From Nature Outlook
Taste is central to our being, but this vital sense is only now becoming clear at the biological level.
Scientists have identified the receptors that respond to the five basic stimuli of sweet, sour, bitter,
salty and umami (savoury), and are now exploring how the brain interprets them. Nature Outlook
Taste reports the latest findings from the front lines of flavour.
http://www.nature.com/nature/journal/v486/n7403_supp/index.html#out
Sensory Science
For millennia, food has been at the center of social events, in times of joy and in times of sorrow.
Protein-energy malnutrition is associated with a significant impairment of cell-mediated immunity,
phagocyte function, complement system, secretory immunoglobulin A antibody concentrations, and
cytokine production. Deficiency of single nutrients also results in altered immune response: this is
observed even when the deficiency state is relatively mild. Of the micronutrients, zinc, selenium,
iron, copper, vitamins A, C, E and B6, and folic acid have important influences on immune
responses. Overnutrition and obesity also reduce immunity. Low-birth-weight infants have a
prolonged impairment of cell-mediated immunity that can be partly restored by providing extra
amounts of dietary zinc. In the elderly, impaired immunity can be enhanced by modest amounts of
a combination of micronutrients. These findings have considerable practical and public health
significance.
http://www.nature.com/ejcn/journal/v56/n3s/abs/1601492a.html
Editorial: Taste
http://www.nature.com/nature/journal/v486/n7403_supp/full/486S1a.html
Taste is more than a sensual experience: it is a signal of nutritional value or danger. It has evolved
as a vital survival mechanism in mammals (see page S16) and driven epic periods of human
history — it was, after all, the quest for spices that helped launch the age of exploration. Yet our
understanding of how taste works has lagged behind the other senses.
In the past decade or so, taste science has been on a roll. In 2000, researchers used the newly
available human genome data to help identify the receptors that respond to bitterness (S2). Since
then, the receptors for sweet, salty, sour and umami (savoury) have also been identified. The
exclusive club of basic tastes might be about to admit new members: carbonation, metallic and
fattiness.
What's more, taste is revealed to be a whole-body experience; taste receptors are found in the gut,
the airways and even on sperm (S7), but the function of many of these sensors remains unclear.
The centrality of flavour to human culture has driven scientists, chefs and the food industry to
experiment with new ways of producing familiar and novel tastes (S14) as well as to create a
scientific style of experimental cooking (S10). And while the link between smell and taste is well
known, studies are showing that the way we experience food is influenced by all five senses (S4).
Individual variation in taste tolerances might help explain why some people tend to be obese
(S12), although scientists still struggle with the question of whether taste is an inherent attribute
of food or a personal psychological construct (S6). However, while much of taste is subjective,
tasting technologies aim to define our eating and drinking experiences with machine-like
consistency (S18).
We acknowledge the financial support of Ajinomoto Co., Inc. in producing this Outlook. As always,
Nature has full responsibility for all editorial content.
Gustatory system: The finer points of taste
As more receptors are defined, researchers will further unlock the mechanics of taste. How the
mind perceives these sensory signals is another matter.
Subject terms:
Cell biology Molecular biology Physiology Structural biology
Take a sip of milk, look in the mirror and stick out your tongue. The tiny pink bumps emerging
from the creamy film coating your tongue are the mushroom-shaped, or fungiform, papillae that
conceal many of your taste buds. These are the gateway to detecting the sweetness of a cake, the
saltiness of a potato crisp, the meatiness of a steak, the bitterness of beer, and the sourness of a
lemon.
The four types of papillae that speckle the tongue (see 'Taste discovery') give it a rough surface
that helps move the food around as we chew. The filiform papillae simply detect texture, whereas
the other three — fungiform, foliate and circumvallate — contain onion-shaped taste buds. Each
taste bud is packed with taste cells, which are capped with sensors for the five basic tastes
(although we may be able to detect other taste qualities too). Contrary to popular belief, every part
of the tongue is sensitive to all five taste qualities — the once common 'tongue map' depicting
specific regions for each taste was based on a misunderstanding in the early 1900s and is wrong.
Figure 1: Taste discovery
MARTIN HARVEY/ GETTY IMAGES
Full size image (298 KB)
In 1931, Arthur Fox, a chemist at US chemical giant DuPont, made a remarkable discoveryafter
accidentally releasing a cloud of fine phenylthiocarbamide (PTC) crystals while transferring the
powder to a bottle. A colleague commented that the compound tasted bitter, but Fox, in the midst
of the powder cloud, tasted nothing. After testing friends, family and colleagues, Fox found that
people are either 'tasters' or 'non-tasters' of PTC (ref. 1).Geneticist L. H. Snyder confirmed Fox's
work and found that non-tasting is a recessive Mendelian trait. Later work uncovered a range of
PTC sensitivity, suggesting the involvement of more than one gene in perceiving this bitter
compound2.
Taste transducer
It took almost seven decades to pinpoint the genes that encode taste receptors. It was widely
believed that each taste cell carried sensors for several, if not all, of the five tastes, with the
signals being decoded in the brain. But this made no sense to neuroscientist Charles Zuker, a
Howard Hughes Medical Investigator now at Columbia University in New York. He couldn't
understand why a cell could carry sensors for both sweet, signalling an energy-rich food, and
bitter, which could warn of spoiled or toxic food — a mix-up could be lethal.
Zuker rejected this 'broad tuning' and assumed that the difference between a sweet and a bitter
taste cell, for example, was the collection of receptors on its surface. He joined forces with
geneticist Nicholas Ryba of the National Institute of Dental and Craniofacial Research in Bethesda,
Maryland, and over the next 12 years they sought to discover the sensors for all five tastes (see
'Taste discovery').
In 2000, using the first draft of the human genome, Zuker and Ryba's team identified the first
taste sensors: a family of G-protein-coupled receptors (GPCRs)3 on chromosome 5. These socalled Taste-2 receptors (T2Rs) can detect bitter tastes from a wide range of different chemical
compounds, requiring a collection of sensors. Some T2Rs can detect only one bitter compound, but
some can respond to more than 50 natural and synthetic bitter chemicals.
Each bitter taste cell carries from 4–11T2Rs — this variety, along with under-lying genetic
variation, is thought to account for an individual's tolerance of bitter tastes. For example, variation
in the gene encoding T2R38 modifies sensitivity to PTC, which correlates with sensitivity to bitter
compounds in cabbage or Brussels sprouts4.
“More than five teams were competing to find the sweet receptor.”
The race was on to find the remaining taste receptors. In 2001, more than five teams were
competing to find the sweet receptor and narrowed the candidates to a different family of GPCRs.
These T1R receptors have a different structure from the T2Rs, including a bulky extracellular region
that interacts with sweet molecules. Zuker and Ryba provided the decisive evidence by
transforming a sweet-insensitive mouse into a sugar-loving one, showing that T1R2 and T1R3
combine to detect natural and artificial sweeteners5. Barely a year later, they also found that the
combination of T1R1 and T1R3 receptors can detect all 20 amino acids found in nature6, sensing a
taste described as umami or savoury (for example, in meat or cheese) that can be chemically
distilled into the common food additive monosodium glutamate. The receptors for sweet, bitter and
umami all use essentially the same signalling molecules to convey nutrient sensing to the brain.
Sour success
In 2006, Zuker and Ryba discovered the PKD2L1 receptor7, which detects sourness — a high
concentration of hydrogen ions, or acidity, found both unripe and spoiled foods. Sour taste cells
also host the Car4 receptor8, which senses carbon dioxide — block this receptor and a carbonated
beverage will taste flat (even though it might still feel bubbly). Finally, in 2010, Zuker and Ryba
found that the epithelial sodium channel (ENaC) detects sodium salt9 (other salts, such as
potassium chloride, probably have other receptors). Instead of GPCRs, salt and sour detection uses
ion channels — proteins that shuttle sodium and hydrogen ions in and out of cells.
Five basic tastes are generally recognized but there may be others, and the hunt for receptors
continues. In 2003, neuroscientist Robert Margolskee, then at the Mount Sinai School of Medicine
in New York, showed that mice lacking the T1R3 receptor couldn't taste artificial sweeteners but
still had an affinity for sugars, especially glucose, and could also perceive umami10. “It said very
clearly to us there are two different sweet mechanisms,” he says.
But receptors don't tell the whole taste story. Zuker and Ryba are using the taste sensors already
discovered to figure out, as Zuker says, “how the brain transforms detection into perception”.
Editorial: Taste = waarschijnlijk dubbel
Taste is more than a sensual experience: it is a signal of nutritional value or danger. It has evolved
as a vital survival mechanism in mammals (see page S16) and driven epic periods of human
history — it was, after all, the quest for spices that helped launch the age of exploration. Yet our
understanding of how taste works has lagged behind the other senses.
In the past decade or so, taste science has been on a roll. In 2000, researchers used the newly
available human genome data to help identify the receptors that respond to bitterness (S2). Since
then, the receptors for sweet, salty, sour and umami (savoury) have also been identified. The
exclusive club of basic tastes might be about to admit new members: carbonation, metallic and
fattiness.
What's more, taste is revealed to be a whole-body experience; taste receptors are found in the gut,
the airways and even on sperm (S7), but the function of many of these sensors remains unclear.
The centrality of flavour to human culture has driven scientists, chefs and the food industry to
experiment with new ways of producing familiar and novel tastes (S14) as well as to create a
scientific style of experimental cooking (S10). And while the link between smell and taste is well
known, studies are showing that the way we experience food is influenced by all five senses (S4).
Individual variation in taste tolerances might help explain why some people tend to be obese
(S12), although scientists still struggle with the question of whether taste is an inherent attribute
of food or a personal psychological construct (S6). However, while much of taste is subjective,
tasting technologies aim to define our eating and drinking experiences with machine-like
consistency (S18).
We acknowledge the financial support of Ajinomoto Co., Inc. in producing this Outlook. As always,
Nature has full responsibility for all editorial content.
Sensory science: Partners in flavour
http://www.nature.com/nature/journal/v486/n7403_supp/full/486S4a.html
Our perception of food draws on a combination of taste, smell, feel, sight and sound.
Subject terms: Developmental biology Psychology
The way we experience food is not limited to the mouth — odour, vision, hearing and even touch
can radically change the taste of food or affect food preference. And it all starts before birth.
Amniotic fluid, the baby's first food, contains glucose, fructose, fatty acids and amino acids, and
newborns are born preferring the sweet taste of mother's milk to other flavours. Since the 1970s,
researchers have known that the introduction of sweet solutions into the amniotic fluid causes the
fetus to swallow more frequently, whereas bitter solutions reduce the rate of swallowing. Similar
reactions are seen in babies: they lick their lips and smile when tasting something sweet, and
wrinkle their noses and flail their arms in response to a bitter or sour taste, precisely the tastes
that warn us to avoid poisons. “A taste for sugar is innate,” says Julie Mennella, a biopsychologist
at the Monell Chemical Senses Center in Philadelphia, Pennsylvania.
But even in the womb, new tastes can be introduced. The odour of garlic, for example, can be
detected in samples of amniotic fluid from women who have eaten garlic pills 45 minutes before.
And babies exposed to garlic in utero are more apt to like the flavour of garlic in the milk of garliceating mothers.
To some extent, a mother can influence her baby's tastes by what she eats during pregnancy and
nursing. In one study, babies of mothers who ate more fruit during lactation were more likely to
accept peaches than babies fed on formula — but the mothers' consumption of string beans did not
have the same effect1. The taste for salt, however, is apparently acquired only with age. Babies
younger than four months are happy to drink either plain water or water with moderate
concentrations of salt, but by the time they are two-and-a-half years old they show a marked
preference for salty water.
A. INDEN/ CORBIS
Researchers have found evidence that pregnant women who consume certain foods — so far they
have tested anise, carrot, mint, vanilla and blue cheese — convey a liking for those tastes to their
infants2. There's a good species-preserving rationale for this behaviour, says Gary Beauchamp,
also at the Monell centre: “When a baby begins to eat solid food, the most logical and safest things
for it to eat are what its mother ate.” Indeed, he adds, this phenomenon suggests a strategy for
improving the population's diet generally: “Exposing infants prenatally and postnatally to the
flavours of things that people aren't consuming enough of could be a good thing to do.”
A sensory milieu
When taste signals are received, various neural pathways go into action: saliva production
increases and stomach secretions are activated. But these pathways do not require food to be in
the mouth. The sound of dishes rattling in the kitchen or a picture of a lobster dinner can make the
stomach rumble.
Vision provides an essential sensory input for food perception, raising expectations and driving
preferences. We unconsciously discriminate between high-calorie and low-calorie foods by sight
alone. Kathrin Ohla, a psychologist at the Monell centre, demonstrated that seeing pictures of highand low-calorie foods provokes responses in different parts of the brain that vary in intensity3.
Without telling subjects the purpose of the test, Ohla showed them pictures of high-calorie foods,
such as lamb chops, salmon, pizza and pastry, alternating with pictures of lower calorie foods, such
as beans, watermelon, yogurt and pasta with tomato sauce. After each picture, she applied a weak
electric current to the subject's tongue, stimulating the brain's taste function without mimicking
any real taste, producing a sensation and metallic taste rather like licking a battery. So the
subjects got the food cue from the picture and were immediately given a neutral taste stimulation.
The EEG measurements of brain activity were clear: the electrical current administered after seeing
a picture of a high-calorie food stimulated a stronger and, according to the subjects, more pleasant
sensation than the same electrical stimulation applied after showing a picture of a low-calorie food.
The neural activity evoked by pictures of high-calorie food was stronger in specific parts of the
brain — the bilateral insula and frontal operculum — than when low-calorie foods were viewed. And
changes in reported taste pleasantness were correlated with the activation of the medial
orbitofrontal cortex.
Even shape can affect taste. David Gal, who studies marketing and consumer behaviour at
Northwestern University in Evanston, Illinois, found that after subjects completed an unrelated task
that involved sorting geometric figures, a piece of cheese with pointed, rather than rounded
corners tasted sharper4. Scientists are only beginning to understand the brain mechanisms
underlying the connections between the visual and gustatory senses.
Sniffing a connection
Odour plays a large role, of course. Chewing releases volatile molecules that travel through the
back of the mouth to receptors in the lining of the nasal passages — quite different from taking a
sniff through the nose. These receptors allow us to identify the combination of sens-ations that
leads to flavour; that is, to know what we're eating. When you eat a strawberry, it may taste sweet
or sour, and you can detect this even while holding your nose. But what disappears is the flavour
— the 'strawberriness' of the food. For that, you need a sense of smell.
CHARLES SPENCE
Tasty tones: tests have shown that when people hear a crunch, a potato crisp seems fresher.
Smell is much more complex than taste. Humans have only a few different taste receptors: the five
that scientists generally agree on (sweet, sour, salty, bitter and umami) and possibly a few others.
But there are hundreds of receptors for odours. The possibilities for mixing and matching taste and
odour are immense, and lead to the wide variety of flavours we perceive that subjectively bear
little resemblance to any of those basic tastes.
Indeed, smell and taste are linked neurologically in a way that no other human senses are. When
we hear and see something at the same time, we are using two senses that have different neural
pathways, and we can easily distinguish between them. Not so with smell and taste. Dana Small, a
psychologist and neuroscientist at Yale University in New Haven, Connecticut, and the Yaleaffiliated John B. Pierce Laboratory, suggests trying what she calls the jelly bean test. Pop a jelly
bean
Perspective: Complexities of flavour
Is flavour an intrinsic objective property, or a subjective experience that varies from person to
person? Barry Smith sorts out the implications.
Subject terms:
Culture Philosophy Psychology
Although we're all familiar with taste, it is surprisingly complex and puzzling. What we call taste
encompasses the combined sensory inputs of taste, touch and smell, as influenced by sight and
sound. The tongue and associated receptors in the mouth detect only salty, sweet, sour, bitter,
savoury and possibly metallic, yet we can 'taste' such flavours as mango, onion, strawberry, mint,
cinnamon and vanilla. Flavours such as these are discovered by the tongue and the nose together,
not by either of these organs alone.
We seldom recognize experiences of pure taste. Holding the nose closed reduces our ability to tell
the difference between pieces of raw apple and raw potato because it prevents odours in the mouth
from reaching the olfactory epithelium at the bridge of the nose. Similarly, people who lose their
sense of smell often report that they can't taste anything, even though when tested they can
detect salt, sour, bitter and sweet (there is no simple test for the savoury taste, umami).
So the quality we are interested in is not taste per se, but flavour. However, trying to define
flavour is far from straightforward. For example, in the introduction to the multidisciplinary journal
Flavour, the editors tell us: “We take flavour to be the experience of eating food as mediated
through all the senses.” The journal also “emphasizes work that investigates the flavour of real
foods”. At first, the editors seem to define flavour as an experience, yet those who study the
physics and chemistry of flavours in food and wine are not investigating psychological experiences;
rather, they are observing and measuring actual physical compounds. To these researchers,
flavours reside in the food and drink we consume.What, I think, the editors intend to focus on is
the multisensory experiences through which we perceive flavours in foods.
Property or experience?
“Psychologists and neuroscientists tell us that flavour is a concoction of the brain”
It's not hard to see why people confuse flavour (the objective property) with the subjective
experience of flavour. Psychologists and neuroscientists tell us that flavour is a concoction of the
brain — the result of the multisensory integration of olfactory, tactile and taste impressions,
modulated by the dynamic time course of a tasting event and the location of sensory stimuli in the
mouth. According to this view, the flavour of a wine, say, is a psychological construct that will vary
from individual to individual as a result of different threshold sensitivities to acid, tannin, sugar,
alcohol, carbon dioxide and sulphur. Lighting conditions, mood and even sounds can affect our
experience of tasting, and wines can be enhanced or distorted by accompanying foods — all of
which suggests that winemakers have little influence over the experience that drinkers of their
wines will have.
However, advances in the science of winemaking suggest otherwise, and they are increasingly used
to improve the perceptible quality of wines. Winemakers strive to find properties such as 'balance'
in a wine — something that drinkers can sense even if they lack the concept of a balanced wine —
and winemakers know many of the factors that affect it.
The central question, then, is this: how should we adjudicate between those who say that flavours
depend on molecular compounds, and those who stress the varying perceptions of individual eaters
and drinkers?
The problem is that analytical chemists struggle to connect the volatile molecules in wines with the
varying perceptions of individual tasters. However, this isn't what they should be trying to do. The
task is to relate the underlying chemical compounds in a wine to the relatively stable flavours they
create, whereas it is the task of psychologists and neuroscientists to chart the complex relationship
between flavours and flavour experiences — explaining why the latter can vary as a result of
conditions internal and external to the taster. Only by recognizing flavours as intermediaries
between the chemical compounds in a wine and our individual reactions to it can we hope to bridge
the two.
Finding the flavour
The right way to view flavours is as configurations of sapid, odorous and textural properties of
foods or liquids that we track using a combination of our senses. The flavour of menthol, for
example, comprises a minty aroma, a slightly bitter taste, and a cool sensation in the mouth
resulting from irritation of the trigeminal nerve (which also causes the hot sensation when we eat
mustard or chillies). For single flavours, such as strawberry, mint and mango, which are easy to
detect, there is little variation between tasters. But for more complex products, like wines, we don't
always detect all their flavours. Our individual flavour experiences, like our other perceptions, are
not always exact guides to reality. Tasting is hard — it requires experience, practise and knowledge
to identify what one is tasting.
Studying the multisensory nature of flavour perception helps us understand how perceptions can
vary across individuals, and within individuals over time, as a result of a variety of factors that
affect our ability to taste. If a wine remains unchanged, we should see these variations as different
ways of perceiving the same flavour, rather than claiming that there are as many flavours as there
are tasters. Where the flavours of a wine evolve in the glass or the bottle, the task of an
experienced taster is to assess its changing flavour profile from the series of snapshots that
individual perceptions provide. Psychology and neuroscience are beginning to show us just how
many factors are involved in individual perception, and with luck we will be able to work out the
conditions that not only diminish, but also improve, our access to the real flavours in our food and
drink.
Neuroscience: Hardwired for taste
Research into human taste receptors extends beyond the tongue to some unexpected places.
Subject terms:
Cell biology Molecular biology Neuroscience
A mouthful of bittersweet chocolate cake with a molten centre can trigger potent memories of
pleasure, lust and even love. But all it takes is one bad oyster to make you steer clear of this
mollusc for life. Neuroscientists who study taste are just beginning to understand how and why the
interaction of a few molecules on your tongue can trigger innate behaviours or intense memories.
The sensors in our mouths that detect basic tastes — sweet, salty, bitter, sour and umami, and
arguably a few others — are only the start of the story (see 'The finer points of taste', page S2).
The way the brain represents these tastes is just as important. Researchers have recently
developed a 'gustotopic map' based on the idea that, just as each taste bud on the tongue
responds to a single taste, so there are regions of the brain that are similarly dedicated1.
The other recent revelation in taste research is that the receptors that detect bitter, sweet and
umami are not restricted to the tongue. They are distributed throughout the stomach, intestine and
pancreas, where they aid the digestive process by influencing appetite and regulating insulin
production. They have also been found in the airways, where they have an impact on respiration,
and even on sperm, where they affect maturation. A better understanding of what they do and how
they work could have implications for treating conditions ranging from diabetes to infertility.
SOPHIE CASSON /THREE IN A BOX
Brain map
After discovering the sensors for the five basic tastes, Charles Zuker, a Howard Hughes Medical
Investigator now at Columbia University, New York, and geneticist Nicholas Ryba, of the National
Institute of Dental and Craniofacial Research in Bethesda, Maryland, embarked on a logical followon project. Their goal, Zuker says, was to determine how the brain “transforms detection into
perception”.
Results of previous studies into taste representation in the brain “have been confusing”, says Ryba.
One of the leading theories was that the gustatory cortex — the primary brain region responsible
for taste perception — was 'broadly tuned', with each neuron responding particularly well to one
taste but still able to respond to others. Moreover, the neurons were thought to be distributed
evenly. This model seemed to be consistent with smell, the other chemical sense, which produces
no recognizable patterns in the brain. Odour recognition is a matter of combinatorial processing:
small differences in firing patterns across large populations of neurons represent a characteristic
smell.
The research that led to these conclusions suffered from poor spatial resolution, however. To
address this shortcoming, Zuker and Ryba's team used functional brain-imaging techniques to
investigate how individual taste qualities stimulate neural activity in the gustatory cortex on a fine
scale. Their tools included two-photon calcium imaging, which reveals processes deeper inside the
brain, and with greater spatial resolution, than conventional single-photon methods. They found
that dropping any of several bitter liquids onto the tongue of an animal that had been
anaesthetized (both for convenience and to replicate conditions in previous studies) consistently
activated the same small group of neurons in the gustatory cortex. These neurons did not light up
in response to sweet, savoury or salty liquids.
The team ran the same experiment for the other tastes and found that sweet, salty and umami
each had its own distinct cluster, or hotspot, of neurons. The only hotspot they couldn't locate was
for sour tastes.
These findings contradict previous ideas about how the brain processes taste. Seeing the first
hotspot was “stunning”, recalls Ryba. “On the one hand it was very surprising: at the level of the
tongue, the sweet and bitter cells and receptors are intermingled, yet at the level of the cortex
they are separated by 2.5 millimetres” — a surprisingly large distance that could span hundreds of
neurons. On the other hand, Ryba can see the logic of this organization: separating the bitter
hotspot from the sweet, for example, means that bitterness can be wired to a brain area that
drives aversion, whereas sweetness can be wired to attraction.
So far the gustatory map is sparse, with just four identified hotspots. But other areas nearby might
also be used for taste coding, possibly involving other senses, speculates Ryba. These areas might
represent mixtures of tastes or perception of flavour.
Finding these hotspots is “a breakthrough” that provides a “basic underlying principle of how the
cortex is organized”, says Susan Travers, a neuroscientist at Ohio State University in Columbus.
Nevertheless, Travers thinks the new map is oversimplified. Previous experiments found that a
neuron “doesn't respond to just one stimulus”, she says. For example, “you would expect the sweet
hotspot to have some smaller responses to other stimuli”, such as salt or umami.
Sidney Simon, a neuroscientist at Duke University in Durham, North Carolina, who specializes in
gustatory physiology, says that Ryba and Zuker's experiments are “technically spectacular” — but
he also has concerns. He says it is strange that they didn't find a sour hotspot, and recommends
that they explore whether it lies in a different area or whether sour detection is dispersed
throughout the brain. He adds that these experiments should be performed on conscious animals,
which, in terms of smell at least, respond differently from anaesthetized ones.
Zuker takes the criticisms in his stride. He's not perturbed by the lack of a sour hotspot and
explains that, unlike the other four tastes, sour — or acid detection — is also involved with pain.
“When you put a drop of acid on your finger it burns, you aren't getting a sour lemony taste,” he
says. So the cortical representation is likely to be more complicated than a single hotspot. Other
tastes don't have this type of conflict, he adds.
As for Travers' comment that the hotspots appear to be too specialized, Zuker concedes that there
may be some sensory crossover, with some cells in the bitter hotspot, for example, also responding
to other tastes. The key, he stresses, is that there is overall topographic segregation, and that the
vast majority of neurons within a hotspot are selective for just one taste.
Zuker and Ryba now plan to explore how taste mixtures are encoded, and then how sweet and
bitter tastes, after being detected by their corresponding cortical fields, can trigger such exquisite
behaviours — from attraction to aversion, from pleasure to rejection.
Out of the mouth
While Zuker and Ryba use receptors to explore how taste relates to emotion, memory and learning,
other researchers hope to explain what taste receptors are doing in other parts of the body.
If taste receptors seem out of place anywhere other than the mouth, this is only because they were
first found in taste buds on the tongue, says neurobiologist Thomas Finger, co-director of the
University of Colorado's Rocky Mountain Taste and Smell Center in Aurora. Taste buds are simply a
way of sensing chemicals, so they can have functions unrelated to detecting the flavour of food.
And they are surprisingly common in the body (see 'The secret lives of taste receptors'), although
their presence is sometimes baffling. “We don't know the function of these receptors in more places
than we do know them,” says Finger.
Box 1: The secret lives of taste receptors: The ability to sense chemicals has whole-body
applications
From
Neuroscience: Hardwired for taste
Taste receptors promote survival by detecting nutritious foods and helping us avoid toxic ones. But
these receptors exist beyond the tongue and digestive system. How do they contribute to the
survival of the species?
In a bid to find out, Thomas Finger, a neurobiologist and co-director of the University of Colorado's
Rocky Mountain Taste and Smell Center in Aurora, has been studying solitary chemoreceptor cells
(SCCs). In 2003, he discovered SCCs in the nasal cavities of rats and mice; more recently, he and
others have found SCCs expressing taste receptors in the human nose. The rodent SCCs contain
T2Rs (bitter receptors) and all the rest of the cellular machinery found in bitter taste cells on the
tongue. In 2010, Finger and colleagues showed5 that bitter compounds that tickle these receptors
trigger apnoea: “they stop breathing, they cough and sneeze,” he says. The receptors, which are
exposed to harmful irritants in the air as well as to compounds produced by bacteria growing in the
nasal cavity, transmit their signal to the trigeminal nerve, which temporarily inhibits breathing.
This response, he hypothesizes, is to stop the irritant from being inhaled deep into the lungs.
It's not only the T2Rs that are found in hard-to-explain places. The T1Rs, responsible for sensing
sweet and umami, exist in the airways too, although what they do there remains unknown. “It is
easier to understand the T2Rs than the T1Rs,” says Finger, “T2Rs always seem to be present in the
context of detecting toxins. The T1Rs seem to have other functions here that are unclear.”
Collections of bitter receptors are also found on tiny finger-like projections called cilia on human
airway epithelium cells. When the researchers stimulated the receptors using bitter compounds,
such as nicotine or quinine, the cilia waved back and forth vigorously, helping to clear the airways
of irritating compounds.
Researchers at the University of Maryland School of Medicine in Baltimore discovered T2Rs on
smooth muscle cells in the human airway6. Exposure to bitter compounds caused these cells to
relax. Testing this effect in asthmatic mice, where smooth-muscle contraction narrows the airways
and obstructs breathing, had the same effect: bitter compounds relaxed the smooth muscle and
improved breathing.
There are many more locations where the presence of taste receptors is enigmatic. A team of
researchers led by Ingrid Boekhoff of the Walther Straub Institut for Pharmacology and Toxicology
in Munich, Germany, recently found7 that mouse and human spermatozoa express T1R1 and T1R3,
the umami receptor. Murine sperm that lack T1R1 had an increased rate of spontaneous acrosome
reaction — a disgorging of DNA that normally only happens when the sperm meets an egg. The
mutant sperm also had higher levels of calcium and a second messenger molecule called cAMP.
Boekhoff and her colleagues suggest that the umami receptor somehow keeps the sperm in a quiet
state by controlling calcium ions and cAMP — which are known to influence the acrosome reaction
— until it reaches an egg.
The bitter receptor T2R5 is also found in the testes8; deleting these cells leads to smaller testes
and a huge reduction in the number of mature sperm. According to Robert Margolskee, associate
director of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, it is not clear what the
bitter or umami receptors are doing in these locations. “But we have the tools to start figuring it
out,” he says. “Check back in five years and we'll have a pretty good idea.” — B.T.
A sperm trying to fertilize an egg. Umami receptors, found on the
sperm's surface, are thought to help control the release of DNA.
It is not surprising that some of the better-understood examples are in the digestive system. The
T1R2/T1R3 sweet receptor is found on K- and L-type enteroendocrine cells in the intestine. These
cells secrete hormones called incretins, which in turn stimulate insulin production. The sweet
receptors neatly explain a phenomenon that had mystified physiologists for more than 50 years:
that eating glucose triggers significantly more insulin than injecting it directly into the bloodstream.
Neuroscientist Robert Margolskee, now associate director of the Monell Chemical Senses Center in
Philadelphia, Pennsylvania, realized that if there were receptors in the intestine that could detect
glucose and trigger the release of hormones, this would provide the missing link for the so-called
incretin effect. In 2007, his hypothesis proved correct as his team found the sweet receptor on L
cells in the human duodenum2 and showed that these cells produce the gastrointestinal incretin
hormone GLP-1, which stimulates insulin production and sends a satiety signal to the brain.
Blocking or deleting these sweet receptors decreases insulin release.
In a further study in mice, Margolskee's team showed that when sweet receptors detect glucose,
the L cells manufacture a glucose transporter that draws the sugar into the cells lining the
intestine. Artificial sweeteners also trigger the glucose transporter and lead to a spike in insulin —
this is a concern, but is unlikely to cause clinical hypoglycaemia (low blood glucose), Margolskee
says.
“Sweet receptors, traditionally associated with just the mouth, were in the gut and essentially
'tasting' the sugar a second time,” says Anthony Sclafani, a behavioural neuroscientist at the City
University of New York. This 'second tasting' triggers glucose transport into the cells and
bloodstream, and the faster this happens, the more insulin will be released. “It's an incredibly
important finding for the control of blood sugar,” he says, adding that it was surprising that
artificial sweeteners, which were thought to influence only the tongue, also trigger changes in the
gut.
Other T1R receptors in the digestive system also play a role in appetite and blood sugar control. In
the stomach, cells carrying the T1R3 receptor, which aids detection of both sugar and amino acids,
secrete the hunger hormone ghrelin when they encounter carbohydrates and protein, encouraging
eating when important nutrients are available.
For bitter tastes, however, the T2R receptors in the digestive system seem to have contradictory
functions. In 2011, Belgian researchers showed that bitter-tasting compounds that reach the
stomach of mice initially trigger the release of ghrelin, stimulating eating as usual3. But after 30
minutes, food intake decreased, as did gastric emptying, keeping the food in the stomach. This
curbs the appetite by prolonging the sense of fullness and satiety — perhaps to prevent the
ingestion of toxic food.
“It's too early to know whether this is a normal satiety response or this is mimicking a response to
a toxin,” says Roger Cone, a biophysicist at Vanderbilt University in Nashville, Tennessee. It seems
counter-intuitive that eating a bitter compound, which is thought to signal a toxin, actually
stimulates the appetite. Yet this is an effect that has been known about for centuries. Indeed, the
Romans drank wine infused with bitter herbs to prime the appetite and prevent overeating. And,
the authors speculate, stimulating bitter receptors in the gut could potentially be used to treat
certain eating disorders.
Bitter receptors are also present in the large intestine. In 2009, researchers at the University of
Shizuoka in Japan showed that activation of T2Rs in the colon of humans and rats stimulates the
secretion of anions, inducing water to enter and causing diarrhoea4 . The authors suggest that this
acts as a defence against dangerous organisms and irritants.
Investigating the function of the taste receptors distributed throughout the body could help
clinicians tackle diseases ranging from eating disorders to diabetes. Digesting all this information
will take time, but it is clear that the function of taste receptors goes far beyond the pleasure of
eating chocolate cake.
Cooking: Delicious science
Chefs are teaming up with researchers to create avant-garde dishes. Is 'molecular gastronomy'
more than a fad?
Subject terms:
Arts Culture Molecular biology
Food science has often focused on nutrition or industrial-scale food and flavour production. But
over the past two decades, a discipline that blends science and cooking has made its way into
universities, restaurants and even home kitchens. Collaborations between scientists and chefs have
made advances in the study of gastronomy and spurred culinary innovations that are opening up
fresh ways of studying something we often take for granted: the enjoyment of a good meal.
This field is often called 'molecular gastronomy', although both scientists and chefs have objected
to the term (see 'Name that cuisine'). Hervé This, a chemist at the French National Institute for
Agricultural Research in Paris, who coined the term in 1988, wanted to establish a new field that
uses science to understand what happens to food when it is cooked.
This conjunction of cooking and science has spawned several developments. First, researchers
have turned the kitchen into a place for serious scientific study, with a growing number of papers
and books detailing the physical and chemical transformations involved in cooking. At the same
time, collaborations between scientists and chefs have helped bring scientific knowledge and
technological innovations into fine restaurants and even homes. And finally, the field has spurred
an interest among both scientists and chefs in moving beyond the physical properties of foods to
understanding the psychology and neuroscience of perceiving and enjoying food.
The science of cooking
The chemical and physical transformations that take place during cooking are complex. The
browning of meat, for example, involves molecular changes produced in a complex set of cascading
chemical interactions known as Maillard reactions. Analyses of foods undergoing Maillard reactions
have shown that the process releases hundreds of compounds, some of which have been
harnessed by the flavour industry to create processed foods that taste better — compounds that
contain the amino acid cysteine provide a meaty smell, for example, and compounds with
methionine enhance the flavour of potatoes.
CLAUS BECH-POULSEN
Lars Williams, head of research at Nordic Food Lab, part of Noma, inspects a variety of vinegar
concoctions.
Many researchers are trying to understand what makes some cooking methods work better than
others. In 2010, Pia Snitkjaer, a PhD student at the University of Copenhagen, described the
investigation of the perceived flavour and chemical composition of meat stock as it is cooked1.
Most of the traditional ways of cooking have been passed down through the generations without
any systematic testing, and molecular gastronomy can pinpoint those that do not yield the best
flavour. Hervé This has debunked the myth that adding hot vinegar to mayonnaise prevents its
decomposition, for example, and that eggs can only be whipped once. Such explorations are also
taking place in restaurant test kitchens and other informal settings, as documented by a sixvolume cookbook Modernist Cuisine (go.nature/ymtjuj) published in 2011 by a team led by Nathan
Myhrvold, formerly chief technologist at Microsoft and now chief executive of the patent licensing
firm Intellectual Ventures, based in Bellevue, Washington. In addition to recipes, the book details
the basic science of cooking, from the properties of water to the principles of heat transfer in pans
(it turns out that copper pans don't transfer heat any better than aluminium ones).
As well as the growing number of scientists interested in studying cooking, many famous chefs
have embraced a scientific approach to creating new dishes. Restaurants such as the recently
closed elBulli in Roses, Spain, The Fat Duck in Bray, UK, Alinea in Chicago, Illinois, and Noma in
Copenhagen, Denmark, have made their reputations through their development kitchens. Chefs at
these establishments have experimented with new methods to create some surprising dishes —
and have sometimes joined scientists as co-authors in published research.
Ferran Adrià was one of the first to take this approach, playing with the physical properties of food.
At elBulli, he used methods such as spherification, in which liquid ingredients are mixed with
sodium alginate and submerged in a calcium bath, resulting in caviar-like spheres that burst in the
mouth. A similar technique creates balls from alcoholic liquids, like the carbonated mojito spheres
created by chef José Andrés for his restaurant Minibar in Washington DC. “We used to cook only by
repeating what we saw without really having a deep understanding of why it was happening,” says
Andrés. Now, he says, the application of scientific principles to cooking has fuelled culinary
innovation. Andrés visited the Fluid Dynamics Laboratory led by John Bush at the Massachusetts
Institute of Technology (MIT), who was mimicking the hydrodynamic properties of aquatic flowers
to create flexible petal-shaped films that, when pulled out of water, enclose a small amount of
water. Andrés and his team have recently developed similar petal-like sheets made of gelatin,
which will enable them to create an edible snack with liquid inside.
Such inventions have stimulated new interest in the science of cooking, and a wave of sciencefocused cookbooks, TV shows and websites bringing these ideas into home kitchens. Amateur
cooks can now buy sous vide machines (which use a temperature-controlled water bath to cook
food slowly in sealed plastic bags), air pumps to make foams, and ingredientssuch as sodium
alginate and xanthum gum to alter the textures and properties of food.
The pleasure principle
For some scientists, the most interesting questions in gastronomy lie not in the chemistry and
physics of food, but in the brain. Peter Barham, a physicist at the University of Bristol, UK, and coeditor of the new journal Flavour, says that much of what people call molecular gastronomy is
simply the application of scientific knowledge about the physics and chemistry of food that has
been known for some time. “What is not well researched is the link between the food that goes into
our mouth and what we think of it,” he says. Barham is one of the authors of a paper2 published in
Chemical Reviews in 2010 arguing for a broad definition of molecular gastronomy as “the scientific
study of why some food tastes terrible, some is mediocre, some good, and occasionally some
absolutely delicious”.
The study of the nature of flavour perception, eating and enjoyment or 'neurogastronomy' is
making many new discoveries, says Barham. It is becoming increasingly clear, for example, that
what we taste depends on the information coming from our other senses (see 'Partners in flavour',
page S4). Chefs who appreciate this phenomenon realize that that they can make meals taste
better by paying attention to the other sensory inputs their customers receive. Heston Blumenthal
and his chefs at The Fat Duck have spent several years collaborating with Charles Spence, an
experimental psychologist who heads the Crossmodal Research Laboratory at the University of
Oxford, UK, who has shown that factors such as background music, plate colour and the materials
used for the cutlery can affect how a dish tastes. After finding that the sound of ocean waves made
a seafood dish taste better, The Fat Duck serves a 'Sound of the Sea' dish that is accompanied by
an iPod playing ocean sounds.
“What makes most people click when you mix gin and tonic?”
How do flavours come together? Per Møller, a sensory scientist at the University of Copenhagen in
Denmark and Barham's co-editor at Flavour, says that cooking presents a wealth of scientific
puzzles — such as the basis of food pairings. “What makes most people click when you mix gin and
tonic?” he asks. One hypothesis proposed by chefs is that foods that go well together have certain
flavours in common — a company called Sense for Taste in Brugge, Belgium, has created a foodpairing database on this premise — but Møller says this hasn't been fully investigated by scientists.
A 2011 study of more than 50,000 recipes found that Western recipes choose ingredients with
shared flavour compounds, whereas East Asian recipes tend to avoid them3.
The ultimate question is why we enjoy certain meals. The components of a recipe matter, but they
must be perceived in the right way by the brain. Molecular gastronomy as a discipline can explain
how food preferences, cravings, reward systems, satiety and even expectations affect the eating
experience. A 2008 study by researchers from Stanford University in California and the California
Institute of Technology in Pasadena, for instance, found that people thought the same wine tasted
better when it was labelled as expensive — and functional magnetic resonance imaging scans
revealed that they derived more pleasure from drinking it4.
Barham believes that the current fad for science-based cuisine will run its course in the next few
years, at least at the top restaurants that made it famous. But even so, he says, “there are aspects
of what we're doing that are going to outlast the fads in the kitchen”. Education is one such area.
Barham argues that cooking offers an engaging — and safe — way to run a classroom chemistry
experiment. “If you can do that, you can also encourage more people to cook at home,” he says, a
skill that he hopes will combat unhealthy eating among young people. But leaving aside such
virtuous aims, studies that integrate cooking, chemistry and nutrition also give an added emphasis
to an aspect of food that ought not be overlooked: pleasure.
Box 1: Name that cuisine: Scientists and chefs find fault with 'molecular gastronomy'
Science has had a big influence on cooking, and cooking has made its way into many laboratories
and journals. But what should we call this hybrid field? In 1992, French chemist Hervé This and
Hungarian physicist Nicholas Kurti organized the International Workshop on Molecular and Physical
Gastronomy in Erice, Italy, which was attended by chefs and
scientists from around the world. The idea was to launch a
scientific discipline devoted to investigating 'culinary
transformations'.
Hervé This later dubbed the field 'molecular gastronomy', a term
that has become linked with unusual and technically innovative
dishes served at top restaurants. He has argued that such work
should be called 'molecular cuisine', and avoid the term
gastronomy, because it represents the application of science rather
than true scientific investigation.
Many chefs also eschew the term 'molecular gastronomy'. In a
2006 open letter published in the UK newspaper The Observer, for
example, food writer Harold McGee joined cooking pioneer Ferran
Adrià and several other chefs to explain that the workshop “did not
influence our approach, and the term 'molecular gastronomy' does not describe our cooking, or
indeed any style of cooking.” — C.H.
Obesity: Insensitive issue
It is becoming clear that links between taste preferences and obesity go beyond simply having a
sweet tooth.
Subject terms:
Disease Neuroscience Molecular biology
What does chocolate ice-cream taste like? A simple enough question, you might think: sweet and
creamy, with a slightly bitter cocoa kick. Delve a bit deeper, though, and the exercise becomes
impossibly subjective, because what you taste when you eat ice-cream is not the same as the next
person's experience. Your tongue and your taste buds are unique, and a sweet taste that seems
strong to you might be almost undetectable to someone else.
Perhaps more importantly, individuals also vary greatly in how pleasurable and satiating they find
ice-cream, or any number of other foods. Could someone's taste perceptions and preferences be a
major influence on their weight?
The emergence of obesity as the world's largest preventable health disorder gives urgency to this
question. Although the drivers of obesity are far more complex than simply a sweet tooth, study
after study suggests that shifting taste preferences are a big part of the puzzle. The latest findings
are forcing us to fundamentally re-examine our understanding of taste perception itself.
Expanding research
Research into a link between an appreciation for sweetness and body weight goes back at least to
the 1950s. And almost from the start, the evidence has been contradictory. A landmark 2006
review1 by Linda Bartoshuk, a sensory scientist at the University of Florida in Gainesville, offered
an explanation for why the results have been so mixed. Bartoshuk argued that a taste described as
“extremely sweet” by a lean person might not seem so sweet to someone overweight, because
their food experiences are different — and ignoring this divide masks the difference in their taste
preferences. “We discovered something that should have been obvious — that if you're fat, you like
sweet and fat better — that's part of what keeps you fat,” she says.
“If you're fat, you like sweet and fat better — that's part of what keeps you fat.”
Although studies such as Bartoshuk's point to a slight preference for sweet foods among obese
people, the link becomes much stronger when a food's fat content is also considered. “There's a
relatively clear link between sweet and fat together — obese people tend to like sweet fatty foods,”
says Lucy Donaldson, who studies taste perception and ill health at the University of Bristol, UK.
“But they also tend to like fatty savoury things,” she adds.
Recent research into links between obesity and savoury preferences has not revealed any clear
relationship with salt or umami taste perception, however. Only one link seems to hold firm. “What
is strongly associated with obesity is not the consumption of carbo-hydrates or sweets — it is the
consumption of fat,” says Yanina Pepino, who studies taste perception and disease at Washington
University in St Louis, Missouri. “The obese crave more fat.”
Fat is not one of the established five tastes (sweet, sour, salty, umami and bitter). However, a
growing group of researchers suspect that the sense of taste is implicated (see 'Hardwired for
taste', page S7). For one thing, the tongue has taste receptors for two of the three macronutrients
we need in our food: sweet corresponds to carbohydrates, and umami indicates protein. “It makes
logical sense that we have some form of taste response to the other macronutrient: fat,” says
Russell Keast, a sensory scientist at Deakin University in Melbourne, Australia. Fat content affects
not only a food's taste, but also its appearance, texture and perhaps even smell. Keast and his
colleagues have devised tests to assess whether people can detect fatty acid in custard when all
sensory cues apart from taste are removed. “The sense of smell and any textural effects are
minimized, and everything is under red light just in case there are any visual differences,” he says.
Just as carbohydrates and protein are revealed by the taste of their breakdown products, Keast
thinks we can taste the breakdown products of fat — fatty acids. And intriguingly, the obese and
overweight seem to be less sensitive at detecting them2. Last year, using their 'sensory-matched
custards', Keast and his colleagues showed that people with low sensitivity for fatty acids tended to
consume significantly more fat in their diet, and have a higher body mass index (BMI), than people
with high sensitivity for flavour3.
Pepino is also researching fatty tastes. “We still cannot claim that fat is a basic taste, but within the
limitations that we do know, the relationship between taste perception and obesity is strongest
with fat,” she says.
Rich taste
The prime candidate for the fatty acid taste receptor is a protein called CD36. The initial evidence
came from rodents: mice genetically engineered to lack CD36, for example, lose their natural
preference for fat, whereas their affinity for other tastes remains unaffected4. Pepino is
investigating similar patterns in people with natural genetic variants that lead to either over— or
under — expression of CD36. “We hypothesized that if you are in the group with the higher
expression level then you will be more sensitive to detect fat at very small concentrations. And
that's exactly what we found,” she says.
Even so, the idea that a 'fat taste' even exists is not universally accepted. Bartoshuk, for example,
questions how fatty acid detection could play a role in sensing dietary fat in the mouth. “Fat
doesn't usually break down until it gets to the stomach,” she says. “And if you taste fatty acids in
their pure form, they are bitter — really nasty. So how is something nasty, that's barely detectable,
going to help you regulate fat at all?”
Keast argues that the explanation lies in the fact that fatty foods naturally contain low levels of free
fatty acids alongside the fat itself. “As the fat level rises, so do the levels of naturally free fatty
acids,” he says.
What's more, the unpleasant taste of fatty acids doesn't become apparent until their concentration
is much higher than our threshold detection level — which is quite low, he adds. As food degrades
naturally, the level of free fatty acids increases “until they can be recognized. This is unpleasant
and indicates the food is past its best.” It's a similar story for other tastes, he adds — too much
salt, for example, turns an enjoyable flavour into something unpleasant.
Cause or effect?
Now the correlation is emerging, the next big question pertains to causation. Does a reduced ability
to taste fat tip someone towards eating an excess of fatty food, or does eating a fat-filled diet
depress our sensitivity to fatty tastes?
The answer probably lies in the middle, with a balance of genetics and environment. “Whether you
like fat depends on many different things, starting with what you're exposed to in utero and as a
neonate, which sets quite a lot of your preferences,” says Donaldson (see 'Partners in flavour',
page S4). But some studies have shown that these preferences are still malleable after childhood.
Change your diet, and your preferences can be partly reset — for better or worse.
The early work on dietary malleability was carried out with salt. Consuming too much salt has long
been associated with high blood pressure and hence an elevated risk of cardiovascular disease.
Reducing salt intake over a period of time diminishes someone's desire for salty food. Donaldson
and colleagues have recently shown with sugar that things can also swing the other way5. A group
of young adults were asked to consume two sweet sports drinks a day for a month in addition to
their regular diet. “We wanted to know if you could change sweet preference in a relatively short
period of time — and we were quite surprised when we did,” Donaldson says. “The people who
didn't prefer sweet things at the beginning preferred sweet things at the end.”
Keast is starting to see similar shifts with fat taste perception6. “If we go on a low-fat diet, our
sensitivity to fats increases,” he says. In one experiment, “both lean and obese people responded
to a very-low-fat four-week diet, becoming orally more sensitive to fat”. Keast is now beginning
longer-term trials to assess whether this increased sensitivity can be translated into a decreased
preference for — and hence consumption of — fatty food.
Obesity is a multifaceted disease with no simple, single cause. But for Pepino, one take-home
message is already becoming clear. “Promoting a healthy diet and a healthy lifestyle should always
be the first line of treatment for obesity,” she says. “For people who want to reduce their fat
intake, they just have to persevere and it will become easier.”
Food science: Taste bud hackers
Scientists and psychologists are trying to trick our mouths and minds into enjoying foods that are
better for us.
Subject terms:
Biochemistry Chemical biology Molecular biology
Tucked away in a biotechnology park in North Brunswick, New Jersey, researchers at Chromocell
are trying to make us change the contents of our kitchen cupboards. They are screening hundreds
of thousands of molecules to find ones that can enhance certain tastes, such as sweet or salty, with
the hope of concocting foods that are kind to our waistlines but still excite our palates.
The business of creating taste enhancers and ingredient substitutes for high-sugar, high-salt and
high-fat foods is decades old. Artificial sweeteners such as saccharin have been in widespread use
since the mid-1900s, for example. But such products have had only modest success. Diet soda, for
example, still makes up only 30% of the overall soda market. And potato crisps fried in olestra, a
fat substitute with molecules too large to be absorbed by the gut, have fewer calories but prevent
the body from absorbing vitamins and nutrients. Consumers complained that these crisps caused
unpleasant side effects.
CHRIS LOSS
The combined hot and cold sensations of chilli and peppercorn might make a tasty substitute to
salt.
People don't derive as much
pleasure from most low-fat,
low-sodium or low-calorie
foods as they do from more
indulgent chocolate mousses
and French fries. “Fat has a
taste and a smell, it can
change an item's taste and
smell, it has a texture and it
changes texture. It's a really
tricky little thing,” says
Jeannine Delwiche, who leads
research into reducing salt,
sugar and fat at multinational
PepsiCo based in Purchase,
New York,. “So when you start
to talk about changing fat in a
food, you're going to be
changing all of those things.”
Which means, she says, it's
very difficult to create a
product that gets it all right.
The food industry is hoping
that an updated
understanding of taste and its
underlying biology will yield
flavoursome formulations that
are better for us than the
products on the shelves today.
Fishing for flavours
One of the major steps forward in taste science in recent decades has been the discovery and
exploration of distinct taste receptors on the human tongue. Our taste buds have separate
receptors for, at the very least, five basic tastes: sweet, salty, sour, bitter and umami (savoury).
Of these, the receptors for sweet, bitter and umami are all members of a family of proteins called
G-protein-coupled receptors (GPCRs). And because GPCRs are well understood, they provide
numerous opportunities for scientists looking for molecules that might trigger them.
Enter Chromocell and Senomyx, based in San Diego, California. These companies have adapted
high-throughput screening systems developed by the pharmaceutical industry to find potential drug
candidates against GPCRs, to identify molecules that interact with taste cells. “The whole idea of
using molecular biology to trick or tweak your taste buds is kind of novel for the food industry,”
says Beverly Tepper, a taste researcher at Rutgers University in New Brunswick, New Jersey.
The screening systems work by running a slew of molecules past a panel of taste receptors to see
what sticks. To find new sweeteners, for instance, researchers take the protein that taste cells
recognize as sweet and express it in a stable cell line. “Then we use this as a fishing net to run
thousands of different compounds against that cell,” says Rudy Fritsch, who leads Chromocell's
flavours and nutrition research. When the instruments indicate that one of the molecules has stuck
to a receptor — much like a key in a lock — then the molecule is worth further investigation.
Senomyx and Chromocell use slightly different systems and maintain slightly different research
goals. Senomyx was the first company to use a high-throughput screening approach to taste
research and is concentrating on synthetic chemicals. Two of its sweet enhancers are already found
in products being sold in test markets in China, Africa, North America and elsewhere.
Chromocell is taking a different tack. Rather than synthesizing potential taste enhancers in the lab,
its researchers are putting more emphasis on using their high-throughput fishing net to catch
natural compounds. This is an approach with popular appeal — a growing proportion of the public
views artificial sweeteners and fat substitutes with suspicion, so many companies are looking
instead for natural products.
Not all researchers believe that taste enhancement needs to come from new molecules isolated in a
lab. Scientists in flavour houses (companies that develop chemicals for the food and drink
industry), food companies and academic institutions are also seeking fresh combinations of existing
tastes and flavours, or are tweaking the properties of existing ingredients, to improve a food's
health profile while maintaining its appeal. They're looking for tastes and aromas that can elicit
either physical or psychological reactions to help enhance flavour.
Playing around with the shape and size of salt crystals, for instance, can help lower sodium intake
but maintain saltiness. Smaller crystals, or those produced in the shape of a pyramid, have more
surface area and pack a bigger punch to the palate. But such crystals are only effective for solid
foods, not in soup or mixtures where the salt is dissolved.
“Scientists and flavour chemists are going to be searching every blade of grass and every leaf in
the Amazon for something that might potentiate taste. And there just aren't many things out there
like that that have been found,” says Chris Loss, a chef and culinary scientist at the Culinary
Institute of America in Hyde Park, New York. Rather than looking for something that tastes
precisely like foods higher in sugar or salt, industry researchers are instead aiming to concoct
healthier combinations that are just as tasty.
Because the brain associates salt with savoury, umami flavourings such as monosodium glutamate
(MSG) — a salt of a non-essential amino acid that is found in many everyday foods, such as cheese
and tomatoes — can enhance a food's taste so effectively that less sodium is needed to achieve the
desired effect. And adding flavours that the brain associates with sugary items, such as vanilla, can
trick us into thinking a food is sweeter than it really is.
Loss has been toying with different combinations that might trick the tongue into thinking it is
tasting salt. “Every taste bud is surrounded by chemosensory receptors that pick up on the
capsaicin in peppers or the cooling effect from menthol,” he says. Because these receptors are in
such close proximity to the ones that sense taste, he wondered if “you could tickle them a little
bit”, stimulating the taste receptors without the actual molecular match.
Along with Szechuan cuisine expert Shirley Cheng, Loss has used a combination of chilli peppers
and a uniquely numbing type of Szechuan peppercorn to create an alternating combination of
heating (chilli) and cooling (peppercorn) effects. He tried to find a combination of these ingredients
that would allow him to reduce a dish's added salt. “We did a sensory test and, while we didn't find
any increase in perceived saltiness, we found that people liked it equally well,” Loss says.
The renaissance in taste enhancement is not limited to processed and prepared foods. Some
scientists are trying to restore taste and nutrition to foods that have gradually had the flavour bred
out of them in favour of other traits, such as high yield and resilience to long-distance travel. “If we
can make produce taste better, people will eat more of it,” says
Harry Klee, a horticultural scientist at the University of Florida in
Gainesville.
Working with other researchers at the same institute — Linda
Bartoshuk, who studies taste and olfaction and Charlie Sims, who
runs sensory testin — Klee has developed chemical profiles of
nearly 200 different varieties of heirloom tomato. Those with the
most diverse range of sugars, acids and volatile chemicals were
presented to panels of 100 consumers. These taste testers noted
down which ones they liked and how strong their preferences
were, allowing the researchers to determine which properties of
the fruits confer the most (and least) desirable flavours. By
analysing the highest- and lowest-ranked tomatoes, the
researchers determined which compounds contributed to overall
taste1. “We could then use that data to extract the ideal recipe of
the perfect tomato, which we've done,” Klee says. “Now, we can
rescue the genetics of what's in the good ones and try to
reincorporate those into commercial tomatoes.”
Scents and sensibility
During the taster panels, Klee and his colleagues also discovered
that the smell tests used to assess a food's appeal had been
going about it the wrong way. It was generally assumed that a
food's odour, sniffed before it was put in the mouth, was enough
to determine its impact on taste — the food industry has been
operating on that assumption for decades. Indeed other research
has shown that when we eat something, the odours that go up to
our olfactory bulb from the back of the mouth are processed in a
different part of the brain to the odours taken in when we put our
nose near something and sniff.
“In previous years, people used the wrong volatiles,” Bartoshuk
says. So she, Klee and Sims used a different method. By sorting
the tomatoes according to taste-tester preference, the
researchers were able to pinpoint the volatile compounds that
actually elicited a response. They used volatiles not just to make
a food smell fantastic, but also to taste fantastic — endpoints not
as closely linked as had been thought. “Now we know what to
look for, we've done it in tomatoes and strawberries, and we're
going to do it in blueberries too,” Bartoshuk says.
Fundamentally, the Florida researchers are working towards the
same goal as their counterparts in industry. “We've tried to
educate people about how to eat healthier diets, but none of us
do it,” Bartoshuk says. “I know perfectly well I have no business
eating spare ribs but I eat them anyway because I like them. And
one way to get people to eat healthier is to make [healthy] foods
that they like.”
Evolutionary biology: The lost appetites
Many vertebrates can detect the same five basic tastes that humans can, but there are exceptions.
Are the differences caused by a change in diet?
Subject terms:
Animal behaviour Evolution Genomics
Ronald Fisher must have been relieved when a chimpanzee at Edinburgh Zoo took a sip of water,
looked him in the eye and spat at him.
It was August 1939, and Fisher was testing whether chimps could taste water laced with a chemical
called phenylthiocarbamide (PTC) that some humans find nauseatingly bitter and others can't
taste. Fisher and his colleagues Edmund Ford and Julian Huxley had been worried that the apes
wouldn't make their preferences known, rendering the experiment pointless.
Instead, about three-quarters of the chimps they tested expressed their displeasure with PTC.
Fisher's team speculated that the variation in sensitivity to bitter tastes was caused by a genetic
mutation shared by humans and chimps, and that natural selection had maintained this diversity in
both species. “Wherein the selective advantages lie, it would at present be useless to conjecture,”
the trio wrote1 in a letter to Nature.
More than 70 years later, biologists are still trying to figure it out. The availability of different
animal genomes has given scientists more insight, culminating in the startling discovery that, for
many creatures, some tastes have no evolutionary benefit at all. Kurt Schwenk, a biologist at the
University of Connecticut in Storrs who studies chemical sensing in lizards and snakes, says: “The
whole story of the evolution of taste is really the evolution of loss of taste.”
The most obvious explanation for the changes is lifestyle. At some point in evolutionary history, a
shift in diet removed the need to sense certain chemicals in food. Evolution is a game of 'use it or
lose it', and genes that do not aid an animal's survival or reproduction are liable to build up random
mutations that destroy their ability to make a working protein. Gary Beauchamp, a geneticist at the
Monell Chemical Senses Center in Philadelphia, Pennsylvania, likens the situation to that of sight in
cave-dwelling fish. A life of darkness eliminated the usefulness of vision, so the fish collected
mutations in genes involved in eye development and eventually lost their sight altogether.
Another example of taste loss lies closer to home: cats cannot taste sweet substances. Beauchamp
noticed this quirk in the 1970s, and in 2005 his team finally found out why2. All cats share a
mutation that disables one of the two genes that build a working sweet receptor, whether for the
tongue, intestine or any other part of the body. Because all felines — from domestic cats to lions —
have an identical mutation, it is likely that the sweet-receptor gene became inactive in their
common evolutionary ancestor. Beauchamp speculates that this ancestral animal moved to a diet
composed of protein-rich meat, devoid of sugary plants, negating the need for a sweet receptor.
Beauchamp's team recently discovered that the inability to taste sweetness is more widespread.
They analysed 12 non-feline species belonging to the order Carnivora, including sea-lions, otters
and hyenas, and identified crippling mutations in the sweet-receptor gene in 7 of them3. What's
more, six species carried unique mutations, suggesting that the ability to taste sweetness had been
lost repeatedly over the course of evolution. Presumably the mutations appeared after each species
or its ancestors switched to a meat-only diet, Beauchamp says. This interpretation is supported by
the team's finding that an omnivorous member of Carnivora, the spectacled bear, still has a
working sweet receptor.
There may be other reasons why tastes are so dispensable. As well as lacking sweet receptors,
dolphins also lack the ability to build working umami (which sense amino acids in proteins) and
bitter taste receptors. One theory posits that because dolphins swallow their food whole,
moderating their intake is irrelevant. However, this theory applies only to the ability to taste food,
and ignores the role of these receptors in other parts of the body (see 'Hardwired for taste', page
S7). There is no evidence that losing taste genes confers any advantages to an animal, although
Schwenk is happy to speculate. “Most carnivores are scavengers as well as predators,” he says.
“They eat a lot of rotting flesh, so it might be good to not taste it so well.”
Pandas, a largely vegetarian member of Carnivora, lack umami receptors. The pandas' bambooonly diet gives them little need to detect proteins, says Jianzhi Zhang, an evolutionary geneticist at
the University of Michigan in Ann Arbor, who reported this lack of receptors in 2010. His team also
discovered that vampire bats have no sweet receptor, and that all bats, including insectivorous
species, lack umami receptors4. Zhang's team has also been unable to find working sweet-receptor
genes in several species of birds, grass-eating horses and omnivorous pigs — a pattern he finds
puzzling.
Devonian tastes
The sense of taste must have carried an evolutionary advantage to have evolved in the first place.
Beauchamp says that taste-sensing systems in the mouth perform two essential tasks: umami and
sweet detection help animals find energy-dense nutrients, and bitter detection helps them avoid
toxic substances. Scientists know far less about the biology and evolution of sour and salt tastes.
One theory is that detecting salts helps an animal control its levels of sodium and other ions,
whereas sour tastes help it avoid the acids in unripe fruit and spoiled foods.
All vertebrates have some ability to avoid toxic chemicals and seek out nutrients. By comparing
animals from different branches of the evolutionary tree, scientists have inferred that taste
probably evolved more than 500 million years ago — before land vertebrates, bony fish, sharks and
lampreys diverged — when their common ancestor, a primitive fish, developed a new kind of cell.
“The super-tasters among the animal world are goldfish.”
Taste buds have been repeatedly tweaked over time to suit various animals' dietary needs, says
Thomas Finger, co-director of the University of Colorado's Rocky Mountain Taste and Smell Center
in Aurora. Many fish are covered with taste buds. “The super-tasters among the animal world are
goldfish,” says Finger. “Goldfish and catfish have way more taste buds than anybody else.” They
have poor vision, and their taste buds, including those on their whiskers, could help them sense
their way to a meal in murky water, he adds.
KEREN SU/GETTY IMAGES
A strict bamboo diet might have stripped the panda of the umami
taste receptors.
The evidence suggests that umami receptors were the first to
develop. In 2008, Zhang's team reported the discovery of genes,
similar to those that encode receptors used by humans and mice
to sense the amino acid glutamate, in the genome of the elephant
shark, a species that branched off from other fish 400 million
years ago. Sharks lack bitter taste receptors, suggesting that
these genes evolved more recently.
Bitter is perhaps the taste that most intrigues evolutionary
biologists. It is the 'Darwin's finch' of taste, elaborated and
customized to suit a species' ecological niche. Humans have 24 or
25 (depending on the person) different bitter receptors, each
recognizing unique combinations of chemicals.
A strict bamboo diet might have stripped the panda of the umami taste receptors.
Toxic bitter compounds come in all shapes and sizes, so it makes sense that the receptors that
recognize them are diverse, says Beauchamp. Bitterness is code for danger, but many bitter
compounds also provide important nutrients. For example, during the lean winter months, the
Japanese macaque supplements its diet by eating willow trees. The bark of the willow contains
salicin, which tastes bitter to many animals. A recent study of the bitter receptor T2R16, which is
common to all primates, reported that the macaque version is the least responsive to salicin5.
Bitter taste evolution, then, is also about distinguishing between different chemicals. “If you go out
into the real vegetable world, what you'll find is that almost everything is bitter,” says Beauchamp.
“An animal that rejects everything that's bitter would be in trouble.”
This principle could explain Fisher's discovery that some individuals, among both humans and
chimps, are unable to taste PTC. Seven decades later, researchers finally identified the gene
responsible for PTC sensitivity, called T2R38, and confirmed that different versions of the gene
largely explain why not everyone can taste the chemical (see 'The finer points of taste', page S2).
Recent studies have refined Fisher's ideas about how the variations in PTC sensitivity evolved. A
team led by Stephen Wooding, an evolutionary biologist at the University of Texas Southwestern
Medical Center in Dallas, repeated Fisher's experiments6 using apples spiked with PTC, instead of
water solutions (perhaps to avoid being spat at). Chimps still differed in their ability to taste the
bitter chemical, but DNA sequencing revealed that chimpanzees that cannot taste PTC have an
entirely different mutation in T2R38 from human non-tasters — their insensitivity evolved
independently.
One of the compounds that T2R38 recognizes, goitrin, is abundant in cruciferous vegetables, such
as broccoli and Brussels sprouts. Goitrin can prevent hypothyroidism, a condition caused by low
iodine intake. Variation in sensitivity to PTC and goitrin might persist because it could help iodinestarved populations avoid hypothyroidism and obtain nutrients from vegetables.
Diet is not the only driver of change in bitter-tasting ability, however. Geneticist Sarah Tishkoff's
team at the University of Pennsylvania in Philadelphia recently tested7 hundreds of individuals from
57 human populations in Central and West Africa. They found both PTC tasters and non-tasters,
but there was no obvious correlation with their different diets.
In the past decade, scientists have discovered sweet receptors in the gut that influence insulin
levels, and bitter receptors in the lungs that can clear inhaled substances. Tishkoff's team suggests
that roles such as these, and not diet, might explain the evolutionary differences in PTC tasting.
Zhang also thinks there is more to the evolution of taste than just flavour. He suggests that
researchers use model organisms, such as mice lacking various taste genes, to understand the
roles of these receptors. “We're finding a lot of mismatches between feeding ecology and tastereceptor evolution,” he says. “Perhaps we still do not have a complete understanding of the
functions of those genes — or of taste.”
Technology: The taste of things to come
Artificial tongues that mimic the human sensory experience could aid the development of better
and more consistently flavoured foods.
Subject terms:
Biochemistry Information technology Structural biology Technology
Imagine a food wholesaler in London deciding whether to buy a new batch of coffee from a farmer
in Brazil. She'd like to taste a sample to see if it will appeal to her customers. So she links to the
supplier over the Internet, makes a selection, turns to a machine on her desk, and voilà! — she's
sipping coffee on another continent.
This scenario — replicating the flavour of not only coffee but a wide variety of food and drink —
isn't all that far fetched, says physicist Seunghun Hong of Seoul National University in South Korea.
“You can actually reproduce those smells or tastes using very cheap chemicals,” he says.
What's lacking for such a device is not the technology to recreate flavours and aromas, but the
data about the specifics of such sensations that could be transmitted from the Brazilian grower to
the British buyer. “We don't have those kinds of quantitative numbers to evaluate your smell and
taste. That's why you cannot say that this coffee tastes the same as coffee ten years ago,” Hong
explains.
But he and other researchers are working to change that, developing artificial tongues that can
respond to flavours in ways analogous to the human tongue. Such sensors could provide quality
control in food and agriculture, record information about taste in ways that are not now possible,
aid the development of new flavours and artificial sweeteners, and help make medicines more
palatable.
Electronic tongues
There are well-established technologies for measuring and reproducing three of the five human
senses — sight, sound and touch — but mimicking our two chemical senses, taste and smell, has
proved more challenging. Kenneth Suslick, a chemist and materials scientist at the University of
Illinois at Urbana-Champaign, says that artificial versions of these senses would have practical
applications and also provide an aid to understanding another aspect of human biology.
Suslick works on both artificial tongues and artificial noses: the noses deal with gases, whereas the
tongues handle liquids (and solids that have been liquefied, which is what happens when we eat).
Human noses, and the devices that mimic them, contain hundreds of different receptors capable of
identifying the complex patterns of thousands of smells. Taste, by contrast, is made up of just a
handful of components — the exact number is disputed, but researchers have agreed on at least
five (see 'Hardwired for taste', page S7).
Taste lends itself to automation because it is fundamentally a chemical process. Sweetness sensors
react to sugar molecules. Sourness is a measure of pH. Saltiness is a measure of the positively
charged ions of alkali metals, notably sodium. And umami, the savoury or meaty taste, is detected
by a receptor for glutamate. Only the fifth major taste, bitterness, is poorly defined, and may be a
catch-all term for several different chemical reactions, Suslick says.
Artificial tongues often rely on measuring changes in electrical potential or current caused by the
target molecule reacting with a receptor. These devices are therefore often called electronic
tongues, or e-tongues, and work in a similar way to the real thing. In the human tongue, various
molecules in food bind to proteins in the taste buds, producing a pattern of electrical signals that
the brain interprets as a particular flavour. Although it might be possible to build a device that
would break down a food into its component chemicals and measure their relative quantities,
creating such a set of instructions for say chocolate ice-cream wouldn't be practical, because a
single food item might contain more than 1,000 different chemical substances. And that's not how
people identify tastes anyway, says Kiyoshi Toko, professor of information science and electrical
engineering at Kyushu University in Fukuoka, Japan.
“Humans don't discriminate each chemical substance,” he says, but rather classify a discrete set of
tastes. He believes there are two additional tastes beyond those widely accepted. One he calls
astringency, which is a form of bitterness caused by tannins. The other he calls pungency — that's
the sting from foods such as hot peppers, and in humans is experienced by receptors for heat and
pain.
Toko has developed an e-tongue that consists of a series of polymer membranes, each coated with
a different lipid, fitted onto a plastic tube and connected to an electrode. When the tube is
immersed in a sample liquid, the taste molecules in the liquid interact with the lipids and change
the electrical potential of the membranes in a characteristic way. The result is a readout that
corresponds to taste.
A company founded to market his technology — Intelligent Sensor Technology of Kanagawa, Japan
— has sold about 300 taste sensing machines in the past five years. The system can examine foods
such as soy sauce, soup and sake for six taste qualities — the five standard ones plus astringency.
The same sensors also produce readings for five aftertastes, mostly variations of bitterness as well
as the 'richness' evoked by umami. He says he doesn't know how to make a sensor to detect
pungency, but he's working on it.
KENNETH S. SUSLICK
Disposable chips produce flavour 'fingerprints'.
Hong has a device that works in a similar way. He built a transistor using carbon nanotubes, which
are highly sensitive to electrical changes, and coated the surface of the nanotubes with a protein —
one that detects bitterness, for example. When substances in the sample bind to the protein, they
send a signal through the transistor, and a change in electrical conductance registers the
bitterness. This technique mimics the human response more closely than some other approaches,
he says, because the proteins he uses are those actually found in human taste buds, which makes
them highly selective for the molecules to be tasted. He produces the proteins by inserting human
genes into Escherichia coli bacteria. “Our device has exactly the same response as the human taste
system,” he says.
Suslick's system, by contrast, relies on colour-changing dyes, rather than electrical changes. His
sensors are designed to measure sweetness. He originally printed an array of commercially
available reagents on an acetate membrane, measuring about 2.5 cm by 4 cm. The sensor can
simply be dipped into a sample liquid, where various substances react with the different pigments
and change colour. A computer compares images of the array taken before and after dipping, and
produces a 'difference map' that shows how much each spot has changed.
He tested his sensor on 14 different natural and artificial sweeteners. Each array produced a
pattern of colours unique to the particular sweetener, providing visible fingerprints for substances
including aspartame, saccharin and fructose. The cheap, disposable sensors gave results in about
two minutes, speed that could be attractive to a beverage company that wants to use the sensors
for quality control. Combining this 'taste chip' with other arrays for sour, bitter, salt and umami
tastes might provide a reasonable mimic of the human tongue, Suslick says.
A flavour sensation
However, neither e-tongues nor colour matrices can completely reproduce the experience of
flavour. “An awfully large fraction of what we call taste is actually smell,” Suslick points out.
“Anyone who's ever had a head cold knows that.” The whole taste experience is based on a
complex interaction of responses by numerous receptors. Each receptor can respond to more than
one chemical, and its response can affect how the brain interprets the responses from other
receptors. The result, whether in the brain or an artificial system, is a complicated feedback
pattern. Researchers use statistical methods from the field of artificial intelligence to map out the
complex response patterns from an array of sensors. Suslick says it's not yet clear how many
points such a map must include to describe an approximation of someone's reaction to taste.
“Ultimately, we would like to take the patterns and see if they can be predictive of human
response,” says chemist Eric Anslyn of the University of Texas at Austin. With a colleague who
studies wine-making at the University of California, Davis, Anslyn has developed a sensor that can
identify different wine varieties. His device can distinguish between a Pinot Noir and a Merlot, for
instance, but it cannot yet discern nuances, such as whether the drink is oaky or fruity.
A crucial part of developing an artificial tasting system is calibrating its results to real human
responses. In Denmark, chemist Anders Malmendal of the University of Aarhus is trying to do this
by using what he calls a 'magnetic tongue'. His technique relies on readings from nuclear magnetic
resonance (NMR) spectroscopy, a common laboratory analytical technique for identifying chemical
composition and concentrations. Often used in metabolomics, which screens biological samples for
the by-products of metabolic activity, NMR is a good way to detect toxic effects of new drugs.
Malmendal, along with colleagues from the University of Naples Federico II in Italy, used NMR
spectroscopy to identify different brands of canned tomatoes. The researchers bought 18 cans at
stores around Naples and used NMR to obtain their chemical signatures, each consisting of 870
variables.
To match these signatures to subjective human taste, Malmendal and colleagues turned to a panel
of 12 human experts. These tasters provided detailed descriptions of flavour, odour and texture,
according to an accepted set of food industry standards. Malmendal's team then matched the NMR
data to the panellists' assessments, so they could use the chemical signature to say what a
particular tomato would taste like to people.
With a large enough library of reference samples, food producers could test a sample and predict
how consumers would react to it. One company, Bruker Biospin of Cologne, Germany, sells a juice
screener that uses NMR spectroscopy and a library of more than 3,000 reference samples of 30
types of fruit from 50 countries. Quality controllers can tell if the juice is fresh or from concentrate,
whether there's added sugar, and whether cheaper mandarin juice is mixed in with the orange
juice — in some cases they even can tell roughly where the oranges originated. In March 2012, the
company introduced a similar system for wine.
Any technique that can consistently distinguish one food sample from another can be used this
way. Build up a library of human assessments of various instances of different foods, and you can
predict the taste of new samples based on how well they match. But Suslick points out two
problems with this approach. Paying experts to assess every variety of every food would be
prohibitively expensive. And in the end, no matter how sophisticated the gadgetry, taste remains a
stubbornly subjective experience. “Can we distinguish between coffees? Yes,” Suslick says. “Can I
tell you what the best coffee is? No.”