The Biology of Taste: Insights into Flavour Perception

By Ethan Sim

“Taste” is a deceptively simple label for a complex biological phenomenon. Strictly speaking, taste (gustation) refers only to what the tongue can detect – sweet, salty, sour, bitter, and savoury (umami) (Sherman, 2019). Yet, the phrase “tasting food” is commonly used to describe the process of flavour perception.

Flavour perception involves more than just the tongue; molecules in food, released during chewing, bind to specific proteinaceous receptors on the tongue and nasal cavity, resulting in a unique pattern of impulses which travel along neurons to the brain (Purves, 2001). The brain then interprets this pattern as a particular flavour: the taste of an apple, for instance. Broadly speaking, specialized cells known as receptor cells have receptors on their surface, and specific compounds in food are able to bind to them in a complementary manner. Such binding alters the shapes of these proteins, activating them. These activated receptors may then bind to and activate other downstream proteins, resulting in a signal cascade which spreads throughout the cell. Ultimately, this causes positively-charged ions to flow into the cell, and an electrical impulse – a chemical message – results. This impulse travels to the brain via neurons, and this is perceived as flavour (Smith and Margolskee, 2006). 

Among these receptor cells, gustatory cells on the tongue are known to be instrumental in flavour perception, but what is less well-known is that their contribution is limited. These cells, which harbour taste receptors, aggregate to form structures known as taste buds. These are located throughout the upper surface of the tongue (Shier, 2016). When taste receptors are activated, the resultant impulses are conveyed along cranial nerves to the gustatory cortex, the taste centre of the brain, and these are perceived as basic sensations of sweetness, saltiness, sourness, bitterness, and savouriness (Sherman, 2019). 

Each gustatory cell possesses many distinct receptor proteins, and is thus capable of detecting a wide variety of compounds; a cell which generates a “sweet” impulse could detect chemicals as diverse as glucose, aspartame (Li et al., 2002), and lead acetate (Couper et al., 2006) – all of these chemicals would taste sweet. However, each cell is only able to trigger the perception of one specific flavour; alone, this sweet cell would not be able to generate “sour” or “bitter” impulses (Berg et al., 2002). Also, a widespread myth, known as the “tongue map”, holds that specific areas of the tongue are solely responsible for the detection of specific flavours. This is untrue – every taste bud comprises one of each type of gustatory cell, allowing all parts of the tongue to detect every type of flavour (Marshall, 2013).

Proof of this receptor-based system can be found in genetic studies – one who lacks a receptor uniquely specific for a compound would be unable to detect this compound in food. An example is given by the OR6A2 gene, which encodes a receptor protein highly sensitive to certain aldehydes in coriander (Eriksson et al., 2012). Those who possess the gene report that coriander tastes unpleasantly soapy, while those without the gene detect no corresponding taste. Additionally, if one were to chemically modify a receptor, rendering it sensitive to a wider spectrum of compounds, the result would be the preponderance of a certain flavour. One such taste modifier is miraculin, a glycoprotein which binds to sweetness receptors, and renders them responsive to certain acids which normally activate only sour receptors (Theerasilp and Kurihara, 1988). As a result, for up to an hour after the introduction of this protein, even lemons taste sweet.

While it is apparent that the tongue is responsible for basic sensations of flavour, further discrimination within these domains, such as between oranges and blueberries, requires the participation of the sense of smell.

Many are aware that smell (olfaction) plays an important role in the perception of flavour, but what is less well-known is that smell permits the perception of subtler differences between flavours. Olfactory receptor neurons (ORNs), which harbour olfactory receptors, are located on the roof of the nasal cavity. They are directly connected to the olfactory bulb, the area of the brain responsible for processing scent information.

While ORNs function similarly to gustatory cells, their level of specificity is much greater.  Every neuron possesses just one type of olfactory receptor, and each type of olfactory receptor responds slightly differently to different scent molecules (odorants); each type of odorant thus activates a unique combination of receptors. Beautifully, ORNs which possess the same olfactory receptor connect to distinct “hubs” within the brain, allowing every type of odorant to form a unique spatial map of neuron activation: a unique flavour image which enables exceptionally fine discrimination between similar flavours (Berg et al., 2002).

However, consciously sniffing one’s food will do little to enhance one’s perception of flavour. Sniffing is an example of orthonasal olfaction – odorants reach ORNs via the nostrils – and it is believed that this pathway merely sets expectations for flavour, rather than itself contributing to flavour perception (Spence, 2015).  Flavour perception is primarily enhanced by retronasal olfaction – when odorants, newly liberated from food during the process of chewing, reach ORNs via the nasopharynx during exhalation. Studies conducted on rats may explain this difference. The gustatory cortex is activated only when feeding takes place, and this occurs in tandem with retronasal, but not orthonasal, olfaction (Blankenship et al., 2019). One could test this by deliberately exhaling while chewing and determining if there are differences in taste.

In summary, the simple process of gustation relies on inputs from taste receptors alone, but the more complex process of flavour perception requires information from both taste and olfactory receptors. Taste receptors, with their limited range of neural outputs, are only responsible for basic sensations, while olfactory receptors, with their remarkable ability to convert odorant molecules into unique flavour images, provide nuance and complexity to flavour. Although this explanation may seem comprehensive, many questions still surround the relationship between taste and the other senses – and, more broadly, the brain’s capacity for multisensory integration. With research indicating that colour (Spence et al., 2010), temperature (Green and Nachtigal, 2015), and even emotion (Noel and Dando, 2015) may affect flavour perception, there remains much to discover about a seemingly instinctual sense.

Further Reading

References

Berg, J., Tymoczko, J. & Stryer, L. (2002) Section 32.1: A Wide Variety of Organic Compounds Are Detected by Olfaction. In: Biochemistry. 5th edition. New York, W H Freeman.

Berg, J., Tymoczko, J. & Stryer, L. (2002) Section 32.2: Taste Is a Combination of Senses that Function by Different Mechanisms. In: Biochemistry. 5th edition. New York, W H Freeman.

Blankenship, M., Grigorova, M., Katz, D. & Maier, J. (2019) Retronasal odor perception requires taste cortex but orthonasal does not. Current Biology. 29 (1), 62-69.

Couper, R., Fernandez, P., Alonso, P. & Ordi, J. (2006) The Severe Gout of Emperor Charles V. The New England Journal of Medicine. 355 (18), 1935-1936.

Eriksson, N., Wu, S., Chuong, B., Kiefer, A., Tung, J., Mountain, J., Hinds, D. & Francke, U. (2012) A genetic variant near olfactory receptor genes influences cilantro preference. Flavour. 1 (22).

Green, B. & Nachtigal, D. (2015) Temperature Affects Human Sweet Taste via At Least Two Mechanisms. Chemical Senses. 40 (6), 391-399.

Li, X., Staszewski, L., Xu, H., Durick, K., Zoller, M. & Adler, E. (2002) Human receptors for sweet and umami taste. Proceedings of the National Academy of Sciences of the United States of America. 99 (7), 4692-4696.

Marshall, P. (2013) The Tongue Map, Real or Not?. The American Biology Teacher. 75 (8), 583-586.

Noel, C. & Dando, R. (2015) The effect of emotional state on taste perception. Appetite. 95 89-95.

Purves, D. (2001) Neuroscience. 2nd edition. Sunderland, Massachusetts, Sinauer Associates.

Sherman, C. (2019) The Senses: Smell and Taste. Available from: https://www.dana.org/article/the-senses-smell-and-taste/ [Accessed 21st August 2020].

Shier, D. (2016) Hole’s Human Anatomy and Physiology. New York, McGraw-Hill Education.

Smith, D. & Margolskee, R. (2006) Making Sense of Taste. 16th edition. Scientific American.

Spence, C. (2015) Just how much of what we taste derives from the sense of smell?. Flavour. 4 (30).

Spence, C., Levitan, C., Shankar, M. & Zampini, M. (2010) Does Food Color Influence Taste and Flavor Perception in Humans?. Chemosensory Perception. 3 68-84.

Theerasilp, S. & Kurihara, Y. (1988) Complete purification and characterization of the taste-modifying protein, miraculin, from miracle fruit. The Journal of Biological Chemistry. 263 (23), 11536-11539.

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