Molecular Gastronomy: Exploring the Science of Flavor (18 page)

ceptions modulate the motor actions that break up food. Chewing causes the

structure of food to be modified, revealing its texture.

Laurence Mioche, Joseph Culioli, Christèle Mathonière, and Eric Dransfield

studied the question of texture in the case of meat (in this case beef), search-

ing for similarities between the sensory perceptions aroused by tasting, the

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mechanical properties of the meat (resistance to compression and cutting), and

the electrical activity of the muscles involved in mastication. The beef was pre-

pared in several ways: Some samples were toughened (to a degree that cooking

did not subsequently counteract) by immediate cooling after slaughter, and

other samples underwent a long aging process at a temperature of 2°c (36°f).

Then the different pairs of samples were cooked at 60°c (140°f) and at 80°c

(176°f). One piece of each pair was analyzed mechanically, and the other was

eaten by trained tasters who judged the elasticity, initial tenderness, overall

tenderness, and length of time in the mouth, which is to say the time needed to

chew the meat before being able to swallow it. During this exercise the physi-

ologists analyzed the process of mastication by recording the electrical activity

of the masseter and temporal muscles.

The Sensation of Toughness

The mechanical measurements corroborated the results of studies that had

been conducted for many years at Clermont-Ferrand. Immediate cold storage

of food after butchering multiplied by a factor of three or four the resistance

to both compression and cutting. Conversely, gradual cooling followed by a

prolonged maturation process diminished both types of resistance. Higher

cooking temperatures greatly increased the resistance to compression but not

to cutting. Finally, differences between the various samples of meat resulted

mainly from the action of myofibrillary proteins (responsible for muscle con-

traction) and the connective tissue, made of collagen, that surrounds the mus-

cle fibers. The physical reactions of the tasters displayed wide variation. The

aging period and cooking temperature had perceptible effects on the process of

chewing, but differences in preparation had little effect on the electrical activity

of the muscles involved.

As they went along the tasters noted their sensations. All of them correctly

identified the toughest meats: The type of muscle, the mode of storage, and

the cooking temperature affected sensory perception in the same way that

they affected mastication. Nonetheless, sensory descriptions did not match

the experimenters’ predictions. For example, the perception of elasticity did

not imply a corresponding initial impression of tenderness. Juiciness, which

tasters associated with an initial degree of tenderness, but not elasticity, was

influenced more by cooking temperature than by the type of storage; the loss of

Tenderness and Juiciness
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juice was not perceived as a loss of juiciness. Today we still do not know exactly

what juiciness is. Is it the quantity of water in the meat and in the mouth? The

quantity of fat? The quantity of saliva secreted in the course of chewing?

The tasters concluded their work by grouping the meats into five classes of

increasing tenderness. The meats they found to be the most tender were those

that had been aged the longest. The toughest meats were those that had been

refrigerated just after slaughter. Lengthening the aging time had a perceptible

effect only in the case of meats cooked at 80°c (176°f). Finally, meats cooked at

the lower of the two temperatures were thought to be more tender than those

cooked at the higher one. Juiciness was found to depend mainly on cooking

temperature and much less on the type of storage or aging or on the type of

muscle. Differences were plain after the first few bites.

The Reliability of the Senses

Mechanical measurements, sensory evaluations, and electromyographic

measurements all yielded the same results, then, with regard to tenderness:

Prolonged chewing is needed to make a judgment. By contrast, juiciness is

best assessed after a few bites, which detect the general characteristics of the

food, causing subsequent mastication to be adapted accordingly.

The Clermont-Ferrand study provided valuable methodological information

as well. It revealed that sensory evaluation is the most effective method for

detecting differences between various samples. The human perception of the

masticatory sequence from beginning to end does a better job of capturing the

sensory properties of the meat than mechanical measurements, and the num-

ber of masticatory cycles is a more reliable measure of elasticity, tenderness,

and toughness, as it is actually experienced in the mouth, than compression

measurements. But the mechanical measurement of relative compression is a

better guide to juiciness. Tasters are known to adapt their style of chewing to

the properties of a particular food, but at which stage of the chewing process

they do this merits further study.

114 | t he physiology of f l a vor

31

Measuring Aromas

Chewing slowly deepens the perception of odorant molecules in cooked

food.

w h i c h a r o m a s d o w e p e r c e i v e when we eat? For a long time this

question could not be answered, for chemical analysis was unable by itself to

determine the concentrations of odorant molecules in the vicinity of the recep-

tor cells in the nose. Andrew Taylor, Rob Linforth, and their colleagues at the

University of Nottingham, working in association with Firmenich (an interna-

tional perfume and flavor research group), have been conducting experiments

since 1996 with a device that shows how aromatic compounds are released

during the mastication of food. The same food, it turns out, smells different

to different people.

Odorants—volatile molecules that stimulate the nasal receptors in passing

upward from the mouth through the rear nasal fossae as food is chewed—are

important components of flavor. Nonetheless, their sensory action is difficult

to analyze because these molecules interact with saliva and with various other

compounds present in foods. Accordingly, the odorant profile of a particular

food cannot be reduced to its chemical composition.

Because molecules can be detected by smell only if they pass into the vapor

phrase, physiologists have sought to measure the concentration of odorants in

the air above foods. But given that the chewing of food, breathing, and saliva-

tion all affect the release of aromas, one cannot rely on this measurement

alone. To identify the active aromas of a food, it is necessary also to measure

the release of odorant molecules while it is being consumed.

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The new method of mass spectrometry devised by Taylor and his colleagues

directly measures the concentrations of odorant molecules in the breath of sub-

jects as they are chewing food. A stream of gas containing the volatile molecules

to be analyzed is pumped into a chamber equipped with an electrically charged

needle that ionizes water molecules. The hydrogen ions that are formed in this

way then transmit their electrical charge to the odorant molecules, which are

attracted by a series of electrically charged plates in a focalization chamber.

From here they are channeled into another chamber for analysis.

The Wisdom of Chewing Slowly

The British chemists first examined how a gel composed of gelatin and sac-

charose releases the volatile components—ethylbutyrate, found in fruits such

as strawberries, and ethanol—that are trapped in it. A tube was placed in a

nostril of each of the subjects (who were nonetheless able to breathe without

difficulty) to capture a sample of the air present in the naval cavity.

The first observation was not surprising. Because molecular concentrations

in the air in the nose vary periodically with the rhythm of respiration, what one

wants to know, for each breath taken and expelled, is the maximum measured

concentration. In the case of acetone, the maximum concentration was the

same for each respiratory cycle because this molecule, which is released by the

metabolism of fatty acids in the liver, is naturally found in the breath.

By contrast, the ethylbutyrate and ethanol detected in the breath came from

the gel alone: The ethylbutyrate was released only during mastication, for

about a minute, whereas the ethanol was released for a longer period of time.

Because ethanol is soluble in water, it dissolves in saliva after having been re-

leased by the rupture of the gel, and only afterward does a part of it pass into

the air. This slow exchange between water and air is stimulated by chewing and

continues even after chewing is finished.

These studies also confirmed what the makers of chewing gum have long

suspected, namely that the release of odorant molecules depends on both the

speed of mastication and the softness of the gum. A comparison of the reac-

tions of three people to the same food (a gel made from gelatin containing

ethanol, butanol, and hexanol) revealed what might be called odorant inequal-

ity: The maximum concentration of molecules in the breath and the time it

took for this concentration to appear varied according to the rate of mastication

116 | t he physiology of f l a vor

for each person tested. The maximum odorant concentrations were lowest in

the case of the most rapid eaters, presumably because they broke up the gel

the least. Brillat-Savarin therefore was right to say, “Men who eat quickly and

without thought do not perceive the [succession of] taste impressions, which

are the exclusive perquisite of a small number of the chosen few; and it is by

means of these impressions that gastronomers can classify, in the order of

their excellence, the various substances submitted to their approval” (Medita-

tion 2,
The Physiology of Taste
).

Measuring Aromas
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32

At Table in the Nursery

Observing the eating habits of small children provides clues to their devel-

oping appreciation of food.

f o o d - m i n d e d p a r e n t s a r e f o r e v e r c o m p l a i n i n g that their chil-

dren like only starches (pasta, rice, potatoes), the blandest cheeses, and taste-

less meats (especially the white meat of chicken). Why do we begin our eating

lives liking such dull foods? How can children be set straight about food before

it’s too late? It used to be that psychologists and sociologists were likeliest to

wonder about the biological motivations that cause parents to despair for their

offspring. Today it is the sensory biochemists who have taken the lead in ex-

ploring the dietary preferences of children and how these change.

An important experiment that only now is yielding its first results began

in 1982 and finished in 1999 at the Gaffarell Nursery of the Dijon Hospital

in France. The children, aged two to three years, were allowed to decide what

they would eat for lunch, with several constraints. The menu contained eight

items: bread, two starters, meat (or a meat-based dish) or fish as a main course,

two vegetables or starches, and two cheeses. Sweets were not offered as part

of the meal because of the children’s presumed attraction to sugar; they were

reserved for snacks instead. The children could have extra helpings but not

more than three of the same food during the same meal. Their minders ac-

commodated and recorded the choices freely made by the twenty-five children

enrolled in the nursery each year. All eight dishes were placed on the table, and

the children were free to serve themselves as they pleased or to eat nothing at

all. In all 420 children were monitored, each for an average of 110 meals.

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The results are being scrutinized by Sophie Nicklaus and Sylvie Issanchou

at the Laboratoire de Recherches sur les Arômes at the Institut National de la

Recherche Agronomique station in Dijon, in collaboration with Vincent Bog-

gio of the medical school at the University of Dijon. They are interested in a

number of questions. What foods did the children eat? Did they show a general

aversion to certain dishes? How are different choices between dishes to be

interpreted? How are variations in choice to be explained: by individual pref-

erences? Sifting through the data, the researchers found evidence in favor of

certain familiar assumptions as well as a number of unanticipated outcomes.

A Taste for the Tasteless

First, as parents have long maintained, children do indeed prefer starches

and meats on the whole. The cheeses they select are almost invariably ones

with minimal taste and a soft texture. Rare are the
amateurs de roquefort
at

the age of two or three—gourmets in short pants one day perhaps, but not

in diapers. It may be a sign of the times that bread was the starch least often