Craving chips and salty snacks one minute and sweetened Cap’n Crunch the next? Here’s the science behind the salivation.
Maybe it’s my blue-collar upbringing, but I’ve always got my eye out for the unsung heroes of the human body. The media clamors on and on about the witty brain and the pump-a-holic heart, and anything to do with our reproductive organs and sex is worth 800 words. Meanwhile, myriad organs, organelles, and structures—sphincters, ossicles, sarcoplasmic reticulae—go about their functions unheralded, the No Name Steaks of the human body. ¶ Taste buds are that way. We are daily dazzled, entertained, and mmmmm’d by taste, and scrumptious foods share magazine-cover duties with supermodels and celebrities. But if we pay so much attention to tasting, how come we know so little about the mechanisms of taste? What exactly is taste? I wanted to know, so—fork in hand—I set off to find out.
If you’re one of these people who’s in the habit of shuttling different foods to the different “taste zones” on your tongue, you can stop now. It makes you look like you’re swilling marbles, and it isn’t helping anyway. The notion that specific tastes are only detected in particular geographic areas of the tongue has been deveined, deboned, deglazed, and, finally, debunked.
And yet the tasting ability of the tongue is definitely geographic, with most of the taste buds at the back. The front two-thirds of the tongue carry a small number of tiny bumps called fungiform papillae, but each of these contain just a few taste buds. Toward the side and back of the tongue are a few linear ridges called foliate papillae, and these contain dozens to hundreds of taste buds. The true taste sensation is found at the very back of the tongue, where three large nodes called circumvallate papillae contain a whopping 2,400 individual taste buds.
But that’s not all, folks! There are another 2,400 taste buds sprinkled throughout the throat. Taste buds in your throat? That’s right. It’s one more reason to swallow carefully, and it’s also why I like to periodically lean back and gargle during a particularly flavorful meal. Wait staff who understand taste physiology aren’t bothered by this.
Taste buds themselves are an onion-shaped collection of taste-receptor cells. Instead of green shoots, tiny hairs called microvilli extend up into the pore of each taste bud to help interrogate whatever morsels fall in. There’s a medical condition called “hairy tongue,” but in this regard, all of our tongues are a little hairy. Chemoreceptors on these microvilli and on the taste-receptor cell itself perform on a lock-and-key basis: When the right chemical—say, sodium—binds to a salt receptor, an electrical impulse is generated that heads north to the brain.
Taste receptor cells live very short lives—10 to 14 days—which means that the life of a taste cell can be quite ephemeral and chancy. They could enjoy a week in Paris dining on prawns and Gruyère, or a week to the Boundary Waters living on granola and beans seasoned with DEET. The best evidence suggests that each individual taste-receptor cell is set up to detect just one of the five basic tastes. That’s right, five. Forget what you learned in junior-high health class. There’s sweet, sour, salt, bitter, and umami.
Yes, umami, from the Japanese character that means “delicious flavor.” Umami allows for the recognition of amino acids, the building blocks of proteins, and it’s the dominant taste of food containing high amounts of the amino acid glutamate: aging cheese, chicken broth, meat extracts and gravies. In humans, there are just two amino acids that evoke umami: aspartate and glutamate (as in monosodium glutamate, or MSG).
Although my umami receptors are fairly yeoman, I’ve got a sweet tooth the size of Dracula’s fangs. This sense of sweetness is thought to be important in detecting the caloric content of foods, and while we identify simple sugars (table sugar, fructose) and complex-sugars (carbohydrates) as tasting sweet, there’s a surprisingly wide variety of unsugary compounds that can trigger a sweet sensation. The best example is the ubiquitous NutraSweet, which combines the amino acids aspartate and phenylalanine to produce a substance that is 180 to 200 times sweeter than table sugar.
If our primal hankering for sweet flavors drives us to secure energy-rich foods like ripe fruit or Cap’n Crunch, then conversely, our ability to taste bitterness safeguards us from poisoning ourselves. Certain plants contain substances that interfere with the neurological system of animals and humans—caffeine, nicotine, and strychnine, for example—and we recognize these compounds as bitter.
The last two taste sensations—salty and sour—are generated by a taste bud’s encounters with single ions rather than larger chemical compounds. The sense of sour is triggered by acid, a hydrogen ion, which is why citric acid can make some fruits taste sour. Although having a sense for sourness is important for detecting unripe foods or spoiled foods, it may be more important in helping us maintain the acid level (pH) of the body, since any significant shift in pH is the metabolic equivalent of throwing gravel in the Cuisinart—biochemical processes come to a grinding halt.
As for salty flavors, there are all kinds of “salts” in the chemical kingdom, but the most important to human physiology is sodium. In fact, a sodium-level drop in the body stimulates secretion of a hormone called aldosterone, which then increases the sensitivity of the sodium receptors in our taste cells. The result? A craving that has us salting down the Cap’n Crunch.
As an ice-cream junkie, I’d like to nominate a sixth taste sensation: fat. We know that fat brings flavor to food and is satiating, which is why many low-fat processed foods have to be doped up with salt and sugar to keep them from tasting like the packaging they’re sold in. While a taste receptor for fat has not been identified, a few studies have shown that nerve centers in the brain light up in response to ingestion of fat. Of course, it could also be a texture issue: Silicone and mineral oil have been shown to produce the same response.
Understanding how taste works, however, doesn’t necessarily mean you can heighten your sense of taste (or lessen your fear of Brussels sprouts). Studies indicate that the ability to taste varies widely among individuals. As much as a 100-fold variation exists across a wide age range in the detection threshold for both sucrose and sodium. Another example: The chemical compound propylthiouracil is intensely bitter to some, but others find it nearly tasteless; the difference seems to be genetic. Which probably lends credence to what you’ve known all along: You’re not a food snob—you’re just a gifted taster.
Craig Bowron is a Twin Cities internist and a regular contributor to Minnesota Monthly.