What is taste? |

Structure and Functions

For many people, the thought of biting into a lemon causes a puckering or a tingling sensation in the mouth. A real taste sensation is evoked even though the actual taste stimulus, a lemon, is absent. This kind of response indicates the power of the sense of taste. Taste is also called the "gustatory sense," a term derived from the Latin word gustatus, meaning "taste." This sense evolved to aid animals in the selection of safe, nontoxic foods. Although loss of the ability to experience taste (ageusia) is not a life-threatening condition, it may indicate the presence of other maladies, some of which are life threatening. A diminished or absent ability to taste may account for loss of appetite or weight loss in some ill persons; for these persons, sufficient and proper nutrition can become a critical issue.

There are five special senses of the human body: gustation, olfaction (smell), vision, hearing, and equilibrium. The organs associated with the special senses take in information from the environment in the form of chemical, light, sound, or mechanical energy and convert that energy into nerve impulses. Nerve impulses are tiny electrical signals that are carried by the peripheral and central nervous systems to the brain, where the information is integrated in order to assess how dangerous or secure an individual may be in any given environment. Taste is a primitive sense, meaning that it need not be taught; it evokes instinctual responses or reactions.

Although taste is not as essential to survival as the sense of vision, it is an important sense for a variety of organisms. Taste preferences can be observed in humans, elephants, monkeys, fish, and even some microorganisms. Yet not all organisms respond equally to the same tastant; one example is sugar. Cats lack taste receptors for sugar; therefore, if a cat eats a sweet food, it does so for other flavors. Just as a cat does not taste sweetness, however, a human might not notice a tastant that a cat recognizes. A tastant is any food or chemical (such as soap) that causes a taste sensation.

Taste is a nerve impulse that is interpreted by the brain. (In organisms lacking a brain, taste is interpreted by some other structure that serves as the center of integration and coordination.) For humans, and for many of the larger land animals, taste sensations begin upon the intake of food or other substances into the mouth. This signal is prompted by the interaction of tastant molecules or ions and chemoreceptors located in specialized regions within taste buds.

Composed mostly of muscle fiber, the tongue not only serves as a receptor site for taste stimuli but also is responsible for the refined and coordinated movements that produce speech—an important form of communication in the human species. Different sections of the tongue are in contact with specific nerves that allow coordinated and specialized movements of the tongue. The palatine section of the tongue is easily seen when the tongue is fully extended from the mouth; it constitutes the front two-thirds of the tongue. The back one-third is the pharyngeal section; the palatine and pharyngeal sections are visibly, but subtly, separated by a transverse groove.

The surface of the palatine section of the tongue is coated with small, closely spaced projections called "papillae." These structures give the tongue the dual properties of being rough and textured while remaining velvety smooth. Papillae are arranged, from the tongue’s tip to the back, in more or less parallel rows that run along the medial groove of the tongue. The medial groove divides the tongue lengthwise into equal halves that are independently coordinated by nerves and muscle working together.

Most papillae covering the tongue are of the filiform type. Unlike the other three papillae forms, filiform papillae do not contain taste buds and thus are not responsive to tastants. Instead, filiform papillae aid in the tearing and grating of food particles. Although filiform papillae are not barbed, they do have a rasping mechanical action that aids in cleansing the body through licking (as observed in cats) and moving food particles about in the mouth.

All other papillae—the fungiform, the foliate, and the vallate—are actively involved in tasting, even though they are not present in great abundance or distributed uniformly over the tongue surface. Fungiform papillae, whose projections are shaped like a mushroom cap, are widely scattered on the tip and lateral sides of the tongue. Foliate papillae mimic the texture and appearance of smooth, folded leaves; they are located on both sides of the tongue, flanking the vallate papillae. The vallate papillae make a semicircular pattern (or a V shape) at the back of the tongue. Located on the palatine region just before the pharyngeal segment of the mouth, vallate papillae resemble rounded, soft cushions. Humans have between seven and twelve of these projections, making the vallate papillae the least abundant form of papillae.

Four tastes have commonly been recognized by humans: sweet, salty, sour, and bitter. These tastes are strongly registered in specific zones of the tongue. Much to the frustration of scientists, however, the recognition of taste is not specific to any particular type of taste-responsive papillae (fungiform, foliate, or vallate). Sweet receptors are plentiful at the tip of the tongue, while salty receptors are grouped together on either side just beyond the tip. Sour sensations are detected more strongly along the middle sides of the tongue, and bitter sensations are detected at the rear.

Since the vallate papillae are located in the region where only bitterness is tasted and the fungiform papillae are located in the regions sensitive to sweetness, one might assume that a match between papillae structure and taste type exists. Scientific studies, however, prove this to be a false correlation. A simple relationship between kind of taste and type of papillae does not exist, nor is there a complete explanation of the anatomy and physiology of the gustatory sense.

Taste-responsive papillae seem to respond to all tastants that enter their taste buds, whether they are classified as sweet, salty, sour, or bitter. The distinguishing factor seems to be a variation in the intensity of the neural message that a tastant induces. The variable levels of nervous impulses caused by a tastant constitute what is called a "taste profile." Taste profiles are mixed neural codes that a taste bud receives from certain tastants.

A tastant molecule causes a nervous impulse to be sent to the brain for interpretation. It has been learned that all three taste-sensitive papillae are fired, to a greater or lesser extent, by all tastant molecules. A neural response is triggered when tastant molecules arrive at specialized sites on the taste buds. Then there is a certain ratio, or firing pattern, that is interpreted as sweet, salty, sour, or bitter. Oddly, this kind of mixed signal can be misread in the brain. For example, if a sweet solution of table sugar and water (tasted and properly identified as sweet) is greatly diluted with very pure water, the sugar solution may be erroneously classified in the brain as salty. The taste sensation is simply able to recognize when something has interacted with a taste chemoreceptor; a weak stimulus may cause a misinterpretation to occur.

For decades, researchers have been on the trail of an elusive fifth taste. While salty, bitter, sour, and sweet were all well-defined tastes, scientists have long suspected the presence of a fifth taste, called "umami." Umami is known more familiarly as the flavor enhancer monosodium glutamate (MSG). In the late 1990s, scientists at the University of Miami School of Medicine found compelling evidence of umami. Molecular biologists there demonstrated that a modified form of a brain glutamate, mGluR4, is a taste receptor for umami. Because its receptor was identified, many researchers now recognize umami as the fifth taste. Umami is difficult to describe, but foods such as Parmesan cheese, steak, seafood, mushrooms, and tomato juice have an umami component to them, though it is generally mixed in with other tastes as well.

Taste buds are relatively large, bulbous-shaped structures located on the tips of fungiform papillae and in the grooves of the foliate and vallate papillae. It is within the taste bud structure that sweet, salty, sour, or bitter begin the journey of becoming distinguished taste sensations. A taste bud is not a wholly sensing bundle; it can be divided into at least three distinct parts—taste (or sensing) cells, support cells, and basal cells.

The number of taste buds in humans ranges from two thousand to nine thousand. About half of these are located in the grooved edges of the vallate papillae, making this a very sensitive taste area of the tongue. Taste buds are abundant in infants and children, but a continuous decline is observed from adolescence throughout adulthood. This explains why adults often like to add rich sauces, gravies, and seasonings to foods; adults need to enhance food so that a greater range of taste sensations is triggered in each mouthful. Children tend to shun sauces and spices because they may experience almost overwhelming taste sensations when they consume adult-prepared foods. In the elderly, low numbers of taste cells can contribute to poor eating habits or food selection, putting them at risk during the most vulnerable stage in adult life. It is often recommended that foods be made readily available and prepared in colorful and aromatically pleasing ways to entice the elderly to eat properly.

Tastant particles must dissolve in saliva in order to cause a taste response that can be identified properly. The minimum amount of tastant that must be present for it to be identified correctly is called the "gustatory stimulation threshold." The requirement that a tastant be soluble in saliva is particularly true of sweet molecules (sugars and carbohydrates) or sour and some salty ions (salts and acids can release charged groups of atoms called "ions" when dissolved in saliva). Bitter substances seem to adhere to lipid sites on the papillae and thus could be thought of as fat-soluble molecules. Sweet, salty, and sour chemicals are generally hydrophilic (water loving), while bitter chemicals tend to be hydrophobic (water hating).

Whether hydrophilic or hydrophobic, tastant molecules or ions must get through the entry point on at least some taste cells located on taste buds. At the entry point of a taste cell, a tiny pore has small taste hair projections that are believed to be the true sensors of taste. The basic premise, according to current theory, is that the ions or molecules of the tastant substance enter the pore and then physically or chemically interact with specialized regions of the taste hairs. This interaction causes an action potential (a nerve impulse) to occur as the permeability of the nerve fibers innervating the taste cell is altered. An action potential will cause a wave of permeability changes all the way up the nerve fibers and into the brain. The medulla oblongata is the first site of the brain to receive the action potentials that will be registered as taste.

The exposure of taste cells to the environment renders them at risk to potential damage. Fortunately, new taste cells are regenerated every seven to ten days; regeneration occurs within the basal cells of the taste buds. It is important to note, however, that this is a regeneration of the taste cells, not of the nerves that innervate them. Olfactory nerves are the only nerve cells in the human body that can regenerate.

Disorders and Diseases

In the medical sciences, gustatory problems are generally not a cause but an effect. A diminished sense of taste (hypogeusia), an alteration of taste (dysgeusia), or the complete absence of the sense of taste (ageusia or apogeusia) is generally a symptom of an underlying pathology. It is rare for true ageusia to be an isolated symptom of a malady; it is even rarer for true ageusia to exist as an isolated physical malady. Yet ageusia does, in fact, exist in human populations. Among some descendants of the Ashkenazi Jews, for example, a double recessive genetic code mandates that taste papillae will not develop, resulting in congenital ageusia.

In discussing when or how noncongenital ageusia, hypogeusia, or dysgeusia can become a problem, it is important to review the critical components of a taste message. Three discrete structures are involved: taste cells, which contain taste hairs at their pores; nerve fibers, which connect the chemoreceptors (taste hairs) to the brain; and the brain itself. Alterations in the ability to taste can originate in any or all three of these discrete steps along the path.

Some pathologies that can cause a miscommunication at the receptor sites include actual physical or chemical damage to the taste buds, such as a burn that covers the tongue’s surface or the ingestion of lye; accidental or therapeutic exposure to radiation; lingual (tongue) or palatal (palate) carcinomas; tumors or lesions of the tongue surface; or a leukemic infiltrate of the tongue surface.

The ability to taste may be altered or lost if damage occurs to certain cranial nerves. Specifically, damage to any of these cranial nerves may be responsible for taste disorders: the facial nerve (the seventh cranial nerve), which carries sweet and salty messages from the front of the tongue; the glossopharyngeal nerve (the ninth cranial nerve), which carries bitter and sour signals; and the large vagus nerve (the tenth cranial nerve), which carries taste sensations from the throat and epiglottis. Damage to these nerves may occur if they are crushed, severed, or pinched or if tumors, lesions, or neural disease interferes with normal function.

Finally, a head injury or malady can account for ageusia, hypogeusia, or dysgeusia. Regions of particular concern include damage to the medulla oblongata, the thalamus, or the gustatory cortex, which is located in the parietal lobe of the brain. Lesions, tumors, or head injuries resulting from sudden or severe impact can give rise to true taste pathology. In these cases, the brain can either no longer identify tastes properly or no longer receive or interpret taste impulses.

Indirect impairment of taste may occur as a result of an imbalance in body chemistry. Such imbalances may result from exposure to or ingestion of trace metal poisons or other toxins, insufficient dietary intake to allow for cellular repair or development, incomplete intake of essential vitamins or minerals, or metabolic imbalances, such as hypothyroidism (an underactive thyroid gland). In addition, taste-modifying pathologies, whose origins are not directly associated at the chemoreceptor sites, can involve allergic or drug reactions.

Other taste disorders that can be clinically assessed include cacogeusia, the alteration of once-pleasant tastes to ones that are repulsive (for example, the perception that all foods taste rotten); phantogeusia, the presence of a taste sensation in the absence of any tastant; heterogeusia, a distortion of tastes for all foods (for example, sweets may taste salty, salts may taste bitter, and so on); and parageusia, an unusual taste distortion of one taste type that does not cause a repulsive taste response (for example, bitter foods may taste salty).

In diagnosing a taste disorder, the physician must first obtain a full medical history from the patient and perform a physical examination. Because of the anatomy involved in taste function, special attention will be given to the head and neck area. General laboratory analysis of kidney, liver, and endocrine function must be performed, as well as a complete blood study. Also, tests may be administered to determine the possible role of allergies in a given pathology.

The issue of taste disorder is often complicated by the common use of terms that have specific meaning in a clinical setting. Three of these troublesome terms are “taste,” “flavor,” and “palatability.” "Taste" means, quite literally, the chemoreceptor response of taste cells embedded in taste buds (located on the papillae), which is caused by tastant molecules. The interaction between the tastant and the chemoreceptors produces a nerve signal that travels from the taste receptor site to the brain. In stringent use of the term “taste,” other factors, such as aroma, texture, or color, should not be considered in the assessment of this sense.

"Flavor" generally means the response of the olfactory and gustatory systems, working in unison, to assess the pleasure or displeasure prompted by a tastant. Smell is fundamentally integrated with taste to the point that people generally salivate more when exposed to an appealing odor, especially if the aroma is associated with a particularly pleasing food, such as freshly baked bread. A common exercise used to demonstrate the close connection between the olfactory and gustatory systems is a blind study in which the subjects close their eyes, pinch their noses shut, and sample uniformly sized cubes of solid foods kept at room temperature. Under such circumstances, most humans cannot distinguish among raw potato, raw onion, white cheese, and peeled fresh apple. Taste-testers may be shocked to discover that raw onions seem much the same as raw apples without visual or aromatic cues. This exercise speaks strongly to the significance of how humans integrate all of their senses in gathering data from the environment.

A loss or a decrease in the ability to smell greatly alters one’s sense of flavor, even though there is no true loss of the ability to experience taste. A common example of this interrelationship between ability to smell foods or beverages and the connected ability to enjoy flavor has been experienced firsthand by anyone who has ever suffered from a severe head cold. When the nasal passages are coated with thick mucus, as generally occurs during a viral cold, eating is no longer pleasurable; foods are generally described as tasting flat or bland. The cold virus does not in any way interrupt the mechanics of the taste cells, nor does it disrupt the neurons associated with the taste cells. What is disrupted during a cold is the ability to smell and, therefore, enjoy the flavor of foods and beverages. Thus, a head cold alters the ability to experience fully all the sensory aspects of the foods or beverages that create a complete food sensation.

"Palatability" describes the association of taste with texture, temperature, and feeling. If a slice of bread is expected to be warm, soft, and sweet but what is ingested is cold, hard, and salty, then palatability is greatly diminished. If a person is truly hungry, however, then the lack of palatability, or even flavor, may be overruled by the greater need for nourishment. In the absence of the ability to see or to smell food (as in blindness or anosmia, respectively), texture and temperature take on heightened importance in the consumption of foods and beverages.

Taste disorders can be quantitatively assessed through various stimuli tests to measure the extent and type of taste disorder present. In addition, magnetic resonance imaging (MRI) and computed tomography (CT) scanning can be used to identify problems that may originate in the central nervous system. Positron emission tomography (PET) scanning can also be used to determine if brain lesions are responsible for a taste disorder. Treatments are as highly varied as the pathologies that cause taste disorders. Those disorders that arise from tumors may be treated by the surgical removal of the tumor. For certain metabolic imbalances, supplements rich in zinc ions may be administered. Some cases require simply restoring the patient to a healthy and balanced diet, while some rare disorders are untreatable.

Perspective and Prospects

Tasting is an inborn sense; it requires no training or skills. So-called acquired tastes are attained by adults mainly as a result of the declining population of taste cells, a natural aspect of the aging process. Because adults cannot sense food as fully as children, they may seek out heightened taste sensations, consuming salty foods such as caviar, drinking strong beverages such as whiskey, or enjoying spicy foods such as curry or hot peppers. Given their divergent taste responsiveness, it is reasonable to expect children to have natural aversions to certain foods, as compared to adults. A child is simply more aware of the mixed flavors of a given food, some of which may be bitter or sour relative to the way in which an adult senses the same food.

Like their primitive ancestors, modern humans let the tip of the tongue sample a new food before actually ingesting it. It is believed, therefore, that sweet receptors evolved to occupy the tip of the tongue to help humans seek out and consume safe foods in nature. Sweet foods, such as carbohydrate-rich vegetables, fruits, and (to some extent) proteins, are generally safe and nourishing. Therefore, humans tend to seek sweet flavors, especially in the infant stages, over salty, sour, or bitter ones. This instinctual drive may account for the powerful attraction many people have for sweet desserts and candies.

Bitterness is detected in the mouth nearer the esophagus. This location seems to prevent humans from naturally seeking bitter foods. Furthermore, it allows for the rejection of a bitter food before it enters the esophagus, where the food would be well on its way to digestion and absorption. This adaptation to the environment aids in human survival. Many naturally bitter substances are poisons or potential toxins. Included in the list of bitter taste sources are caffeine-containing tea leaves and coffee beans, cocaine, nicotine, almond bitters, and lye (sodium hydroxide). At the turn of the twenty-first century, scientists identified a new family of genes that encode proteins that function as bitter taste receptors. The discovery opens the way for the identification of additional receptors that detect bitter and sweet tastes and also gives researchers new probes with which to trace the wiring of the taste perception pathways into the brain itself.

In 2002, scientists reported the discovery of a new taste receptor that recognizes most of the twenty naturally occurring amino acids. They theorize that this receptor was evolutionary important because it helps humans select foods rich in these essential nutrients.

Researchers also continue to investigate the influence of genetics on food preferences, diminishment of tasting ability with age, regeneration of taste and smell receptors, relationship between the sweet taste receptors and glucose regulation, and better means of diagnosis and treatment for taste disorders.

Taste, or, more specifically, flavor, is a big business. Flavor houses—companies that use modern chemistry to craft synthetic additives to enhance or imbue flavor—generate billions of dollars in revenue every year from their work crafting flavor for food producers. According to an October 30, 2014, article from the Wall Street Journal, consulting firm Leffingwell & Associates reported that, in 2014, American flavor houses are expected to bring in $4 billion in revenue. This number is nearly twice the $2.5 billion in revenue generated in 2003. The favoring industry first began to grow during World War II, when the government worked to develop ways to make military rations less perishable and nutritious as well as better tasting. Over time, progress in the fields of chemistry and food production have led to a booming flavor industry. According to the Journal, one flavor house—Synergy Flavors in Illinois—has about 80,000 different flavoring formulas in their catalog.

Taste is a special sense that facilitates the ability of an organism to survive in or interact with an environment. More than this, however, taste provides the body with great sensory pleasure. Serving as both a tool to survive and as a pleasure-seeking sensor, taste is a unique and enriching sense.

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