Essential oils have gone from being an obscure aspect of botany to an all round marketing ‘good thing’. You can even by washing powder now that trumpets that it contains essential oils. What actually are they?
Well different people have different ways of looking at them, and there are various definitions around. Basically they are the volatile oils that you can extract from a plant, usually with a distinctive odour. The extraction technique here is almost always steam distillation – though there are other ways of getting them out.
So from a chemist’s point of view they are oil soluble extracts that have managed to survive a high temperature extraction. The bulk of these oils are made up of the terpene family, or similar. Terpenes have lots of uses. Some are handy as solvents. Others have intriguing structures. So chemists tend to find essential oils interesting.
Biologists also take an interest in essential oils, but come at them from a slightly different perspective. Plants are living creatures, and as such they all need to have a basic tool kit to stay alive. They need to generate energy, and produce the building blocks to grow leaves, stems, twigs, fruit, etc. The whole of the plant kingdom has a basic machinery, its metabolism, that it needs to stay alive. All plants have this in common and all of them share the same set of basic metabolites to do this. Analyse any plant any time and you’ll always find this primary set of things like carbohydrates, sugars etc.
But plants also produce their own distinctive chemicals that are particular to their own lifestyle. For example, a lemon tree living in a sunny place full insects produces a pungent oil that repels them, particularly from the peel of its fruit. Biologists refer to these kind of ‘specials’ as secondary metabolites. They are often characteristic of particular species and are sometimes only produced under particular environmental conditions. A good example is rosemary. This is a common enough plant in English gardens, and it does produce a certain amount of oil. But it only produces large quantities when it is stressed by dry conditions, so we need to import it from Spain where miserable wet summers are less common. The chemical composition of the oils varies with the weather too.
Plants produce secondary metabolites for specific reasons, often to attract or repel other species. The pungency of rosemary oil is an adaptation to repel voracious goats. Rose oil is intended to attract insects – and only incidentally impresses humans. The calming effect of lavender oil might well be intended to slow down predators grazing on the plant.
So secondary metabolites can have biological effects. Essential oils, which are basically secondary metabolites concentrated, have been a rich source of therapeutic agents over the centuries and research continues into them. This means they can be dangerous as well as beneficial. The skin sensitising potential of bergamot oil is a good example. This was traced back to a particular component which is now removed, so that particular problem was solved. But it is as well to remember that natural doesn’t mean automatically safe.
But having said that, all secondary metabolites are produced in living systems and they do tend to be structures that can be broken down relatively easily. This means that they are unlikely to accumulate in large quantities in the environment. So I think essential oils do deserve their green image. And they also deserve, and will no doubt receive, the attention of chemists, biologists and pharmacists. We’ve still got lots to learn about their benefits and their risks. And apart from that, they are just intrinsically interesting.