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Systems making sense:
taste and smell in winetasting
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 round strips
of blotting paper that have been soaked in a solution of propylthiouracil (a
thyroid medication, known more simply as PROP). The audience are surprised:
science lectures aren't usually this interactive. Each person is told to place
the paper on their tongue, and the result is surprising. One-quarter taste
nothing at all. Of the others, most find the paper to taste quite bitter, and a
sizeable minority experience an intense bitterness that is extremely
unpleasant. What Bartoshuk is illustrating is the individual variation in the
ability to taste bitter compounds. Her research 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 group are exquisitely sensitive to
PROP and certain other bitter compounds. This hard-wired difference 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 their tongues, with
non-tasters having relatively few.
What does this mean? Well, it seems
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 extend¾albeit less
dramatically¾to other flavour sensations. According to Bartoshuk,
"Supertasters perceive all tastes as more intense than do medium tasters and
non-tasters". These research findings could have significant implications for
the way we approach wine. Bartoshuk certainly thinks so: "It is important for
wine makers to test their wines on all three groups." She adds, "It would be
interesting to see if we could find systematic differences in preferences for
specific wine types across the three populations."
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 specialized taste buds on the tongue. We perceive
just five different tastes: sweet, salty, bitter, sour and a fifth taste known
as 'umami'. This latter term is a Japanese word that loosely translates as
'meaty', or 'savoury', and refers to the taste of amino acids (the chemical
building blocks of proteins) such as glutamate.
But taste provides us
with relatively little information compared with the sense of
smell¾known as 'olfaction' in the trade. Whereas there are just five
basic tastes, we can discriminate among thousands of volatile compounds
('odorants'). Indeed, much of the character and interest in wine stems from the
complex odours detected by the olfactory system. How does olfaction work? Our
olfactory epithelium, located in the top of our nasal cavity, contains
olfactory receptor cells, each of which express just one type of olfactory
receptor. Each of these receptors is tuned to recognise the particular
molecular structure of different odorants.
So where does the sense of
touch kick in? The brain uses touch localize to 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 such 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 found.
Bartoshuk
emphasises that it is important to distinguish between 'retronasal'
'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 are often diminished. Work in her lab involving the
anaesthesia of taste shows that the intensity of taste also plays a role in the
intensity of retronasal olfaction. The implications? "If this is so, then
supertasters with their more intense taste sensations may also experience more
intense retronasal olfaction". Taste and smell therefore overlap.
Since
olfaction is a key element of wine tasting, do individuals show pronounced
inter-individual variation in their ability to smell, just as they do with
taste? 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 just been looking at the way the tongue
and olfactory epithelium detects the chemical environment they are exposed to.
For us to use this information, the brain has to interpret it and pull out 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 review, 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 very key here, and we take
particular notice of what we have learned 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 peoples' ability
to detect it. Dr Charles Wysocki, an expert in olfaction from 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. Wysocki comments, "If a large enough
sample of people is tested¾say 20¾the range in sensitivity to a
single odorant can be 10 000-fold on a single day." Others put this figure a
little lower. Dr David Laing, from the University of Western Sydney, suggests
that in a sample of 100 people, "You could expect a variation of about 100
times between the most and least sensitive persons." Still, a significant
difference, and enough to explain the reported differences in perception of the
cork taint compound trichloroanisole. 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 times¾for example fewer than 10."
Another olfaction researcher, Professor Tim Jacob from 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 is where individuals are completely
unable to detect certain odours, a condition known as specific anosmia. An
example of this familiar to diabetes doctors is the ability to smell ketones,
found in the breath of patients with poorly controlled diabetes. This ability
is an all-or-none phenomenon, with about a quarter of doctors failing to detect
this smell. Anosmias such as this¾and it is not clear how many there
are, or whether any relate to odours commonly found in wine¾are usually
genetic in origin. Fascinatingly, though, Wysocki points out that sometimes
environmental exposures to certain odours can influence gene expression,
turning on receptors in the olfactory epithelium. "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 that previously unaware of.
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 in his research 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.
So the picture emerging is 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 two 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." She
maintains that, "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 peers, we humans
have a pretty poor sense of smell. Our olfactory epithelium covers just
one-fifth of the area of that in cats. And dogs can distinguish between the
smell of clothing worn by non-identical twins. And as for discriminating
between a number of aromas in a complex mixture, such as wine¾well,
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 a complex one (e.g. smoke)." Worth
bearing in mind next time you are tempted to write a flowery tasting note?
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