Eating and drinking are a truly multisensory experience – flavour occurs when input from all senses is combined. Not just from two or three senses; like when admiring the sea or the botanical garden; when skiing on a cold winter day; or when playing with a kitten or a puppy. (See our blog for Multisensory processing.)
In this blog post, I have invited Dr Xinni Xua,b and Professor Thomas Hummela to explain the relationship between smell and taste (known as chemical senses), and the other senses too. Dr Xinni Xu specialises in ear, nose, and throat conditions, both as a clinician and a researcher. Professor Thomas Hummel’s research focuses on the diagnostics and treatment of olfactory / gustatory loss, the mechanisms involved in irritation of the upper airways, the olfaction in neurodegenerative disorders, and the interactions between the olfactory, trigeminal, and gustatory systems. Between them, they have published over 800 articles, books, book chapters, and abstracts. And received several awards for their work (e.g., the Young Surgeon Award and the European Chemoreception Research Organization (ECRO) for “Excellence in Chemosensory Research”).
aSmell & Taste Clinic, Department of Otorhinolaryngology, Universitätsklinik Dresden. bDepartment of Otolaryngology – Head & Neck Surgery, National University Hospital, Singapore.
Let’s start with a simple exercise. Think about one of your favourite foods. Why is it your favourite?
Perhaps some of the immediate things that came to mind were that you like the way it tastes, or that its flavour appeals to you. But are taste and flavour the same thing? These terms are often used interchangeably in common language. Biologically speaking, they mean different things.
Not just a matter of taste
Humans can recognise five fundamental tastes: sweet, sour, salty, bitter and umami (savoury). In the last decade, the taste of fat (oleogustus)1 and even water2 have been added to this list. The taste buds containing taste receptor cells that are responsible for detecting these tastes are located mainly on the tongue, but they can also be found in other areas such as the soft palate, epiglottis and upper esophagus.
The scientific meaning of taste, of gustatory function, is therefore rather narrow. Food is much more than just these few categories. For example, vanilla and chocolate ice cream both taste sweet, but at the same time, you would not say that they “taste” the same. Your ability to smell is what distinguishes between the two flavours.
The nose knows
There are two ways that odour molecules reach the olfactory receptors high up in the nose. The first is by sniffing through the nose, which is called orthonasal olfaction. The second way is from the back of the mouth and through the back of the nose. This is termed retronasal olfaction. Retronasal olfaction plays a bigger role in flavour perception than orthonasal olfaction3 . The next time you eat, for example something fruity, try chewing first with your nose pinched and focus on the taste. Then, after some 15-20 seconds, release your nose. With the first part of the experiment, the experience may have come across as somewhat flat. In the second part of the experiment, the added dimension of smell from retronasal olfaction creates the perception of flavour. One reason why the sense of smell adds body to flavour is because humans can detect thousands of distinct odours, if not more, compared to our ability to detect only a few types of taste4. We have about 400 different receptor types for smell5. In addition, combinatorial encoding occurs in olfaction, in which each odourant is recognised by varying combinations of olfactory receptors6. This makes our olfactory sense significantly more intricate than the gustatory sense.
Another aspect of smell is that it is directly linked with emotions – neurologically speaking. The olfactory bulb, a structure at the base of the brain, receives olfactory signals from the nasal olfactory receptors. The olfactory bulb projects neurons directly to the limbic system, including the amygdala and hippocampus, which are responsible for regulating emotions, memory and behaviour. This is why odours can involuntarily evoke memories of events past and feelings associated with them. This experience has a name: the Proustian effect, coined after the French author Marcel Proust. In his tome “À la recherche du temps perdu (Remembrance of things past)”, he famously reflected on how a morsel of madeleine soaked in tea brought forth a heady rush of vivid memories from the past and whimsical nostalgia.
“Mouth-feel”
A big part of food experience is how food feels in the mouth. This is known as somato-sensation. Receptors from the trigeminal nerve in the oral cavity are responsible for this, and they include mechanoreceptors to detect texture and tactile stimulation, thermoreceptors to detect temperature and nociceptors detect noxious stimuli. Mouth-feel is what allows us to savour the creaminess of a pudding, the crunch of crisps, the prickle of carbonated drinks and the spiciness of curries. These sensations can influence our preference or rejection for foods of certain temperatures or textures. Somato-sensation becomes even more important in improving the hedonic quality of a meal for people who have lost their sense of smell or taste.
In short, flavour is the synthesis of individual tastes, smells and feels into an overall perception, modified by mastication sounds during eating, the appearance of the food and numerous environmental factors7. Yet at the same time, flavour is more than the sum of its parts, because it is shaped by our individual experiences, cultures and preferences. Hence the reason why the flavour of your favourite food appeals to you is really a matter of personal taste.
See our blog for Activities; especially 40-42.
Some suggestions for further listening, reading, and watching:
Madeleines and memories – the Proust Effect
Overview of Smell and Taste Disorders
This is What it’s Like to Live in a World Without Smell
What is life like without smells
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1Running, C. A., Craig, B. A., & Mattes, R. D. (2015). Oleogustus: The Unique Taste of Fat. Chemical Senses, 40(7), 507-516. DOI: 10.1093/chemse/bjv036
2Zocchi, D., Wennemuth, G., & Oka, Y. (2017). The cellular mechanism for water detection in the mammalian taste system. Nature Neuroscience, 20(7), 927-933. DOI: 10.1038/nn.4575
3Hummel, T., & Seok, H. S. (2016). Orthonasal and retronasal perception. In: E. Guichard, C., Salles, M. Morzel, and A.-M. Le Bon (eds). Flavour: From food to perception [pp 310-318]. Oxford: Wiley Blackwell. DOI: 10.1002/9781118929384.ch13
4Dunkel, A., Steinhaus, M., Kotthoff, M., Nowak, B., Krautwurst, D., Schieberle, P., & Hofmann, T. (2014). Nature’s Chemical Signatures in Human Olfaction: A Foodborne Perspective for Future Biotechnology. Angewante Chemie International Edition, 53(28), 7124-7143. DOI: 10.1002/anie.201309508
5Mainland, J. D., Keller, A., Li, Y. R., Zhou, T., Trimmer, C., Snyder, L. L., Moberly, A. H., Adipietro, K. A., Liu, W. L. L., Zhuang, H., Zhan, S., Lee. S. S., Lin, A., & Matsunami, H. (2014). The missense of smell: functional variability in the human odorant receptor repertoire. Nature Neuroscience, 17(1), 114-20. DOI: 10.1038/nn.3598
6Firestein, S. (2001). How the olfactory system makes sense of scents. Nature, 413(6852), 211-218. DOI: 10.1038/35093026
7Spence, C. (2020). Multisensory Flavour Perception: Blending, Mixing, Fusion, and Pairing Within and Between the Senses. Foods, 9(4), 407. DOI: 10.3390/foods9040407
