Burning sensation: Licking the mystery of taste and pain


Remember the Tongue Map? There it sat, a disembodied organ on the pages of high school science textbooks, neatly divided into tiny states labeled "sweet" or "sour" or "salt" or "bitter." It was just bizarre enough to stick in the mind and give most Americans an idea of how taste works.

Scientists in the last few years have unleashed their test tubes and number-crunching computers on the complex phenomena of taste and flavor, which is taste plus aroma plus the tactile sensations produced by eating food.

One investigator is Dr. Barry Green, a sensory psychologist at the Monell Chemical Senses Center in Philadelphia. While probing the interaction of stimulus and response during eating, Dr. Green has concluded that an important, highly pleasurable and largely neglected part of what we call "flavor" is actually pain.

Until recent years, it was commonly believed that humans detected individual tastes through the taste buds, those little bumps on the front of the tongue. Close, but wrong in these three respects:

* First, those little bumps are not taste buds but taste papillae, in which the taste buds are housed.

* Second, while they are most prevalent along the front of the tongue, these papillae are found throughout the oral cavity ("You can actually taste with your upper palate," Dr. Green says).

* And finally, up to three-quarters of the sensory receptors in these papillae are not taste buds. They are nerve fibers that sense touch, temperature -- and pain.

More than just neighbors, the taste buds and the nerve fibers are actually quite literally intertwined, says Dr. Linda Bartoshuk, a professor at Yale University School of Medicine who is a leading expert on taste perception.

"There is this odd business about the pain fibers in the tongue," Dr. Bartoshuk says, "which is that they are actually wrapped right around the taste buds. The physical relationship between them could not be closer."

Understanding the geography of pain and taste sensors in the mouth sheds new light on some of the more puzzling characteristics of spicy foods, like chili peppers, horseradish, black peppers and mustard. For example, it helps clarify why we experience many of these substances as being hot even when they are physically cold.

"When you are exposed to a chemical irritant like capsaicin or mustard oil, it doesn't stimulate all the pain fibers, just a subset of them," Dr. Green says. "But since that particular subset also includes sensors that respond to painfully high temperatures, you perceive the pain as heat."

In the often-macho world of hot-food lovers, the burning question is not so much "Why do I think it's hot?" as "How come he can eat hotter food than I can?" At least part of the answer lies in a unique phenomenon called transient desensitization, which Dr. Green has explored.

As aficionados know, the burn of chili pepper and its fiery cohorts continues to increase as food is eaten. But chilies also have a curious ability to relieve pain, and capsaicin has even been used as a topical analgesic. Dr. Green has found that to activate this pain-numbing desensitization, the chilies have to be left alone.

"As long as you keep eating chilies, their effect keeps building," he explains. "But take a break -- even as little as two to five minutes, depending on the individual -- and when you go back to them, you are somewhat desensitized."

Because desensitization lasts for hours, he says, "the person who eats a lot of hot food on a regular basis is probably going around at least partly desensitized all the time."

Lurking just over the horizon in all these experiments, though, is this central question: Why would anyone other than a confirmed masochist not only willingly eat, but also aggressively seek out, food that causes pain?

Dr. Green is not the first academician to address this phenomenon. In a now-famous study done in 1980, Paul Rozin of the University of Pennsylvania coined the phrase "constrained risk" to describe the body's roller-coaster ride on the symptoms of physical danger that chilies elicit. Mr. Rozin also hypothesized that as in long-distance running, eating chili peppers may release endorphins, the body's own euphoric, painkilling drug.

While saying he admires these theories, Dr. Green also offers a down-to-earth answer that addresses not just chilies, but all painful foods. "The easiest explanation for why people like pain from their food," he said, "is simply that it adds a whole new dimension to flavor. "People put the wonderful subtlety of French food on a pedestal," Dr. Green adds, "but maybe subtlety is not the be-all and end-all." Comparing French food to a symphony and Mexican or Thai cuisines to jazz, he suggests that there is something for every mood.

"Sometimes you want to be excited by complexity," he says. "A symphony might be more beautiful, but good jazz or rock and roll is more fun."

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