Understanding flavour: Who will win the race to be the sixth basic taste?

Our ability to perceive flavours is based on a complex mix of stimuli, including receptors on taste buds found on the tongue, roof of the mouth, and back of the throat, in addition to our perception of touch, smell receptors, colour, and even indications of temperature.

This complex mixture of signals can have dramatic implications for the way we believe our food tastes - and can have important implications on the eating and buying behaviours of consumers.

But what about our basic tastes? There are just five of those, right?​

Basic tastes, often refer to the signals detected by taste receptors located on the tongue, mouth, and pharynx. However, according to Michael Tordoff, a behavioural geneticist at the Monell Chemical Senses Center, USA: “There is no accepted definition of a basic taste."​  

In humans, five taste qualities have so far been identified: sweet, salty, sour, bitter, and umami.

Yet as new technologies in genetics and molecular biology develop, researchers have begun to propose even more additions to these five basic tastes. Indeed, it has been suggested that there are more than ten individual 'basic' tastes. And vying for acceptance as the sixth, or more, basic taste are 'tastes' including calcium, kokumi, piquance, coolness, metallicity, fat, and carbon dioxide.

The taste race​

Researchers are continuing to discover new receptors and new pathways for our taste perception at a rapid rate - below are a few of the tastes that may be vying for a spot as the sixth, seventh, or eighth taste.

1. Piquance​ - or spiciness - is a well known taste sensation, that in some Asian cultures is considered to be a basic taste. Historically, however, food scientists have not classified the hot burning sensation as a taste because the taste is triggered by touch rather than taste receptors. The key piquancy receptor (TRPV1) acts like a molecular thermometer, sending a signal of hotness to the brain when exposed to piquant substances such as capsaicin - just as it does when exposed to a thermally hot substance.

2. Coolness.​ In direct opposition to the sensation from spicy foods is that of the cool, fresh, sensation from peppermint or menthol. Again, this relies on touch rather than taste receptors - known as TPRM8 - to give a sensation of coldness at normal oral temperatures.

There is an argument that temperature sensation - both in the genuine sense and in the receptor trickery of piquance and coolness, deserve to be part of the basic taste groupings.

3. Metallicity.​ This is a taste occurring when metals like gold or silver are added to foods - something that is a custom in some Asian and European cultures. Although usually tasteless, such garnishes are sometimes reported as having a distinctive flavour - however no metallic-taste receptor has been found, and some speculate that an electrical conductivity influence or other mechanism may be at work.

4. Carbon Dioxide​ is a very strong candidate for the sixth taste. The gas gives liquids a zing and fizz when dissolved. This tingling was long thought to result from bubbles bursting on the tongue, and had therefore been consigned to the touch category, however in 2009 Science​ paper presented a strong case for dedicated, taste bud-based carbon dioxide sensors. The mouse model found that an enzyme called carbonic anhydrase 4, which appears on sour taste-sensing cells, specifically detects carbon dioxide.

5. Calcium​ is an element that is vital for our bodies- being a key player in muscle contraction, cellular communication and bone growth. 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.

6. Fat​ perception has  been thought to be a result of our mouth's ability to sense its creamy texture. However, it has also been suggested to have its own distinctive taste. Researchers have shown that mice can taste fat, and a 2010 study suggested that humans can too - though at varying degrees of sensitivity.

7. Kokumi.​ Translated as 'mouthfulness' and 'heartiness', Kokumi has been suggested to be the sixth basic taste by the same researchers at Japanese firm Ajinomoto, who helped convince the taste world of the fifth basic taste, umami, a decade ago.

The team suggest that certain compounds, including the amino acid L-histidine, glutathione in yeast extract and protamine in fish sperm, or milt – interact with calcium receptors on our tongue. The result is an enhancement of flavours already in the mouth, they suggest.