Taste, or gustation, is one of the two senses triggered by chemical stimuli (the other is olfaction). A person has approximately 10,000 taste buds. Most are on the tongue, but some are located in the back of the throat.
Grouped together in bumps or papillae on the surface of the tongue, the taste buds contain receptors that respond to four basic tastes: sweet, salty, sour, and bitter. (It has also been proposed that monosodium glutamate (MSG) produces a fifth taste, called “umami,” that enhances other tastes.) Each receptor responds most strongly to one or two of the four basic tastes and slightly to the others.
The receptors that are sensitive to bitter substances are located on the back of the tongue. Beginning at the tip of the tongue and progressing to the rear on each side are overlapping receptors for sweet, salty, and sour tastes.
Although the number of basic tastes registered by human taste receptors is extremely limited when compared with the hundreds of odors that can be identified by olfactory receptors, the taste buds work together to send a unique pattern of impulses to the brain for each substance tasted. As any gourmet or wine taster will attest, a wide range of patterns can be created by mixing and blending the four primary tastes in different combinations.
As food is chewed, its chemicals act as the stimuli for taste, breaking down into molecules, mixing with saliva, and infiltrating the areas that contain the receptors. Activation of the taste buds triggers nerve impulses that travel to the brain and are there transformed into sensations of taste.
Because of their relatively “toxic” environment, taste buds live short lives, being replaced about every ten days. The sense of smell often works in conjunction with our sense of taste by combining sensations to achieve the perception of flavor. In fact, the olfactory sense actually contributes more to the perception of specific flavors than does the sense of taste.
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This phenomenon is commonly demonstrated in people whose sense of taste becomes dulled by colds. It has also been investigated in laboratory research, including tests in which subjects detected little taste in such strong substances as peppermint, onions, and cinnamon when their noses were congested.
When a person eats, chemical stimuli taken in through chewing and swallowing pass through an opening in the palate at the back of the mouth and move toward receptor cells located at the top of the nasal cavity, where they are converted to olfactory nerve impulses that travel to the brain, just as the impulses from olfactory stimuli taken in through the nose. The olfactory and gustatory pathways are known to converge in various parts of the brain, although it is not known exactly how the two systems work together.
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Another way to regard the relationship between taste and smell is as two component parts of a perceptual function identified as the “flavor system,” which also includes temperature and tactile receptors. Warm foods seem tastier because warming releases additional aromas from the mouth to the olfactory receptors.
Warm foods also seem sweeter, although temperature has no effect on the perception of salty foods. A food’s tactile properties (how it feels in one’s mouth) influence perception of its flavor, hence distinctions such as that between smooth and crunchy peanut butter.
Pain receptors are even included among the mouth’s nerve endings involved in flavor perception, and may account for some of the appeal of hot and spicy foods. A person’s nutritional state can influence perceived tastes, as well as the desire for particular foods: salt deficiency and food deprivation increase the desire for salty foods.
The sweet properties of saccharin and aspartame were discovered by accident in laboratory settings, and researchers are now actively working on developing new artificial sweeteners to allow consumption of sweet foods that are low in calories.