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Tip of the Tongue: Humans May Taste at Least 6 Flavors

Milk

We cook, therefore we are. Over the millennia, humankind – hardly content to eat plants, animals and fungi raw – has created a smorgasbord of cuisines.

Yet for all our sophistication in the kitchen, the scientific understanding of how we taste food could still use some time in the oven. Dating back to ancient Greece and China, the sensation of taste has historically been described as a combination of a handful of distinct perceptions. Western food research, for example, has long been dominated by the four "basic tastes" of sweet, bitter, sour and salty.

In recent decades, however, molecular biology and other modern sciences have dashed this tidy paradigm. For example, Western science now recognizes the East's umami (savory) as a basic taste. But even the age-old concept of basic tastes is starting to crumble. 

"There is no accepted definition of a basic taste," said Michael Tordoff, a behavioral geneticist at the Monell Chemical Senses Center in Philadelphia. "The rules are changing as we speak."

Our ability to sense the five accepted categories comes from receptors on our taste buds. These tiny sensory organs appear mostly on the tongue, the roof of the mouth and in the back of the throat.

The sense of touch also plays a key role in experiencing taste, as evidenced by the strong opinions on crunchy versus smooth peanut butter. Smell, too, impacts our tasting abilities. Just ask anyone with a stuffed-up nose picking away at what seems to be a plate of bland food. [Supertaster vs. Nontaster]

In the mouth itself, though, food scientists continue to discover new receptors and new pathways for gustatory impressions to reach our brain. Here are some taste sensations vying for a place at the table as a sixth basic taste. 

1. Calcium

The element calcium is critical in our bodies for muscle contraction, cellular communication and bone growth. Being able to sense it in our chow, therefore, would seem like a handy tool for survival.

Mice seem to have it figured out, kind of. Recent research has revealed that the rodents' tongues have two taste receptors for calcium. One of those receptors has been found on the human tongue, though its role in directly tasting calcium is not yet settled, said Tordoff.

Calcium clearly has a taste, however, and counterintuitively most mice (and humans) don't like it. People have described it as sort of bitter and chalky – even at very low concentrations. Tordoff thinks our calcium taste might actually exist to avoid consuming too much of it.

An over-sensitivity to calcium-rich foods such as spinach could help explain why four out of five Americans don't get enough calcium. "There is a strong relation between people not liking vegetables and calcium," said Tordoff.

As for milk and other calcium-loaded dairy, the calcium in it binds to the fat, so we don't taste the mineral all that much, Tordoff noted.

2. Kokumi

That calcium receptor might also have something to do with an unrelated sixth-taste candidate called kokumi, which translates as "mouthfulness" and "heartiness." Kokumi has been promulgated by researchers from the same Japanese food company, Ajinomoto, who helped convince the taste world of the fifth basic taste, umami, a decade ago.

Ajinomoto scientists published a paper in early 2010 suggesting that certain compounds, including the amino acid L-histidine, glutathione in yeast extract and protamine in fish sperm, or milt – which, yes, they do eat in Japan, and elsewhere – interact with our tongue's calcium receptors.

The result: an enhancement of flavors already in the mouth, or perhaps a certain richness. Braised, aged or slow-cooked foods supposedly contain greater levels of kokumi.

If all that sounds a bit vague, it does to Western scientists also. Ajinomoto representatives have visited Tordoff's group "and given us foods they say are high in kokumi – but we have no idea what they're talking about," he said. "Kokumi may be something that the Western palette is not attuned to."

3. Piquance

Spicy-food lovers delight in that burn they feel on their tongues from peppers. Some Asian cultures consider this sensation a basic taste, known in English as piquance (from a French word). Historically, however, food scientists have not classified this undeniable oral sensation as a taste.

That's because certain piquant compounds, such as capsaicin from peppers, directly activate our tongue's touch, rather than taste-bud, receptors. The key piquancy receptor is called TRPV1, and it acts as a "molecular thermometer," said John E. Hayes, a professor of food science at Penn State.

Normally, nerves with this receptor send a signal of hotness to the brain when exposed to substances around 107.6 degrees Fahrenheit (42 degrees Celsius), the heat pain threshold for humans. Capsaicin fits into this the TRPV1 receptor and lowers the activation temperature to 95 degrees Fahrenheit (35 degrees Celsius) – cooler than body temperature.

Hence, "all of a sudden the receptor is sending signals to brain about 'oh, hot!'" said Hayes, though the food itself is not necessarily hot temperature-wise. These TRPV1 receptors appear all over the body, which is why exposed mucous membranes in the nose or the eyes also feel the burn of pepper spray, for example.

4. Coolness

At the opposite end of taste sensation from piquance's peppers is that minty and fresh sensation from peppermint or menthol. The same trick of sensory perception is at work here – activated touch receptors, called TPRM8 in this case, fool the brain into sensing coldness at normal oral temperatures, said Hayes.

As touch sensations, both piquance and coolness are transmitted to the brain via the trigeminal nerve, rather than the three classical nerves for taste. "The set of nerves that carry the burn and cooling sensation are different than from taste sensation," said Hayes. [10 Fun Brain Facts]

Still, there is an argument that temperature sensation, both in the genuine sense and in the confused-brain phenomenon of piquance and coolness, deserves to be in the pantheon of basic tastes. Interestingly, Germanic people dating back to 1500 had considered heat sensation as a taste, Hayes said, and the modern debate over temperature's status is far from over.

5. Metallicity

Yet another controversial "taste" is our registering of metals, such as gold and silver, in the oral cavity. Some Asian cultures place gold and silver leaf, as it's called, atop curry dishes and candies, while Europeans fancy a bit of these metallic foils on pastries. The silver foil garnish is known as "vark" when used on Indian sweets, as in the picture above.

Although usually tasteless, such garnishes are sometimes reported as having a distinctive flavor. Researchers have shown that this sensation might have something to do with electrical conductivity, in effect giving the tongue a little zap. "If you cut a copper penny in half, expose the zinc core and put it on the tongue, you get a whopping metallic taste," said

Harry Lawless, a professor emeritus of food science at Cornell University. "It's like a little battery, with a drop of saliva – you get about 550 millivolts."

Lab tests have failed to turn up a metallic-taste receptor, Lawless said, and it remains unclear if electrical conductivity or something more is going on for those shiny culinary embellishments. "We're leaving the door open," Lawless said.

6. Fat

The jury is still out on whether our tongues can taste fat, or just feel its creamy texture. Clearly, many of us enjoy fatty foods, from well-marbled steak to pretty much fried anything.

"Fat is a tremendous source of calories," said Linda Bartoshuk, a physiological psychologist at the University of Florida "Eating fat is encouraged by our brains to have us survive."

Mice can taste fat, research has shown, and it looks like humans can too, according to a 2010 study in the British Journal of Nutrition. The study revealed varying taste thresholds for fatty acids – the long chains that along with glycerol comprise fats, or lipids – in participants.

Intriguingly, the subjects with the higher sensitivities to fat ate fewer fatty menu items and were less likely to be overweight than those with low sensitivity.

Bartoshuk, who was not involved in the research, noted that fatty acids "tend to taste bitter in the mouth," and she thinks touch fibers in the taste buds sense the creamy thickness of non-broken-down fat globs instead. 

7. Carbon Dioxide

Yet another strong sixth taste candidate: carbon dioxide (CO2). When dissolved in liquids, this gas gives soda, beer, champagne and other carbonated beverages their zingy fizz. [Infographic: All About Champagne]

That familiar tingling was thought to result from bubbles bursting on the tongue, and had therefore been consigned to the touch category. "It's tricky because CO2 was always considered a trigeminal stimulus," said Tordoff.

Researchers presented a strong case for dedicated, taste bud-based carbon dioxide sensors in a Science paper in 2009. They found that an enzyme called carbonic anhydrase 4, which appears on sour taste-sensing cells, specifically detects carbon dioxide in mice.

Further evidence comes from a drug called acetazolamide, often taken by climbers to avoid altitude sickness. Acetazolamide blocks the activity of carbonic anhydrase 4. Upon reaching the summit and cracking a beer or popping a bottle of bubbly, climbers have reported that the beverages taste boringly flat.

Thus, for those celebrating this New Year's with a traditional glass of champagne, take delight in the range of tastes – whether official or not – that our tongues and brains affords us. 

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