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 the 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!
ESSENTIAL OIL DEFINITIONS
Why that is the case will become apparent from definitions of essential oils. Let’s see some of them:
What those definitions have in common is that essential oils are characterised by the method they are produced: only when the plant material from a particular aromatic plant species is distilled or expressed can we talk about essential oil. Thus they are, strictly speaking, not plant products but distillation products.
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),
- 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 (pelargonium a.k.a. geranium), 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.
When an insect, for instance, touches a glandular trichome on a leaf, it ruptures like a water balloon, releasing defence compounds into its surrounding.
A scheme of a glandular trichome of a peltate (sessile) type. VOCs are produced in the cytosol (sesquiterpenes) and in plastids (monoterpenes) of secretory cells and released between the cell wall and cuticle (waxy outer layer of epidermal cells). The cuticle sparates from the cell walls and forms a bubble-like structure where VOCs are stored. Often, other types of compounds are produced in addition to volatiles, such as polysaccharides, proteins, fatty acids, lipids and flavonoids.
Another major type of defensive VOCs in leaves are green leaf volatiles (GLVs). These are alcohols, aldehydes and esters of fatty acid origin with a characteristic ‘green’ smell of freshly cut grass (e.g., cis-3-hexenol a.k.a. leaf alcohol), produced almost universally by green plants.
The synthesis of GLVs drastically increases within seconds after leaves are infected or mechanically wounded. When released from damaged tissues, they act to deter invading organisms directly, to attract natural enemies of invading herbivores, and as a communication signal within and between plants to induce further defence mechanisms – an example of how plants can communicate with each other (Scala et al. 2013). GLVs are often present in essential oils and hydrolats of leafy materials, however in minimal amounts.
Roots exude a variety of nutrients such as sugars and amino acids, as well as volatile and non-volatile secondary metabolites: the aim is to selectively attract beneficial (symbiotic) organisms and deter pathogens and herbivores. VOCs released from root secretory cells have been studied primarily for their anti-herbivore and antimicrobial effects.
However, sesquiterpenes released from the roots of vetiver (known for its highly prized essential oil) act as food for certain bacteria. These bacteria, in turn, can grow in the vicinity of the roots, preventing colonisation of other, potentially pathogenic microorganisms (Junker and Tholl 2013). Moreover, these root bacterial communities in large part determine the vetiver oil composition, as it was found that in non-colonised roots the complexity of essential oil is drastically reduced (Del Giudice et al. 2008).
In fleshy fruits such as apples and berries, although VOCs aren’t actively emitted, their main role is to attract fruit-eating herbivores, which help to disperse the seeds. Typically, fruit VOCs are characterised by a large proportion of esters of non-terpenic origin, but other groups of volatiles are also present. Specific blends of fruit VOCs contribute to their characteristic smell and taste, which is further enhanced by sugars. In fleshy fruits as well as in dry fruits such as cardamom or black pepper, VOCs are produced and stored in single secretory cells.
In seeds from the Apiaceae plant family (e.g. coriander, caraway, carrot), VOCs – predominantly terpenic compounds and phenylpropanoids – are stored in secretory ducts, tubular cavities under the seed surface. The role of these VOCs is again defence against microbes and herbivores and also to prevent the growth of other plants (known as allelopathy).
In bark and wood, VOCs either comprise a significant proportion of resin, which is stored in resin ducts and secreted when the plant is wounded, or are dispersed throughout the wood. In the former case, essential oils are distilled from the collected resin (e.g., myrrh and frankincense), whereas in the latter case, trees must be cut and their wood chipped or powdered to obtain the oil. Among them, natural resources of sandalwood (Santalum album), rosewood (Aniba parviflora) and oud (Aquillaria sp.) are endangered due to their unsustainable exploitation.
Before proceeding, one more important thing to remember: plant volatiles do not flow or circulate in plants, they’re not any circular system analogue, and they certainly aren’t any unique essence or even a plant’s soul.
They are there because they have specific biological functions for the plants that produce them. The fact that they are natural doesn’t mean they are harmless. Plants don’t care if the substances they produce are toxic or beneficial for us humans, who use them for our own specific needs. It is our responsibility to use them safely.
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 (Williams, 2008).
The presence of water – usually traces – may cause further hydrolysis of esters. Another type of chemical modification is polymerisation, where many similar molecules bind together to form large linear chains, causing an increase of viscosity, darkening, haziness and reduction of odour intensity.
In certain cases, however, ageing is desired. Some essential oils, such as patchouli, sandalwood, vetiver, oud, etc. gradually lose terpenic, earthy, smoky and other harsh notes and become increasingly mellower, creamier and sweeter, ageing much like a good wine.
Regardless of whether all these changes are desired or not, the essential oil is a new product that is co-produced by the plant, man and environment.
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ISO 9235:2 – Aromatic natural raw materials
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