Molecular Gastronomy: Exploring the Science of Flavor (17 page)

the intracellular electrical potential reaches a certain threshold. Nonetheless,

an upper limit appears to exist as well: Frogs’ egg cells equipped with the vr1

receptor die after a few hours of continuous exposure to capsaicin, evidently

because the inflow of calcium ions is excessive.

The loss of sensitivity observed in spice lovers seems to result from the

death of sensory fibers. This would explain the paradoxically analgesic effect

of capsaicin in the treatment of viral and diabetic neuropathies and of rheu-

matoid arthritis, where by killing pain neurons it helps reduce the sensation

of pain.

Finally, the ucsf team showed that rapid increases in temperature trigger

ion currents in the vr1 receptor analogous to the currents triggered by capsa-

icin. The vr1 channel therefore turns out to be both a chemical and a thermal

sensor, which is why eating spicy dishes makes the mouth feel as though it is

on fire.

Hot Up Front
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28

The Taste of Cold

Cooling and heating the tongue arouse the perception of tastes, even in the

absence of food.

t h e p h y s i o l og y o f f l a vor is riding a wave of fresh discoveries. In

recent years physiologists have elucidated the molecular bases of the sensa-

tion of spiciness, described the mechanisms in the papillary cells of the

mouth responsible for taste perception, and at long last, in April 2000,

identified the first receptor for a taste molecule. It was expected that the

analytical methods that led to this last discovery would help researchers find

other receptors, but a great surprise lay in store: thermal tastes. Variations

in the temperature of the tongue alone are enough to cause tastes to be

perceived.

When we eat, various molecules stimulate the olfactory cells of the nose,

the papillary cells of the mouth, and receptors that register levels of spici-

ness and a set of mechanical and thermal sensors. How do these different

perceptions interact to form the synthetic experience of flavor? It was long

supposed that the pieces of information obtained by the various sensory cells

traveled upward up from neuron to neuron until they reached the higher

centers of the brain, which then gave them a joint interpretation, thus creat-

ing the sensation of flavor. A few years ago, however, physiologists came to

realize that this account was inadequate: Sensory information has already

been combined by the time it reaches the first neuronal relay station, which

is to say that sensory integration begins in the tongue.

106 |

Heating the Tongue

To determine the physiological consequences of this integration, Ernesto

Cruz and Barry Green at Yale University investigated the effect of the tempera-

ture of foods on taste perception. Using small thermodes (devices whose tem-

perature is regulated by means of electric currents) to stimulate the tongues

of subjects, they discovered—rediscovered, actually—the “thermal tastes”

aroused by heating and cooling the tongue. This effect had been discovered in

1964 by G. von Békésy, but the observation was part of a theory that was later

discredited and so failed to receive the attention it deserved.

The reactions were not identical for all subjects, but for a large proportion

of them, heating the tip of the tongue up to 35°c (95°f) produced a slight sweet

sensation, and cooling it down to 5°c (41°f) elicited a sour sensation or, in the

case of one person, a salt taste. Heating the back of the tongue produced only

a weak sensation of sweetness, but cooling this region often gave rise to more

distinct sensations, variously described as bitter or sour. A few subjects tested

perceived none of these thermal flavors.

Plainly this curious phenomenon called for further scrutiny. The Yale physi-

ologists sought first to quantify the relationship between temperature and the

intensity of sensations: The intensity of the sweet flavor, at the front of the

tongue, increases as the temperature rises, and the sour taste caused by cool-

ing becomes saltier as the temperature falls. Because the subjects often re-

ported differences in taste perception between the center and the sides of the

tongue, Cruz and Green sought to make these impressions more precise. They

found that for all subjects the perception of thermal sweetness was greatest in

the tip of the tongue and that thermal sourness was most clearly perceived on

the sides of the tongue.

Paradox Elucidated

How are these results to be interpreted? We know that the nerve fibers of

two cranial nerves innervate the gustatory papillae. The fibers of the chorda

tympani are ramified in the papillae and transmit information about tastes to

the brain, whereas the fibers of the lingual branch of the trigeminal nerve termi-

nate in the papillary epithelium, near the taste buds, and transmit information

The Taste of Cold
| 107

about temperature, pain, irritation, and pressure. Note that thermal sensors and

taste receptors are found near one another in the mouth.

It seems plausible to suppose that changes in temperature activate the re-

ceptors responsible for the normal coding of the perception of tastes. In this

hypothesis, it ought to be possible to block thermal tastes by stimulating taste

receptors. Conversely, if the perception of thermal flavors is not caused by the

normal taste receptors, inactivating these receptors will have no effect on the

perception of thermal taste. Tests are under way.

What are cooks to make of this discovery? One does not taste with the tip

of the tongue alone, so the effect of thermal tastes is weak in ordinary eating

situations. On the other hand, it is a simple matter to determine whether you

are capable of perceiving such flavors: Just place an ice cube against the tip of

your tongue or stick your tongue in a glass of warm water.

108 | t he physiology of f l a vor

29

Mastication

Understanding how we chew our food will change how we think about

cooking.

d r . j o h n h a r v e y k e l l o g g, he of the breakfast cereals, advocated a hy-

gienic regime based on relentless mastication. His ideas echoed an ancient

East Asian tradition according to which each mouthful of whole-grain rice was

to be chewed 100 times.

Why do we chew in the first place? Everyone knows that mastication breaks

up food into smaller pieces—small enough that, having also been lubricated

by saliva, they easily descend into the digestive system. Jons Prinz and Peter

Lucas at the Odontological Museum in London have identified another func-

tion. Without knowing it, we chew until particles of food are bound together

by saliva into a compact mouthful that can be swallowed in such a way as to

minimize the risk that small bits take a wrong turn down into the windpipe.

For each food, then, there is an optimal number of masticatory movements.

In asserting that “animals feed, man eats,” Brillat-Savarin sought to do away

with the animal side of our nature—the very thing that upset the
Précieuses
of

mid–seventeenth-century salons in Paris, who made a fashion of mousses be-

cause they eliminated the need for “the unsightly act of mastication.” And yet

who wants to forgo the pleasures of a piece of crusty bread? A sticky dumpling?

A crispy piece of bacon? If we are to enjoy the full range of pleasures that the

culinary world offers, we must frankly accept our humanity and turn our physi-

ological peculiarities to the advantage of our weakness for good food.

| 109

Chewing divides food into pieces of smaller diameter than that of our phar-

ynx. Nonetheless, we normally go well beyond what is necessary for this pur-

pose. As mammals that expend a great deal of energy, we chew our food in

order to increase the surface area accessible to digestive enzymes. Indirectly,

then, mastication accelerates the assimilation of nourishment.

Prinz and Lucas devised a model to explain how salivation causes the par-

ticles formed by chewing to cohere. Their model takes into account the two

main forces exerted on masticated food: adhesion between its parts and the

adhesion of these parts to the inside of the mouth. These forces depend on the

secretion of saliva and the quantity of juice squeezed out of food by the act of

chewing.

Small pieces of food are broken up less thoroughly than big pieces. On the

other hand, the number of fragments into which a mouthful of food is divided

by chewing depends on the mechanical characteristics of the food in question.

To simplify the modeling problem, the British physiologists assumed that each

piece of food is divided into spherical particles and calculated the total surface

forces holding them together.

Furthermore, Prinz and Lucas assumed that these particles agglomerate

when the force causing them to adhere to one another is greater than the force

causing them to adhere to the wall of the mouth. Using computer calcula-

tions of these forces and incorporating values for various other parameters

drawn from studies of human physiology, they then determined the cohesion

of mouthfuls of food after 150 masticatory cycles for two foods having very

different properties: raw carrots, which are broken up very slowly, and Bra-

zil nuts, which are broken up much more rapidly. Computation showed that

the cohesion of the masticated food is initially low, then rapidly increases and

reaches its highest point after twenty cycles. After that point it diminishes as

the particles become smaller and smaller.

To test the proposed model, the calculated degree of cohesion was com-

pared with the cohesion actually measured in mouthfuls of food spit out after

having been chewed. The agreement of theory with practice was good, but the

actual number of masticatory cycles was a bit higher than the number calcu-

lated, no doubt because we are not only machines for absorbing nourishment:

“The Creator, in making man eat in order to live,” Brillat-Savarin observed,

“persuaded him by appetite and rewarded with by pleasure.” Because we take

110 | t he physiology of f l a vor

pleasure in eating, we prolong our enjoyment by chewing longer than is strictly

necessary in order to make food particles cohere.

Model and Cuisine

What can we learn from the model for culinary purposes? Depending on

their physical characteristics, foods need a greater or lesser degree of mastica-

tion. The addition of compounds that make saliva more liquid (tannins, for

example) or increase the concentration of liquids extracted by the teeth has the

effect of reducing cohesion, which ought to lengthen the amount of time spent

chewing and so add to the enjoyment one takes from a dish. Could this be why

gourmets drink wine (which contains tannins) with their meals?

Thickening agents, on the other hand, ought to accelerate the absorption of

food into the digestive system. The use of such agents, particularly in diet prod-

ucts, creates a marketing problem: The shorter the time that food is chewed,

the fewer the number of odorant and taste molecules that are released.

More generally, the hypothesis that the body automatically detects the ideal

cohesion of mouthfuls of food ought to be a source of fresh ideas for the

cook who wants to find new ways to combine sticky, gluey, dry, or absorbent

ingredients.

Mastication
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30

Tenderness and Juiciness

Chewing is what allows us to enjoy the juiciness and tenderness of meat—

though for different periods of time in each case.

l e s s t h a n a d e c a d e a g o tenderness was thought to be the most im-

portant sign of a good piece of meat. After all, the true gourmet detests tough

meat. But how does one tell tenderness and toughness apart? It had been

forgotten that meat is not butter and that texture is one of its fundamental

qualities. Toughness was confused with a lack of juiciness and the need to

chew for a while before swallowing. To elucidate the relationship between the

physical structure of meats and their texture, Institut National de la Recher-

che Agronomique biologists in Clermont-Ferrand analyzed the mastication of

samples of meat prepared in various ways.

Studies of the texture of meat have long been hobbled by the mistaken idea

that the texture of a food is the same thing as its consistency, which is a micro-

structural property. Texture has to do instead with the psychological reaction

to the physicochemical stimuli aroused by mastication. (For example, water is

a liquid, but if you land on it outstretched from a height it can feel as hard as

concrete: the texture of water varies depending on whether it can be displaced

beneath a falling body, but its consistency is always the same.) Sensory per-