Taste

Abstract

Taste is a type of sensation: the chemical sense that detects molecules dissolved in the mouth and sorts them into a small set of basic qualities—sweet, sour, salty, bitter, and umami. Each quality is detected by its own receptors on the taste cells of the tongue and carried to the brain along a dedicated labeled line, so that a sweet molecule and a bitter one excite different cells and different cortical fields rather than a shared code. This organization, worked out largely through the molecular biology of the taste receptors, explains how five qualities arise from distinct detection systems. This article sets out the basic tastes and their receptors, how taste signals reach the brain, and how taste is measured, distinguishing taste as the sensory ability from taste perception, its downstream interpretation.

Keywords: taste, gustation, taste receptor, labeled line

A ripe strawberry and a dose of quinine could hardly feel more different in the mouth, and that difference is not an accident of culture but a fact of biology: the strawberry's sugars read as something to seek, the quinine's bitterness as something to spit out. Taste is the sense that makes food edible or dangerous before a single calorie is absorbed, a chemical gatekeeper at the entrance to the body. For most of the twentieth century its qualities were catalogued but its machinery was unknown; only in the last twenty-five years were the receptors for each basic taste identified, turning a sense once described by tongue maps and folklore into one of the clearest examples of how molecules become perception (Chandrashekar, Hoon, Ryba, & Zuker, 2006; Yarmolinsky, Zuker, & Ryba, 2009).

What Taste Is

Taste, or gustation, is the sense that detects chemicals dissolved in saliva and renders them as the basic qualities of sweet, sour, salty, bitter, and umami. It is one of the two chemical senses, the other being smell, and it is defined by MeSH as the sense that detects the qualities of substances taken into the mouth, the function served by the taste receptor cells of the tongue. As a sensory ability it sits alongside vision, hearing, touch, and smell as a channel through which the nervous system registers the world; where this article treats taste as that sensory channel, the richer business of recognizing, judging, and preferring particular tastes is the domain of taste perception.

Taste is often confused with flavor, but the two are not the same. Flavor is the whole experience of food in the mouth, built from taste proper together with smell, texture, temperature, and the mild irritation of chemesthesis; taste is the narrower chemical sense that supplies the five qualities. Those qualities are not arbitrary. Each is tied to a nutritional or defensive problem the body must solve: sweet signals energy-rich sugars, umami the amino acids of protein, salt the sodium needed for fluid balance, while sour and bitter warn of unripe or spoiled food and of the many toxins that happen to taste bitter (Breslin, 2013). The sections that follow build the sense from its receptors upward: first the basic tastes and the molecules that detect them, then the route from taste cell to brain, then how taste is measured.

Types of Taste

In the Medical Subject Headings vocabulary, taste (the parent concept examined here, a form of sensation) has one narrower descriptor, shown in Table 1. The relation is one of measurement rather than kind: the child is not a sub-variety of taste but the quantity psychophysics uses to pin the sense to a number. MeSH is an indexing vocabulary built to organize the biomedical literature, so its hierarchy reflects how research on taste is catalogued, not a strict biological taxonomy of the sense; the entry below is included because it is a live topic on this site.

The Basic Tastes and Their Receptors

The modern account of taste rests on a single organizing fact: each basic quality has its own detection system, and those systems are molecularly distinct. Sweet and umami are sensed by receptors of the T1R family, G-protein-coupled receptors that work in pairs; Nelson and colleagues identified the sweet receptor as a combination of two such proteins, T1R2 and T1R3, which together respond to sugars and artificial sweeteners (Nelson et al., 2001). Zhao and colleagues then showed that a different pairing, T1R1 with T1R3, is the umami receptor that detects the savory taste of glutamate, so that the same small family supplies two of the five qualities through different combinations (Zhao et al., 2003). Umami itself was named a century earlier, when Ikeda identified glutamate as the substance behind the savory taste of seaweed broth and argued that it was a basic taste in its own right, long before any receptor was known (Ikeda, 2002).

Bitter is sensed by a second family, the T2R receptors, and here the logic differs. Instead of one or two receptors, humans carry about twenty-five T2R types, and between them they respond to thousands of structurally unrelated bitter compounds, which is what a defensive sense against a chemically diverse world of toxins requires. Salt and sour are detected not by G-protein-coupled receptors but by ion channels: sodium enters salt-sensing cells through an epithelial sodium channel, which Chandrashekar and colleagues identified as the basis of the appetitive salt response in mice (Chandrashekar et al., 2010), while sour is detected by the proton channel OTOP1, a mechanism Zhang and colleagues traced across vertebrates from the platypus to the human (Zhang et al., 2019). The overarching scheme, that distinct cells and receptors detect distinct qualities, was set out in the reviews that consolidated the field (Chandrashekar, Hoon, Ryba, & Zuker, 2006; Yarmolinsky, Zuker, & Ryba, 2009). The first demonstration lets the reader choose a tastant and trace its dedicated line from receptor to taste cell to the brain.

From Taste Cells to the Brain

The receptors sit on taste receptor cells bundled into taste buds, and the cells are not all alike (Chaudhari & Roper, 2010). Roper and Chaudhari describe three functional types: Type I cells, which play a supporting, glia-like role; Type II cells, which carry the T1R and T2R receptors for sweet, umami, and bitter; and Type III cells, which detect sour and relay signals from their neighbors (Roper & Chaudhari, 2017). A striking feature of the Type II cells is how they signal: they have no conventional chemical synapses, and instead release the transmitter ATP through a large-pore ion channel. Taruno and colleagues identified that channel as CALHM1 and showed that without it the sweet, bitter, and umami signals never leave the taste bud, establishing an unusual, non-vesicular route out of the receptor cell (Taruno et al., 2013).

Figure 1 sets out the labeled-line scheme the rest of this section defends: each basic quality has its own receptor, its own taste cell, and its own territory in the gustatory cortex, so that a quality is fixed by which line is active rather than by the stimulus itself.

Figure 1

Five Labeled Lines, from Receptor to Cortex

From the taste bud the signal travels along the gustatory nerves to the brainstem and thalamus and on to the gustatory cortex, and the central question is whether the qualities stay separate along the way. The evidence favors labeled lines. Chen and colleagues mapped the gustatory cortex and found a gustotopic map, with separate spatial fields for sweet, bitter, umami, and salty, so that each quality is represented in its own cortical territory rather than by a distributed code (Chen, Gabitto, Peng, Ryba, & Zuker, 2011). Lee and colleagues then proved the lines causally by rewiring them: manipulating the connection between the periphery and these cortical fields changed which taste an animal perceived, showing that the quality is fixed by which line is active, not by the stimulus itself (Lee, Macpherson, Parada, Zuker, & Ryba, 2017). Within the bitter line, though, a combinatorial breadth remains: the second demonstration shows how roughly twenty-five bitter receptors can, between them, flag an enormous number of distinct bitter compounds.

Table 2 sets out the five basic tastes, their receptors, and the signal each one carries.

Measuring the Sense

Because taste experience is private, measuring it requires tasks that externalize it, and the central measure is the taste threshold. A detection threshold is the weakest concentration of a tastant a person can reliably tell from plain water; a recognition threshold, always higher, is the weakest concentration whose quality can be named. Both are found by presenting a tastant across a series of concentrations and locating the point where the psychometric function, the curve relating the probability of a correct response to concentration, crosses the halfway mark between chance and certainty. The third demonstration runs such a task, letting the reader set a concentration and read off the predicted probability of detection.

Threshold is only half the story, because tastes above threshold differ enormously in how intense they feel, and here taste reveals some of the largest individual differences in all of sensory psychology. Bartoshuk and colleagues showed that sensitivity to the bitter compounds PTC and PROP is genetically variable: non-tasters barely register them, while a group she named supertasters find them intensely bitter, a difference tied to the density of fungiform papillae on the tongue and to inherited receptor variation, and one that colors the perceived intensity of many other tastes (Bartoshuk, Duffy, & Miller, 1994). Measuring taste therefore means measuring two things: the threshold at which a taste is first detected, and the scaled intensity it reaches once present, which can vary several-fold from one healthy person to the next.

Worked Example

The central facts of taste become concrete when the numbers are worked, and the demonstrations reproduce this arithmetic. Take the labeled lines first. There are five basic qualities, each with its own detection system, so the peripheral code is not a rich combinatorial space but a small set of parallel channels: sweet, sour, salty, bitter, and umami, five lines that stay distinct from receptor to cortex. This is why rewiring a line changes the perceived quality while leaving the stimulus unchanged, and it is the logic the first demonstration traces.

Now the one place taste is combinatorial, the bitter system. If each of the roughly twenty-five T2R bitter receptors either does or does not respond to a compound, the receptors act like a row of binary switches, and the number of distinct on-or-off patterns is two raised to the number of receptors. Twenty-five receptors therefore yield two to the twenty-fifth, which is 33,554,432 possible patterns, an enormous space from a single taste quality. A more conservative count asks how many compounds could be distinguished if each activated a fixed-size subset of receptors: the number of distinct k-receptor patterns drawn from twenty-five is the binomial coefficient twenty-five-choose-k. For k equal to 3 this is 2,300, and for k equal to 5 it is 53,130. Either way the bitter line alone can flag far more compounds than the other four qualities combined, which is exactly what a broad chemical alarm requires. The second demonstration computes these counts as the receptor number and subset size are varied.

Finally the detection threshold. Model the probability of detecting a tastant as a logistic function of log concentration, p equals one divided by one plus e to the minus (x minus m) over s, where x is the log concentration, m is the threshold, and s sets the steepness. With the threshold m at zero and s equal to one half, a concentration one log unit below threshold gives p equal to about 0.12, at threshold p is exactly 0.5, and one log unit above threshold p is about 0.88. The halfway point of the curve defines the measured threshold, and each log unit on the axis is a tenfold change in concentration. The third demonstration plots this curve and reads the detection probability at the concentration the reader sets.

Discussion

Taste entered the modern era as a sense whose qualities were described but whose mechanism was a mystery, and it leaves that era as one of the best-understood examples of molecular sensory coding. The identification of the receptors, T1R pairs for sweet and umami, the T2R family for bitter, and ion channels for salt and sour, gave each quality a physical basis, and the discovery of the gustotopic map and the rewiring experiments established that the qualities are carried by labeled lines kept separate from the periphery to the cortex (Nelson et al., 2001; Zhao et al., 2003; Chen, Gabitto, Peng, Ryba, & Zuker, 2011; Lee, Macpherson, Parada, Zuker, & Ryba, 2017). The old tongue map of regional taste zones, still printed in textbooks, is simply wrong: all qualities can be tasted across the whole tongue, and what varies is receptor and cell type, not location.

Two features keep taste from being a solved problem. The first is the cell biology of the taste bud, where an unconventional, synapse-free route of transmission through the CALHM1 channel shows that even the exit of the signal from the receptor cell holds surprises (Taruno et al., 2013; Roper & Chaudhari, 2017). The second is the gulf between taste as a clean five-channel code and the messy, variable, strongly hedonic experience of flavor, into which smell, texture, and large genetic differences in sensitivity all feed (Bartoshuk, Duffy, & Miller, 1994; Breslin, 2013). The sense that guards the mouth is molecularly simple at its receptors and psychologically rich at its edges, and the distance between the two is where the psychology of taste now lives.

Current Directions

The most consequential recent advance has been the resolution of the last unexplained quality, sour. For decades the sour receptor was uncertain, with several candidates proposed and discarded. Zhang and colleagues established that the proton channel OTOP1 is the sour sensor and traced it across vertebrates, from the platypus to the human, giving sour the same molecular footing the other four qualities already had and completing the receptor-level account of the five basic tastes (Zhang et al., 2019). The finding also reframed sour as acid sensing by a proton-permeable channel rather than a conventional receptor, a mechanism distinct from every other taste.

A second active front is the reach of central taste coding. The demonstration that taste qualities occupy separate cortical fields, and that rewiring those fields changes what an animal tastes, turned the labeled-line hypothesis from an inference into a manipulable system (Chen, Gabitto, Peng, Ryba, & Zuker, 2011; Lee, Macpherson, Parada, Zuker, & Ryba, 2017). The open questions now run downstream: how the separate quality channels are combined with smell, reward, and internal state to drive what an animal actually eats, and how learning reshapes those connections. Taste, long studied at the tongue, is increasingly a problem of how the brain uses a clean peripheral code to make a messy decision about food.

Common Misconceptions

Different regions of the tongue taste different qualities.

The tongue map is a myth. All five basic qualities can be detected across the whole tongue; what differs between taste cells is which receptors and cell type they carry, not where they sit (Chandrashekar, Hoon, Ryba, & Zuker, 2006).

Taste and flavor are the same thing.

Taste is the narrow chemical sense of the five qualities; flavor is the whole experience of food, built from taste together with smell, texture, and temperature. Most of what is called the taste of food is actually its smell (Breslin, 2013).

There are only four basic tastes.

Umami, the savory taste of glutamate, is a fifth basic taste with its own receptor, identified first by Ikeda in 1909 and confirmed molecularly a century later (Ikeda, 2002; Zhao et al., 2003).

Taste and taste perception are the same thing.

Taste is the sensory ability to detect tastants; taste perception is the downstream recognition, judgment, and preference built on it. The two are closely linked but treated as distinct (Yarmolinsky, Zuker, & Ryba, 2009).

Glossary

Basic taste.

One of the five elementary qualities taste resolves—sweet, sour, salty, bitter, and umami—each with its own detection system on the taste cells.

Bitter.

The quality detected by the roughly twenty-five T2R receptors, tuned between them to thousands of structurally unrelated compounds, functioning as a broad warning against toxins.

CALHM1.

The large-pore ion channel through which Type II taste cells release ATP, the unconventional, synapse-free route by which sweet, bitter, and umami signals leave the taste bud.

Chemical sense.

A sense that detects molecules rather than energy; taste and smell are the two chemical senses, taste responding to dissolved tastants.

Detection threshold.

The weakest concentration of a tastant a person can reliably tell from plain water, located where the psychometric function crosses the halfway mark between chance and certainty.

Flavor.

The whole experience of food in the mouth, built from taste together with smell, texture, temperature, and chemesthesis; distinct from taste, which supplies only the five qualities.

Fungiform papilla.

A mushroom-shaped structure on the front of the tongue that houses taste buds; its density varies between people and is higher in supertasters.

Gustotopic map.

The spatial organization of the gustatory cortex into separate fields for each basic taste quality, the central evidence that taste is coded by labeled lines.

Labeled-line coding.

The principle that each taste quality is carried by its own dedicated cells and pathway, kept separate from receptor to cortex, so the active line fixes the perceived quality.

OTOP1.

The proton channel that detects sour by admitting hydrogen ions into Type III taste cells, identified as the sour sensor across vertebrates from the platypus to the human.

Psychometric function.

The curve relating the probability of detecting a stimulus to its intensity; its halfway point between chance and certainty defines the detection threshold.

Sensation.

The registration of physical stimuli by the sensory receptors, the broad category under which taste is classified as a chemical sense.

Supertaster.

A person with unusually high sensitivity to bitter compounds such as PROP, linked to a high density of fungiform papillae and to inherited receptor variation.

Taste bud.

A cluster of fifty to a hundred taste receptor cells housed in the papillae of the tongue, containing the Type I, II, and III cells that detect the basic tastes.

Taste perception.

The downstream recognition, judgment, and preference built on taste, as distinct from taste as the sensory ability to detect tastants.

Taste receptor cell.

A sensory cell within the taste bud that carries the receptors or ion channels for one or more basic tastes and signals to the gustatory nerves.

Taste threshold.

The lowest concentration of a tastant that can be detected, or recognized by quality; the psychophysical measure of sensitivity to a basic taste.

Umami.

The savory taste of glutamate and related amino acids, detected by the T1R1 + T1R3 receptor; named by Ikeda in 1909 and the fifth basic taste.

Key Researchers

Linda M. Bartoshuk

(b. 1938). University of Florida; pioneered the psychophysics of human taste, discovering supertasters and the genetic variation in PROP and PTC bitter sensitivity, and developed the scaling methods that made individual differences in taste intensity measurable. Google Scholar - Wikipedia - Wikidata

Kikunae Ikeda

(1864-1936). Tokyo Imperial University; identified glutamate as the substance responsible for the savory taste he named umami in 1909, establishing umami as a distinct basic taste a century before its receptor was found. Wikipedia - Wikidata

Stephen D. Roper

. University of Miami; studies the cell biology and synaptic signaling of the taste bud, resolving how the receptor and presynaptic taste cells transmit signals to the afferent nerves, and co-authored the definitive review of taste-bud cells, signals, and synapses. Google Scholar

Nicholas J. P. Ryba

. National Institute of Dental and Craniofacial Research; with Charles Zuker, co-discovered the mammalian receptors for sweet, umami, and bitter taste and the labeled-line logic of taste coding, from the peripheral receptor cells to the gustotopic map in the brain. ORCID - Faculty Page - Google Scholar

Charles S. Zuker

(b. 1957). Columbia University (Zuckerman Institute, HHMI); led the molecular dissection of the mammalian taste system, from the sweet, umami, and bitter receptors through the CALHM1 release channel to the hard-wired gustotopic map and the demonstration that taste identity can be rewired centrally. Faculty Page - Wikipedia - Wikidata

Frequently Asked Questions

What is taste?

Taste, or gustation, is the chemical sense that detects molecules dissolved in the mouth and sorts them into the basic qualities of sweet, sour, salty, bitter, and umami. It is one of the two chemical senses, the other being smell, and it is served by the taste receptor cells of the tongue (Chandrashekar, Hoon, Ryba, & Zuker, 2006).

How many basic tastes are there?

Five: sweet, sour, salty, bitter, and umami. Each has its own class of receptor or ion channel, and umami, the savory taste of glutamate, is the one most recently confirmed, though it was first proposed by Ikeda in 1909 (Ikeda, 2002; Zhao et al., 2003).

What are the receptors for taste?

Sweet and umami use T1R receptor pairs, bitter uses about twenty-five T2R receptors, salt uses an epithelial sodium channel, and sour uses the proton channel OTOP1. Each quality is detected by its own molecular system (Nelson et al., 2001; Zhang et al., 2019).

What is labeled-line coding?

It is the principle that each taste quality is carried by its own dedicated cells and pathway, kept separate from the tongue to the cortex. The gustatory cortex has separate fields for each quality, and rewiring a line changes the perceived taste (Chen, Gabitto, Peng, Ryba, & Zuker, 2011; Lee, Macpherson, Parada, Zuker, & Ryba, 2017).

Is the tongue map real?

No. The idea that different regions of the tongue taste different qualities is a myth. All five basic tastes can be detected across the whole tongue; what varies is the receptors and cell types the taste cells carry, not their location (Chandrashekar, Hoon, Ryba, & Zuker, 2006).

Why do some people find foods more bitter than others?

Sensitivity to bitter compounds such as PROP and PTC is genetically variable. Supertasters, who have a high density of fungiform papillae and particular receptor variants, find these compounds intensely bitter, while non-tasters barely register them (Bartoshuk, Duffy, & Miller, 1994).

What is the difference between taste and flavor?

Taste is the narrow chemical sense of the five basic qualities. Flavor is the whole experience of food, combining taste with smell, texture, and temperature; much of what seems to be the taste of food is actually its smell (Breslin, 2013).

How is taste measured?

Chiefly by thresholds, the weakest concentration a tastant can be detected or recognized at, and by scaling the intensity of suprathreshold tastes. Intensity scaling reveals some of the largest individual differences in sensory psychology (Bartoshuk, Duffy, & Miller, 1994).

References