Molecular Gastronomy: Exploring the Science of Flavor (15 page)

found that the four cerebral taste areas were not systematically inverted be-

tween the two groups. By contrast, activation of the insula differed according to

handedness. This area is composed of two regions. The upper part is activated

in both hemispheres, for right-handers as well as left-handers; the lower part

is activated unilaterally in the subject’s dominant hemisphere. The perception

of taste therefore is lateralized in a way that is analogous to language use and

motor activity.

A third series of studies compared subjects’ reaction to molecules that have

only taste and to molecules that have both taste and astringency (or pungency).

This time the activated areas were analogous, which explains why flavor—the

overall sensation that is registered in the course of eating—is so all-encom-

passing and so difficult to describe: The brain constructs a global sensation

through the synthesis of signals coming from various types of receptors.

90 | t he physiology of f l a vor

23

Papillary Cells

The functioning of the cells that allow us to perceive the taste of foods is

discovered.

i n 1994, r i c h a r d a x e l a n d l in d a b u c k at the College of Physicians

and Surgeons of Columbia University in New York announced the discovery

of proteins in the membrane of nasal cells that capture odorant molecules and

make olfaction possible. The news caused a great stir, but it failed to satisfy

researchers interested in the related problem of taste. Two years later another

team of biochemists at Columbia, Gwendolyn Wong, Kimberley Gannon, and

Robert Margolskee, published the results of a study of gustducin, a protein

found in the papillary cells of the tongue that had been cloned in 1992 but

whose function was unknown. They observed that inhibiting the synthesis of

gustducin in the taste cells of mice caused these animals to lose their aver-

sion to bitter flavors and, still more surprisingly, their sensitivity to sweet

molecules.

Taste begins when a sapid molecule binds with receptors or ion channels

in the membrane of a papillary sensory cell. Once the electrical potential of the

papillary cell is sufficiently modified as the result of a series of reactions, the

cell commences to excite neurons, which, little by little, convey information to

the brain.

Not all taste molecules act in the same fashion. Whereas hydrogen ions

(sour taste) and sodium ions (salt taste) act directly on the channels of taste cell

membranes, immediately modifying the electrical potential of the cell by add-

ing their electrical charge to its total charge, compounds of sweet, bitter, and

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other tastes (licorice, for example) bind to molecules known as receptors—no

doubt proteins—that are located in the cell membrane, in contact with the

extracellular environment.

It is thought that these receptors are paired with other previously identi-

fied proteins, the G-proteins, which trigger the emission of molecules known

as second messengers that act within the cell. Nonetheless, the receptors are

evanescent because they form only a weak bond with taste molecules. This is

inconvenient from the scientific point of view, but, gastronomically speaking,

it is an advantage: If taste molecules formed too strong a bond with receptors,

we would not perceive the rapid succession of tastes in a dish.

Before the discovery of these receptors, Margolskee and his colleagues had

begun investigating G-proteins, which are particularly abundant in the taste

cells of the tongue. Using a method of genetic amplification involving the poly-

merase enzyme, they multiplied the number of genes of the alpha-subunits

of several G-proteins, notably gustducin, which is uniquely expressed in such

papillary cells.

These studies confirmed similarities between taste and vision. Gustducin

was found to resemble a class of G-proteins found in the receptor cells of the

eye known as transducins. Moreover, the Columbia team detected the transdu-

cin that is specific to the cones and rods of the eye in taste receptor cells.

The Eye and the Papilla

The resemblance proved to be enlightening. For if papillary cells function

like the cells in the eye, then gustducin and transducin activate an enzyme that

diminishes cyclic adenosine monophosphate production. In this hypothesis,

the shortage of this second messenger would either modify the ion channels of

the cell membrane and associated enzymes or disrupt the exchange of calcium

ions between the inside and outside of the cell.

To test this hypothesis, the Columbia team inactivated the gene that codes

for the alpha-subunit of gustducin and studied the behavior of mice born with

this inhibited gene when they were offered various sapid solutions to drink.

At the same time the neurobiologists recorded the electrical signals from the

chorda tympani, a branch of the facial nerve that conveys gustatory informa-

tion to the brain. The reactions were normal for salt and sour flavors but much

weaker for bitter compounds such as quinine sulfate and denatonium benzo-

92 | t he physiology of f l a vor

ate and for sucrose (ordinary cane sugar) and a normally very intense synthetic

sweetener.

Why was the perception of bitter and sweet not completely nullified? The

neurophysiologists reasoned that because transducin plays a role, along with

gustducin, in the perception of these tastes, the fact that it was not eliminated

meant that they continued to be perceived, albeit to a lesser degree. Therefore

their next experiment will investigate the consequences of inhibiting the genes

for both transducin and gustducin.

Papillary Cells
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24

How Salt A‡ects Taste

Salt transforms and softens bitter and sweet avors.

t r u e g a s t r onom e s h a v e t wo g r e a t f e a r s: gout and a diet with-

out salt. To guard against gout they abstain, at least occasionally, from gamey

meats; but against a salt-free regime they find themselves powerless and dread

the doctor who prescribes it. This fear is doubly well founded. Gary Beau-

champ and his colleagues at the Monell Chemical Senses Institute in Philadel-

phia have shown that the absence of the salt taste is not the sole inconvenience

of this regime. Without salt, agreeable tastes forfeit their prominence, and they

are unable to prevent disagreeable tastes from asserting themselves.

In earlier chapters I examined the action of salt on the texture of foods

without discussing its taste. Salt is important because it increases the ionic

strength of aqueous solutions, making it easier for odorant molecules to sepa-

rate themselves from food. This is why unsalted soup has no flavor and why

adding salt amplifies its odor, which is an important part of flavor. Sodium

chloride is also a taste molecule that stimulates the papillary receptors. Does it

have other virtues from the point of view of flavor? Does it really bring out the

flavor of a dish, as some maintain?

In examining these questions experimentally, Beauchamp and his col-

leagues did not limit themselves to sodium chloride but also tested other

salts such as lithium chloride, potassium chloride, and sodium aspartate.

They sought to make sense of a paradoxical state of affairs: Whereas most

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psychophysiological studies test pairs of tastes and succeed in showing that salt

either suppresses the accompanying taste or has no effect on it, every gourmet

knows that unsalted foods lose much of their interest. Cooks who add salt to

their pie dough—even a pinch in the case of a sweet pie—do so not in order to

make the dough salty but to give it flavor.

Filtering Tests

The Monell Institute team of psychophysiologists wanted to know wheth-

er salt selectively filters tastes, weakening unpleasant tastes while enhancing

pleasant ones. Convinced that it was not enough to examine pairs of tastes,

they compared aqueous solutions containing one or more of three substances:

urea (bitter), sucrose (table sugar), and sodium acetate. There were reasons

for choosing these three: Sucrose added to urea softens its bitterness, and so-

dium acetate contributes sodium ions without imparting too salty a taste. Ten

subjects were asked to evaluate the intensity of bitter, sweet, and other sapid

sensations produced by combinations of urea, sugar, and salt in different con-

centrations (three for urea and salt, four for sugar).

As predicted, sodium acetate reduced the bitterness of urea. What gastro-

nomic empiricism did not predict, however, was that salt masked the bitterness

much more effectively than sugar. Mixtures of sugar, urea, and salt turned out

to be sweeter and less bitter than unsalted mixtures of urea and sugar. More-

over, in strong sugar concentrations, the sweet character was increased by the

addition of sodium acetate, probably because salt offsets the weakening of the

sweet intensity caused by the bitterness of urea. Consistent with the hypoth-

esis, the addition of sodium acetate by itself to sugar, in the absence of urea,

did not increase the intensity of the sweet taste.

These studies were conducted for many other compounds and showed that

sodium ions selectively suppress bitterness (and probably other disagreeable

tastes as well) while intensifying agreeable tastes. It is therefore a question not

of bringing out a single basic taste but rather of modifying the proportions of a

combination of tastes. Adding salt to a variety of dishes—vegetables (both bit-

ter ones, such as endive, and sweet ones, such as carrots and peas), certain fatty

foods, and meats—may have become habitual because there is an unconscious

wish to eliminate unpleasant tastes and to reinforce the natural sweetness of

How Salt A¤ects Taste
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many foods. The recent experiments seem also to explain why some coffee lov-

ers put a pinch of salt in the filter: to remove the bitterness of caffeine.

It is not yet known how the stimulation of taste receptors produces these

effects, but we do finally know why salt-free diets make us wince.

96 | t he physiology of f l a vor

25

Detecting Tastes

Discovery of a molecular receptor for a fth taste.

o n e o f t h e h o l y g r a i l s o f p h y s i o l o g y is finally in our hands. For

decades physiologists sought to explain how the cells of the gustatory papillae

detect taste molecules. It was supposed that the surface of these cells contains

proteins called receptors, to which the taste molecules attach themselves, but

these receptors proved to be elusive. Attempts to extract them from papillary

cells in solution were unsuccessful because receptors form a weak bond with

taste molecules. Compensating for this experimental difficulty is a physiologi-

cal advantage: If the bond were strong, receptors would be stimulated for long

periods of time by a single molecule, and the resolution of individual tastes

would be low. In that case gastronomes would be forced to savor their food

in slow motion. Focusing on the phenomenon of weak molecular bonds, in

February 2000 physiologists at the University of Miami identified one of the

sought-after receptors, associated with the taste called umami.

It was long believed that the mouth is capable of detecting only four tastes,

but the matter had been examined only cursorily. In 1908, Kikunae Ikeda at

the Imperial University of Tokyo established that glutamate (the ionized form

of an amino acid, glutamic acid) produced a particular sensation that was nei-

ther salt, sugar, sour, or bitter. After decades of struggle against conventional

wisdom the notion of a fifth taste came to be accepted, in large part on ac-

count of the growing popularity in Western countries of Asian cuisine, which

uses a great deal of monosodium glutamate. Moreover, it was shown that even

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animals detect this taste, perhaps because glutamate is present in many foods

that are rich in proteins (which are chains of amino acids), such as meat, milk,

and seafood. The detection of tastes is important because it signals satiety.

One does not cease eating because one’s stomach is full; one stops because

the brain, alerted by the sensory system, notifies the organism that a sufficient

quantity of food has been consumed.

From Mouth to Brain

In searching for glutamate receptors, Nirupa Chaudhari and his colleagues

at the University of Miami took as their starting point the results obtained ten

years earlier by Annick Faurion at the Laboratoire de Neurobiologie Sensorielle

in Massy. Because glutamate is a neurotransmitter, which is to say a molecule

that is exchanged between neurons in the brain (on being released by one

nerve cell it binds to a receptor on the surface of a neighboring nerve cell), it