About the science of fragrance

Fragrances are made up of complex mixtures of volatile molecules that interact in complex ways with receptors in the nose to create specific scent experiences. The way the millions of odour chemicals combine within the nose to produce those experiences is still not well understood but, suffice to say, that even a tiny addition or deletions or even a change in the concentration of a single component can have a dramatic affect on what you sense.

The perception of fragrance involves a complex interplay between the chemical signals from the nasal (“olfactory”) receptors and their interpretation by the brain. Just as some scientists now believe there are no such things in the physical world as “colours” - they are constructed in the mind to help us interpret our world - I believe that fragrance is constructed within the brain as a reaction to the signals from our olfactory receptors. This means what we experience is innately individual and linked strongly to our knowledge, experience and memories, yet at the same time is something we share through common experiences with others, just as we all share the concept of “red” or “yellow”. Trying to discern whether two people have the same internal perceptions of odour is as difficult as trying to determine whether they experience “blue” identically. In the end, it doesn’t matter, as long as it is consistent and two people agree that something smells like strawberry, no matter what is happening inside their nose and brain.

The connection between our sense of smell and the brain is directly through the amygdala and hippocampus, which are responsible for our memories and emotions. This is unlike any of our other senses, which are first “handled” by the thalamus and then channelled from there to other parts of the brain. This may be why people react so quickly and emotionally to fragrances and why sometimes a fragrance can bring a strong memory to the fore, seemingly out of nowhere. Because of this direct connection, people can react very differently to the same odour. For one person, the smell of eugenol, a component of cloves and cinnamon, conjures up fond memories of Christmas or mulled wine. For another, it might dredge up unpleasant memories of childhood visits to the dentist, where eugenol is used for the relief of pain (and, centuries ago, cloves were used to treat dental caries). Careful addition of lemon oil to a scent gives it a refreshing and uplifting zest but if overdone, it simply evokes household cleaners. The art of the perfumer is to blend components in just the right way to give the desired effects.

Fragrances comprise mixtures of chemicals - whether they are complex, naturally derived extracts or pure aromachemicals that are synthesised in the lab. All of the odour chemicals involved must be volatile, so will have molecular weights of 400 at most. While the art of perfumery is about blending these components to achieve balance, there are also physical laws that ultimately determine how mixtures will behave. Put simply, molecules are more or less oil- or water-loving, have differing evaporation rates, reactivities, interactions and levels in the air at which they can be detected by the human nose. One certainly does not need to be a chemist to be a perfumer but some knowledge of chemistry can add to our understanding of how and why fragrances behave the way they do. How quickly an aromachemical will disappear from a smelling strip, how different bases such as skin creams might change the profile of a fragrance because of the molecules relative preferences for oil or water, or how components might react or change with time can be understood through the chemical and physical sciences. And for the past couple of decades or more, sophisticated techniques like gas chromatography-mass spectrometry (GC-MS) have allowed us to determine in detail the chemical makeup of finished perfumes and natural extracts, alike. Nonetheless, science cannot now, nor maybe ever, unravel our personal reactions to fragrances - that key aspect remains an art, and we are the better for it.