The old "tongue map" from our elementary school textbooks has been roundly debunked. Experimental confirmation of "umami" expanded Westerners' traditional four basic tastes—sweet, sour, salty, and bitter—into five. But did you know those 5 basic tastes might actually be 6 . . . or 7, 8, or more?

Advances in the technology and techniques available to researchers have led to significant new discoveries in taste perception. Receptors have been discovered in the last few years for "tastes" long assumed to be entirely smell or texture dependent. What tastes have you been tasting your whole life without even knowing it?


Image credit: Unknown/National Cancer Institute. Public domain.

Your tongue isn't just for tasting. Like your skin, your tongue registers general senses like touch, pain, pressure, and temperature. It is these non-tastes that give food its texture and lend hot peppers their "burn".


Up until around the '90s, it had been assumed that fat was just another texture. Sensations like "greasy", "creamy," and "oily" would be registered by variations in touch and pressure, conveying a sense of how fatty the food was. Several studies backed that assumption up, showing that texture really was a big part of sensing fat in food.

But in 2005, researchers discovered long-chain fatty acid taste receptors on mouse tongues (and, in 2012, on human tongues too). Further research showed that fat-taste sensitivity varies from person to person . . . and that sensitivity to fat in food is correlated with fat intake . . . people who taste fat more intensely eat less of it, and have a lower BMI on average.


Image credit: Spiff/Wikimedia Commons. Public domain.

To fend off high-altitude sickness, mountain climbers can take drugs called carbonic anhydrase inhibitors. They work well, but have an disappointing side effect known as the "champagne blues" . . . when the successful climbing expedition reaches their summit and uncorks a celebratory bottle of champagne, the medicated climbers can no longer taste the fizzing bubbles in their bubbly.


If a bubbly drink didn't taste bubbly, then researchers figured perhaps the "fizzy" taste had nothing to do with the bubbles themselves. Which made it less surprising when scientists recently announced finding a taste receptor for carbonation.

Carbonic anhydrase 4 (CA4) is used by the body to help maintain pH. Researchers discovered that CA4 also appears on the the tongue, signaling our sour taste buds in the presence of dissolved CO 2. (Carbonation also acts on the non-taste senses in the mouth, so the "fizz" of soda water is likely a combination of taste and non-taste both.)

So why would humans (or other mammals) need to taste carbonation? The researchers suggest that "tasting" might be an accidental side effect, that the CA4 is really there for another reason (maintaining pH in the acid-sensing sour taste buds, for example). Or perhaps a surprising carbonated flavor helped warn our ancestors away from rotting fruit, just as fruit flies seek out the taste of CO2 in the air to lead them to rotting fruit.

(Trivia time: insects and other arthropods can also taste carbonation. Also, while nearly all arthropods taste with their mouthparts and feet, fruit flies can also taste carbonation with their wings.)


Photo credit: Adam Gillum/USDA. Public domain.

Calcium is the fifth most abundant element in the ocean, and it's also the fifth most abundant element in the human body. Our oceangoing fishy ancestors could get all the calcium they needed just by swimming . . . but when tetrapods finally escaped the ocean, they found themselves in a dangerously calcium-poor environment with no way to absorb it except through diet. And when you need to eat something to survive, being able to taste it is a big help.


It seems we can taste calcium, from bullfrogs to birds, and from rodents to humans. Scientists have known for decades that animals sought out "calcium licks" just like they do salt licks, and that some calcium-deprived animals would choose calcium over calories when forced to pick one or the other. But a calcium taste was only discovered in the last decade, first in bullfrogs (2007), then rats (2008), and now humans (2012).

One interesting feature of calcium: it only tastes good in low concentrations. Too much calcium tastes not as good, apparently to humans and other animals alike. (In fact, high calcium content may be somehow connected to some people's distaste for certain vegetables, like spinach, kale, and collard greens). Since calcium (like salt) is necessary in low doses but potentially dangerous in high doses, it makes sense that too much calcium (like too much salt) would taste repulsive.

Every one of these newly-discovered tastes was trumpeted in news reports as "the sixth taste". Yet the old sweet/sour/salty/bitter/umami paradigm remains. Why? Perhaps this quote (from the paper announcing the discovery of a calcium taste receptor) offers the best explanation (emphasis mine):

We are often asked whether calcium is a basic taste, akin to sweet, sour, salty, bitter and umami. Our demonstration of a receptor in the oral cavity fulfills a universally accepted criterion for a basic taste but there is little consensus about what other criteria must be met. For example, is a specific taste quality or a central representation required? The "basic taste" concept is under fire. It cannot deal well with observations that the tastes of complex carbohydrates are bland to humans but avidly preferred by many other species, that dozens of receptors all produce a unitary sensation of bitterness, or that there is apparently no region encoding sour taste in primary taste cortex. Without a better definition we cannot determine whether calcium is a basic taste.

I'm reminded of the early 2000s, when the discovery of many new "planets" beyond Neptune (like Haumea, Makemake and larger-than-Pluto Eris) suddenly threw the definition of "planet" into question. By 2006, "planet" had been redefined to accommodate the new discoveries.


With our expanded understanding of how our tongues and brains deal with chemical perception, perhaps "taste" is due for a similar clarification. We might be looking at a different set of tastes in our future. Umami, sweet, and salty seem safest . . . perhaps bitter or sour will end up being a taste-Pluto.