Let’s now consider the sensation of “taste” (or “the palate”). Certain soluble molecules of food pass through saliva and bind to receptors in the oral cavity, mainly found in the hundreds of taste buds that coat our tongue *.
It’s said that humans perceive at least five kinds of flavours, all explained by evolution. Bitter, which alerts us to the presence of the potentially toxic alkaloids in certain plants. Acid, which tell us that a fruit is unripe or contaminated by (often acid-generating) microbes. Sweet, which indicates the food’s energy level. Salt, a sign of minerals essential to maintaining the electrolyte balance of our organism. And the more recently discovered umami, which signals the presence of amino and nucleic acids (mainly glutamate, inosinate and guanylate), essential constituents of our organism.
But that’s probably not a comprehensive list, because other flavours could be added to it, as long as we identify the receptors responsible for them. That’s the case for fat (fatty acids), starch (long carbohydrate chains), liquorice (glycyrrhizic acid), metal (iron, copper, etc.), and various other compounds that scientists suspect interact with our taste buds.
From a physiological point of view, taste resides in the small reddish blisters (visible to the naked eye) that line our tongue, and which contain one to several hundred taste buds, each composed of many cells with specific receptors on their surface. Thus, umami, sweet or bitter flavours are detected by the interaction of various compounds (peptides, amino and/or nucleic acids, sugars, sweeteners, salts, alkaloids) with membrane proteins (of which there are many and which are coupled to G proteins, allowing the transfer of information inside the cell). Meanwhile, acidic or salty flavours are provoked by fluxes (H + proton and Na + ion respectively) in ion channels **.
We use the word ageusia when the tongue loses its ability to taste and dysgeusia when taste perception is distorted, two pathologies often due to surgery (such as tooth extraction) or drug treatment (including certain chemotherapies).
* Note that the “tongue map” found in several popular works, indicating the zones supposedly gifted with the ability to detect different flavours (sugar on the tip, bitter at the bottom, acid on the sides, salty in front) is obsolete. Researchers have shown that, although some areas may be a little more sensitive than others, our entire tongue can discern various flavours.
** The perception of flavours can be triggered by very diverse compounds. For instance, in sweet compounds, we find sugars (such as sucrose, fructose or glucose), as well as amino acids (glycine), polyols (such as sorbitol and xylitol), vegetable proteins (like thaumatin, with a sweetening power 3,000 times higher than sucrose) and peptides (for example aspartame). In addition, many compounds can activate several receptors. This is the case with certain peptides, which trigger both umami and bitter perceptions, or with certain sweeteners (such as stevioside from stevia, a plant native to South America) which is both sweet and bitter. Finally, there are interactions between flavours. Which is how, for example, we can lower the perceived acidity of lemon juice by adding sugar, while its actual acidity (in pH terms) does not change! Even more surprising, there are coupling magic tricks like the one performed by miraculin, (a glycoprotein from the berries of synsepalum dulcificum, a West African tree), which does not have a sweet flavour in itself, but which lends one to everything that is ingested, thus “softening” acidic or bitter foods. The effect can last more than an hour!