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The Wormwood Society

Tourist in a taste lab – taste research

A fascinating account of the genetic science behind why some people abhor sugar in their absinthe, and why others can’t drink it without.

THE BRAIN IS THE MASTERMIND OF FLAVOR BUT TONGUES ARE WHERE IT STARTS-AND SOME ARE FAR MORE SENSITIVE THAN OTHERS.

A GROUP OF US, some scientists but mostly not, have just had dinner
together at the New York Academy of Sciences. We’ve eaten well–broiled
salmon with spears of asparagus, a leafy salad, goblets of green-tea,
ice cream, wine and coffee for those who want them–and settled down
for the evening’s lecture. The speaker, from Yale University, is Linda
Bartoshuk, a specialist in human taste. She hands us each a small
packet containing what looks like a Communion wafer. It’s a piece of
filter paper saturated with a compound called propylthiouracil, known
in taste circles as PROP. We’re instructed to put the papers in our
mouths. As my saliva wets it, a nasty bitterness blooms. My neighbor,
too, is making a face that says yuck. Then Bartoshuk asks for a show of
hands. How many of us tasted something? How many of us didn’t? Of
course, Bartoshuk knows the punch line: Typically, a quarter of the
audience tastes nothing. This routine never fails to impress an
audience. Jaws go slack as hands shoot up in answer to both questions.
How can something be tasteless to some people and so unpleasantly
bitter to others? If we trust our senses to reflect the real world, the
answer seems unnerving: We may think we all ate the same dinner tonight
but we don’t all live in the same taste world.

Actually, researchers began suspecting as much in the 1930s, after
a chemist who was making a batch of a compound called
phenylthiocarbamide (PTC) let a puff of the crystals fly into the air.
A lab colleague, who must have swallowed some of the airborne crystals,
noted how bitter they were. The astonished chemist, who tasted nothing
himself, became the first to describe "taste blindness" to the bitter
compound. Of course, taste blindness to PTC or its chemical cousin PROP
might just have been a scientific curio–so-called nontasters do
respond to other types of bitterness. But, as it turned out, nontasters
respond to all types of bitterness less intensely than tasters, and the
degree to which people taste PROP can serve as a general indicator of
their overall taste capacity.

Judging from family studies, the inability to taste PROP is genetic
and most likely due to a recessive gene. That would fit rather nicely
with Bartoshuk’s finding that there exists a subset of PROP-tasters
supersensitive to bitterness. She calls them super-tasters. Looking at
the three groups, you see just the sort of patterns you’d expect for a
recessive gene. Roughly 25 percent of people tested with PROP don’t
taste it, consistent with two recessive genes; 50 percent are tasters,
consistent with one recessive and one dominant gene; and 25 percent are
super-tasters, consistent with two dominant gene copies, one from each
parent. The distribution is slightly sex-skewed. More women than men
are super-tasters, perhaps because bitter is the skull and crossbones
of the natural world, and evolution once favored mothers with superior
poison-detecting systems.

What’s fascinating, says Bartoshuk, is that tongue anatomy spells
out the differences among these groups. Tasters have more taste buds
than nontasters, and super-tasters have the most. Because taste buds
are surrounded by nerve endings that sense not only taste but pain and
touch, super-tasters, perhaps not surprisingly, have a more sensational
taste repertoire. Bitter tastes bitterer, salt a bit saltier, sour
sharper, and some sweets sweeter. Fat feels fattier, gums thicker, and
alcohol and chili burn more fiercely. The inside of a nontaster’s mouth
makes up "a very small world compared to the super-taster’s," says
Bartoshuk, a non-PROP-taster herself. But because the super-tasters’
sensory realms are so intense, they may avoid strong tastes–especially
bitter ones like grapefruit, coffee, beer, or broccoli–and thus
actually shrink their dietary horizons.

Intrigued by her discoveries, I persuaded Bartoshuk to let me visit
her lab for a two-day immersion in taste. One experiment in particular
had caught my attention. A surgeon injects anesthetic through an ear to
knock out a cranial nerve that runs into the tongue. ("Sure you want to
do this?" a friend asked, envisaging my face with some kind of lopsided
palsy.) "If you’re lucky," Bartoshuk said, "you may get a taste
phantom," a taste that appears for no apparent reason. I had a flicker
of recognition. Every once in a while a metallic taste sneaks up on me,
as if iron had somehow leached into my mouth. Here was a chance for me
to confront my phantom.

LINDA BARTOSHUK’S LABORATORY is on the second floor of a redbrick
building in the Yale University School of Medicine complex. It differs
little from any other equipment-crammed lab, except for the food
paraphernalia–jelly-bean jars, hard candies, bottles and bottles of
Tabasco, jalapeno, and other hot sauces. And then there are the
magnified pictures of tongues. The photographed tongues are strikingly
individual, some lightly patterned and others crammed with bumps.

Bartoshuk is an affable, outgoing, and generous woman in her early
sixties. She likes a good laugh. This morning, though, she’s all
business. After a few minutes’ casual conversation, she snaps on her
are missing," Bartoshuk says. "Now, no big deal. It suggests some
trigeminal damage. The most likely cause would be a mild dental injury.
Did you ever have a shot of novocaine that nailed a nerve?"

Not that I recall.

She counts the fungiform papillae on the monitor. "Hard to know
whether you’re a high medium-taster or a low super-taster. You’re on
the borderline, one of those people for whom we need a gene test, which
is an area of research that’s getting very exciting. You see here, look
at the density of the fungiforms," she says, pointing to a region on
the front-right of my tongue, where the fungiforms crowd together much
more tightly than in other places. "There it’s high, more
characteristic of a super-taster tongue."

Then she points to widely spaced spots on the screen. "On my
tongue–I’m a non-PROP-taster–the fungiforms would be like polka dots,
here and here and here. But notice over here" she says indicating a
barren patch on my tongue’s left half. "We don’t see very many. There
should be more fungiform papillae over here."

Next Bartoshuk ponders my fungiformless spot. "Something has
clearly happened to your trigeminal nerve," she says. In humans the
upkeep of the fungiform papillae isn’t done by the front taste nerve;
it’s done by the trigeminal nerve. "This is the nerve that dentists try
to deaden when they’re doing a lot of drilling work. And it’s very easy
to accidentally penetrate it with the anesthetic injection" she adds.
"It could have happened when you were a kid, a long time ago."

The nerves going to the mouth are vulnerable to many kinds of
insults: dental work, ear surgery, knocks to the head, whiplash, common
viral infections, and the like. For anatomic reasons, the taste nerve
serving the front of the tongue, the chorda tympani, takes the brunt of
viral hits. As the nerve travels between the brain and the tongue, it
passes right under the eardrum, the tympanic membrane. So during an ear
infection, or an upper respiratory infection affecting the ears, a
virus sometimes infiltrates this taste nerve and knocks it
temporarily–or even permanently–out of commission.

Lucky for us, our sense of taste has a system of compensation.
Normally the nerves of the tongue inhibit one another. The taste nerve
in the front, for instance, inhibits the taste nerve at the back, and
vice versa. Injure the front nerve, and there’s a release of inhibition
on the signals from the back nerve, which makes up for the deficit.

"So when taste in the front goes down, taste in the back goes up,"
says Bartoshuk. Our system for maintaining taste works so well, she
adds, that "many people who have local taste losses aren’t aware of
them until we test them." But sometimes there’s a cost. A loss in one
nerve can lead to exaggerated responses in the disinhibited nerve, or
even conjure up sensations that seem to have no real-world cause.

These "taste phantoms" says Bartoshuk, "are sensations in the
absence of stimulation" They come in all four of the classic tastes, as
we might expect, as well as the sour-mineral taste we interpret as
metallic. The most common "release-of-inhibition" phantoms turn up at
the back of the tongue when the taste nerve up front sustains some type
of damage. My metallic visitor tends to make an appearance when I have
a stuffy cold or flu, probably because the invading virus has made it
into the chorda tympani.

It’s possible to induce phantoms like this experimentally by
mimicking injury to the chorda tympani with a carefully aimed,
nerve-deadening anesthetic injection. The technique came about by
accident, Bartoshuk recalls. She and John Kveton, an otolaryngological
surgeon at Yale University School of Medicine, were walking away from a
lecture together. "John made a comment about a patient who’d had an
anesthetized chorda from an eardrum injection. And I said, `You can do
that?’" From a taste researcher’s point of view, that injection
represents a vast improvement on the traditional lingual block or
dental injection. Unlike the block, the eardrum injection doesn’t
deaden touch. And it numbs only one taste concentration like a light.
Studying taste is tricky, time-consuming work. And having to coax human
volunteers through batteries of tests–as opposed to pointing
electrodes at rats’ tongues or manipulating cells in a dish–doesn’t
make the task any easier. Although she is not a physician, Bartoshuk
also investigates severe taste problems in patients referred to her by
their doctors.

First, Bartoshuk performs a checkup–a spatial test of my
tongue–by painting tastes on it one section at a time. Using cotton
swabs as brushes, she dips into various concentrations of sodium
chloride, paints the salt solutions on my tongue, and asks me to rate
their intensity on a scale of zero to 100. Then she brushes on sucrose
to test my sense of sweet, citric acid to test sour, and quinine to
test bitter. She’s aiming for little bumps called papillae on the
tongue’s front, sides, and back. Those on the front are the so-called
fungiforms, because they look like button mushrooms. The foliates,
supposedly leaflike, show up as a reddish series of folds on each edge
near the back. Way, way back, practically down the throats of some
people, lie the circumvallates, which stand like round moated towers
across the tongue’s surface in an inverted V.

We call the bumps of our papillae taste buds, but they’re more like
Xs marking taste-bud spots: Our taste buds, for the most part, are
nested into them and much too minuscule to see. Special receptor cells
poke out of these tiny organs to catch the sweet, salty, sour, and
bitter molecules that land in the mouth. When tasty molecules stimulate
the receptors, they in turn stimulate nerve endings inside the tongue,
and the messages ricochet along nerves to the brain.

"So this is essentially a neurological test for nerves in your
mouth" explains Bartoshuk. "I’m putting solutions on areas of the
tongue where I know which nerve is innervating the tissue, and I want
you to evaluate what you’re perceiving."

The nerves to which she’s referring are the two main cranial nerves
for taste that run from the brain to the tongue’s front and back. When
she dabs sweet onto the fungiform papillae, she knows she’s stimulating
the taste nerve at the front: the chorda tympani. When she puts sweet
onto the circumvallates, she’s testing the taste nerve at the tongue’s
back: the glossopharyngeal. In addition, by dabbing on pure alcohol or
capsaicin, the fiery chemical in chilies, she can test the nerve for
touch, called the trigeminal, which sends little, pain-sensitive fibers
to the papillae. It’s as if she were examining a fuse box,
systematically checking the switches and wiring of my taste system.

As Bartoshuk continues to dot my tongue with tastes, I suddenly
comprehend the inaccuracy of those textbook tongue maps–the ones
showing sweet corralled at the tip, salt and sour on the sides, and
bitter on the back. We clearly taste them all over. "The tongue maps
are wrong," Bartoshuk says flatly.

Judging from the way I rate tastes, especially bitter tastes, I’m a
relatively "strong responder" says Bartoshuk. "You’re not going to have
a nontaster tongue, that’s for sure. The question is just how many
fungiform papillae you have." A typical nontaster’s tongue has few
fungiforms, a super-taster’s tongue packs loads of them, and a
medium-taster’s tongue falls somewhere in between.

To make the papillae more visible, Bartoshuk stains my tongue’s
surface a ghoulish blue with vegetable dye (it delineates the
fungiforms, which stay pink, from other, bluish tongue tissue). Then,
because the tongue muscle tends to fidget, I’m asked to hold mine
between two little plates of clear plastic. A video camera is trained
on the setup, and suddenly an image of my tongue, magnified 10 times,
lurches onto the video screen.

"You have an area of your tongue where the fungiform papillae
nerve–the one serving the tongue’s front, and only on the injected
side. A collaboration was born.

Kveton has a steady, deft touch and a calm, unflappable manner to
match. His specialty is operating on acoustic neuromas, a type of tumor
that grows next to the brain, eventually involving the taste nerve. I
feel in good hands as he injects my right ear the following morning.
The needle pushes into the skin inside the ear canal, close to where
the nerve runs under the eardrum, and then the injected lidocaine
washes over the nerve. After a brief recovery period, Bartoshuk tests
the right tip of my tongue with a salt solution. I can feel a little
drag from the cotton-topped stick on my tongue, a bit of coolness from
the moisture, but there’s no taste–none whatsoever. The nerve’s
completely out, so Bartoshuk gets busy with her tests. "Sip some
water," she instructs. "This will often precipitate a phantom if you’re
going to get one." It doesn’t take long.

My phantom drifts in gradually, as if it needs time to assemble all
its parts. It comes in as sour at the back of my mouth, on the left,
and then slowly the characteristic metal notes fill out. I’m oddly
pleased, considering that the ferrous taste usually spoils food and
drink. There’s pleasure in recognizing something familiar, of course,
and pleasure in the control that comes with understanding. But the
pleasure also comes from sheer wonder.

"Notice the metallic is contralateral–opposite–to the side of the
anesthesia?" says Bartoshuk. "That’s incredibly important because it
has to be the brain that’s doing that. The left and right halves of the
tongue are innervated separately. The first time signals from the two
halves interact is in the brain. So when we do something on one side
and the other side is affected, we conclude it happened in the brain."

MY PHANTOM WAFTS IN AND OUT, finally fading for good as the
anesthesia wears off. In the meantime, Bartoshuk has something else up
her sleeve.

She shows me how the brain uses touch to "place" taste in the
mouth. She brushes salt around the tip of my tongue, going from my
right no-taste side to my left tasting side. And as expected, I taste
nothing until salt hits the left. But when she reverses direction, and
swabs salt from left to right, something counterintuitive happens. The
salt I can taste for real on the left swooshes over to the right
side–which, in fact, can’t taste a thing. My brain produce a
continuing illusion of taste on the right, because it’s continuing to
get touch signals there. As Bartoshuk puts it: "The taste sensation
follows the touch path." Feeling is believing.

Much of what we commonly refer to as taste, of course, consists of
smell, the aroma of food in our mouths. It’s not sweetness that makes a
banana banana-y; it’s the scent. When we’re eating a banana, however,
we’re not aware of smelling its scent up the back chimney of our noses.
It feels as though we’re sensing it inside the cavernous laboratory of
our mouths, as part and parcel of an overall flavor. Again the brain
appears at work here, referring food-aroma to the mouth by the
sensations of touch and taste.

Valerie Duffy, a taste researcher at the University of Connecticut
who collaborates with Bartoshuk, has come up with a simple
demonstration of how taste is crucial for anchoring food-smells in the
mouth. With my tongue anesthetized on the right, I’m to take a spoonful
of banana yogurt, move it around my mouth, and tell Bartoshuk if
banana-y flavor–the aroma-taste combo–is coming more strongly from
any part of my mouth. And sure enough, I can’t perceive banana very
well on the side of my tongue with no taste–but on my tasting side, I
sense banana loud and clear. "Yeah, you got it," says Bartoshuk. And
with that we call it a day.

Taste-illusion experiments like these show how we taste with our
brains. But when we eat, the brain isn’t just melding taste, smell, and
touch to give us the intricately flavored and textured experience of
food. It recalls the pleasures, and displeasures, associated with the
foods we’re eating. "Sweetness is a deep biological pleasure," says
Bartoshuk. It signals safe calories in nature, and the pleasure’s
probably hardwired. But most of the pleasures of eating are
conditioned, learned by experience. "Does a gourmet enjoy his expensive
chocolate mousse more than I enjoy my Hershey’s bar? I don’t think so.
I like my Hershey’s bar a lot," Bartoshuk says with a big, infectious
laugh.

So when it comes to pleasure, it probably doesn’t matter much
whether we are super-tasters, medium-tasters, or nontasters. Our brains
will work to calibrate pleasure to our particular sets of chemical
senses, because it wants to keep those calories coming in so we’ll
survive. Our noggins will make sure we enjoy our food to the max–our
max. It really is chacun a son gout.

DYEING TO KNOW

To check the density of your fungiform papillae-and thus your taste
sensitivity-paint your tongue with blue food coloring. Then punch a
1/4-inch hole in a piece of paper and place the hole over the tip of
your tongue. The hole’s edge should be on the tip of the tongue; its
center should be on the midline of the tongue. Now use a magnifying
lens to count the fungiforms-which should remain pale against the
blue-stained tissue. Super-tasters can have as many as 50;
medium-tasters, 15 to 30; nontasters, as few as 10.

If you don’t care for the dye test, consider your food preferences.
Super-tasters tend to avoid bitter foods like black coffee and
grapefruit, and they dislike very sweet, high-fat desserts. But there
is no hard-and-fast rule about likes and dislikes, cautions Bartoshuk,
because food preferences are often learned.

COPYRIGHT 2000 Discover
COPYRIGHT 2000 Gale Group

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