We usually consider our essential oils merely as a reflection of the processes in plants, assuming that plants ‘produce’ and ‘contain’ essential oils, and that distillation is a method to extract them in a pure and concentrated form. After all, they smell just like the plants they come from, right? But only up to a moment when we compare them side by side.
Welcome to the world of plant volatiles…
Although we have the same plant species, even the same chemotype growing in same conditions, the composition of aromatic molecules in a fresh plant will always be somewhat different compared to its essential oil. You don’t need a chemical analysis to notice the difference: essential oils typically smell less fresh and have a “cooked” or terpenic nuance.
Actually, essential oils as such do not exist in the plants!
WHAT IS THE AROMATIC ‘SUBSTANCE’ THAT PLANTS PRODUCE?
It is the volatile fraction of aromatic plants, termed the plant volatiles or volatile organic compounds (VOC in short). In line with the systems biology era, the term volatilome is also gaining recognition (Maffei et al. 2007). It comprises the total fraction of plant-produced volatile molecules.
In aromatic plants, there are typically about 1% of VOCs in the dry material, but their percentage varies considerably, ranging from close to zero to 17% (clove buds) or more than 70% in dried resins.
Based on their biochemical origin, the major groups of VOCs are (Dudareva et al. 2013):
- Terpenes and their oxygenated derivatives terpenoids (hemi-, mono-, sesqui-, di-terpenes and terpenoids),
- Phenylpropanoids,
- Fatty acid derivatives (such as green leaf volatiles),
- Amino acid derivatives (aldehydes, acids, alcohols and esters),
- Compounds that do not belong to those groups and are limited to specific plants.
VOCs are synthesised in all parts of plants: flowers, leaves, roots, stems, trunks, barks, fruits and seeds. They are stored in specialised structures, such as various types of secretory cells, cavities and ducts, glandular trichomes and epidermal cells. From there they are released into the environment by different mechanisms, which depend on their biological role.
THE PLANT VOLATILES: WHERE, WHAT AND WHY?
The two fundamental biological functions of VOCs are communication and protection. Their biological roles, chemical composition and release patterns depend highly on which part of the plant they are synthesised.
Flowers release the highest amounts and diversity of VOCs. Their natural role is obviously to attract pollinators from a distance, but also to defend flowers from microbial pathogens and florivores such as ants and other insects. Known defensive floral compounds include linalool and β-caryophyllene, both of which commonly contribute to the overall floral scent (Muhlemann et al. 2014). It has been noticed in several plants that during the flowering phase the production of volatiles shifts towards monoterpenes (with a concomitant decrease in sesquiterpenes), which is presumably linked to pollinator attraction due to increased volatility (Figueiredo et al. 2008).
Emission of floral VOCs often oscillates in sync with daily rhythms, peaking when pollinators are most active. Thus many flowers whose pollinators are nocturnal animals such as moths or bats, also release highest amounts of VOCs at night. Flowers usually don’t have specific structures for VOC storage but produce and emit them from areas of specialised epidermal secretory cells, sometimes called osmophores. In some cases, however, they are stored in secretory glands (e.g., chamomile or clove).
As opposed to flowers, in leaves and stems VOCs have mainly defensive role and are released locally when a plant is attacked by a pathogen or a herbivore. Terpenoids and phenylpropanoids are typically stored in:
- various types of glandular trichomes in Lamiaceae (aromatic herbs such as basil, lavender, sage, etc.), Asteraceae (chamomile, helichrysum), Geraniaceae, Solanaceae (tomato) and Cannabaceae (cannabis) families
- resin ducts of conifers such as pine, fir and spruce of the Pinaceae family and juniper, cypress and cedar of the Cupressaceae family
- single secretory cells in the leaves of aromatic grasses (Poaceae) such as lemongrass or citronella of the genus Cymbopogon, and in bay laurel leaves of the Lauraceae family.
- secretory cavities in myrtle, tea tree and eucalyptus leaves of the Myrtaceae family, and in leaves of lemon, orange, bergamot, mandarine, combava and other species of the genus Citrus.
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SO WHAT’S IN THE BOTTLES, THEN?
Not only is there a huge variety of VOCs, but also their composition is highly dynamic. It reflects continuously changing factors within plants and their environment: developmental stage and specific organs, daily rhythms, climate, season, soil, water status, microbial infections, grazing, etc. The composition can vary significantly even between neighbouring glands of the same type and on the same plant due to intrinsic variability in their production (Schmiderer et al. 2008).
The distilled essential oil is thus like a frozen image in time, with a combined contribution of all those environmental factors and internal variability, an averaged contribution from millions of individual glandular cells from the plant material.
The essential oil is a mixture of tens or hundreds of different compounds that reflect:
- Processes in the plants
- Processes that occur during distillation
- Post-distillation processes
Let’s see some of these processes in more detail. Depending on whether and how they differ from the plant volatiles, we can separate essential oil compounds into four classes.
- Compounds that were present in the plant before harvest and distillation and depend on the plant species, part of the plant, developmental stage and specific environmental conditions – as already discussed.
- Compounds that are differentially distributed to the non-polar (essential oil) and polar (hydrolat) fractions of the distillate. Not all VOCs are distributed into the non-polar fraction of the distillate because some are polar molecules, which means that they are soluble in water and can be found predominantly in the hydrolat fraction. Phenylethyl alcohol is a typical example. It is present in significant amounts in many flowers, such as rose, but there are minute amounts of it in the essential oil. It is, however, the main aromatic compound in the rose hydrolat and absolute.
- Compounds that form during distillation due to chemical modifications enhanced by the presence of water and heat. One type of reactions is hydrolysis, cleavage of chemical bonds caused by water. Esters, sugar-bound and protein-bound molecules (which may produce sulfuric compounds with “still” notes) are prone to hydrolysis (Williams, 2008).
A typical example is hydrolysis of linalyl acetate (a monoterpene ester that is abundant e.g. in lavender, bergamot and clary sage), which will partially convert to linalool (monoterpene alcohol) and acetic acid during distillation.
Apart from hydrolysis, a variety of other chemical modifications may occur, such as hydration and dehydration (addition and cleavage of water, respectively), or decarboxylation (cleavage of carbon dioxide). A typical example of such modifications is chamazulene, which forms from matricin and hues some essential oils, such as German chamomile or yarrow intensely blue.
- Compounds that form after distillation due to reactions caused by light, heat and oxygen. Terpenes, phenols and aldehydes are prone to oxidation, and some of the oxidation products may cause irritations, loss of fresh smell, development of off-odours (e.g., due to the formation of carboxylic acids from aldehydes), and darkening of the oil. Typical examples are limonene oxide and (+)-carvone, oxidation products of limonene, which is abundant in many essential oils, most notably citruses