Molecular Gastronomy: Exploring the Science of Flavor (48 page)

fondue, but also of cream. Just as chilling and whisking crème fraîche at the

same time yields Chantilly cream, beating a cheese béarnaise over a bed of ice

cubes will give you Chantilly cheese.

Imaginative cooks are sure to take advantage of this innovation. Didier

Clément, chef at the Lion d’Or in Romorantin, has already proposed a goat

cheese Chantilly made with Chavignol, accompanied by caramelized shallots.

Rather than caramelize the shallots in the usual way with sucrose (ordinary

sugar) and a bit of water, Clément was inspired by chemistry to substitute

fructose, which produces an original flavor that puts one in mind of candied

grapes.

Chantilly Rescued

Let’s finish with true Chantilly cream, which will be a perfect accompa-

niment for your dessert. It is simple to make because one has only to whip

very cold cream. Whisking introduces air bubbles into the emulsion, and the

fatty droplets crystallize on the outside of the bubbles, stabilizing them. Alas,

whisking by hand often is replaced by the electric mixer, which brings with it

a greater risk of turning the cream into butter (the result of the fatty droplets

fusing and air being lost in the process).

Yet even in the event of mishap all the ingredients of a good Chantilly

cream are still present. It is merely a matter of reconstituting them. Simply

heat a tablespoon of water in a pan and add the butter. Then put the pan on ice

and whisk it once more; the cream comes back. Happy holidays!

330 | a c uisine f or t omor r ow

100

The Hidden Taste of Wine

Adding enzymes to grape juice releases its avors.

g r a p e s a r e l i k e a l a z y s t u d e n t who can do better. In addition to

odorant volatile compounds (members mainly of the class of terpenols—lin-

alol, geraniol, nerol, citronellol, alpha-terpineol, linalol oxides, and terpenic

polyols—whose very low threshold of olfactory perception plays an important

role in giving wines their typicity), grapes also contain, in much greater quan-

tities, terpenic glycosides, molecules composed of terpenols bound to sugars.

These molecules are precursors of terpenols, but unfortunately they do not

contribute to the flavor of wines.

Claude Bayonove and his colleagues at the Institut National de la Recherche

Agronomique Laboratoire des Arômes et Substances Naturelles in Montpellier

wanted to know whether the aromatic qualities of wines could be intensified

by dissociating the two parts of these precursors (sugars and terpenols) by

means of acids or enzymes. Because enzymatic hydrolysis seemed more prom-

ising than chemical treatment, in part because it gives a more natural aroma,

they began by characterizing grape enzymes that release terpenols from their

precursors.

Working with glycosidic extracts from grapes of the Muscat of Alexandria

variety, the Montpellier team added thirty-four commercially developed enzy-

matic preparations (pectinases, cellulases, hemicellulases, and so on) to see

whether some of them formed terpenols from their precursors. Five proved

to be effective, releasing linalol or geraniol depending on the case. All the

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effective preparations contained beta-glucopyranosidase and alpha-rhamno-

pyranosidase or alpha-arabinofuranosidase as active components, which were

shown to carry out the enzymatic hydrolysis of the terpenic glycosides of the

grape in two stages.

This analysis was followed by an in vitro replication of this hydrolysis using

rhamnopyranosidase, arabinofuranosidase, and glucopyranosidase. These en-

zymes released not only the desired odorant terpenols but also norisoprenoids,

volatile phenols, and benzylic alcohol, all compounds with very low perception

thresholds and an agreeable smell.

The Enzymes of the Grape

The second stage in the glucopyranosidase-mediated hydrolysis of glyco-

sides limits the release of terpenols from both the grape and the wine, for

natural enzymes have little effect on the monoglucosides of tertiary alcohols

(linalol, terpineol) that have a nonglucosidic part (called aglycone). By contrast,

the beta-glucosidase found in yeasts used in winemaking shows weak activ-

ity for linalyl-beta-glucoside, one of the principal glucosides of the Muscat of

Alexandria grape.

Whereas this grape exhibited noticeable beta-glucosidase activity, it pre-

sented only very weak rhamnopyranosidase activity and no activity whatever

for arabinofuranosidase, which blocks metabolism in the first stage and limits

its overall action on the grape’s terpenic glycosides.

Finally, because the grape’s glucosidase is unstable and inactive at the level

of acidity found in the must and in the wine, it does not seem to be a good

candidate for glycoside hydrolysis in either one. Would plant enzymes or mi-

croorganisms do a better job of hydrolyzing the terpenic glycosides than the

enzymes of the grape? Plant enzymes hydrolyze only the glycosides of primary

alcohols, such as geraniol, nerol, and citronellol; the beta-glycosides of tertiary

alcohols, such as linalol and alpha-terpineol, were hydrolyzed by only one of

the two enzymes studied, though with greater difficulty.

Today the Montpellier physical chemists are studying exogenous enzymatic

preparations created by commercial researchers at Gist-Brocades, S.A., from

the stock of legally approved microorganisms, seeking to identify organisms

that display higher levels of activity at the temperatures and sugar and acid

concentrations found in the juice of grapes and in musts.

332 | a c uisine f or t omor r ow

Professional tasters are sensitive to the presence of intensified flavor in the

juice of fruits or of “enzymed” wines, but the use of enzymatic preparations re-

mains largely unexplored. The discovery of new glycosides (such as a recently

identified apiosylglycodide) and corresponding enzymatic processes will be of

particular importance in improving exogenous preparations.

The Hidden Taste of Wine
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101

Teleolfaction

Waiting for a new form of telecommunication.

r e c a l l t h e b o o m i n p o p u l a r i t y that classical music underwent in

the late nineteenth century, when anyone who had access to a telephone could

appreciate the virtuosity of the great performers. At the time the transmission

of visual images seemed a utopian dream, but only a few years after Alexander

Graham Bell patented his famous device Paul Nipkow was awarded a patent

in Germany for an apparatus that would transmit such images. This time the

beneficiaries were the followers of Polymnia, Terpsichore, Erato, Melpomene,

and Thalia.

What realms of communication are left to conquer? The transmission of

tactile stimuli is now being mastered, thanks to the design of special gloves

fitted with piezoelectric crystals that register or exert pressure. But smells?

Flavors? The delay in developing teleolfaction and telegustation is a source of

frustration for gourmets.

Olfactory Stimulation

Olfactory and gustatory sensations arise from the binding of odorant and

sapid molecules with receptor cells in the nose and mouth. Two possible meth-

ods for transmitting such sensations at a distance may be entertained. One

could imagine encoding the electrical activity produced in the brain by smell

and taste in a series of signals that would stimulate the brain of the receiver by

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means of electrodes. This is what André Holley and Anne-Marie Mouly at the

University of Lyon have been trying to do, hoping to be able to condition rats

through excitation of the olfactory bulb.

Alternatively, it might be possible to analyze mixtures of odorant and taste

molecules in the same fashion as colors, associating them with arbitrarily se-

lected stimuli. Then, once the electronically encoded information has been

transmitted and received, the basic molecules of these stimuli would be com-

bined to reproduce the initial sensations.

Sensors called artificial noses might be useful components for the realiza-

tion—still a remote prospect—of such systems. Originally developed by the

food industry, such sensors are already used in certain processing plants to

provide an objective evaluation of the volatile compounds emitted by food.

Several types exist.

Articial Noses

At the Institut National de la Recherche Agronomique (inra) station in

Theix, Jean-Louis Berdagué and his colleagues developed a mass spectrometry

system capable of analyzing all volatile compounds in a given sample. Samples

are first thermically decomposed in a heated cup. After fragmentation and ion-

ization, the smoke molecules pass into a mass spectrometer, where magnetic

and electric fields bend the trajectories of the various molecules in proportion

to their mass and electrical charge. Finally, a sensor identifies the quantity of

each kind of fragment. In this way the Theix researchers were able to obtain a

unique electronic signature for any given mixture. The results are truly won-

derful: Such a signature makes it possible to pinpoint the origin of a particular

oyster along the coastline of France. What gastronome could match this feat?

Another method involves the use of doped semiconductors on which vola-

tile compounds are reversibly adsorbed, diminishing the electrical resistance

of the semiconductor. At the inra laboratory in Dijon, Patrick Mielle and his

colleagues are studying networks of sensors composed of a ceramic substrate,

heated by a resistant element, that is coated with a semiconductor such as tin

oxide and doped with zinc, iron, nickel, or cobalt oxides. These sensors react

differently to the various adsorbed molecules. Whatever the exact mixture of

volatile compounds, contact with the sensor network produces a distinctive

electrical signature that is then processed by a computer.

Teleolfaction
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The practical application of these networks remains problematic. First,

because the signals of the various sensors slowly drift over time, the Dijon

researchers devised a rapid transfer system in which compounds are mea-

sured and characterized by the sensors and their signatures fed into the net-

work with only a slight delay. Second, the sensors were observed to react quite

differently, depending on the temperature. This contingency, which must be

controlled for during the measuring process, turns out to be an advantage be-

cause measurements made at several temperatures make it possible to obtain

different reactions from a single sensor, in effect multiplying the number of

sensors in the network.

Finally, because recording the sensors’ signals and recognizing the signa-

tures of the various compounds make heavy demands on the system’s limited

processing power, in practice the number of sensors must be limited to fewer

than a dozen. Mielle and his colleagues therefore proposed recording the sig-

nals at various instants after the injection of the samples into the measuring

cell, proportionally increasing the number of data points generated.

The measuring cells being tested today capture 80% of the gaseous phase

molecules, and networks consisting of six sensors that record measurements

at four temperatures are able to characterize mixtures of volatile compounds

in about ten seconds. The capacities of the human nose are no longer quite as

unrivaled as they once were.

336 | a c uisine f or t omor r ow

Glossary

Some readers may ask why a glossary is needed that denes terms such as

butter, egg white, milk, caramel,
and
chocolate.
Let me respond to this perfectly reasonable

question with an anecdote. The Hungarian physicist Leo Szilard used to edit a journal. One

day he ran into his friend Hans Bethe, who asked him why he edited a journal. “Because,”

Szilard replied, “I want God to know the facts.” “But don’t you believe He already knows

the facts?” “Yes, but I want Him to be informed of
my
version of the facts.”

In what follows I want to give you my idea of foods, molecules, and the reactions that

take place in cooking. But rather than give a comprehensive set of strict definitions—
omnia

definitio pericolosa!
—let me share instead my thoughts about a few specific terms.

acetic a cid: One of a number of acids present in vinegar. Vinegar also owes its taste to

malic acid, formic acid, and tartaric acid, among others. The essential step in the trans-

formation of wine into vinegar involves the oxidation (caused by microorganisms of the

species
Mycoderma acetii
) of the wine’s ethyl alcohol (or ethanol) into acetic acid.

acidity: A food may be said to be acid, or sour, in the mouth when it causes a sensa-

tion analogous to that produced by vinegar or lemon juice. Adding sugar changes this