Fragrances are volatile compounds capable of readily converting from a liquid state to a vapour state at room temperature. These molecules of vapourized liquid are what we perceive as smells; lighter than air, they drift into our noses and engage our olfactory receptors.
There are many types of fragrance molecules, but we will focus on essential oils, which are abundant in the natural world.
Essential oils are aromatic compounds produced by many plants, and they are of interest to us because of their fragrances: The smells of different essential oils can alter our brain chemistry, impacting our emotional and mental states.
Essential oils are complex mixtures of terpenes and other aromatic or aliphatic compounds, produced as secondary metabolites in specialized secretory tissues of aromatic plants.
Plants produce essential oils for a variety of biological purposes: to cool off by way of oil evaporation, to attract pollinators, to make themselves unpalatable to insects and animals, to ward off disease, even to make the soil around them toxic to other plants, thus reducing competition for sunlight, moisture, and nutrients. Essential oils may be present in the plants’ flowers, leaves, roots, or bark.
After the plants have been harvested, the essential oils can be extracted using different methods such as steam distillation, expression (physical crushing of essential oil glands situated in fruit rinds or the outermost waxy layer of fruit peels), microwave-assisted extraction, solvent extraction, or enfleurage (transfer of the essential oil from flower petals to fat).
In general, the most common method is steam distillation, but expression is the method used most frequently to obtain essential oils from the peels of citrus fruits such as oranges, lemons, or bergamot.
Essential oils have found their way into everyday life, notably into foods, beverages and confectionery items; into personal care products (soaps, toothpastes, mouthwashes, deodorants, bath lotions and shampoos), perfumes, other cosmetics, and pharmaceutical formulations. Essential oils are added to make such products more attractive or to mask the taste or smell of less pleasant ingredients.
Green consumerism and the “naturals” trend have provided a fresh impetus for the use of plant essential oils, particularly in the skincare beauty industry. In addition to their fragrances and natural marketing image, essential oils also bring comprehensive active compounds to modern skincare products. For example, essential oils can serve as natural preservative agents, due to their antimicrobial properties. They can also provide additional benefits to skin such as anti-acne effects, anti-aging effects, skin lightening, and sun protection.
Essential oils may contain anywhere from a few to more than 100 single molecular structures. However, essential oils are usually natural mixtures of about 20–70 components, with two or three of the major components being present at fairly high concentrations and other components present in trace amounts.
The contribution of a single compound to an oil’s fragrance does not solely depend on its concentration though; it also relies on its odor threshold, which is determined by its structure and volatility. Therefore, minor components derived from oxidation or degradation reactions may have a strong impact on an oil’s fragrance if their aroma values are high enough.
Essential oil components can be roughly classified into three families: lipophilic terpenoids, phenylpropanoids, and short-chain aliphatic hydrocarbons. Lipophilic terpenoids are the most frequent and characteristic constituents. Among them, allylic, mono-, bi-, tricyclic mono- and sesquiterpenoids make up a major part of essential oils.
In general, monoterpene hydrocarbons are less influential on the fragrance of the essential oil than their oxygenated counterparts, which are highly odoriferous. Having a concentration of about 90%, monoterpenes are the most abundant component in essential oils. They have a great variety of structures, but geraniol/nerol, linalool, citronellol, citronellal and citral are the most important terpenes to an oil’s fragrance.
The chemical compositions of essential oils are heavily dependent on physiological (plant organ, ontogenesis), environmental (soil composition, weather conditions), and genetic factors. Other factors such as geographical variation, plant characteristics (which species, whether cultivated or wild), harvest or postharvest conditions, production parameters (oil production methodology, distillation parameters), and other parameters (storage condition, storage time) also impact the compositions of essential oils.
Essential oils are classified as top, middle, or base notes according to their odorous characteristics, diffusion rate and volatility.
The top notes are the most volatile oils and are therefore the first perceptible odors. In other words, they are detected first and fade first. As such, top notes are responsible for a product’s first impression on customers. These are light scents, usually lasting 5–10 minutes, but they may remain for a maximum of 30 minutes. Bergamot, juniper, cinnamon, and gardenia essential oils are all top notes.
Middle notes tend to be spicy or floral and give body to blends; they can remain for up to one hour. Ylang-ylang, geranium, lavender, jasmine, and clove essential oils are all middle notes. The base notes give a fragrance depth and last the longest, remaining for up to several hours. Myrrh, vanilla, sandalwood, and frankincense essential oils are all base notes.
Most common essential oils have been well tried and tested; their safety levels have been determined. As such, it is known that some essential oils are more likely to cause adverse skin reactions than others.
The presence and concentration of a relatively potent allergen is a major factor in allergic contact dermatitis, and the oxidation of essential oil constituents can increase the risk of adverse skin reactions because the resulting oxides and peroxides are generally more reactive. Therefore, the proper storage of essential oils is important to the preservation of their effectiveness and the reduction of adverse reactions.
Photosensitization may occur when an essential oil with a photosensitizing constituent is applied to skin and then exposed to sunlight or ultraviolet light. For instance, furanocoumarins are photosensitizers that may be present in expressed citrus peel oils and angelica root, rue, parsley leaf, or marigold essential oils.
The most common furanocoumarins are psoralen and bergapten. Inflammatory skin reactions such as pigmentation, blistering, or even severe skin burns can occur when furanocoumarins are applied to skin and exposed to ultraviolet light. We have detailed this situation in an earlier article about phytotoxicity for safe, natural ingredients.
Want to know more about fragrances? Visit the Fragrance Zone at in-cosmetics Global
Enjoyed this article? Get more by subscribing to our newsletter!
Feeling inspired to see ingredients and trends in action?
Then why not visit one of the in-cosmetics events around the world?