Johns Hopkins Magazine - September 1996 Issue

A Primer on Taste

By Elise Hancock
How come food doesn't taste good when I have a cold?

Because most of what we call "taste" is in fact smell, triggered by odor molecules from our food and drink. Some molecules we smell in the air, from the plate or as the fork approaches; others vaporize as we chew, then rise into the nasal passages at the back of the mouth.

Tastebuds alone can detect only sweet, sour, salty, and bitter. "If you lick a pink ice cream cone," says Donald Leopold, an otolaryngologist at Hopkins's Bayview Medical Center, "your tongue tells you it's cold and sweet and smooth, but your sense of smell tells you it's strawberry. Probably 80 percent of what you eat, you appreciate through your sense of smell." That's why if you have a cold, you could mistake a bite of onion for apple.

If smell is so crucial to taste, why do we have tastebuds?

Authorities agree that tastebuds have high survival value. They screen our food for important molecules that do not vaporize-- salt, for instance--and therefore cannot be smelled.

Hungry Forefather: "Hmm. Is this good to eat?" He nibbles. "Hey, it's sweet!" Kneeling, he crams berries into his mouth. Sweetness marks glucose, a good source of energy.

Ten feet away, Foremother makes a face and spits. The root she just sampled tastes bitter, a characteristic of many poisons. She moves over to the berries.

Humans and other animals are overwhelmingly attracted to sweetness, and it is the taste that degrades least in old age. We also crave salt, a universal bodily need; wars have been fought over salt. As for sour, the taste of high acidity, Kenneth Johnson, scientific director of Hopkins's Krieger Mind/Brain Institute, suggests it's a marker for hydrogen ions.

Some neuroscientists would add a fifth basic taste: delicious, a translation of the Japanese word "umami." Delicious is defined by the taste of MSG, a flavor-enhancer that was first isolated from a seaweed the Japanese had been adding to food for centuries. And what is MSG? monosodium glutamate, an amino acid. As a rule, the taste "delicious" signals amino acids, building blocks of protein, therefore good for you.

And there you have it: As with sex, crafty Nature has put the pleasure principle to work. In general, what tastes good will keep the species going. What does not, may not.

Isn't there a flavor "hot"?

No. The hot, spicy flavor produced by chili peppers is caused by chemical irritation or burning on pain receptors. It's a sensation of pain, one that's endured (often enjoyed) by people of many different cultures. After salt, chili is the most popular seasoning in the world.

So tell me about tastebuds. Where are they, and how do they work?

Go to a brightly lit mirror and take a good look at your tongue, something you may not have done since fourth grade. You'll see bumps scattered on the surface, small round ones at the front and sides, larger ones in the back. These are technically called papillae, from the Latin word for bumps.

Each bump has from one to several hundred tastebuds (visible only with a microscope), and each tastebud has 50 to 150 taste receptor cells (taste cells). In all, most people have 2,000 to 5,000 tastebuds.

So, in comes a forkful of food. Gnash, gnash, chop, chop, work the tongue, pause to savor. Specialized receptors all over the mouth take note of texture, temperature, and nippy spices, while each taste cell reacts to the nearest molecule of food. It reports its verdict--sweet, sour, bitter, or salty--by firing an action potential, launching an electrical signal that eventually reaches the brain.

As the map below shows, receptors near the tip of the tongue are especially sensitive to sweetness. Salty and sour tend to be sensed on the sides of the tongue, and bitterness at the rear. But of course sweet can be tasted somewhat at the back of the mouth, bitter at the front, and so on.

Nor are the taste cells themselves rigidly specialized. Experiments show that while each has a taste it consistently prefers, 9/10ths report on two or more of the basic tastes. Taste cells also vary in their threshold; a cell might prove impervious to sucrose, but respond to the much sweeter saccharine.

These variations help the nervous system sort flavors out with some precision. Tea, for instance, causes rapid firing from receptors for both acidity and bitterness, plus sweetness signals depending how much sugar you put in.

The tongue's message to the brain, then, is more like a symphony than a single tune-up A. The action potential from each taste cell--let's be conservative and say some particular mouth has 300,000 of them--zings up the nerve axons into the brain stem, the thalamus, and finally the cortex. The cortex also gets reports about temperature and texture and delivers the conscious experience of the bite: Strawberry ice cream, but let's not buy this brand again. It's too sweet.

What role does the palate play in taste?

For most of us, not much, says Leopold--though it's true that not all taste receptors are on the tongue. The throat has some, as does the palate, which wine-tasters can exploit. "Appreciating the wine in a different place does something to the way people perceive it," he says. "But you have to be pretty good to get it."

How can I enhance my sense of taste?

Eat slowly, and work the food around. The longer you chew, the more taste cells get exposed; and as food becomes more liquid, additional molecules vaporize, rising into the olfactory zone. Taste will intensify and become more complex, giving you the full "taste chord" that the cook intended.

Also, maximize variety. Don't eat all the potatoes, then all the meat. Instead, sample a little of this, a little of that. Even a one-pot meal will offer many different tastes, textures, and combinations.

The biological explanation is that taste and smell, like all senses, react strongly to change and surprise, but stop responding to stimuli that stay the same. ("Another piece of potato? All right already! I got it.") That's why wine-tasters wait 15 to 45 seconds between sips, a strategy that also works with food.

What happens to taste as people age?

Unlike smell, taste holds up well, according to research by James Wieffenbach, an NIH researcher who has collaborated with Leopold in studying patients at Hopkins's Gerontology Research Center. "Taste is amazingly robust with age," says Wieffenbach. Studying 170 subjects over 10 years, he found only insignificant losses in their ability to detect sour, bitter, or salty tastes, and no loss in tasting sweetness. For weak and diluted flavors, oldsters actually prove more sensitive than the young.

That's not to say age has no price. Even though the mouth itself is generally protected, small patches of taste cells on the tongue may quit, and a lifetime of colds and ear infections can damage the nerve axons for taste as they pass through the middle ear. Yet people seldom notice, Wieffenbach says, because the system compensates. Other taste cells step up their activity, and the brain generalizes from the all-over food sensations of touch, smell, and taste. "The sensory system doesn't label things," he says. "We just appreciate the objects in our mouth."

Complaints about taste do rise in old age, nevertheless. The well-known decline in the sense of smell seems to be the major cause, but Wieffenbach cautions doctors against just assuming that's the problem, especially when the complaint is not loss of taste but bad tastes. Nose drops or mouth infections may be the culprits--and fixable. Furthermore, even healthy elderly people may be taking medications. Some medicines can diffuse through the walls of blood vessels and stimulate the bottom of the taste cell. "Ugh!"

In Leopold's experience, those few patients who've lost their sense of taste suffer much more than the many with loss of olfaction. "If you lose your sense of smell," he says, "pizza may taste like salty cardboard, but you get used to it. People adjust." Not so those who lose their sense of taste. Even though taste contributes very little to flavor, those who lose it "are much more anxious and concerned, and they don't get over it."

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