Molecular Gastronomy: Exploring the Science of Flavor (35 page)

of openness is high. Lebon and his colleagues tested this assumption in the

vineyards of Alsace, which are semicontinental (and so quite different from

those of Bordeaux and the Loire Valley), in an area lying between Witzenheim

and Sigolsheim, near Colmar.

The soils and parent rock of Alsace had already been extensively studied but

never on the scale of the experiment now contemplated. Over an area of 1,750

hectares (4,325 acres) Lebon and his colleagues carried out a hectare-by-hectare

survey to determine homogeneous pedological units. On the basis of various

criteria for characterizing types of soils, they distinguished some thirty homog-

enous units. A pit was dug in each of these zones to analyze soil types.

Landscape and Climate

Lebon and his colleagues made climatological measurements at stations set

up near pits dug in vineyards in which the Gewurtztraminer grape was culti-

vated and observed that local climates varied little, on an annual average basis,

from unit to unit. Significant variations occurred only on shorter time scales.

During periods of inclement weather, for example, the air temperature was

found to depend only on altitude; in clement weather the diurnal temperature

varied as a function of altitude, declivity, orientation, the height of the hori-

zon from east to west, and the thermal properties of the soils. Taken together

these measurements define mesoclimates, which is to say climates on the local

scale of a slope or valley bottom, for example. Comparing the various landscape

234 | investigations a nd mod el s

indices with the climatological data made it possible to refine the notion of
ter-

roir
. Temperature turned out to be important in determining the mesoclimate,

but not as important as the landscape.

What makes a
terroir
good for growing wine? The inra agronomists showed

that in Alsace, more than in the Bordeaux region or the Loire Valley, the prin-

cipal differences result from variations in the maturity of the grapes at the mo-

ment of harvest. In all the wine-growing regions studied, water nourishment

conditions played a major role in determining, among other things, the length

of time between the fruiting of the vine and the ripening of the grapes. Matura-

tion comes late when water is plentiful because the vine produces leaves rather

than berries. When the supply of water is insufficient, maturation is delayed

as well, but the adage that the vine must be made to suffer if it is to produce

grapes is only partly true: The supply of water should decline both moderately

and regularly.

Now that the notion of
terroir
has been validated in Alsace, it remains to

study the relationship between a wine-growing site’s total natural environment

and the quality of the wines it produces. Attempts to characterize aromas are

under way: Alex Schaeffer and his colleagues at the inra station in Colmar

have already observed a high degree of variability in terpene alcohol content

and the oxides of these alcohols.

The
Terroirs
of Alsace
| 235

70

Length in the Mouth

Enzymes in saliva amplify an important aromatic component of wines made

from the Sauvignon Blanc grape.

b i o c h e m i s t s a r e v e r y i n t e r e s t e d in the methods used for making

wines, but they have rarely explored the physiology involved in tasting them.

Recent results suggest that this situation may be changing. In wines made

from the Sauvignon Blanc grape an odorant molecule has been found whose

effect is registered only when the enzymes in saliva have separated it from its

precursor. A few moments are needed, then, for the aroma to be perceived.

In 1995, Philippe Darriet and Denis Dubourdieu at the Institut d’Œnologie

de Bordeaux discovered a molecule that gives Sauvignon wines a boxwood or

broom note. Significantly, this simple molecule, whose skeleton is composed

of only five carbon atoms, contains a sulfur atom. Darriet and Dubourdieu,

observing that it appears during alcoholic fermentation, managed to identify

its precursor. Additionally, they observed that the frequency with which this

precursor is transformed into an odorant molecule depends on the strains of

yeast responsible for fermentation. The Bordeaux chemists studied a class of

enzymes capable of releasing the wine’s distinctive aroma, reasoning that if

they could identify the particular enzyme at work in the case of Sauvignon they

would have thereby determined the structure of the precursor.

In a related study, Claude Bayonove at the Institut National de la Recher-

che Agronomique station in Montpellier examined glycosidases, enzymes that

dissociate odorant molecules of the terpene class from the sugar molecule to

which they are bound. The molecules considered in the Montpellier study do

236 |

not produce the odorant compound analyzed in connection with the Sauvi-

gnon grape because this compound is not bound to a sugar.

Amino Acids and Sulfur-Based Aromas

Darriet and Dubourdieu looked at enzymes that break the bonds between a

carbon atom and a sulfur atom to see whether one of them causes Sauvignon’s

aroma to appear. They were interested in an enzyme called lyase, produced

by the intestinal bacterium
Eubacterium limosum,
that breaks down the sul-

fur derivatives of cysteine, an amino acid. The compound in Sauvignon was

released in vitro from a nonvolatile extract of its must through the action of

some product present in the medium obtained by grinding up
Eubacterium

limosum
. They concluded that the precursor of the aroma in the must contains

cysteine.

These results confirmed, first, that the Sauvignon grape has an aromatic

potential that is brought out by the vinification process. This means that the

wine maker must choose yeasts that are capable of developing the taste latent

in the grape. The great gastronome Curnonsky (1872–1956) was famous for

demanding that foods have “the taste of what they are.” Chemistry is a way of

satisfying this requirement.

The Origin of Length

The Bordeaux chemists also showed that the aroma’s precursor is found

in significant quantities in the wine itself, the aromatic molecule remaining

bound to the nonaromatic part. The operation of enzymes in the saliva, which

during tasting detach the cysteine from the sulfur aroma after a few seconds,

is the chief mechanism underlying the notion of length, which is to say the

length of time the sensation produced by a wine remains in one’s mouth. A

special unit, the
caudalie,
has even been introduced to quantify the length of

time this sensation persists once the wine has been swallowed. Certain Bor-

deaux wines produce a sensation that lasts several seconds (or
caudalies
) and,

in a few exceptional cases, is capable of reasserting itself: Having disappeared,

the sensation comes back. If the sulfur molecule discovered by the Bordeaux

researchers does not actually explain this peacock-tail effect, it is the first to

yield a length whose origin is understood.

Length in the Mouth
| 237

What are the practical consequences of this discovery? Because the adul-

teration of wines is forbidden by law, oenologists (who often work for produc-

ers) will seek to optimize the concentration of a given aroma during vinifica-

tion. But if you don’t have a lot of money and are willing to indulge in a little

amateur chemistry, you can try adding the aroma to wines in your collection

that don’t have enough of it. After all, the law doesn’t forbid lengthening the

duration of one’s pleasure.

238 | investigations a nd mod el s

71

Tannins

The development of tannins diminishes the astringency of wines.

h o w d o e s w i n e a g e ? Gourmets have long complained of the lack of sci-

entific interest in the role of tannins in the development of red wines. Tannins

are astringent substances that are abundant in young wines and that change

as the wine ages, giving it an adobe color, smooth taste, and powerful aroma.

Over time, tannins are said to soften up. With the aid of modern analysis tech-

niques, Joseph Vercauteren and Laurence Balas at the University of Bordeaux

revived a topic of research that had largely been abandoned since the work of

Yves Glories in 1976 and elucidated several chemical transformations whose

existence had previously been suspected by oenologists.

Tannins are what create the sensation of astringency when one chews rose

petals, for example, or drinks a wine that is too young. By forming complexes

with the lubricating proteins in saliva they prevent these proteins from playing

their natural role, so that the mouth feels dry. Tannins are found in the woody

parts of plants as well as fruits. The alcoholic solution that is formed in the

process of making red wines, in particular, extracts tannins from the pip, skin,

and stalk of the grape.

Thirteen years after Glories wrote his thesis on the “coloring matter of

wines,” in 1989, Vercauteren and Balas analyzed the behavior of tannins in the

course of vinification. Every other day they took samples from two red wines

(Château Cérons and Château Baron Philippe de Mouton Rothschild) in order

to monitor variations in tannic concentration. Using ethyl acetate to extract the

| 239

tannins from these wines, they obtained paradoxical results. Whereas certain

oenologists believed that maceration must be prolonged beyond two weeks

in order for tannins to accumulate, giving the wine body and a chewy sensa-

tion, the Bordeaux chemists discovered that the mass of recovered tannins for

the three grapes tested (Merlot, Cabernet Sauvignon, and Cabernet Franc) was

greatest around the tenth day of vinification. Why this contradiction? Was it

because tannins, being reactive molecules, are transformed into compounds

that are difficult to extract when they are put into an ethyl acetate solution?

This initial finding made it clear that a fresh start had to be made, synthe-

sizing the constituent elements of tannins and studying their chemistry in

the hope of being able to identify them in the wines. Vercauteren and Balas

therefore sought to develop a method for hemisynthesizing condensed tan-

nins, which is to say for creating flavonol-based compounds derived from the

parent ring structure of flavan that contain several hydroxyl (–oh) groups. Taxi-

foliol, extracted from the bark of the Douglas fir, contains several preponderant

compounds whose chemical structure was subjected to analysis.

The question needing to be answered, then, was how flavonols bond with

condensed tannins. The simple case of flavonolic dimers, formed from the

combination of only two flavonols, was already difficult enough, for its two

subunits can bond in two different ways. The Bordeaux chemists compared

the structure of the synthesized compounds and various natural tannins by

nuclear magnetic resonance imaging, but this method did not disclose the

two types of bonds. By contrast, transforming the tannins into peracetates by

replacing –oh hydroxyl groups with –oocch acetate groups yielded a general

3

method for determining the structure of condensed tannins.

Armed with this result, Vercauteren and Balas sought next to determine

whether flavonols and glycosylated tannins (tannins bonded with sugars) were

present in the wines they had selected for examination. They knew that the

glycosylation of certain hydroxyl functions stabilizes polyphenols (the class of

molecules to which tannins belong), but condensed tannins had never been

observed in glycosylated form in either grapes or wine. Nonetheless, they

suspected that the diminishing astringency of wines over time resulted from

an intensification of tannin–sugar bonds. Resorting once again to a method

that had served them well in the past, they synthesized glycosylated tannins,

analyzed their characteristics, and then sought to identify these molecules in

the wines.

240 | investigations a nd mod el s

Because the compounds extracted by means of ethyl acetate were rich in

two simple flavonols, catechin and epicatechin, the chemists studied its glyco-

sylation using glucose, the sugar found in grape juice. Four glycosylated flavo-

nols were shown to be highly insoluble in ethyl acetate, which corroborated the

initial hypothesis: The reason that flavonol and glycosylated tannins had not

been found in either the grape or the wine was that the extractive solvents were

unsuitable to the task. Subsequent analysis of these hard-to-extract compounds

indicated that at least three glycosylated tannins are present in both the grape

and the wine, thus establishing that the polymerization and glycosylation of

tannins are two of the mechanisms responsible for the aging of wines.