Molecular Gastronomy: Exploring the Science of Flavor (28 page)

a genetic basis.

More than 200 samples from various regions in France and Italy were ana-

lyzed. The Montpellier biologists studied satellite dna sequences, which dif-

fer substantially between species of the same genus, and observed no genetic

variability in the samples of black truffles. They also compared black truffles

with summer truffles (
Tuber aestivum
) and Burgundy truffles (
Tuber unciatum
).

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Differences were found between the black and summer truffles, as expected,

but the species boundaries between summer and Burgundy truffles turned out

to be fuzzy. Subsequent studies confirmed that the latter two types must be

considered distinct varieties—what biologists call a species complex.

How is the genetic homogeneity of the
Tuber melanosporum
species to be

explained? It is believed that the last Ice Age trapped a small population of

black truffles next to the Mediterranean, along with the trees on which they

develop (the mushroom forms a subterranean root network, of which the truf-

fle is only the reproductive organ). Because the black truffle matures during

the winter, from November to February, its propagation was confined to the

most southerly zones. During the later climatic warming, the black truffle is

thought to have recolonized the regions where its favored trees first developed,

as weather conditions permitted. Ten thousand years would have been enough

time for the species to reestablish itself in southern Europe but not enough

time for it to evolve.

During the same period, by contrast, summer and Burgundy truffles,

which mature in spring and fall, respectively, are thought to have been con-

fined to a more northerly area. Certainly the fact that they are found in coun-

tries further to the north and east of France proves that they are able to tolerate

colder climates. The current genetic diversity of species therefore results from

the fact that present-day truffles are descended from a numerous and varied

population.

A Himalayan Trufe

Genetic studies completed these results by illuminating the problem of Chi-

nese truffles, which every year swindlers try to pass off as French truffles. In

1996, Marie-Claude Janex-Favre and her colleagues at the University of Paris–

VI studied the Chinese truffle, which initially had been assigned to the species

Tuber himalayense, Tuber indicum,
and
Tuber sinense
. These truffles come from

the foothills of the Himalayas, where they are harvested at an altitude of about

2,000 meters (6,500 feet), at least a dozen centimeters (four or five inches)

below ground. They are easily confused with the French truffle, and the cost of

harvesting them is far lower.

The Chinese truffles have a very irregular, bumpy surface. Reaching as much

as 7 centimeters (almost 3 inches) in diameter, they are covered with low scales

186 | investigations a nd mod el s

displaying an inverted pyramid form with a square base. This general appear-

ance is almost identical to that of a particular kind of Périgord truffle that is

covered with large flat scales and devoid of marked protrusions. The spores of

the two types of truffles nonetheless look different under the microscope. Does

this difference in appearance result from a difference in developmental condi-

tions or from speciation? Genetic studies carried out by Delphine Graneboeuf

and her colleagues at the inra station in Clermont-Ferrand have established

that they belong to different species. The mild taste of Chinese truffles there-

fore is a consequence of genetic rather than environmental factors.

In the case of black truffles, although environmental factors are now known

to be responsible for differences in gastronomic quality, science has not yet

explained how the soil and climate exert their influence. Biologists are busy

trying to find out.

Truºes
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54

More Flavor

Odorant molecules are trapped so that they may be better perceived.

s o m e c l a s s i c r e c i p e s a p p e a r p a r a d o x i c a l . To make a
salmis de

canard,
for example, one removes the breasts and thighs of the duck and roasts

them. The cook then makes a sauce by cooking the scraps and bones of the

duck in water together with vegetables and aromatic herbs. Isn’t the second

step redundant? No, because in the final dish the odorant and taste molecules

contributed by the duck are retained for different lengths of time by the meat

and the sauce before being released. As a result, the flavor of the dish lingers

in the mouth longer.

The problem of trapping odors has received additional attention with the

widespread use of extrusion cooking technology by food processing compa-

nies. This technology, borrowed from the polymer industry, involves a screw

that turns inside a cylindrical barrel, as in a meat grinder, only here the screw

is tapered (so the distance between the edges of its blades decreases from the

top of the screw shaft to the bottom). A mixture of solids and liquids placed

in this apparatus is forcefully compressed—so much so that the water that is

suddenly released evaporates, causing the food to be “puffed.” Cocktail crack-

ers typically are made using this method.

The puff is achieved at the expense of flavor, however, for the food’s odorant

molecules are carried away with the evaporated water. Commercial produc-

ers often spray aromas on the extruded crackers afterwards. But because this

is a costly expedient, they are interested in finding ways to trigger chemical

188 |

reactions during the extrusion process that will produce odorant molecules or

otherwise to trap the volatile molecules.

How can molecules be trapped? In the sauce that accompanies the roast

duck, for example, the long cooking of the scraps and bones in water with

vegetables has the effect of extracting the gelatin present in the skin, tendons,

and bones, producing a decoction that, as chemists very well know, extracts the

odorant and taste molecules present in animal and vegetable matter. Odorant

molecules therefore are found in two distinct physicochemical environments:

in the meat, where they are dissolved for the most part in fats, which are them-

selves dispersed between the muscle cells; and in the sauce, where they are in

a liquid solution. In the mouth these molecules are released in different ways,

so that the flavor of the duck lasts longer.

Retention in Solution

Let’s try to figure out why this is so. If the duration of a dish’s flavor in

the mouth has to do with the release of odorant and taste molecules, how

can this release be controlled? By controlling the environment of the odorant

molecules.

In their pure state these molecules are highly volatile, and the pressure of

saturated vapor increases with temperature. The cook is able to vary the de-

gree of volatility by redistributing the odorant molecules into environments of

different temperatures. The molecules can also be placed in solution. In this

case their volatility depends on the solvent used (whether water, alcohol, or oil)

because the molecules bind to a greater or lesser degree with the molecules of

the solvent (thus saturated and unsaturated oils differentially retain odorant

molecules in solution).

Playing with Molecular Interactions

Cooks can slow down the evaporation of volatile molecules further by put-

ting them in the presence of larger molecules, with which they bond. For ex-

ample, we know that iodine turns food containing starch blue because the

amylose molecules in the starch (long chains composed of linked glucose mol-

ecules) wrap themselves around the iodine in a helix. More generally, in water

solution, the amylose wraps around hydrophobic molecules, many of which

More Flavor
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are odorant molecules. Amylose is by no means the only molecule that wraps

around in this fashion: Gelatin does the same thing, hence its usefulness in

sauces. More generally, well-chosen flexible polymers bond with odorant mol-

ecules to retard their release.

Compartmentalization is another, more radical means of retaining mol-

ecules. For example, fines herbes (a mixture of fresh chopped herbs used in

cooking that typically includes parsley, chervil, tarragon, and chives) release

their odorant molecules only when their cells are ruptured by chewing. Emul-

sions, foams, gels, and pasta are systems of the same type. Similarly, cooks

may soon be able to use liposomes, which are sorts of artificial cells created by

the assembly of molecules analogous to those of cell membranes. This ques-

tion is being studied as part of a European Union project devoted to the innova-

tive transfer of technology for culinary purposes.

In addition to microcompartmentalization at the cellular level, there are

various forms of macrocompartmentalization. Consider the farces, or stuff-

ings, used in classic cooking, in which aromatic meat and vegetable mixtures

are placed beneath the skin or inside the bodily cavities of fowl and other ani-

mals. Odors can also be retained if they are made to penetrate foods: The flavor

of a meat that is marinated or of a meat that cooks in a fragrant broth is en-

riched by the odorant molecules of the marinade or broth. Related techniques

include decoction, infusion, and maceration.

There are many methods, then, for retaining aromas to one degree or an-

other so that they are released at different moments as one eats a dish. The

greatest cooks are able to create waves of flavor that call to mind the peacock-

tail effect of the great wines of Bordeaux.

190 | investigations a nd mod el s

55

French Fries

A new kind of potato for frying, packaged raw, absorbs less oil than frozen

fries.

m a n y o f t h e f r i e s s e r v e d t o d a y in restaurants in France come out

of a vacuum-sealed bag. Home cooks still prefer to use fresh potatoes because

they soak up less oil. Will traditional practice give way to the new technique

of packaging sliced potatoes raw under a controlled atmosphere, developed

in 1997 by Patrick Varoquaux and his colleagues at the Institut National de la

Recherche Agronomique (inra) station in Montfavet?

In cafeterias and restaurants at least there is no avoiding the superior con-

venience of precut and processed potatoes because the large volumes con-

sumed take extensive advance preparation. Another complication is that po-

tatoes darken upon exposure to the air once they have been cut up because

slicing releases enzymes and associated substrates that otherwise are shut up

in separate compartments in the cells of the potato. In the presence of oxygen

these molecules react and form brown compounds similar to the ones that

cause our skin to tan in the summer.

Before Varoquaux’s work, this enzymatic browning prompted the makers

of prepared fries to offer precooked products: The potato sticks were peeled

and sliced, then dried and deep-fried in oil (often palm oil, cheaper than other

kinds), and finally frozen. For the final cooking they could either be deep-fried

again, in which case the microscopic fissures created during freezing caused

them to absorb a lot of oil; or reheated in the oven, in which case they ended

up being too dry.

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Atmosphere of Fries

In their search for ways to remedy these defects, the inra chemists experi-

mented with the idea of packaging raw sliced potatoes under controlled atmo-

spheres—what are now known in Europe as
quatrième gamme
fries—taking

care to monitor signs of browning during fabrication.

To minimize browning, a number of steps are followed. First, the potatoes

must be carefully peeled, preferably under a stream of water, so that the cel-

lular structure is not damaged. For this reason the blades of the stainless steel

knives used to cut the potatoes into sticks must be kept as sharp as possible.

Next the individual sticks are kept at a temperature of about 4°c (39°f) so

that the metabolism of the intact cells is slowed down as much as possible.

After draining by either centrifugation or ventilation, the potatoes are treated

with an inert gas, in the absence of oxygen, in a perfectly sealed packet. In this

way the sticks can be preserved for 10 days, still at 4°c (39°f), without altera-

tion (in the course of storage, however, the tissues of the potato accumulate

sugars that cause the fries to darken during cooking, by reactions analogous

to those that brown the crust of bread). The flavor and texture of fries that are

cooked later nonetheless resemble the flavor and texture of fresh French fries:

The proportion of oil absorbed is similar, much less than in the case of frozen