The Surprising Secrets of Taste and Smell

You bring the glass to your nose, and a magnificent bouquet wafts up - exquisite! Then, after the sip swirls around your mouth, the taste washes over your senses like a wave of pleasure. But here's the twist: what you smell and what you taste are often the same experience. So, why do they sometimes feel so different?

When it comes to tasting - whether it’s wine, beer, or spirits - the two key actions are, unsurprisingly, nosing and tasting. But once we subtract the basics - sweet, sour, salty, bitter, umami, and mouthfeel - what's left is aroma (olfaction). Though we haven’t pinpointed exactly how much of 'taste' is olfaction, the general consensus is that it’s most of it. Which may come as a surprise to many.

Understanding the difference between the aromas we inhale from the air around us and those we detect inside our mouth is key to mastering flavour. Scientific research reveals that these two processes are not only unique in their mechanisms, but also offer deeper insight into the tasting experience. Armed with this knowledge, you can elevate your sensory skills and refine the experiences you offer your guests.

It’s time to dig into the science of taste and smell. Understanding how and why we perceive aromas differently will change the way you taste forever.

Taste, Aroma, and Finish

The tongue’s repertoire is fairly limited - it detects sweetness, bitterness, sourness, saltiness, umami, and possibly fat too. Mouthfeel is also constrained, largely dealing with chemical (chemesthesis) and touch (somatosensation) sensations. For more on this, check out our article: Mouthfeel - The Unspoken Hero.

Aroma detection, or olfaction, is an entirely different beast. We don’t have an exact count of how many odours the nose can detect, but let’s just say it’s an astronomical number. So when we talk about the taste of a drink, what we’re mostly describing are the aromas. Think coconut, vanilla, banana, apple, toffee, pencil shavings, blackberries, pineapple - these are all smells rather than ‘tastes’.

The same goes for the 'finish' - the lingering flavours that swirl around long after the main event has passed. These too are mostly aromas. So, when we discuss a drink's taste at a tasting, the bulk of our experience is actually olfaction. However, when we taste, our attention is drawn to taste sensations and mouthfeel. So much so that we often do not even consider the leading role that aromas play. So to help understand them, let’s dig a little deeper.

Orthonasal Olfaction vs Retronasal Olfaction

We’re all familiar with the front-facing appendage we associate with sniffing flowers, whisky, or fresh rain on dry earth. This is orthonasal olfaction - smelling odour molecules from the air around us.

But when we take a sip or chew something, the aroma molecules are released inside the mouth and travel up into the nasal cavity. This is called retronasal olfaction.

Both types of olfaction detect the same volatile molecules and share the same receptors and processing centres. But here’s the fascinating part: the experience can feel radically different. Our sense of smell is unique because it’s the only dual-sense. It detects both external and internal odours. It’s remarkable how the brain can tell whether an odour comes from the air or from inside the mouth - and it processes each source differently. As sensory science has identified.

Researchers Small et al (2005) exposed participants to a selection of odour molecules, one at a time - lavender, butanol (solventy), farnesol (light floral), and chocolate. By exposing the participants to the odour molecules through a tube they were able to isolate orthonasal olfaction (through the nose) from retronasal olfaction (from the mouth). Using brain imaging they could see which areas were utilised for each type of olfaction. So what did they find?

Wanting vs Liking

Actually, there was no significant difference between the brain activity for three of the odour molecules. But the only food-associated odour stimulated different areas of the brain to the others. When the chocolate odour was experienced orthonasally, it sparked up the same parts of the brain as the other odours. However, when the same chocolate odour was experienced retronasally it was quite different.

In this instance the chocolate odour appeared to excite brain areas that are associated with hedonic experiences, pleasure-seeking. Those that are involved in the reward pathways of food and help us to decide how much we like a flavour. An idea that is supported by Berridge (1996) who suggests that there are two functional components to odour recognition.

The first component, ‘wanting’, is involved with appetite, incentives and motivation. The go-and-find-some-food function. The second component, ‘liking’, is involved with pleasure and palatability. The how-yummy-is-this function. What does this mean?

Smelling is Evolution

In essence the neuroscience of the olfactory system suggests that orthonasal olfaction is tasked with finding things that smell appealing, and it helps motivate us to seek beneficial foods. It activates our dopamine pathways which are deeply rooted in ‘motivation’.

On the other hand, the retronasal olfaction from food or drink inside the mouth is tasked with how much we enjoy the flavour, encoding the experience into memories. This activates our opioid and benzodiazepine pathways - the ones that make us feel good. But from an evolutionary perspective there are anomalies.

For example, it’s common that smoky odours are more pronounced retronasally versus orthonasally. Which seems odd because surely smoky odours are associated with danger rather than pleasantness. Perhaps we have some deep-rooted programming that tells us to trust cooked food (which would naturally taste smoky over a fire) because it is both safer to consume and easier to digest.

Given that orthonasal and retronasal olfaction share the same aparatus (it’s only the delivery of the volatile molecules that changes) how does the brain know which is which?

How Does the Brain Do That?

The olfactory epithelium in the nasal cavity is where our odour receptors hang out. Whether they are exposed to odour molecules from the external atmosphere i.e. through the nostrils, or internally from the mouth, in theory it should not make any difference to the odour receptors. It’s the same olfactory receptors receiving the same odour molecules.

But when aromas are experienced from the outside air, according to Small et al, they stimulate various regions of the hippocampus (memory formation and retrieval), amygdala (emotions), and caudolateral orbitofrontal cortex (decision-making and expectation).

When the chocolate odour was presented retronasally it stimulated the perigenual cingulate (emotion regulation, decision-making), medial orbitofrontal cortex (evaluation of rewards, sensory pleasure), and posterior cingulate (memory integration).

So how does the brain differentiate between the two sources of the odours? How does it send the same nerve signals to different physical areas in the brain based on if the odour is experienced orthonasally or retronasally? And also, how does it distinguish food-related odours from non-food-related odours at an early stage?

It’s been suggested there is a gating mechanism that is triggered by stimulation within the mouth - perhaps due to tongue movement, jaw movement, or gustatory stimulation. This mechanism could instruct the brain on how the nerve signals should be processed. Or it could simply be that the olfactory receptors react differently under the two conditions. There may well be other explanations, but it highlights how much more there is to discover and learn.

One thing for sure is that it makes a difference whether the aroma comes from a food-related odour or a non-food related odour. Out of the four odours in the Small et al study, it was only chocolate that evoked a different response. This points towards the role of retronasal olfaction in associating aromas with foods. Thereby learning what we should consume or not consume and using this information to establish an aroma memory to drive behaviour when seeking more food (or drink) in the future.

We must also consider that flavour molecules change within the mouth. For example, enzymes in saliva cause reactions that break down molecules into different molecules. These may have different odours, as is the case with lipid oxidation where fatty acids can be broken down into new compounds. Plus the impacts of ph and temperature difference will also play a role. The chemistry of tasting is far more complex than most of us realise, however, the Small et al study removed such impacts from their experiment by delivering the volatile molecules directly to the olfactory epithelium through tubes.

Are We Designed for Retronasal Olfaction?

The human oral cavity is also quite unique within the animal kingdom. We have a much shorter distance between our palate and the cavity that leads to the nasal cavity. The result is that odour molecules can travel from the mouth to the nasal cavity in humans more freely than say in a dog or a horse. Is this a chance of evolution that has shaped our olfactory experience? Or have we evolved in this way for that very reason? Nobody knows!

It’s also an evolutionary benefit that we are more efficient at detecting odours when breathing out as opposed to breathing in. Air travelling into the body through the nostrils flows into a larger diameter area than the nostrils themselves. This enables a relatively unimpeded flow from the outside air into the lungs. Conversely, when we breathe out, the small diameter of the nostrils creates a back pressure that results in turbulence within the nasal cavity. This turbulence creates greater opportunity for volatile molecules to interact with our olfactory receptors, thereby increasing how much aroma we can detect.

Another interesting difference between orthonasal and retronasal olfaction has been identified in the habituation time of each. What’s olfactory habituation? Olfactory habituation refers to the decrease in perceived odour intensity over time when the same odour molecule is continuously presented. It’s a sensory adaptation process in which the brain becomes less responsive to a persistent odour, making the sensation of the odour gradually diminish or even disappear.

In essence it’s a type of olfactory fatigue that you may have experienced through a prolonged nosing experience. A 2021 study by Xiao et al found that olfactory habituation, or the reduction in perceived odour intensity over time, occurred for both orthonasal and retronasal pathways, but orthonasal habituation occurred faster. In essence, orthonasal odour detection ‘fatigued’ faster than retronasal. While the reasons are not well understood, it could be that orthonasal olfaction has an inherent ability to reduce the intensity of a perceived odour over time to enhance its ability to detect new odours that could signify danger, for example.

The Key Takeaway

So the next time you stick your schnozzle into a glass, spare a thought for how we smell odours in different ways and how most of taste is in fact olfaction. Understanding how orthonasal and retronasal olfaction have evolved differently helps shape one’s approach to tastings. Not all ‘smelling’ is equal, it would seem. Some odours are more prominent on the ‘nose’, whilst others are more prominent on the ‘palate’. But approaching them from the mindset of aroma helps to create clarity around flavour as a whole.

Insights such as these are beneficial to ambassador teams and visitor centre teams by enhancing the team member’s abilities to explain flavour and create more meaningful experiences for their guests. These insights are also invaluable for production and sensory panel teams, whereby grasping the nuances of nose and palate aid the assessment process and help to reduce the impacts of olfactory fatigue. Want to elevate your team? Get in touch here to see how a bespoke sensory training course can create next-level skills and abilities for you.