Children learn about taste in grade school -- out of the five senses, it seems like one of the simplest. There are no cones, rods or lenses. There are no tympanic membranes or miniscule bones. Yet scientists know less about taste than they know about sight and hearing -- senses that are far more complex. Why is something seemingly so rudimentary so complicated and controversial? Why is taste so mysterious?
To start with, most people confuse taste with flavor. Taste is a chemical sense perceived by specialized receptor cells that make up taste buds. Flavor is a fusion of multiple senses. To perceive flavor, the brain interprets not only gustatory (taste) stimuli, but also olfactory (smell) stimuli and tactile and thermal sensations. With spicy food, the brain will even factor in pain as one aspect of flavor.
Testing sensation is also a subjective science -- taste perhaps more subjective that most. Some people have inherited genetic traits that make certain foods taste disgusting. Others, called supertasters, have abnormally high concentrations of taste receptors. To their heightened palates, bland food tastes perfectly flavorful. And, as we all know, food tastes differently to different people -- we don't all like the same flavors.
In recent years, scientists have expanded the definition of taste, allowing one, and possibly two, primary tastes into the original canon of four -- sour, bitter, sweet and salty. They've challenged the tongue map, the biology-class staple that charts distinct regions of taste. Food scientists have even tampered with taste receptor cells, blocking or stimulating them in an effort to cut sweeteners and salt out of food without sacrificing flavor.
In this article we'll learn about the physiology and psychology of taste.
Sensation to Perception
Taste begins with sensation in the form of electrical impulses. Sensations, however -- responses to stimuli like pressure, light or chemical composition -- become perceptions like touch, vision or taste only when they reach the brain.
Different stimuli activate different sensory receptors. Chemical stimuli activate the chemoreceptors responsible for gustatory and olfactory perceptions. Because taste and smell are both reactions to the chemical makeup of solutions, the two senses are closely related. If you've ever had a cold during Thanksgiving dinner, you know that all of the subtlety of taste is lost without smell.
In some species, however, the two chemical senses are practically one. Invertebrates like worms do not have distinctions between gustatory and olfactory receptors. They instead differentiate between volatile and nonvolatile chemicals.
In humans, the chemoreceptors that detect taste are called gustatory receptor cells. About 50 receptor cells, plus basal and supporting cells, make up one taste bud. Taste buds themselves are contained in goblet-shaped papillae -- the small bumps that dot your tongue. Some papillae help create friction between the tongue and food.
Every gustatory receptor cell has a spindly protrusion called a gustatory hair. This taste hair reaches the outside environment through an opening called a taste pore. Molecules mix with saliva, enter the taste pore and interact with the gustatory hairs. This stimulates the sensation of taste.
Once a stimulus activates the gustatory impulse, receptor cells synapse with neurons and pass on electrical impulses to the gustatory area of the cerebral cortex. The brain interprets the sensations as taste.
In the next section, we'll learn about the primary tastes and how taste gives us clues about what we eat.
The Primary Tastes
Until recently, scientists have accepted four basic tastes. You know them well -- sweet, salty, sour and bitter. They are the building blocks of flavor and at the root of other tastes. Each primary taste triggers a particular gustatory receptor (although receptors can, and frequently do, respond to multiple tastes). The basic tastes went unchallenged for years, perhaps because of their familiarity -- name another taste that is as distinctive as one of the four.
In the early 1900s, however, a Japanese scientist sought to detect another taste -- that of the savory seaweed common in Japanese cooking. Kikunae Ikeda eventually isolated glutamic acid as a distinct fifth taste -- one with its very own gustatory receptor. Ikeda named this fifth taste umami, a Japanese word meaning delicious, savory taste. You can taste umami in meats and tomatoes.
Researchers continued to study umami throughout the 20th century. An important breakthrough came in 1985 when scientists trying to mimic the controversial, flavor-enhancing substance MSG failed to replicate the taste with any combination of the basic four.
But because Ikeda's study on taste was not translated into English until 2002 and because the taste of glutamic acid is subtle and less common in Western food, umami has only recently entered the taste canon. Now that the gate is open, however, it's unlikely that scientists will ever be so secure in the limits of primary taste. French researchers even identified a potential gustatory receptor for fat. Fat could actually be the sixth taste.
The primary tastes gave early humans clues about what food was good to eat and what was harmful. Sweet foods usually had calories. Salty foods had important vitamins and minerals. Sour foods could be healthy, like oranges, or spoiled, like rotten milk. Bitter tastes were often poisonous. The enhanced flavor of processed food could signify nutritional value that isn't actually there, but our preferences have remained. We still crave and respond to our ancestral favorites, even to our detriment.
So if there are at least five primary tastes, what's up with the tongue map? In the next section we'll learn about the biology-book mainstay and why it might be completely wrong.
The Tongue and Regions of Taste
Just as scientists are reexamining the basic tastes, they are also redefining the tongue map. The tongue map breaks the tongue down into regions of sensation -- bitter in the back, sour on the sides, salty on the front edge and sweet at the tip. Umami researchers have claimed that the tongue's posterior is important for detecting the fifth taste.
But for everyone who remembers arguing the tongue map as a grade-schooler, insisting they could perceive salt at the back of the tongue or sour at the tip, news that the tongue map is flawed at best must come as sweet vindication.
A German scientist named D.P. Hanig developed the tongue map in 1901 by asking volunteers where they could perceive sensation. Other scientists later corroborated his findings but charted the results in such a way that areas of lowered sensitivity looked like areas of no sensitivity. By 1974, Virginia Collings determined that while the tongue did have varying degrees of sensitivity -- some areas could perceive certain tastes better than others -- there was no real truth to the strict tongue map. Although taste receptors usually react strongly to a single taste, many respond to multiple gustatory stimulations. People can perceive taste anywhere there are taste receptors.
Scientists are also learning more about the shocking diversity of taste sensitivity. In the next section we'll learn about an acute sense that you actually might be glad not to have.
Usually, it's great to have heightened senses like 20/20 vision or sharp hearing. But a heightened sense of taste, no matter how delicious it might sound, is really no joy. Supertasters are people with two or sometimes just one dominant allele for the gene TAS2R28. And although they can perceive more nuanced flavor in food than nontasters, they often find common foods too bitter, sweet or spicy.
In the 1930s, a scientist at DuPont discovered that people had varying degrees of sensitivity to the chemical PTC (phenylthiocarbamide). For some, PTC tasted shockingly bitter, but for the mystified minority, PTC had no taste at all. Due to concerns about PTC's safety, scientists began studying people's reactions to PROP (6-n-propylthiouracil), a synthetic compound used in thyroid medicine. For nontasters, PROP had no flavor; for tasters, it was unpleasant and for supertasters, PROP slapped the tongue with an intense bitterness.
In 1991, Linda Bartoshuk, then of Yale Medical School, coined the name "supertasters" for the people with acute PROP sensitivity and noticed that they had a denser covering of fungiform papillae than nontasters. She linked the number of taste receptor cells to supertaste.
For supertasters, coffee, hoppy beer and vegetables like Brussels sprouts might be too bitter; cake and ice cream might be too rich and chili peppers might be too hot. There are, however, a few advantages of super taste-sensitivity.
Beverly Tepper, a scientist at Rutgers University, discovered that, at least among women in their 40s, supertasters were 20 percent thinner than nontasters. With their heightened sensitivity to sugar and dairy fats, supertasters are less likely to crave junky foods. They actually eat less food overall -- but, unfortunately, they also skimp on leafy vegetables. Tepper saw no correlation between tasting and weight in men [source: Flaherty].
With such stunning links developing between taste and body mass, scientists are eager to study taste receptors as a possible factor in obesity. Yet just as flavor is more than taste, taste is more than a genetic impulse. People's food preferences and eating habits are largely based on what they grew up on and even what their mothers ate while pregnant.
To learn more about taste, cells and other related topics, sample the links on the next page.
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More Great Links
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