The discovery of a new taste cell deepens our understanding of this complex and adaptive sensory system.
Years ago, my father took a huge slurping bite of what he thought was a fat navel orange and immediately gagged. Spitting it out, he later recalled, it was the most poisonous piece of fruit he’d ever tasted—or so he thought. What’s wrong with this orange? he asked his companion. It’s actually a grapefruit, she said. And with that, the horrible taste of the rotten fruit turned delicious in his mouth, almost like the flick of a switch.
For some time after I heard this story, I wondered whether it would it be possible to do the same thing deliberately. If the brain can be fooled into shaping our experience of taste by accident, could you also trick it on purpose—by taking a pill or painting a few drops of solution on your tongue before a meal? Can science turn the experience of eating the worst Soylent Green into one of the best crème brûlées ever?
This anecdote is germane to a group of researchers at the University of Buffalo in upstate New York. They recently discovered a new type of taste cell—a discovery that has somewhat upended what we thought we knew about the human taste system.
A taste of evolution
The taste system helps accomplish one of life’s oldest needs: analyzing chemicals on the tongue and detecting flavors that indicate whether something is food or foul. Even the most ancient prokaryotic organisms to emerge from the primordial ooze on the tangled banks of wherever needed to find food, and the prevailing idea in biology today is that the modern mammalian taste system evolved to serve that most fundamental of needs.
We are adept at detecting certain flavors of things in our environment that contain the nutrients we need and at detecting more abhorrent flavors of things that would make us sick. We detect sweet things quite well because sugars are a key chemical by-product of carbohydrates, which we typically consume as a basic food in our diet. We need sodium and chloride to maintain cardiovascular homeostasis, so we detect salt quite well.
And in genomic tribute to our forgotten, foraging past, humans are able to taste bitter flavors with finely honed accuracy. The human genome has no fewer than 25 different subtypes of Tas2r genes to detect bitter tastes—less than frogs, which have 50 such genes, or mice, which have 35, but far more than chickens, which have only three bitter taste receptors.
“The taste system is just absolutely required for all organisms to be able to consume the nutrients that they need and avoid the things that would make them sick,” says Kathryn Medler, a biologist at the University of Buffalo in New York who led the team that discovered the new taste cell.
As a fundamental tool for survival today, taste seems almost unnecessary for the modern human—at least anywhere one can drive past a thousand restaurants to a 24-hour grocery store bursting with food choices. We did not evolve with such abundance—but rather in a wild, dangerously fluctuating world full of frequent famine and strange, bitter things that may be toxic.
As a result, says Paul Breslin, a nutritional scientist at Rutgers University and researcher at the Monell Chemical Senses Center in Philadelphia, “we’re a food chemistry analytical machine.”
Cats, cows, koalas, and dolphins
Consider the cat. Cats are carnivores, and they have no need in their diet for carbohydrates or sweet things, except perhaps the occasional non-critter bon-bon. This isn’t merely a question of preference—it’s evolution. Cats actually don’t even have sugar receptor genes.
“They don’t even taste it—they’re taste-blind to sweet,” says University of Miami biologist Steve Roper, another expert in the field who published a review on the current state of knowledge on taste buds a few years ago. If a cat licks something sweet, Roper added, it’s because they detect the fat in that food and not the sugar.
Cats do have a highly developed sense of savory taste—dubbed umami in Japanese (delicious savory taste)—and this makes sense because they are consuming protein all the time. What would it be like to eat meat with a cat’s tongue? It’s impossible for science to say for certain, but one can imagine for a sensory system specifically tuned to taste savory flesh, it must be delicious.
On the other hand, you would never want to eat a salad with a cow’s tongue because cows eat grass all day—something they would never be able to do if they were actually able to taste it the way we do. Unlike humans, with our dozens of bitter receptors, cows are highly insensitive to bitter foods. They still detect it, but apparently not so well.
“In some respects, the taste system is like a mirror of [our] environment,” says Maik Behrens, a biologist at the Leibniz Institute for Systems Biology in Freising, Germany. He has studied the taste systems of multiple animals, publishing a paper a few years ago that explored the ability of chickens and frogs to taste bitter flavors.
Lots of creatures in the animal kingdom have reached the point over time where they have more or less one thing in their diet. Koalas are known to mostly eat eucalyptus, pandas are primarily bamboo eaters, and sea lions are bona fide pescatarians. According to experts, you see the same trend in lots of animals like these: a refinement of their taste systems to suit their niche.
“That’s why we taste bitter better than bottlenose dolphins,” which actually have no taste receptors at all, Behrens says. “Apparently they simply swallow their fish without tasting a lot.”
If you want to enjoy some sushi, don’t try it with a dolphin’s tongue
That ape is alive and well within us
If an animal’s sense of taste is informed by what’s to eat in the environment in which they live, how they find that food in the first place matters. The more an animal forages, the more receptors it will need to detect potentially dangerous bitters.
“If you don’t know what you’re going to eat,” says Breslin, “you have to engage in this process of evaluation.”
Foodies often tout the learned aspect of taste, a sense of refinement-through-experience. But fundamentally, our taste preferences are hard coded from the beginning. There is evidence that fetuses taste foods in utero, and newborn infants will coo at sweet and pucker at sour flavors placed in their mouths.
“We love sugar for the same reason the chimpanzees do—that other apes do,” Breslin says. “Just like the gorillas, chimpanzees, bonobos, and orangutans, which are all largely fruit eaters, we were meant to be sugar eaters.”
“That ape is alive and well within us,” he added.
Reached for comment on the discovery of the new type of taste cell, Medler explained how it did not behave the way we expect taste cells to behave.
Decades ago, there was thought to be a single type of taste cell that responded to all tastes. But over the years, that concept was slowly refined as we discovered different taste cells. Scientists then classified taste cells into different types, but the sense was still that a single cell would detect a single taste.
“That’s how it has stood for many years,” says Roper, the biologist from the University of Miami.
But what Medler and her colleagues recently discovered was a subset of these cells that were actually broadly responsive to four of the five basic taste qualities and seem to be just as important for the perception of those tastes.
“It’s a very elegant study,” says Julie Mennella, a biologist at Monell Chemical Senses Center who studies taste also but was not involved in Medler’s research. The study involved characterizing isolated batches of broadly receptive taste cells for their electrical signaling as well as extensive behavioral experiments in which mice placed in special cages with different bottles of solution were trained to seek out certain bottles that offered tastes they like. Medler’s team showed that when mice lose their broadly responsive taste cells, the animals basically behave like they can’t taste bitter, sweet, and umami.
The quest for new tastes
Not everyone agrees with the current breakdown of taste into five key flavors, and some scientists have proposed that additional tastes could exist, and some of these potential new tastes have been identified.
“There are quite a number of additional candidates out there,” says Purdue University nutritional scientist Richard Mattes, who coined the word oleogustus (fatty taste) to describe one of these proposed new sensory flavors. Calcium, carbon dioxide, starch, and short-chain carbohydrates also have all been variously proposed as possible new tastes. “But they are by no means accepted broadly in the scientific community,” Mattes says.
The evidence for fat as a taste is overwhelming, Mattes says. There is at least one receptor found on human taste cells, called CD36, that can detect fat. There is also a mechanism for it to transduce signals to the brain, and these signals seem to be tuned specifically for fat. There are also sensory experiments where people can sensitively detect fat on their tongues.
“It’s not just sour, salty, bitter, or umami—people can, under the right testing conditions, say, ‘Oh—no! This is not like any of those others,” Mattes says. “In my mind, it’s a pretty strong case.”
The evolutionary argument is that humans have the ability to detect a specific type of fat known as free fatty acids, which are present in rancid food. The food industry has been aware of this for decades and goes to great lengths to eliminate free fatty acids from food. The reason why olive oil is cold pressed, for instance, is to reduce the temperature-dependent oxidation of the oil, which produces these free fatty acids and fouls the taste of the oil.
The last time a new flavor was added to the list was 20 years ago, when umami was added after the discovery of its receptor—but that was almost a century after Japanese chemist Kikunae Ikeda first proposed it in 1909. The discovery of the new type of taste cell by Medler and her colleagues is not quite as profound as Ikeda’s, but experts say it’s not all about new tastes. Identifying these new cells is an important step in our efforts to understand the basic features of human taste because it reveals new details, like how taste cells communicate with each other—and ultimately how our sense of taste works.
“The hope is that this study is going to help us understand better how taste signals are sent back to the brain,” Medler says.
COVID, chemo, shingles, and spit
Ultimately, in human physiology, it always seems to come back to the brain, and that’s certainly the case with taste. The mind exerts a huge influence over how we perceive foods because it combines taste cues with signals from our other senses and integrates them with memories and other things.
For that reason lots of the tricks you can do with taste have to do more with how taste signals are processed than with flavors themselves. If you hold your nose, close your eyes, and eat a jellybean, you won’t be able to tell if it’s strawberry, lemon, or grape because you won’t have the right olfactory input. Color, which can have nothing at all to do with taste, is nevertheless a strong driver of flavor perception. When people taste white wine that’s been dyed red but otherwise left unaltered, they will describe its taste using more robust adjectives as if it were a red.
The experience my father had with the grapefruit/orange was another example of this—and a dramatic example of an abrupt change in flavor brought about by new information.
But taste is not all in your mind. It’s highly physically adaptive as well. The proteins in saliva have been seen to change according to the diet of rats, and when rats are fed a diet of bitter foods, their saliva upregulates proteins that start interacting with bitter compounds, blocking them from interacting with their bitter taste receptors. The same thing is seen in people. Giving almond milk to people will, over time, change their saliva profile, and they come to crave it more.
Other things can also physically affect taste. Shingles can impact the gustatory nerves that connect taste cells to the brain and cause people to lose their sense of taste entirely. A common side effect of COVID-19 is the loss of smell and taste, and a group at Harvard showed earlier this summer that this was probably due to coronavirus genes interfering with the human olfactory mucosa.
Aging can lead to the loss of taste because as people age, their cell turnover process may become impaired. The result can be profound. Many older people who experience taste cell degradation suffer from loss of appetite.
“It’s a big problem with old people becoming malnourished,” Medler says. “When the taste system is out of whack, it can really affect our consumption.”
Head injuries and radiation therapy can also interfere with the taste system and change the taste of foods—as can ear surgeries and tooth extractions. And the loss or distortion of taste is often cited as an unwanted side effect of chemotherapy and other medicines, which can impact both the quality of life as well as treatment outcomes.
“The number one reason for non-compliance for a medicine is because of how that medicine tastes,” Mennella says.
Hacking your tongue
So the million-dollar question in the end is: If the human sense of taste can be abruptly changed through infection or injury—or modulated over time by things like almond milk—can it also be hacked?
There are lots of potential health applications for such a hack: restoring temporary taste loss, formulating oral drugs so they don’t taste so bad, altering the chemistry of food to make it more palatable, reducing salt content without affecting taste, and sweetening foods with lower amounts of sugar and calories. Then there is the purely culinary question of changing taste for artistic or hedonic effect. Could we find ways to modulate flavors and enhance the experience of eating by hacking the tongue or the food? Could a chef tinker with all new options to delight the senses and direct a dining experience?
“Dr. Medler’s work does not directly lead to an ‘A-ha!’ moment that way,” Roper says. The work is significant, and it gives us more information, he says, but we still have a long way to go to understand the taste system. Asked whether it will be possible to deliberately, practically alter the taste of a food in this way, the experts seemed skeptical it will happen any time soon.
“Will people on the ship on its way to Mars licking a tube of some toothpaste-looking stuff and then eating a Soylent say, ‘Oooh, this tenderloin is extremely tasty?” Breslin asked. “That’s going to be a really, really hard thing to do.”
Roper echoed that sentiment in equally whimsical terms by comparing our understanding of the taste system to the ability to hear but a few notes of a musical composition—it won’t be until we hear whole chords that we’ll be able to say whether it’s Beethoven or Stravinsky, he says.
Or for that matter whether it’s an orange or grapefruit.