Our webinar on 23 March 2021 brought together two leading researchers in smell and taste: Professor Barry Smith, Director of the Centre for the Study of the Senses at the University of London, and Alexander Fjaelstad, Associate Professor at Aarhus University and the University of Oxford.
Over the course of the hour they took us through a fascinaing exploration of the sensory tapestry that is smell, taste and flavour.
When is a taste a smell?
Barry explained, “When we talk about taste, or tasting our food, we’re really always talking about at least three things: taste, touch, and smell.” Combined, these senses give us our perception of flavour.
When people say “I can't taste anything”what they are really missing is their sense of smell.
Taste, as detected by the tongue, picks up the basic components of sweet, salty, bitter, sour, and umami. It is the sense of smell that allows people to pick up more complex flavours, and when this sense is lost people often don’t recognise that they can still taste the ‘basics’ as they’ve never had this sense in isolation before. Many people find that they become more sensitive to taste as they get used to it.
Why is a sense of smell necessary to detect flavour?
The flavour of food is far more complex than ‘sweet plus sour’, for example. In order to detect the complex flavours of food, our olfactory system needs to play a role. ‘Flavour’ is in fact a mixture of taste and smell.
There are two pathways by which odour particles reach our olfactory epithelium - the area high up inside the nose that contains olfactory nerves and sends signals about smell to our brain.
The first pathway is from the outside, and up into your nose when you inhale or actively smell something.
The second is through the back of your throat when you’re eating – as you chew, odour molecules are released and move up from your mouth to your nose from the back. This is further helped by swallowing, which pushes these molecules up. They reach the olfactory epithelium and the nerves detect the various odours and send these signals through to your brain. As you’re getting smell sensations in the nose, but via the mouth, the brain plays a trick on us – it makes us feel as though those tastes are actually coming from the tongue. This is known as ‘oral referral’.
We experience food differently whether these molecules reach our olfactory epithelium from inside or outside of the head due to these two different pathways. Some things can smell disgusting – for example a mature parmesan cheese – but taste delicious when in the mouth. This could mean that these odour molecules are processed and understood differently by the brain depending on the route that they took to get there.
What role does touch play?
The sensation of touch, and how food feels, also plays an important role in our experience of food and flavour.
There is a nerve in our faces called the trigeminal nerve. This is activated by many different triggers, and is the cause of the feeling when you eat wasabi, for example, or experience peppermint as ‘cool’.
The trigeminal nerve also detects the temperature of food, spices within it and many other textures. Many people report that the feel and texture of food becomes much more important when they are experiencing smell and taste loss.
The challenges of measuring sensory perception
Many people report loss of taste when it is actually smell loss and a loss of flavour detection. Others may report still being able to detect the flavour of some things, but what they’re actually recognising is the ‘pattern’ of feeling that the trigeminal nerve produces when exposed to that food – perhaps something that is spicy or cooling.
It is difficult to tell the difference between the loss of smell and taste, and to detect the severity of loss ourselves. Researchers are not able to rely on what people self-report. The complex sensory experience makes it difficult for people to easily tell the senses apart themselves. As each person has a different level of perception, self-reporting makes it impossible to compare objectively against others.
Two tests - Sniffin Sticks and the UPSIT - are recognised as standard tests of olfactory function for research purposes, but testing still happens in the lab for most people. It’s more difficult to measure the function of the trigeminal nerve as it’s very sensitive and is activated in response to so many things. fMRI scans that show areas of activation in the brain in response to a stimulus also add to this picture of how the brain interprets the world around us.
Researchers continue to test and observe to better understand how smell, taste and flavour detection work, how they are interlinked, and how the brain processes each of these inputs. New understanding is being developed all the time that informs the way we think and treat smell disorders.