Taste and flavour: tickling the taste buds
Senses of smell and taste must have been vital to the survival of early humans trying to distinguish food from poison, so it is astonishing how poorly understood these senses are.
Most of us probably grew up with the old concept of 4 types of taste receptors on our tongues - salt, bitter sweet and sour, while the discovery in 1909 of the umami (or savoury) receptor was largely ignored till around 1980s (probably because the original paper was published in Japanese).
Finally, in 2000, the first taste receptors for bitter compounds were identified, then sweet, umami, sour (sour taste cells also host the Car4 receptor that senses carbon dioxide in fizzy drinks, discovered in 2009) and finally salt as recently as 2010.
Researchers continue to look for more too. Recent discoveries include taste receptors in our trachea, intestines and even in testicles - researchers speculate that this may be to do with linking food status to reproductive health.
Taste is detected by the bumps or papillae that speckle the tongue. There are 4 types: the filiform papillae simply detect texture, whereas the other 3 - fungiform, foliate and circumvallate - contain onion-shaped taste buds. Each taste bud is packed with taste cells, with sensors for the five basic tastes.
Receptors to all 5 tastes all over tongue
The once common belief that different taste buds were mapped to different places on our tongue is plain wrong: every part of the tongue is sensitive to all five tastes.
What makes a good taster?
This leads on to exploring differing taste thresholds in tasters, which is important to help us understand why our preferences for wine vary so much. It's fairly widely known that tasting sensitivity is divided roughly into super-tasters, normal tasters and non-tasters.
Super-tasters are notably more sensitive to bitterness, tested using a compound called 6-n-propylthiouracil (PROP), which tastes intensely bitter to super-tasters, mildly bitter to normal tasters and flavourless to the rest. About 25% of us are believed to be super-tasters and the trait is more common in women, while a further 25% are non-tasters.
It was also believed that this sensitivity to bitterness was linked to the number of taste papillae on your tongue, but recent research in Denver, Colorado tested 3,000 people and found no link at all between number of papillae and perception of bitterness with PROP. Another long-held belief busted!
With wine tasting - especially with levels of phenolic bitterness (tannins, polyphenols and many more complex compounds) common in wine, it is really useful to know where you fit on the scale. Similarly perceptions of sweetness, sourness and umami (savoury) characters also vary.
The 'feel' of wine in the mouth
Taste is also about physical tactile sensations. 'Mouth-feel' is a term that sums this up nicely as it literally means how the wine feels when you taste it. Sensations you might have noticed include whether the wine gives a drying sensation - or astringency, and whether it seems 'fat' and rounded.
Taste is also commonly confused with physical (or somatosensory sensations) such as the cool of menthol or the heat of chili peppers.
How does the ability to smell affect taste?
Even less well known is how many of us are anosmic or physically unable to smell certain aromas simply because we don't have the right receptors in our noses. We have something like 400 different olfactory receptors but the exact combination varies widely.
Another piece of recent research suggests that the old myth that humans could only detect around 10,000 different odours is also wrong. Scientists at Rockefeller University in USA now reckon their research proves we can distinguish around 1 trillion different odours.
Total loss of smell is rare but researchers now think that all of us have aroma 'blind spots'. Even for aromas as common in our diet as vanilla, up to 3% of people can't smell it at all, while a massive 30% can't smell androstenone, which is a key component in the attraction of truffles.
Our sense of smell is so critical because much of what we perceive as taste is in fact odorants detected in our noses, and it is the combination of taste and smell that makes up flavour.
How does the brain turn detection into perception?
Scientists are now working on trying to understand how smell and taste are perceived in the brain. Techniques like gas chromatography can identify molecules responsible for odour and taste, but not what happens when these compounds hit the nose - in other words how the brain turns detection into perception.
Research groups are using genetically engineered mice which produce fluorescent proteins when a neuron (nerve cell) is activated by a particular odour to produce a 'barcode' of the exact pattern of receptors activated.
So far results have been published for musk and eugenol (best known from cloves but also found in oak-aged red wine).
Other experiments have used functional magnetic resonance imaging to see where neurons fired in the brain in response to different tastes being dropped onto the tongue.
So far four distinct and clearly separate 'hotspots' of nerve cells have been identified in the cerebral cortex - covering sweet, bitter, salt and umami, but a sour hotspot, if one exists, remains elusive.
A link between smell, taste & emotions?
Scientists would also like to understand the link between smell, taste and emotional or behavioural responses - such as the lifelong aversion to oysters that can be triggered by just one bad experience, or what makes a food or drink attractive.
This may be useful to stimulate appetite in the elderly as both taste and smell acuity are known to decline as we age.
Another area of much research is the close link between memory and smell. The speculation is that this may be because the olfactory bulb in our noses seems to have direct links to the amygdala and the hippocampus parts of the brain, which are closely associated with emotion and memory respectively.
Clearly, there is still much more to discover about how our senses of smell and taste really work, but in the meantime, we can all enjoy a bit of practical research by pouring a nice glass of wine! Cheers