Essential oils and sustainability in cosmetic industry: a brief review
Cosmetics have been an integral part of personal care and hygiene since ancient times. The employment of natural products as sources of nourishing and moisturizing agents for the skin, substances with pleasant odor or beautifying agents that highlight facial features is well-known.
Through the years, technological and scientifical development has led to different ways of cosmetics’ production and content of ingredients in order to keep up with the current needs of the market. Cosmetic products have been upgraded regarding their efficacy and purposes of use and their variety has grown with respect to consumers’ needs. For expanding the shelf-life, various substances that can act as preservatives (e.g. parabens) are incorporated into cosmetics. However, in recent years, there has been a shift towards more natural products which are composed, if not entirely, at least to the greatest percent, of natural substances. This trend is attributed both to the negative effect of synthetic antioxidants and preservatives used in cosmetics, as well as the adoption of an ever-increasing environmentally friendly mindset by consumers. In the light of these events, companies need to create new, synthetic-free, innovative products that are effective and high-quality, in order to stay ahead in a highly competitive market.
To this respect, although essential oils from aromatic and medicinal plants are key components of cosmetic products, and especially perfumes, their use tends to increase in the recent years. Due to their lower toxicity compared to similar synthetic fragrances, their natural origin and their antimicrobial and antioxidant activity, they are great alternative ingredients in cosmetic products. This diverse group of natural volatile compounds present a wide spectrum of biological properties, including antibacterial, antifungal and antioxidant activities. So far, essential oils have been mainly used as fragrance ingredients due to their characteristic and intense odor. It is estimated that more than 3000 essential oils are of commercial importance and used in flavor and cosmetic industries (Dreger & Wielgus, 2013; Yang et al., 2020).
2. Essential oils from aromatic plants
Essential oils are secondary metabolites that are produced and secreted naturally by plants under specific conditions through a series of complicated molecular interactions, in order to protect them from diseases and herbivores (Tariq et al., 2019). They are mixtures of volatile compounds which are divided into three broad categories based on their chemical structure and the metabolic path that they follow for their production. In particular, they are divided into terpenes (e.g. p-cymene, limonene), terpenoids, which are oxygenated derivatives of terpenes (e.g. thymol, carvacrol) and phenylpropanoids (e.g. eugenol, vanillin) (Hyldgaard, Mygind, & Meyer, 2012). Essential oils are stored in cavities, epidermal and secretory cells, canals or glandular walls of plants and are obtained mainly by hydro-distillation from various parts of them (flowers, stems, leaves, roots, etc.) (Tariq et al., 2019).
In terms of their biological properties (e.g. antimicrobial, antioxidant, anti-inflammatory) essential oils are highly heterogenous. These properties are dependent on the type and amount of their main individual compounds (Nazzaro et al., 2013). Among these compounds, the group of terpenoids are widely active against a range of microorganisms. One of the most important and effective antimicrobial volatile compounds that belongs to the group of terpenoids is carvacrol. This phenolic terpenoid is hydrophobic as it consists of a highly active hydroxyl group and can act a cationic carrier in membranes, transporting H + inside the cytoplasm and K + outside (Ultee, Bennik & Moezelaar, 2002).
2.1. Antimicrobial activity
Essential oils have the ability to act as natural antimicrobial agents. The wide variety of their individual compounds is responsible for the number of cellular targets of their antimicrobial activity that have been described. To this respect, different essential oils present different antimicrobial activity against different bacteria. Their way of action is based on a series of biochemical reactions that eventually lead to cell death (Gutiérrez-del-Río, Fernández & Lombó, 2018). Due to their lipophilic nature, essential oils can easily cross both the cell wall and the cytoplasmic membrane of bacteria. Their effect on Gram-negative and Gram-positive bacteria is different owing to the different structures of cells in each group. In Gram-positive bacteria, the presence of lipoteichoic acid as part of their cell wall facilitates the penetration of hydrophobic compounds in the cell. On the other hand, the absence of this structure and the presence of an outer membrane in Gram-negative bacteria renders them more resistant to the action of essential oils (Tongnuanchan & Benjakul, 2014).
Another way of antimicrobial activity of essential oils is through their action on the formation of various fatty acids, phospholipid layers and polysaccharides which eventually affect the membrane by increasing the permeability. The destruction of the cell wall and cytoplasmic membrane is fatal for bacteria as it leads to ion leakage into the extracellular space, reduction of the membrane potential, collapse of the proton pump, depletion of the ATP reservoir and loss of macromolecules that will ultimately cause cell lysis (Oussalah et al., 2007). Essential oils can also cause cytoplasm aggregation as well as direct damage to cellular macromolecules (Burt, 2004). Finally, there are mechanisms of action of essential oils that have not yet been fully understood, such as inhibiting the action of certain enzymes vital to the bacterial cell (Gutiérrez-del-Río, Fernández & Lombó, 2018).
2.2. Antioxidant activity
Apart from their antimicrobial activity, essential oils present significant antioxidant activity which is related to their composition. In particular, the phenolic and terpenoid compounds present in essential oils act as antioxidants in order to maintain balance in the redox potential. An antioxidant can be characterized as any component that significantly inhibits or slows down the oxidation of the substrate even in lower concentration than the latter. The constituents of essential oils can act either by preventing the oxidation of sensitive biological molecules by free radicals, by scavenging them, or by reducing their formation through inactivation (Bonner & Arbiser, 2014). In addition, they have the ability to form chelating complexes with transition metals (e.g. copper or iron) thus inhibiting their pre-oxidative action (Liyana-Pathirana & Shahidi, 2006). Among the main compounds of essential oils, thymol and carvacrol have the strongest antioxidant activity, which has been associated with their phenolic structure and their redox properties are fundamental in the decomposition of both peroxides (Burt, 2004).
Generally, the antioxidant activity is related to the main active compounds found in different essential oils, such as eugenol in clove essential oil, carvacrol in oregano essential oil and thymol in thyme essential oil. However, other compounds as well have been reported to have antioxidant activity, such as camphor, eucalyptol, terpinene and methyl-babicol, but larger amounts are required to achieve a similar antioxidant effect (Teixeira et al., 2013).
3. Market demand and production of essential oils for the cosmetic industry
Essential oils are mostly used in cosmetic products as fragrances, in perfumes, skincare and haircare products and toiletries. According to Cosmetics Europe, the European trade association for the cosmetics and personal care industry, the corresponding European market is the largest in the world with a value of €76.7 billion in 2020. Based on data from the same association, the categories of toiletries, haircare and skincare products comprise about 70% of the total European cosmetics market. This large market provides a good opportunity for essential oils exporters in developing countries, which is expected to grow even more in the following years. Additionally, there is a growing market for natural and organic cosmetics in Europe valued at €3.64 billion in 2018. The increasing consumers’ demand for high-quality natural products is responsible for the consequent growing demand for niche essential oils in the fragrance and personal care industries. The leading importers of essential oil production in Europe are Germany, France, United Kingdom, Italy and Spain.
Key market drivers of high essential oil demand include the growing consumers’ awareness regarding the side effects of synthetic ingredients in cosmetics and their replacement with natural and alternative ones, the additional properties of essential oils, such as antimicrobial and antioxidant activity as well as environmental awareness involving adoption of eco-friendly and sustainable practices. In addition, the growing popularity of relaxation techniques and alternative medicine practices such as aromatherapy boosts the overall demand. In the socio-economic sector, improved lifestyles and higher living standards combined with increasing disposable consumer income further increase the demand for natural cosmetic products for the purposes of overall body care and well-being. Last but not least, sales of essential oils are closely related to consumer education. Since more consumers understand the way of action and use of essential oils, the demand is greater and the sales are growing (Barbieri & Borsotto, 2018).
4. Natural vs sustainable: a current challenge
The basis of employing naturally derived compounds for the purposes of health and well-being has been the core of folk medicine for many years. Natural compounds have been proven to offer promising medicinal properties with little or no side effects and are potential resources of effective drug molecules (Ahmed et al. 2017; Shishir et al. 2019). As natural products, essential oils include different groups of bioactive compounds, such as terpenoids, terpenes and polyphenols, which present remarkable pharmacological activities, including antimicrobial, antioxidant, anti-inflammatory, anti-carcinogenic, anti-aging, anti-allergenic, anti-hyperpigmentation, moisturizing, and UV protective (Charles Dorni et al. 2017). Many plants are being used for the production of essential oils that are further utilized as active ingredients in cosmetic products either for their fragrant properties or for their biological activities. In terms of innovation in the market, new plant sources are being explored and harnessed by the cosmetics industry to create a different combination of ingredients (Ahmed et al., 2020). Additionally, as mentioned in section 3, the current market needs for natural cosmetics as well as the environmental concerns have led to an increase in the use of essential oils as cosmetic ingredients.
Nowadays, the term sustainability is linked to almost every aspect of production, consumption and end-life options regardless of the product. The environmental crisis, along with the financial and health crisis has led to a shifting of mindset regarding the methods of production, exploitation of natural sources as well as consumption and end-of-life options. The idea of sustainability lies in three different parameters: social, economic and environmental and the main aim of sustainable development is meeting the current population needs without compromising the respective needs of future generations (Bom et al., 2019). However, a naturally derived, renewable source does not mean it is sustainable. A clear example is the use of essential oils in cosmetics.
The production of essential oils is quite demanding, taking into account the natural sources used for growing the respective plant materials (water, minerals, sun etc.). Many factors, including soil and climate conditions, can actively affect the crop, while the resources being used are further increased in the case of non-native crops. Additionally, the sustainability of the essential oil production cycle is strongly doubted as there is a need for enormous quantities of plant materials in order to produce some liters of essential oil. In fact, some essential oils are not easily extracted as they are stored in tiny cavities or ducts within the plant and are not being externally secreted. In some other cases, the yield of essential oil is limited depending on the type of the plant material. For example, rose petals provide a typical yield of 0.006%, while bay leaf typically provides a 3% yield during distillation. The need for differentiation in the cosmetics industry leads to harvesting of wild plants that cannot be easily cultivated which is harmful for natural equilibrium. For example, the production of rosewood and sandalwood essential oils have led to massive deforestations (Bom et al., 2019). In fact, Aniba rosaeodora, also known as pau-rosa, from which rosewood oil is extracted, as well as Cedrus atlantica (Atlas cedar) are listed as endangered species in the IUCN Red List of Threatened Species, while Santalum album (Indian sandalwood) is listed as vulnerable. Furthermore, the majority of essential oils are strongly volatile and usually, products scented by essential oil mixtures end up carrying exactly the same scent usually due to the potent affinity of some of them (e.g. lavender) and the high evaporation rates of some others (e.g. citrus notes).
Although synthetic fragrances are not naturally derived, in terms of sustainability, they have advantages over essential oils. The main difference between these two lies to the fact that essential oils are solely produced from plants while fragrances are made with chemical compounds. In this way, essential oils are naturally volatile, so the corresponding perfumes last less than fragrances. Synthetic fragrances can be selected targeting exactly to the desired final result. They carry a well substantiated safety profile and can be considered sustainable, as there is no harm to natural resources in producing them. They may contain some fixatives in order to remain intact in the product used during its entire shelf life. Despite their non environmentally friendly way of production, fragrances can present stronger scent, last longer, cost less and follow sustainable way of production (O'Lenick and Matson, 2011).
5. Essential oils as ingredients for sustainable cosmetics
Cosmetics consist of a wide variety of products that can be classified based on their purpose of use. To this respect, Rieger (2000) has proposed seven different categories of cosmetics including skin care, hair care, cleansing, improvement of odor and decorative cosmetics. Essential oils and their individual compounds, owing to their odoriferous characteristics, are normally present as ingredients in all categories. The aims of incorporation of essential oils in these products include the improvement of the scent of the product (odoriferous agents), the protection of the product against microbiological spoilage and oxidation (antimicrobial and antioxidant agents), the contribution in a cooling effect in products such as toothpastes (cooling agents) as well as the enhancement of their pharmacological profile. In the case of the latter, essential oils are added due to their biological properties (anti-inflammatory, anti-diabetic, anti-carcinogenic effect) and the products are characterized as cosmeceuticals (cosmetic + pharmaceutical) (Mamusa et al., 2021). The variety of properties that essential oils provide can actively modify the lack of sustainability related to their production.
More specifically, cosmetic industry should ensure the overall and complete utilization of essential oils in cosmetic products, covering all the possible ways of use. The combination of ways of action of essential oils in a cosmetic product should be carefully planned based on the purpose of use of the respective product. For example, tea tree essential oil can be added in a skincare product (e.g., face cream) with triple action; antimicrobial activity against Propionibacterium acnes, pleasant scent and preservative agent. Based on the above, an overview regarding the current applications of essential oils in cosmetics is presented. In some cases, different properties of essential oils are already taken into consideration and they are being applied in one product aiming to different ways of action.
Essential oils and their compounds have been widely incorporated in perfumes for centuries due to their strong fragrant properties. The great variety of essential oils employed in perfumery is indicative of the different combinations of fragrances and perfumes available on the market in order to suit every taste. An estimated number of 300 different essential oils deriving from 3000 plant species are being employed in the perfumes and flavors industry (Sharmeen et al., 2021).
Based on the characteristics regarding odor, volatility as well as diffusion rate in the air, essential oils are classified as top, middle and base notes in a perfume. Top notes are usually perceived first by the consumer and for a short period of time (about 5-10 minutes) since they are highly volatile. The aim of top notes is the contribution the freshness of the blend of the perfume as well as the positive impression of the consumer. For this reason, essential oils usually used as top notes in perfumes are bergamot, lemon, eucalyptus and orange essential oils that are light and give a fresh feeling (Irshad et al., 2020). Middle notes are the ones that give body to the blend and last a bit longer (up to 1h). These essential oils tend to be spicy or floral, such as cinnamon, clove, geranium, jasmine and lavender essential oils (Sharmeen et al., 2021). On the contrary, base notes are responsible for the main fragrance of the perfume and are the ones that last the longest compared to the other two categories. These are usually the least volatile essential oils with intense odor that remain even up to several hours and are responsible for the ‘’depth’’ of the perfume. Some examples of base notes are patchouli, sandalwood, myrrh, frankincense and vanilla essential oils.
As previously noted, the scent of essential oils is attributed to their unique and complex mixture of individual volatile compounds. This mixture can be easily altered as it depends on many factors. For example, based on the type of the plant species, the harvest area and time of the year, the part of the plant used for the essential oil production as well as the type and time of extraction/distillation process, essential oils with different composition are produced. On the other hand, some compounds are almost solely responsible for the scent of some essential oils and are usually linked with the respective essential oil. A characteristic example is limonene, a terpene found in all citrus-derived essential oils (orange, lemon, bergamot, bitter orange etc.) and has a characteristic fresh, citrus odor.
There are different types of perfumery products based on the amount of alcohol and essential oils used for their production. For example, in perfumes alcohol is the main ingredient in the formulation, whereas in eau de types of perfumes, essential oils are mostly used. For this reason, these products usually present a slightly yellow coloration owing to their natural oils’ color. Based on the percentage of essential oil they contain, perfume types can be classified as eau de parfum, eau de cologne, eau de toilette, etc. (Sharmeen et al., 2021), as shown in Table 1.
3.2. Skin care
Essential oils can also be added to skin products, such as creams, lotions and sunscreens, either in bulk form or in combination with a carrier oil. These products are available for everyday use in order to moisturize or cleanse the skin as well as prevent and treat different skin conditions such as sunburns, dermatitis, acne, inflammation, aggravation, etc. (Abelan et al., 2020). Essential oils are believed to be absorbed through the upper part of the dermis and their use plays a crucial role on the physiological balance of the skin owing to their lipophilic character. The application of products containing essential oils lead to the formation and maintenance of a protective film in the outer part of the skin that prevents the insertion of harmful microorganisms while preserving the natural skin microbiota (Lohani, Mishra & Verma, 2018).
Regarding skin care products, the properties of essential oils have created the basis for a variety of potential applications, apart from the ones mentioned above. Essential oils are widely used also in aromatherapy, they are incorporated in cosmeceuticals, and they are employed as complementary and alternative medicines for different conditions. One of their main biological properties, the remarkable antioxidant capacity, is of great importance for cosmetics industry. More specifically, essential oils can be used as key antioxidant components in different cosmetic products for the prevention of skin damage and delay of skin aging process.
Lately, their employment as natural antioxidants in cosmetics and cosmeceuticals has increased, as they are favored over synthetic antioxidants butylhydroxytoluene and butylated hydroxyanisole, due to their natural origin (Lohani, Mishra & Verma, 2018). Moreover, individual essential oils’ compounds, depending on their structure, are able to enhance the penetration of drug molecules through the skin barrier. For example, hydrocarbon terpenes, such as limonene, is preferred in combination with lipophilic drug molecules, whereas terpenes with polar groups, such as menthol, provide a penetration enhancement effect for hydrophilic drug molecules (Abelan et al., 2020). Based on the therapeutic target of different skin products, these compounds can be used accordingly to achieve maximum effectiveness.
3.3. Hair care
Regarding hair care, essential oils can also be incorporated in a variety of hair products. Due to their pleasant aromas, they are usually incorporated in shampoos, in order to offer a fresh and natural aroma in these products. Similar to skin care products, apart from the pleasant scent, essential oils can affect cellular function in the skin after topical application, while providing antioxidant, anti-inflammatory, and antimicrobial benefits on the scalp. In this way, hair shaft density is increased, hair bulb is cleansed and moisturized, and the entire bulb/stem system is strengthened.
Currently, there are several cosmetic products containing essential oils available on the market which promote therapeutic actions on both scalp and hair shaft conditioning (Abelan et al., 2020). Following a pattern of penetration similar to the skin care products, the bioactive ingredients of the essential oils can quickly penetrate in the scalp offering a number of advantages such as moisturization of the hair roots, nourishment of the deep hair follicles, stimulation of hair follicle growth, hair strength and removal of unwanted metabolites that block the pores.
Examples of essential oils used in hair products are presented in Table 2.
3.4. Natural preservatives
A novel use of essential oils in cosmetics is their application as natural preservatives in cosmetics for the replacement of synthetic ones that have been found to be harmful for human health. Esters of 4-parahydroxybenzoic acid (PHBA), commonly known as parabens, are the most used preservatives in cosmetic industry (Nowak, Jabłonska & Ratajczak-Wrona, 2021).
Since essential oils present potent antimicrobial and antioxidant activities and they are generally recognized as safe (GRAS), with applications already in cosmetics and food industries, they have a great potential as substitutes of parabens in cosmetic products. However, the complete elimination of synthetic preservatives from cosmetics and the replacement with natural ones are very challenging tasks. A number of disadvantages, as well as technical difficulties need to be faced by companies regarding the shelf-life expectancy and preservation of cosmetics in case of total replacement of chemical preservatives with natural compounds, such as essential oils.
To this respect, the method of nano-encapsulation has been employed in order to enhance the stability and prolong the activity of these bioactive compounds in cosmetics. Despite the advantages of this method, the overall costs of materials and manufacture very often exceed the cost of traditional and well-known methods of preservation (Nowak, Jabłonska & Ratajczak-Wrona, 2021).
Essential oils are one of the main ingredients in cosmetics, dermo-cosmetics and perfumes, with great importance in the corresponding fields. Although they are naturally derived, their production is far from sustainable practices. To this respect, a standardization framework regarding the sustainable production and utilization of essential oils is one of the main sections that cosmetic industry should focus on in order to keep up with the demands of the market as well as the environmental crisis.
Currently, one solution regarding the conscientious use of essential oils in cosmetics is aiming to the combination of their properties and the exploitation of their full range of uses. In this way, the environmental footprint and impact will be indirectly decreased which are responsible for a sustainable way of living.
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