The science of taste

In his first article for Kitchen Geekery, Alex will explain the science behind our tastebuds and dispel a few myths in the process.

A common misconception about taste is that there is a “tongue map” which illustrates the differences in sensitivity across the human tongue for the basic tastes.

The supposed tongue taste map

The tongue map dates back to research by a German scientist named D.P. Hanig, published in 1901, after he chose to travel the Far East. As he was not familiar with Japanese cuisine, Hanig used volunteers to measure the sensitivity of certain parts of the tongue to the four known basic tastes.

Years later in 1942, Edwin Boring, a psychology historian at Harvard University used the data from Hanig's subjective experiment and calculated real numbers for the levels of sensitivity from the raw data. The numbers that were calculated only expressed sensitivity to taste in a relative manner but they were misinterpreted by other scientists to assume that the areas of lower (relative) sensitivity were areas of no sensitivity. Thus, the modern tongue-map was born.

Scientists did not dispel this notion of a tongue-map until 1974 when a scientist named Virginia Collings confirmed that all tastes are equally discernible across areas of the tongue. We now know that there are five primary taste sensations: salty, sour, sweet, bitter and umami.

  • Sweet - usually indicates energy rich nutrients
  • Umami - the taste of amino acids
  • Salty - allows modulating diet for electrolyte balance
  • Sour - typically the taste of acids
  • Bitter - allows sensing of diverse natural toxins

Our ability to taste comes from molecules (tastants) released during food intake, which stimulates special sensory cells in the mouth and throat. Humans’ sense taste via taste buds, which are numerous sensory cells clustered together. Each cluster contains about 50-150 taste cells.

These chemical tastants dissolve in saliva and are then able to enter the taste buds through pores. They then interact with either with ion channels or the proteins on the surfaces of taste receptors. This triggers stimulation of chemical signals which causes them to be transmitted to the brain via the central nervous system.

Different taste receptor cells are linked to the various taste perceptions, for example:

  • Sweet tastants trigger responses in taste cells by binding to TIR2 and T1R3 proteins.
  • Bitter tastants bind to T2R receptors.
  • Umami tastants bind to T1R1 and T1R3 receptors.
  • Sour and Salty tastants flow through ion channels.

It's in the brain where specific tastes are identified. As each taste cell has a receptor which responds to one of at least five basic taste qualities, different cells are depolarised, different neurones are contacted and the brain is able to differentiate between the tastes and the magnitude of each taste is calculated.

You may notice that Sweet and Umami use one of the same protein (TIR3) but they do not cause the same taste to be simulated. This is because the tastants are different. Whereas sweet tastants tend to be Mono/Di-saccharides (maltose, sucrose, fructose, glucose, etc), umami’s tastant is glutamate, an amino acid which is present in fish, meat, vegetables, fermented goods and breast milk.

These tastants have different chemical structures and thus interact in a different manner with the same protein. This is why they are able to produce different tastes - the threshold levels for detection of the chemical tastants for the five basic tastes are shown below.

Taste Common chemical tastant Threshold value for detecting taste
Sweetness Sucrose 3.42g sucrose
Saltiness NaCl 0.58g NaCl
Umami Glutamate 0.10g glutamate
Sourness HCl 0.033g HCl
Bitterness Quinine 0.0026g quinine

Why does the sensitivity vary so much?

As seen above, it requires extremely small amounts of HCl and quinine to trigger the detection of sourness and bitterness. This is because HCl and other sour tastants are present in foods that have begun to go off and it was advantageous for our hunter-gatherer precursors to be able to detect that. The ones that could detect the tastants at small amounts would not then suffer the illnesses that came with rotten food and as a result would be able to reproduce, passing down their genes via Natural Selection.

The same goes for bitter foods. The ability to perceive bitter tastants was advantageous as there are numerous natural bitter compounds, of which a large number of which are known to be toxic. The ability to detect bitter-tasting and therefore toxic compounds at extremely low thresholds was considered to provide an important protective function.

The umami taste is one which researchers are still trying to find evolutionary reasoning for. Glutamate is the most abundant amino acid in nature and like other amino acids, is used by the body for growth. By that logic, the ability to taste umami is most likely down to our need for glutamate and other amino acids, the building blocks of proteins in the body.

Berries

Sweet tastants are detectable at low values because when one thinks of a hunter-gatherer scenario that was all too common in times gone by, food that contained sufficient calories for the survival of a group was relatively scarce. Therefore, the ability to perceive sweet tastants would be vital as it would draw the hunter-gatherers to foods with high energy content which could be broken down by the body.

Salty foods contain electrolytes, which help your body with homeostasis – the regulation of its internal environment. Electrolytes are important because they are what your cells use to maintain voltages across the cell membranes in order to carry electrical impulses (nerve impulses, muscle contractions) to other cells. When you exercise heavily, you lose electrolytes in your sweat, particularly sodium and potassium.

Since the body is two-thirds water, it is incredibly important that the body have enough electrolytes and this is the reasoning behind the relatively higher threshold value of the salty tastant. Hunter-gatherers would lose large amounts of electrolytes through sweat and diseases (like diarrhoea), so the ability to taste salty foods would be vital for survival.

Sources: Oxford Journals, Journal of Young Investigators, Canadian Broadcasting Centre and Colorado State University.

Comments

Annette Hottenstein replied on June 24, 2014 at 01:15

1

Fabulous article.  I am a sensory scientist by training and found this information helpful for an upcoming presentation I am giving.  Really good quality of information and the science is brilliant!

Kudos!

Annette Hottenstein, MS, RD

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