A Matter of Taste

The tongue, the nose, and the brain all work together to notice flavors.

Have you ever tried the jelly bean test? It’s an experiment that proves the importance of the nose in the sense we call “taste.” Hold your nose closed and put a jelly bean or some other fruit-flavored candy in your mouth. Chew the candy. You’ll taste sweetness and maybe a little sourness but not much else. Then open your nose. Suddenly, you’ll get the full force of the fruit flavor. Chewing releases molecules, which are groups of atoms stuck together, in the candy. In the mouth, these molecules trigger basic tastes: sweet, salty, sour, and bitter. Odor molecules also float from the back of the mouth up into the nose.  

The Chemical Senses

The little bumps on your tongue are called papillae. They hide tiny taste buds. Inside the taste buds are even smaller structures called receptors. When a sweet food molecule reaches a sweet receptor, it fits into it like the last piece of a jigsaw puzzle. The same is true for a bitter food molecule—it has a different shape that matches a different receptor. When a molecule locks into a matching receptor, a chemical signal goes to the brain that says, “Sweet!” or “Bitter!”

With salty and sour tastes, the process is a little different; it’s not shape-based. Still, these taste molecules stimulate receptors on the tongue, causing a signal to travel to the brain. The signal reveals there’s something salty or sour in the mouth.

Meanwhile, in the nose, odor molecules bind to some of the many receptors there and send their own signals to the brain. These pathways to the brain are separate from the taste buds’ paths. When the brain combines the signals from the tongue and the nose, it figures out the food’s identity from its flavor. It might announce, “Strawberry!” or “Broccoli!” or “Dill pickle!”

What we think of as taste is really a combination of input from our taste buds and input from the aroma detectors in our nose. Scientists call taste and smell the chemical senses. The job of our taste buds and the odor receptors in our noses is to detect chemicals—in our food or in the environment.  

The End of the Tongue Map

Psychologist Linda Bartoshuk of the University of Florida has been studying taste and smell most of her life. She remembers, “When I was in college, my father developed lung cancer, and one of the things that really bothered him was that food didn’t taste right.” This can result from cancer treatments, but at the time she didn’t understand the cause. Bartoshuk thinks it may have been this experience that, in graduate school, drew her to study taste.  

At the time, a diagram appeared in many textbooks and articles showing that receptors for sweet taste are in the front of the tongue, bitter in the back, and salty and sour on the sides of the tongue. Bartoshuk knew this was wrong, as did many others. She wondered where the mistaken map had come from. It turns out that an American psychologist had misunderstood a table published by a German researcher decades earlier. The table showed sensitivities to sweet, salty, sour and bitter tastes relative to areas of the tongue. The American published his mistaken interpretation of the table in 1942, and the tongue map was born. It still shows up in some books and articles. In fact, taste buds on all parts of the tongue have receptors for all tastes. You can test this by dipping a Q-tip in saltwater or lemon juice or a sugar solution and touching the front, sides, and back of your tongue with it.

Bartoshuk once visited an elementary school class where the students had tested the tongue map they saw in their textbook. She asked if they’d gotten the same answers as the book. They admitted they hadn’t. The students thought they must have messed up. Bartoshuk assured them that the book was wrong and they were right. 

Beyond the Tongue

The four basic tastes are hardwired in humans. Bartoshuk says, “If you take a newborn baby and put a drop of sugar on its tongue the baby will look happy. The love of sweet is hardwired in the brain, present from birth.” Something bitter might make a baby cry, sour causes a baby’s mouth to pucker, and a mild salt taste is appealing to a baby. All these reactions make sense.

Sweet tastes, like those of ripe fruits, suggest a food that provides needed energy. Every calorie (a measure of food energy) was important to our ancient hunting and gathering ancestors. Bitter tastes, on the other hand, could indicate poison. Many plants produce bitter toxins to keep from being eaten. Sourness might mean a fruit is unripe and indigestible or even spoiled, or it could point out a dangerous acid. Salt is something we need (in moderation) for our muscles and nerves to work properly.

The nose and tongue are the first stops for the detection of chemicals in our food. But we actually have taste receptors throughout our digestive system. In addition to taste receptors for sweet, salty, sour, and bitter, our tongues also have receptors for umami, which is the savory flavor associated with cooked meat. The taste receptors for umami detect glutamate, an element naturally found in proteins. Protein molecules are too large to stimulate the taste or odor receptors in the mouth and nose. But in the stomach, digestion breaks them into their parts. Glutamate slips into the receptors there, and a signal goes to the brain that says, “Protein!” Our bodies need protein. We learn to associate the smell of glutamate-containing food with something good. The aroma molecules from roast chicken or grilled steak tell us we’re getting protein even before the glutamate receptors in the stomach know it.

Bartoshuk says, “It’s one thing to detect chemicals in the mouth. That’s very useful for taste. But there are a lot of other potential uses of chemical detectors in the body. Sometimes Nature just uses the same receptors.” Sweet receptors in the gut can tell the pancreas to release insulin, a hormone that keeps blood sugar levels healthy. Bitter receptors in the gut can tell the body not to absorb molecules that might be toxic.  

A Volatile Situation

In 2005 Bartoshuk accepted a position at the University of Florida. She says, “I started hanging out with colleagues in horticulture who wanted Florida fruits and vegetables to taste better.” One colleague wanted to make tomatoes taste better. She explains that the food industry has a standard way of approaching a problem like this. She says, “Let’s say you want to make a new chocolate cake mix. Well, you find out everything that goes into it, and you make every different chocolate cake possible with those ingredients.” Then volunteers taste the cake and report what they like. Comparing people’s likes with the various ingredients in the cakes they tasted, she says, helps you see how to put together the best chocolate cake ever.

She adds, “That’s exactly what we wanted to do with tomatoes. And we did it.” Tomatoes don’t have separate ingredients like cakes, but they do have a number of chemical components. To make a better tomato, growers may focus on breeding varieties with lots of the chemical components people like best. For this experiment, they measured the sugars, acids, and aroma molecules of 80 varieties of tomatoes. Volunteers tasted the tomatoes and rated flavor intensity, sweetness, and sourness, and how much they liked them. Researchers could see which chemical components made people like tomatoes more.  

Making Fruit More Appealing

Aroma molecules are known in taste research as volatiles. Those are the odor molecules that travel from the back of the mouth to the nose. When the brain puts taste signals and aroma signals together, they can affect each other. For example, some volatiles can make a sweet taste.

Bartoshuk says, “It turns out that fruits have many, many such compounds in them… Take a fruit like a strawberry. You bite into it and it tastes sweet, and you think you’re tasting sugar. To some extent you are, but the volatiles in the strawberry can also make that sugar message two to three times as intense. We know about 100 volatiles that will enhance sweet.”

Try the jelly bean experiment again, but this time bite into a strawberry or an orange or another sweet fruit with your nose closed. After you’ve chewed it, open your nose and see if the fruit tastes sweeter to you than it did with your nose closed. If it does, you’re sensing the sweet-enhancing volatiles.

Fruit growers have bred crops for larger yields—more apples or peaches per tree, for example. It takes energy for plants to make more fruit. That leaves less energy for increasing sugar content. However, it costs a plant little energy to produce volatile compounds that make a fruit seem sweeter. Bartoshuk and her colleagues found that some varieties of fruits have more sweet-enhancing volatiles than others. This is especially true of older varieties, ones that weren’t bred to have higher yields.

The scientists plan to cross varieties with lots of sweet-enhancing volatiles with varieties that produce a lot of fruit. A resulting peach might taste super sweet while being low in sugar. Bartoshuk hopes that what they’re doing will result in a wider variety of healthy and delicious choices in the grocery store.

Her enthusiasm for this project is obvious. “We have a whole new world to explore. We don’t know what the rules are yet,” she says. “The thing that gets me up in the morning to go to work is: how does this work?”

Spoken like a true scientist.