Essential oils, essentially chemistry
They have become quite popular recently, yet they are not new. At all. Essential oils have been among us for some time, in fact, it is known that Egyptians used precursors of them for medical and religious purposes, like waxes and ointments. Distillation pots that date back to 3500 BC have been found in Tepe Gawra, former Mesopotamia. Nowadays, it is said that the most famous drink in the world includes in its formula a secret blend of essential oils (1). I am talking about Coca Cola. But what are they, essentially?
Essential oils are widely used in some common beverages.
An essential oil is an aromatic liquid, normally obtained from steam distillation of different parts of plants containing volatile oils. They can also be obtained by mechanical processes in citrus fruits. Most come from the leaves of the plant, like eucalyptus or mint; others come from the flowers, like lavender or orange. But also, from other parts of the plant like rhizomes or the wood. If we would look at these parts with a microscope, we would see some globules, called secretory cells, where the plant volatile oils are stored. The plants produce them as a defence mechanism against pathogens or harmful insects, and for attracting pollinators(2).
Essential oils have a characteristic chemical composition for each plant species, which includes hundreds of small and complex molecules, mostly terpenoids. Sometimes we can have groups of plants that belong to the same species and have the same morphological characteristics, but produce different molecules. Therefore, they will have a different chemical profile, and are called chemotypes of this species (3). Not all the species produce different chemotypes, but we must pay attention when they do: if their chemical profile is different, their properties can be different too. In fact, it is this complex chemical profile of each essential oil what will determine its characteristic smell and its properties.
Different essential oils and chemotypes have a characteristic chemical profile.
Actually, many properties have been attributed to the essential oils. They have been widely used for the food industry as flavourings and as cosmetic ingredients (4). It goes without saying that perfumery would not be the same without them. But lately they have become very popular for their therapeutic properties. And it is not for less: they are said to be antimicrobial, antioxidant, anti-inflammatory… as time goes by, their list of properties continues to grow. But far from being magic, all this can be explained establishing a relationship between the molecules present in an essential oil and their biological effect.
For example, tea tree oil is commonly accepted as an antimicrobial essential oil. In fact, we can usually see it as an ingredient in acne cosmetic products, used to combat acne related bacteria Propionibacterium Acnes (5). In this case, it has been seen that tea tree oil can actually break the cell wall of bacteria, mostly for the contribution of the tea tree’s major component, a molecule called terpinen-4-ol (6). However, it is not all good news: tea tree oil is known also for its sensitizing potential. Some molecules like ascaridole or terpinolene, present in tea tree essential oil, can cause contact dermatitis when applied in high concentration (7).
Different essential oils can have different properties: tea tree is commonly used for acne.
It is important to remember that essential oils are complex mixtures of hundreds of molecules that can have biological effects in our body. Well used, the essential oils represent a valid alternative for many disturbances, but they can as well be harmful when misusing them (8). In the end, natural products imply chemistry, and they are not necessarily safer or harmful, but we have to understand their mechanisms in order to know how to use them.
(1) The real thing: The essential ingredients in Coca-Cola - Lush Fresh Handmade Cosmetics UK https://uk.lush.com/article/real-thing-essential-ingredients-coca-cola (accessed Nov 13, 2019).
(2) Sharifi-Rad, J.; Sureda, A.; Tenore, G. C.; Daglia, M.; Sharifi-Rad, M.; Valussi, M.; Tundis, R.; Sharifi-Rad, M.; Loizzo, M. R.; Oluwaseun Ademiluyi, A.; et al. Biological Activities of Essential Oils: From Plant Chemoecology to Traditional Healing Systems. Molecules. 2017. https://doi.org/10.3390/molecules22010070.
(3) Botanical Names and Chemotypes: Do you Know What’s in Your Bottle? https://phytovolatilome.com/botanical-names-and-chemotypes/ (accessed Nov 13, 2019).
(4) William Charles Evans. Trease and Evans Pharmacognosy, 16th edition.; Elsevier, 2009.
(5) Tea Tree Oil for Acne: Does It Work and How to Use It Safely https://www.healthline.com/health/skin/tea-tree-oil-for-acne#research (accessed Nov 13, 2019).
(6) Li, W. R.; Li, H. L.; Shi, Q. S.; Sun, T. L.; Xie, X. B.; Song, B.; Huang, X. M. The Dynamics and Mechanism of the Antimicrobial Activity of Tea Tree Oil against Bacteria and Fungi. Appl. Microbiol. Biotechnol. 2016. https://doi.org/10.1007/s00253-016-7692-4.
(7) de Groot, A. C.; Schmidt, E. Tea Tree Oil: Contact Allergy and Chemical Composition. Contact Dermatitis. 2016. https://doi.org/10.1111/cod.12591.
(8) What Are Essential Oils (and What Are They Good For)? https://greatist.com/live/what-are-essential-oils#1 (accessed Nov 13, 2019).