Sweet, salty, sour and bitter — every schoolchild knows these are the building blocks of taste. Our delight in every scrumptious bonbon, every sizzling hot dog, derives in part from the tongue’s ability to recognize and signal just four types of taste.
But are there really just four? Over the last decade, research challenging the notion has been piling up. Today, savory, also called umami, is widely recognized as a basic taste, the fifth. And now other candidates, perhaps as many as 10 or 20, are jockeying for entry into this exclusive club.
“What started off as a challenge to the pantheon of basic tastes has now opened up, so that the whole question is whether taste is even limited to a very small number of primaries,” said Richard D. Mattes, a professor of nutrition science at Purdue University.
Taste plays an intrinsic role as a chemical-sensing system for helping us find what is nutritious (stimulatory) and as a defense against what is poison (aversive). When we put food in our mouths, chemicals slip over taste buds planted into the tongue and palate. As they respond, we are thrilled or repulsed by what we’re eating.
But the body’s reaction may not always be a conscious one. In the late 1980s, in a windowless laboratory at Brooklyn College, the psychologist Anthony Sclafani was investigating the attractive power of sweets. His lab rats loved Polycose, a maltodextrin powder, even preferring it to sugar.
That was puzzling for two reasons: Maltodextrin is rarely found in plants that rats might feed on naturally, and when human subjects tried it, the stuff had no obvious taste.
More than a decade later, a team of exercise scientists discovered that maltodextrin improved athletic performance — even when the tasteless additive was swished around in the mouth and spit back out. Our tongues report nothing; our brains, it seems, sense the incoming energy.
“Maybe people have a taste for Polycose,” Dr. Sclafani said. “They just don’t recognize it consciously, which is quite an intriguing possibility.”
Dr. Sclafani and others are finding evidence that taste receptors on the tongue are also present throughout the intestine, perhaps serving as a kind of unconscious guide to our behavior. These receptors influence the release of hormones that help regulate food intake, and may offer new targets for diabetes treatments, Dr. Sclafani said.
Many tastes are consciously recognized, however, and they are distinguished by having dedicated sets of receptor cells. Fifteen years ago, molecular biologists began figuring out which of these cells in the mouth elicit bitter and sweet tastes.
By “knocking out” the genes that encode for sweet receptors, they produced mice that appeared less likely to lap from sweet-tasting bottles. Eventually, the putative receptors for salty and sour also were identified.
In 2002, though, as taste receptors were identified, the evidence largely confirmed the existence of one that scientist had been arguing about for years: savory.
Umami is subtle, but it is generally described as the rich, meaty taste associated with chicken broth, cured meats, fish, cheeses, mushrooms, cooked tomatoes and seaweed. Some experts believe it may have evolved as an imperfect surrogate for detecting protein.
Since then, researchers have proposed new receptor cells on the tongue for detecting calcium, water and carbonation. The growing list of putative tastes now includes soapiness, lysine, electric, alkaline, hydroxide and metallic.
“The taste field has been absolutely revolutionized,” said Michael Tordoff, a biologist at the Monell Chemical Senses Center. “We’ve made more progress in the last 15 years than in the previous 100.”
One candidate for the next basic taste appears to have emerged as the front-runner: fattiness. The idea has been around for a while, and many scientists thought it was not a specific taste, more like a texture or an aroma.
But researchers recently identified two taste receptors for unsaturated fats on the tongue. And fat evokes a physiological response, Dr. Mattes has found that blood levels of fat rise when we put dietary fat in our mouths, even without swallowing or digesting it.
Hours after a meal, the taste of fatty acids alone can elevate triglyceride levels, even when the nose is plugged. But fat, like umami, does not have a clear, perceptible sensation, and it is hard to distinguish a texture from a taste.
Dr. Mattes says that fat may have a texture that we like (rich and gooey) and a taste that we don’t (rancid).
If so, the taste may serve as part of our sensory alert system. When food spoils, he notes, it often contains high levels of fatty acids, and the taste of them may be “a warning signal.”
Although there is still no consensus beyond sweet, salty, sour, bitter and savory, the research makes clear there is more to taste than a handful of discrete sensations on the tongue. Before long, scientists may have to give up altogether on the idea that there are just a few basic tastes.
“If you’re talking three, four, five, six, you can still call it a pretty exclusive club,” Dr. Mattes said. “If you start getting beyond that, is the concept really useful?”