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Published:  23 July, 2008

The recent recognition of umami as the fifth taste has stimulated further research into the range and sensitivity of the human palate. So how exactly are people's wine tasting capabilities determined by the complex interaction of tongue, nose and brain? Jamie Goode takes a trip through the human tasting zone

Linda Bartoshuk, Yale University professor and respected authority on the science of taste, is giving a lecture. Midway through, she interrupts her presentation and begins to hand out strips of blotting paper that have been soaked in a solution of propylthiouracil (a thyroid medication, known more simply as PROP). The audience is surprised: science lectures aren't usually this interactive. Each person is told to place the paper on their tongue, and the result is surprising. A quarter taste nothing at all. Of the others, most find the paper tastes quite bitter, and a sizeable minority experience an intense bitterness that is extremely unpleasant. What Bartoshuk is illustrating is the now well documented individual variation in the ability to taste bitter compounds. Her research, building on an accidental discovery in the 1930s, has shown that people can be separated into three different groups according to their ability to taste PROP: 25% of the population are PROP non-tasters, 50% are medium tasters and the remaining 25% are supertasters. The latter are exquisitely sensitive to PROP and certain other bitter compounds. This sensitivity is thought to be genetic, although no gene has yet been implicated. Anatomically, Bartoshuk has shown that supertasters have an extremely high density of taste papillae - the structures that house the taste buds - on the tongue, with non-tasters having relatively few. This means that individuals in each group are living in different taste worlds' to the others. Although the main difference between these populations relates to bitter-tasting ability, the taste differences also extend - albeit less dramatically - to other flavour sensations. According to Bartoshuk, Supertasters perceive all tastes as more intense than do medium tasters and non-tasters'. As you would imagine, these research findings could have significant implications for the way we approach wine. Bartoshuk certainly thinks so: It is important for winemakers to test their wines on all three groups,' she says. It would be interesting to see if we could find systematic differences in preferences for specific wine types across the three populations.'

The three degrees of separation If we are going to interpret how these results relate to wine tasting, we'll need a grasp of the basic science involved. What we commonly think of as taste' or flavour' is actually a complex mix of three different sensory inputs: taste, smell and touch. Strictly speaking, the sense of taste involves just the inputs from specialised taste buds on the tongue. We can perceive just five different tastes: sweet, salty, bitter, sour and umami'. The latter is a Japanese term that translates loosely as meaty' or savoury', and refers to the taste of amino acids (the chemical building blocks of proteins) such as glutamate. The receptors for these different tastes are spread more or less evenly across the tongue. This may come as a surprise to those familiar with the tongue map beloved of school biology texts, which shows sweet, salty, bitter and sour flavours to be localised to different regions. Another of Bartoshuk's contributions to the taste field has been to expose this map as one of the scientific world's most enduring myths. It's based on German research from the early 20th century, which showed very small differences in sensitivity to the different tastes around the perimeter of the tongue. An influential mistranslation of this study in the 1940s assumed that where sensitivity to the different tastes was at a minimum, it was absent altogether. The result? A diagram showing that bitter, salty, sweet and sour are detected in different regions, which despite being wrong is still being widely taught to students of wine. Educators please take note! Taste provides us with relatively little information compared with the sense of smell - known as olfaction' in the trade. Whereas there are just five basic tastes, we can discriminate among many thousands of volatile compounds, or odorants'. Indeed, much of the character and interest in wine stems from the complex odours detected by the olfactory system: our taste buds alone give limited detail. So how does olfaction work? Our olfactory epithelium, located in the top of our nasal cavity, contains olfactory receptor cells, each of which expresses just one type of olfactory receptor. Each of these receptors - and there are hundreds of them in humans - is tuned to recognise the particular molecular structure of different odorants. It's not clear how we can discriminate among thousands of different odours with only a few hundred different receptors, but it appears likely that there is some sort of combinatory processing going on. So where does the sense of touch kick in? The brain uses touch to localise flavours perceptually. When you put a piece of steak in your mouth, the input from both the taste buds and the olfactory epithelium is combined in the brain in such a way that you think this information is coming from where you can feel the steak to be in your mouth. Likewise, take a swig of wine and the taste sensation appears to come from the whole mouth, not just where the taste buds are located. Bartoshuk emphasises that it is important to distinguish between retronasal' and orthonasal' olfaction. Orthonasal olfaction refers to what we typically call smell. When we sniff an odour, it moves through the nostrils into the nasal cavity, where it is detected by the olfactory receptors. In contrast, retronasal olfaction occurs when we chew and swallow food, or slurp a wine. Odours are forced behind the palate and into the nasal cavity by a back-door route. We think that the two forms of input are even analysed in different parts of the brain,' says Bartoshuk. We have evidence that taste plays an important role in telling the brain that the odour is coming from the mouth and should be treated as a flavour.' She has found that in patients with taste damage, flavours (including the assimilation of information from the nasal receptors, even if they aren't damaged) are often diminished. Work carried out in her laboratory involving the anaesthesia of taste shows that the intensity of taste also plays a role in the intensity of retronasal olfaction. If this is so,' says Bartoshuk, then supertasters - with their more intense taste sensations - may also experience more intense retronasal olfaction.' Taste and smell therefore overlap. We've seen that there is good evidence that people can be separated into different groups according to their ability to taste. Given that olfaction is a key element of wine tasting, how much do individuals differ in their ability to smell? This question is harder to answer because of the increased complexity of olfaction, but the answer seems to be a qualified Yes', although to a much-reduced degree than is true for taste. This is where we need to appreciate the significant role of the brain in processing the information detected by our senses. So far, we have been looking only at the way the tongue and olfactory epithelium detect the chemical environment to which they are exposed. For us to use this information, the brain has to interpret it and isolate the useful bits from the midst of all the noise, a function known as higher-order' processing. It's a complex field of psychology, and one where experiments that provide firm answers are rare. For the purposes of this article, it's sufficient to note that the brain does quite a lot to the information that it receives from the tongue and nose. The role of learning is key here, as we take particular notice of what we have learnt to be relevant information and ignore what we think is unimportant. An example of the latter is habituation'. Repeated or constant exposure to an odour reduces people's ability to detect it. Dr Charles Wysocki, an expert in olfaction at the Monell Chemical Senses Center in Philadelphia, thinks that this could relate to the performance of professional wine tasters. If individuals are constantly exposed over a lengthy session, they become less sensitive to odorants that repeat themselves, such as oak.' It is clear that people differ in their sensitivity to different odours. According to Wysocki, If a large enough sample of people is tested - say 20 - the range in sensitivity to a single odorant can be 10,000-fold on a single day'. (Or, in layman's terms, the most sensitive individual in the group could be 10,000 times more sensitive to a single odorant than the least sensitive group member.) Others put this figure a little lower. Dr David Laing, of the University of Western Sydney, suggests that in a sample of one hundred people, You could expect a variation of about one hundred times between the most and least sensitive persons'. This is still a significant difference, and enough to explain the reported differences in perception of the cork taint compound trichloroanisole (TCA). But Laing adds that, as with all things biological, the natural distribution of sensitivities means that many of us differ in sensitivity by only a few times - for example, fewer than ten'. Another olfaction researcher, Professor Tim Jacob of Cardiff University, supports this idea. It is possible that we do each have different smell universes, but it is remarkable that we agree about smells to the degree we do.' A more extreme variation arises whereby individuals are completely unable to detect certain odours, a condition known as specific anosmia. An example of this applies to the (in)ability of doctors to smell ketones, found in the breath of patients with poorly controlled diabetes. This ability is an all-or-nothing phenomenon, with about a quarter of doctors failing to detect this smell. Anosmias such as this - and it is not clear how many there are, or whether any relate to odours commonly found in wine - are usually genetic in origin. Occasionally, environmental exposures to certain odours can influence gene expression, turning on receptors in the olfactory epithelium. As Wysocki points out, Some people who cannot smell androstenone [a pig pheromone found in pork meat] can be induced to perceive its odour by repeated, short exposures to the odorant over a few weeks'. The implication for wine tasting ability here would be that we can learn to detect new smells of which we were previously unaware. Again, scientists are not clear how widespread this faculty is. Our noses are quite temperamental performers. According to Jacob, women have a heightened sense of smell at ovulation. Appetite will also stimulate smell, making us more perceptive when we're hungry. There are centrifugal neuronal pathways leading from the brain to the olfactory bulb which modulate odour perception,' says Jacob. These act as a sort of gate, allowing more or less information through.' Jacob also suspects that humidity affects the perception of smell, and he has noticed as yet unquantified seasonal and weather-associated differences. Intriguingly, some odours can also counteract others, such that small quantities can cancel the smell experience of another, unrelated odour. Age also modulates the senses of taste and smell, although in different ways. There is a clear loss of smell with age, and while there is a much smaller loss in taste ability over a lifetime, it affects men and women differently. Males show a steady decline in the ability to taste bitter substances, whereas women show a sharp decline in this ability at the menopause.

Conclusion The picture emerging is clearly a complex one. We see that there are significant individual differences in tasting ability, with three distinct populations each living in different taste worlds'. We also see that there are complex and less clear-cut individual differences in the sense of smell, with the senses of taste and smell overlapping to a certain degree. But how do these rather surprising results relate to wine? Would supertasters make the best wine tasters? No,' says Bartoshuk, there is too much learning involved. Much of the skill of a wine expert comes from learning the odour complexes produced in wine. We know that learning plays a very important role in the naming of odours.' Jacob agrees that learning is crucial: The inexperienced person does not have a smell vocabulary. This hugely restricts their ability to describe and define odours.' Even for wine experts, a common problem is the impoverished language we have for describing tastes and smells. In Jacob's opinion, A large part of the wine taster's skill comes from being able to develop some sort of classification system and then to associate words/categories with smells.' Finally, a humbling thought for those of us who evaluate wine professionally. Judged by our mammalian friends, we humans have a pretty poor sense of smell. Our olfactory epithelium covers just one fifth of the area of that found in cats. And dogs can distinguish between the smell of clothing worn by non-identical twins (identical twins produce the same scent). And as for discriminating between a number of aromas in a complex mixture, such as wine - well, we're just not very good at it. According to Laing, Humans can only identify up to a maximum of four odours in a mixture, regardless of whether the odours are single molecules (e.g. ethanol) or complex ones (e.g. smoke).' Worth bearing in mind next time you are tempted to write a flowery tasting note.