ingredients but also encourages more environmentally friendly production models, reducing environmental impact and addressing the demand for products that are effective, safe, and eco-friendly. This approach aligns with the principles of a circular economy and sustainability. Consumers increasingly care about what they apply to their skin, and ingredients from olive and coffee by-products are becoming a key point of differentiation. The extraction methods for these by-products are becoming more efficient and environmentally friendly, however the consistency of olive and coffee by-products as ingredients remains a challenge. Despite that, some industries have already commercialized some products using hydroxytyrol and caffeine as ingredients, with promising results and consumers approval.

Future research should prioritize optimizing extraction methodologies, establishing standardized extracts with uniform bioactive profiles, and increasing the number of clinical and dermatological investigations. Standardizing bioactive content is crucial to maintain consistent quality across batches, considering the seasonal and geographical variability of raw materials. Further research on formulation compatibility is also imperative to evaluate stability and interactions with other cosmetic constituents. Moreover, addressing industrial scalability and the cost-efficiency of eco-friendly extraction methods such as UAE, MAE, DES, and EAE remains a significant challenge requiring further exploration. Quantitative data demonstrating carbon savings and estimates of waste reuse within the cosmetic industry would further substantiate sustainability claims.