The Chemical Sense

“Smell is a potent wizard that transports you across thousands of miles and all the years you have lived.”  – Helen Keller

What is your favourite smell?  I have many: the smoky, velvet, roasted smell of freshly brewed coffee (I have a pot in front of me), the fragrant hops at Brakspears’ brewery at Henley (no longer existing), and the sharp earthy smell of freshly cut grass are three that often come to my mind when thinking about smells that I love.  One smell I really cannot bring myself to love is the durian, although many of my friends swear by it.

Smell is primeval

Of the five main senses (the other four will be covered separately), smell is the most primeval.  Although we have the remarkable ability to detect around 20,000 different smells by some estimates, our sense of smell is quite poor compared with many other animals.  For example, humans have around 12 million olfactory (smell) receptor cells, while cats typically have five times this number, and dogs 1 billion or more!

In less evolved animals, smell is the primary survival sense, although vision has become dominant for humans.  Even so, one of the primary functions of smell is to signal to our brains if we should approach or avoid something in our environment, and many primary emotions, such as disgust, are intimately linked to smell.   Our acceptance or rejection of different smells is culturally learned rather than innate (unlike taste).  For instance, while I love the smell of strong cheese, to many people, particularly in Asia, the smell is more like vomit and rejected (in fact the two smells are similar chemically).

In evolutionary terms, our brains evolved from our sense of smell – we think because we smelled!  The brain evolved from the olfactory bulb in the limbic (basic emotional reaction) system of fish: smell was originally just a basic chemical detection system.  Smell remains the only sense with direct access to our brain.  The nerve endings of smell receptors literary go straight from the top of the nose into the olfactory cortex, and the olfactory cortex is directly linked to the brain’s emotion and memory systems.  Other senses are mediated through the thalamus (which is also responsible for language), and importantly are integrated before reaching memory while smell has a direct stand alone route.  Interestingly, we tend to forget that we have two nasal cavities, which do act in stereo like our eyes, and can be used to detect the direction of smells.

The world is made of atoms

Smell is also primitive in that it is “the chemical sense”.  Richard Feynman was once asked if there was a single sentence which summarised all humans understood about the world from science, and his answer was “the world is made of atoms.”  Smell is very much a contact sport, where the receptors in our nose have to come into direct contact with molecules of whatever we smell, and these particles are dissolved in a watery solution so that they can be absorbed by the mucous membrane in our nose.

Although we still don’t fully understand how smell works, the most widely believed explanation is that the shape of molecules (or part of those molecules) provides a “key” used by our olfactory receptors to measure “fit” to specific smells.  Because of this, it can be the case that very different chemicals with similar shapes smell the same, while chemicals which differ only slightly (ie different isomers) may have different shapes and hence very different smells.  Like the colour system, some have conjectured that there are “basic” smells.  In one system there are eight: camphorous, fishy, malty, minty, musky, spermatic, sweaty and urinous.   Another commonly used system developed by Henning almost 100 years ago uses six basic smells: flowery, putrid, fruity, spicy, resinous, burned (see below).

Poignant land mines

Diane Ackerman uses a beautiful poetic phrase to describe the sense, “Smells detonate softly in our memory like poignant land mines, hidden under the weedy moss of many years and experiences”.   Smells are not translated into language before they are stored in memory or activate our emotions, which may be one reason why, despite our detection abilities, we find it hard to describe smells in detail (in addition to the inadequacy of language itself).  However, they are directly linked into our emotions and memories, and can therefore trigger very strong emotional responses.  Although memories which are triggered by smell are very powerful, work by Rachel Herz and others has shown that they are not more accurate – they just seem much more vivid and therefore make us more confident that we are right.

Proust famously wrote about his “madeleine moment”, and others have written similarly about the evocative nature of smell.  This is probably because of their strong link to emotions and also the role of context in both smell and memory.  Memory (as we have discussed before) is based on an emotional experience linked to a very specific context.  Smell operates in the same way – the perception of a smell is highly dependent on the context in which we smell something, and smells which are evocative are usually linked to very positive emotional experiences (such as visiting Brakspears’ brewery or helping my father mow the lawn) or very negative emotional experiences (for example, I remember vividly suffering from food poisoning on a picnic when I was seven or eight years old, and didn’t eat another cornish pasty for 20 years!

The importance of context may also be why aromatherapy works for some people, through association of particular smells with specific moods and positive experiences.  Apart from its role in tasting food and drink (see below), smell also plays a role in sexual selection.  Our body odours reflect our immune system, and we typically prefer the odour of partners who smell “different” to us – meaning that they have different immune systems making them more compatible.  If you want to choose the right partner, make sure that they are not wearing perfume or cologne on your first dates!

Nothing accounts for taste

Although it is common to talk about taste and flavour interchangeably, taste is technically a different (and much more basic) sense linked to actual ingestion of food.  Flavour description comes from both taste (on the tongue) and smell (through the front of the nose and also via the back of the nose when we chew food).  Most of us are very poor at describing flavour, just as we are poor at describing smells, even though we are generally good at detecting them.  However, there is considerable individual difference in ability to detect flavours, both qualitatively and quantitatively.  This is because not everyone has exactly the same set of olfactory receptors (so you may be able to smell things that I cannot and vice-versa), and we also have different numbers of receptors.  Those with more receptors typically may perceive things as more intense than those with less receptors.

For these reasons, consumers (ie all of us) are typically very poor at describing the sensory characteristics of products.  In product development, this means that there is a very important role for trained “expert” panels, who have been selected for their ability to describe sensorials, and are trained to evaluate them in a consistent and reliable way.  Such panels are able to describe the nature of products much more accurately than any consumer(s) can do, even when grouped together in large samples.

This leaves consumers to focus on their emotional reaction to products, something which they are very well able to do.  However, one final lesson from our understanding of the chemical sense is that reactions to products are very context dependent.  The same smells may have completely different perceptions and meanings when placed in different contexts.

Therefore, my main advice when testing new products is to always think about the context in which products are evaluated.  This could include, but is not limited to, the location, the branding or product description, the packaging and containers used, how the product is prepared and presented and what other products it is presented with.

For example, although my coffee smelt great in a nice glass pot in my living room, the same coffee smell in a factory canteen or a coffee shop may have very different connotations.  Smell is critical to our reactions to new products, and smell depends on context.


A Natural History of the Senses by Diane Ackerman (1991)

The Brain Book by Rita Carter (2009)

The Secret of Scent: Adventures in Perfume and the Science of Smell by Luca Turin (2007)

See What I’m Saying: The Extraordinary Powers of our Five Senses by Lawrence D. Rosenblum (2010)

The Scent of Desire: Discovering our Enigmatic Sense of Smell by Rachel Herz (2008)

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