Taste

Definition

Taste is one of the five senses (the others being smell, touch, vision, and hearing) through which all animals interpret the world around them. Specifically, taste is the sense for determining the flavor of food and other substances.

Description

One of the two chemical senses (the other being smell), taste is stimulated through the contact of certain chemicals in substances with clusters of taste bud cells found primarily on the tongue. However, taste is a complex sensing mechanism that is also influenced by the smell and texture of substances. An individual's unique sense of taste is partially inherited, but factors such as culture and familiarity can help determine why one person's favorite food made be hot and spicy while another cannot get enough chocolate.

The primary organ for tasting is the mouth. Clusters of cells called taste buds (because under the microscope they look similar to plant buds) cover the tongue and are also found to a lesser extent on the cheek, throat, and the roof of the mouth. First discovered in the 19th century by German scientists Georg Meissner and Rudolf Wagner, taste buds lie on the bumps and grooves of the tongue (called the papillae) and have hairlike extensions (microvilli) to increase the receptor surface of the cells. Four different pairs of nerves are involved in the tongue, which helps explain in part why the sense of taste is a robust one, and not easily knocked out by disease or trauma.

Genetic and other factors affecting taste

Scientists have also discovered that genetic makeup partially accounts for individual tasting abilities and preferences for specific foods. According to Yale University researchers, some people are genetically programmed to have more taste buds and, as a result, taste more flavors in a particular food. (The number of taste buds varies in different animal species. For example cows have 25,000 taste buds, rabbits 17,000, and adult people approximately 10,000.) In general, a person's ability to taste can lie anywhere in a spectrum from poor to exceptional, with the ability to sense tastes increasing in proportion to the number of taste buds present. The difference in the number of taste buds can be extreme. Researchers have found anywhere from 11 to 1,100 taste buds per square inch in various young people tested. They have also found that women tend to have more taste buds than men and, as a result, are often better tasters. How well people taste greatly affects what they like. Studies at Yale, for example, revealed that children with fewer taste buds who are classified as poor tasters liked cheese more often than exceptional tasters, who experienced a more bitter sensation, probably because of increased sensitivity to the combination of calcium and the milk protein casein found in cheese.

Despite the important role that taste buds play in recognizing flavors, they do not work alone in providing the experience of taste. For example, the amount of naturally occurring salt in saliva varies; with the result that those with less saliva can better taste the saltiness of certain foods than others, who may end up adding salt to get a similar flavor. The smell and texture of foods are also important contributing factors to how people perceive a food to taste and whether or not they like it. Food in the mouth produces an odor that reaches the nose through the nasopharynx (the opening that links the mouth and the nose). Since smell is much more sensitive to odors than taste is to flavors, people often first experience the flavor of a food by its odor. The texture and temperature of food also influences how it tastes. For example, many people would not think of drinking cold coffee, while others will not eat pears because of a dislike for the fruit's gritty texture.

The predilection for certain foods and tastes is not determined merely by biology. Culture and familiarity with foods greatly influence taste preferences. The Japanese have long considered raw fish, or sushi, to be a savory delicacy. Until the 1990s, few Americans would have enjoyed such a repast. As the number of Japanese restaurants grew along with the sushi bars they often contained, so did Americans' familiarity with this delicacy, resulting in a new taste for it.

Function

Taste's primary function is to react to items placed in the mouth. For most foods and substances, saliva breaks down the chemical components which travel through the pores in the papillae to reach the taste buds. These taste buds specialize primarily in processing one of the four major taste groups: sweet, sour, salty, and bitter. Because the four taste groups may not describe all taste sensations, other proposed tastes include metallic, astringent, and umami. Umami is the oral sensation stimulated by monosodium glutamate.

Taste occurs when specific proteins in the food bind to receptors on the taste buds. These receptors, in turn, send messages to the brain's cerebral cortex, which interprets the flavor. The actual chemical processes involved for each major taste group vary and involve various mechanisms. For example, salty and sour flavors occur when saliva breaks down sodium or acids, respectively. The chemical constituents of foods that give bitter and sweet tastes are much more difficult to specify due to the large number of chemical components involved.

Although certain taste buds seemed to have an affinity for one of the four major flavors, continued research into this intricate biological process has revealed a complex neural and chemical network that precludes simple black and white explanations. For example, each taste bud actually has receptors for sweet, sour, salty, and bitter sensations, indicating that taste buds are sensitive to a complex flavor spectrum similar to the way vision is sensitive to a broad color spectrum grouped into the four major colors of red, orange, yellow, and green. Particular proteins of taste are also under study, like gustducin, which may set off the plethora of chemical reactions that causes something to taste bitter and sweet.

Taste buds for all four taste groups can be found throughout the mouth. A common but mistaken tongue diagram shows areas labeled with basic tastes, such as sweet at the tip of the tongue while bitter is at the back. While specific kinds of buds tend to cluster together, the four tastes can be perceived on any part of the tongue and to a lesser extent on the roof of the mouth. Bitterness does appear to be perceived primarily on the back of the tongue because of several mechanisms.

Role in human health

Taste helps people determine whether potential foods are palatable. It also plays a major role in appetite. People constantly regenerate new taste buds every three to 10 days to replace the ones worn out by scalding soup, frozen yogurt, and the like. As people grow older, their taste buds lose their fine tuning because they are replaced at a slower rate. As a result, middle-aged and older people require more of a substance to produce the same sensations of sweetness or spiciness, for example, than would be needed by a child eating the same food.

Common diseases and disorders

The inability to taste is so intricately linked with smell that it is often difficult to tell whether the problem lies in tasting or smelling. An estimated two to four million people in the United States suffer from some sort of taste or smell disorder. The inability to taste or smell not only robs an individual of certain sensory pleasures, it can also be dangerous. Without smell or taste, for example, people cannot determine whether food is spoiled, making them vulnerable to food poisoning. Also, some psychiatrists believe that the lack of taste and smell can have a profoundly negative affect on a person's quality of life, leading to depression or other psychological problems.

The reasons for taste and smell disorders range from biological breakdown to the effects of environmental toxins; but a clear precipitating event or underlying pathology is often lacking in taste disorders. Here are some of the more common ones:

• Cold and flu are the most common physical ailments that can assault the sense of taste and smell. Allergies and viral or bacterial infections can produce swollen mucous membranes, which diminish the ability to taste. Most of these problems are temporary and treatable.
• Medications, including those used in chemotherapy for cancer treatments, can also inhibit certain enzymes, affect the body's metabolism, and interfere with the neural network and receptors needed to taste and smell.
• Neurological disorders due to brain injury or diseases like Parkinson's or Alzheimer's can cause more permanent damage to the intricate neural network that processes the sense of taste and smell.
• Twenty to 30% of head trauma patients suffer some degree of smell disorder, which can in turn affect taste.
• Exposure to environmental toxins like lead, mercury, insecticides, and solvents can also severely hinder the ability to smell and taste by causing damage to taste buds and sensory cells in the nose or brain.
• Aging itself is associated with diminished taste and smell sensitivity.

Resources

BOOKS

Beauchamp, Gary, and Linda Bartoshuk. Tasting and Smelling: Handbook of Perception and Cognition, 2nd ed. San Diego: Academic Press 1997.

Goldstein, E. Bruce. Blackwell Handbook of Perception. Malden, MA: Blackwell Publishers Ltd, 2001.

Macbeth, Helen. Food Preference and Taste: Continuity and Change. Oxford, England: Berghahn Books 1997.

Nagel, Rob. "The Special Senses." In Body By Design: From the Digestive System to the Skeleton. Edited by Betz Des Chenes. Farmington Hills, MI: Gale Group, 2000.

PERIODICALS

Smith, David V., and Robert F. Margolskee. "Making Sense of Taste." Scientific American, (3 March 2001) 〈http://www.sciam.com/2001/0301issue/0301smith.html〉.

OTHER

BiblioAlerts.com. "NeuroScience-in-Review: The Sense of Taste." Paid subscription service for reports in science and technology. 〈http://preview.biblioalerts.com/info/com.biblioalerts_biblioalerts_CRE000312.html〉.

Kimball's Biology Pages. "The Sense of Taste." 〈http://www.ultranet.com/∼jkimball/BiologyPages/T/Taste.html〉.

MEDLINE plus. "Health Information." 〈http://www.nlm.nih.gov/medplus〉.