The Ex-Tongue Map and the Main Characteristics for Identifying Basic Tastes

There are many myths that we still hold in our memories. However, today, science can help us unravel or better understand some facts and rumors!

One of the greatest discoveries made, especially after the advancement of genetic engineering techniques, is related to taste. Through these, scientists were able to identify the mechanisms of action of the substances that provide the five basic tastes (sweet, salty, bitter, sour, and umami).

Humans have four types of taste buds on their surface: fungiform, foliate, circumvallate, and filiform. Filiform taste buds are involved only with somatosensory sensations, that is, those related to touch, temperature perception, mechanical sensations, or even pain, caused by substances present in peppers. The other three contain taste buds. Each taste bud is composed of taste cells, which have receptors* (note 1) for the five basic tastes. Researchers have found that these receptors are spread throughout our tongue and, contrary to popular belief, there is no “tongue map” where each region allows the sensation of a particular taste.

Trivedi (2012) mentioned studies that confirmed this fact in his publication in the taste supplement of Nature magazine. The article described how the “tongue map” was introduced in the early 20th century, but this fact has been considered obsolete since 1931, when American chemist Arthur Fox, of the DuPont company, made a remarkable discovery by accidentally releasing fine crystals of phenylthiocarbamide (PTC)* (note 2) during a transfer to another container and found that the compound’s bitter taste could be perceived throughout the tongue, not just at the back, as the “tongue map” indicated.

In addition, he found that this compound produced an extremely bitter taste in some people, but in others, it was completely tasteless (FOX, 1932). This fact was also evaluated by geneticist Lawrence H. Snyder, who confirmed Fox’s assertion, revealing that there were Mendelian traits in a recessive gene for so-called “non-tasters” (those who could not taste substances). These publications marked the first step toward sensory analysis research, differentiating “tasters” from “non-tasters.” Today, it is also known that there is not just one recessive gene, but several. Therefore, this also reveals that there is no point in “mapping” human tongues, as individuals can experience all five tastes, each in their own way, as everything depends on their genetic makeup (TRIVEDI, 2012; FOX, 1932; SNYDER, 1931; WOODING, 2006).

Mechanisms of Action of Taste Substances

Despite Fox and Snyder’s observations, it was only decades later that the receptors for basic tastes were truly unraveled. Different research groups have published numerous scientific articles in the race to discover the mechanisms of taste action.

Umami taste, specifically, is known to have been first noticed by Dr. Kikunae Ikeda in 1908, but it was only in the 1990s that research groups identified these receptors. One of the groups was that of scientist Nipura Chaudhari, initially from the University of Colorado, who indicated, in 1965, that there were taste cell membrane receptors for the main umami substance, glutamate (CHAUDHARI et al., 1996).

In addition to Chaudhari’s group, the group of American neuroscientist Charles Zuker (Howard Hughes Medical Investigator / Columbia University – New York) and geneticist Nicolas Ryba (National Institute of Dental and Craniofacial Research in Bethesda, Maryland) distinguished themselves in the detection of taste receptors. First, in 2000, they detected receptors for bitter taste, and called them T2R* (obs. 4) (CHANDRASHEKAR et al., 2000). In 2001, the same group identified the receptors for sweet taste (T1R2 and T1R3), both for sugars naturally occurring in foods and for sweeteners. Furthermore, in 2002, they also managed to identify other receptors responsible for the umami taste sensation, T1R1 and T1R3 (NELSON et al., 2002; CHAUDARI & ROPER, 2010).

Science can debunk many myths that still circulate in news and textbooks. Therefore, we should always be aware of the latest developments…
More details about taste receptors can be found in the article “The Perception of Taste.”

Notes

Note 1: Receptors: proteins present in the membrane of cells/organelles that interact with specific substances, promoting signal transduction (chemical reactions inside the cell), resulting in specific mechanisms of action.

Note 2: Phenylthiocarbamide (PTC): a protein found in some vegetables of the Cruciferae and Gramineae families, which can be very bitter to some people and tasteless to others.

Note 3: Metabotropic receptors: These receptors are associated with signaling molecules, known as G proteins and/or second messengers, responsible for triggering a sequence of biochemical events that, in the case of tastes, can activate neurons for their identification.

Note 4: The term “TR” comes from the English term “taste receptor”; the numbers correspond to the classifications for identifying each receptor.