Taste, or gustation, is the sensation that occurs when a substance in the mouth, a tastant, reacts chemically with receptors to send a signal to the brain. Taste is distinguished from flavor, a term that generally describes a broader experience, which includes gustation, olfaction, trigeminal nerve stimulation (texture, pain, temperature), and even sight and sound. There are five basic categories of taste: bitter, sweet, sour, salty, and umami (a Japanese word that loosely translates as “savory” in English). Other possible basic tastes, such as fatty acids, calcium, carbon dioxide, and even water, are being investigated.
Taste has evolved to help humans distinguish between nutritious and toxic foods. Foods with sugar have evolved to taste good because sugar is essential for survival. Umami tastes delicious because proteins are essential. Salt is an essential nutrient that must be consumed in a specific amount to maintain proper levels in the body; this is why a little bit of salt tastes good, but too much salt tastes bad. Sour appears to have both aversive and pleasant qualities that function to alert us when foods have spoiled or when plants are not yet ripe, but also to encourage the consumption of sour fruits that contain important vitamins and nutrients. Bitterness has evolved to be a generally aversive taste because most foods that taste bitter tend to be poisonous. However, the bitter molecules in many edible plants are toxic only in high concentrations and these edible plants contain anticarcinogenic nutrients. Thus, it makes sense that these edible plants taste pleasant to many people.
Taste stimulants are detected via taste organs, called taste buds, which are located in the mucosa of the tongue, soft palate, inner cheeks, pharynx, and epiglottis. It is important to note that taste buds are not the bumps on a person’s tongue; these are called papillae. Each person has about 3,000—10,000 taste buds, most of which are on the surface of the tongue (Bartoshuk & Snyder, 2013). Each taste bud is a cluster of 50—100 elongated epithelial cells arranged like the segments of an orange. The cells are renewed about every 12 days. They are divided into three main groups: Type I are support cells; Type II are the receptor cells, which contain the G protein—coupled receptors that bind sweet, bitter, and umami tastants; and Type III react to sour stimuli, which involve ion channels. Type III are also known as presynaptic cells because, unlike Type II, they have identifiable synapses with the nerve fiber and release neurotransmitters, such as serotonin and GABA (gamma-aminobutyric acid), onto the afferent nerve fiber. The mechanism that Type II cells use to excite the nerve fiber is unknown; however, it is speculated that ATP (adenosine triphosphate) acts as a transmitter. The cells that contain receptors for salty taste have not been clearly delineated.
The receptor proteins are located in the plasma membrane of long microvilli called gustatory hairs that extend from the taste cells through a taste pore to the surface of the papillae. The taste pores are filled with saliva, which transports the dissolved tastants to the receptors. The epithelial taste cell then generates impulses in the sensory nerve fibers that innervate them.
The flavors of foods and beverages are perceived through both the taste and olfactory systems. In fact, the sense of smell is the most important chemical sense for flavor to be distinguished. This is why food does not taste as flavorful when a person has a stuffy or runny nose from a head cold, flu, or sinusitis. This sidebar focuses on isolating one of the five basic tastes, umami.
Umami is a Japanese word that is best translated into English as “savory.” It detects amino acids that are appetizing, such as meat, mushrooms, tomatoes, and cheese. Humans have taste receptors that are specific for L-glutamate, an amino acid that is found in high-protein foods and that binds to the umami taste receptors. The most common salt form of L-glutamate is monosodium glutamate (MSG). It is a common misconception that MSG is bad for humans and that it causes headaches, allergies, and childhood obesity. In fact, most taste researchers agree the notion that MSG causes sickness in humans is unfounded.
1 Lay’s® Potato Chip—Classic
1 Doritos® Tortilla Chip—Nacho Cheese flavor
You will need to eat the potato chip first so that you will saturate the salt receptors and be able to isolate the umami receptors that the Doritos® Tortilla Chip will stimulate. These brand-named chips are necessary for the experiment to work the best, as each chip has the same amount of salt. For best results, make sure your nasal passage is clear and that you can take a deep breath. Place the Lay’s® Potato Chip in your mouth and eat it very slowly. You should take about a minute to dissolve the salt and potato chip in your mouth before you swallow the chip. Swish the dissolved salt and melted potato chip throughout your mouth—as you have taste buds on your cheeks, inside of lips, and roof of the mouth—and then swallow. With one hand plug your nose. Note: you can breathe through your mouth during this experiment. Place the Doritos® Tortilla Chip in your mouth and chew about 5—10 times with your nose still plugged. If you have completely saturated the salt receptors in your mouth, then the Dorito® chip should not have any taste. Unplug your nose, as this will allow retronasal olfaction to take place. Instantly, the cheesiness of the Dorito® should be detected as the MSG should have bound to your isolated umami receptors.
Jennifer L. Hellier
Most taste buds are contained in papillae. There are three types of papillae: the relatively large circumvallate, found at the back of the tongue in an arc; foliate, found on the edges of the medial tongue; and fungiform, found at the tip of the tongue. The taste pores containing the protrusions of the microvilli are located on the surface of the fungiform papillae and buried in the sides of the circumvallate and foliate papillae. In order for food molecules to be tasted, they must be dissolved in saliva because only then can they flow into the taste pores.
The popular tongue map that shows each of the five tastes being perceived on a certain region of the tongue, specifically bitter on the back, sweet on the tip, is a successfully propagated myth that was started in 1942 by a textbook author who misunderstood a research paper on the subject. Contrary to this popular belief, all five tastes can be perceived anywhere there are taste buds.
To the Brain
Taste information travels to the brain via three cranial nerves. The chorda tympani branch of the facial nerve (cranial nerve VII) carries impulses from taste receptors in the anterior two-thirds of the tongue; the glossopharyngeal nerve (cranial nerve IX) from the tongue’s posterior third and pharynx; and the vagus nerve (cranial nerve X) from the few taste buds on the epiglottis and lower pharynx. These three cranial nerves synapse in the solitary nucleus located in the medulla. From there, impulses are transmitted to the ventral posterior medial nucleus of the thalamus and then to the gustatory area of the cerebral cortex in the insula lobe. The orbitofrontal cortex receives projections from the insular cortex, as well as information about touch, temperature, and smell, suggesting it is an integration area for flavor. The three nerves partially inhibit one another. This provides a means to retain taste perception if one nerve is damaged, for the other two nerves will have amplified signals. When a nerve is damaged, this delicate inhibition process is tampered with and this can sometimes result in “phantom taste.”
The lingual branch of the trigeminal nerve (cranial nerve V) has nerve endings surrounding taste buds. It sends sensory information about touch, temperature, and pain. These nerves can be irritated by certain chemicals, such as capsaicin, and send a signal of pain and heat to the brain, giving us the sensation of spiciness, or pungency. Menthol works the same and elicits a feeling of coolness. These are both chemesthetic sensations.
Theories of Transduction
There are two theories that explain how taste sensations are encoded from receptor to brain: the labeled line hypothesis and pattern coding theory. Labeled line theorizes that each individual nerve fiber encodes only one of the five basic tastes. Pattern coding, also known as across fiber coding or population coding, theorizes that nerve fibers receive information from more than one of the five tastes, and the overall pattern of many nerve fibers firing is interpreted by the brain to be a particular taste or tastes.
In fungiform papillae, the nerve fibers that innervate taste cells branch so they innervate multiple cells, and each cell can also be innervated by multiple different fibers. Recordings show that the taste cells innervated by branches of the same taste fiber have similar specificities to tastants. This supports the labeled line theory. However, recordings also show that nerve fibers can respond to more than one taste class, evidence supporting the pattern coding theory. A general consensus is beginning to arise that both theories are partially correct: nerve fibers are “best” at transducing one taste. So while they may weakly respond to another taste, “sucrose” best fibers, for example, are best at sending the “sweet” message to the brain.
Bitterness is a “bad” taste that is believed to have developed to preserve life. Thus, when a person or animal eats something that is bitter, they do not like the taste and spit it out. Substances that are bitter include coffee, beer, aspirin, quinine, peels of citrus fruits, and many vegetables. There are about 25 different known receptor proteins that bind with bitter tastants. They are called the T2R receptors and are encoded from a family of genes called Tas2r.
Sweetness is the taste of simple carbohydrates, which are the body’s source of energy. It is important for survival and eating sweet foods signals reward pathways in human brains so that everyone is encouraged to eat these energy-rich foods. There are about 25 variants of the T2R bitter receptors but only 3 variants of the T1R G protein—coupled receptors for sweet and umami. This makes sense, for while there are many different molecules that are poisonous, there are only a few different carbohydrates that are biologically important to humans.
Umami is a savory taste that is elicited by the amino acid glutamate that is found naturally in meat, aged cheeses, and tomatoes. In its salt form, glutamate is the flavor enhancer monosodium glutamate, or MSG. There was controversy about the safety of MSG, but the consensus today is that aside from a few sensitive people, MSG is safe.
Saltiness is the taste elicited by salts, which are molecules made up of oppositely charged particles. Table salt, or sodium chloride (NaCl), dissolves into positive and negative ions, Na+ and Cl−. Sodium ions enter taste cells through sodium ion channels and depolarize the taste cells, causing them to transduce a signal to the afferent nerve fiber.
Sour is the taste of acid, such as hydrogen chloride (HCl) or organic acids, like lactic and citric acids. Sour foods include many citrus fruits, vinegar, yogurt, and spoiled dairy (cheese and sour cream). Acids release hydrogen ions (H+) that interact with ion channels on taste cells and depolarize them. The exact mechanisms of H+ detection, however, are not completely understood.
See also: Bitter Sensation; Chorda Tympani Nerve; Cranial Nerves; Facial Nerve; G Proteins; Glossopharyngeal Nerve; Interrelatedness of Taste and Smell; Salty Sensation; Sensory Receptors; Sour Sensation; Sweet Sensation; Taste Bud; Trigeminal Nerve; Umami; Vagus Nerve
Bartoshuk, Linda M., & Derek J. Snyder. (2013). Taste. In D. W. Pfaff (Ed.), Neuroscience in the 21st century, from basic to clinical (pp. 781—813). New York, NY: Springer Science+Business Media.
Mueller, Ken L., Mark A. Hoon, Isolde Erlenbach, Jayaram Chandrashekar, Charles S. Zuker, & Nicholas J. Ryba. (2005). The receptors and coding logic for bitter taste. Nature, 434, 225—229.
Yarmolinsky, David A., Charles S. Zuker, & Nicholas J. Ryba. (2009). Common sense about taste: From mammals to insects. Cell, 139(2), 234—244.