Molecular Gastronomy: Exploring the Science of Flavor (36 page)

Tannins
| 241

72

Yellow Wine

Sotolon is the principal molecule that ˆves
its characteristic

avor.

i n 1 9 9 1 , Patrick Étiévant and Bruno Martin at the Institut National de la Re-

cherche Agronomique (inra) station in Dijon began to analyze a wine known

as
vin jaune
(yellow wine) that is produced only in the French Jura. The specific

flavor of this wine results from the practice of maturing it in barrels for several

years under a thick veil of yeast of the species
Saccharomyces cerevisiae
. A similar

wine is made in Alsace, Burgundy, and in the town of Gaillac, in the Tarn, where

it goes by the name
vin de fleur
or
vin de voile
. Its only near equivalents outside

France are the sherries of Spain and the Hungarian tokay. The Dijon chemists

wanted to know which molecules are responsible for its distinctive flavor.

This wine and ones like it contain hundreds of volatile components, a tenth

of them aromatic, so that identifying the molecule that produces a particular

aroma is not easy—a bit like picking out a guilty person among some 300

suspects. In the early 1970s some researchers believed that solerone (4-acetyl-

gamma-butyrolactone) was the chief odorant molecule of yellow wines, but in

1982 Pierre Dubois, also at the Dijon station, found it in red wines as well.

Solerone had an alibi.

The next suspect was sotolon (4.5-dimethyl-3-hydroxy-2[5h]-furanone),

a molecule constructed around a ring of four carbon atoms and one oxygen

atom. Because sotolon and solerone are found in minimal concentrations in

vins de voile
and are chemically unstable, the Dijon chemists searched for ways

to extract them more efficiently.

242 |

Sotolon Uncovered

The most direct method of analyzing molecular extracts from wine is chro-

matography. One begins by injecting a sample into a solvent, which is then

vaporized. Next, a polymer-lined tube is inserted that captures the various

compounds of the gaseous mixture in varying proportions, with the separated

compounds settling at the bottom of the tube. The chemists’ first task was to

devise a variant of this technique in order to identify the compounds present

in minimal quantities in the complex mixtures of yellow wine.

Chromatograms of the wine samples were then compared with those of

pure solutions of sotolon and synthetic solerone. Sotolon was found to be pres-

ent in 40–150 parts per thousand in sherries. Solerone seems to be less char-

acteristic of yellow wine, but its concentrations are higher in sherries, which

explains why it was first found in these wines. Finally, sensory tests of the

separated parts showed that tasters did not perceive solerone, in the concentra-

tions in which it is found in Savagnin (the grape from which yellow wine is

made), either in the wines themselves or in the laboratory solutions. Solerone

therefore was unquestionably not the culprit in the case of the
goût de jaune
.

Beginning in 1992, the chemists devoted all their energies to the search for

sotolon, whose presence had been observed in cane sugar molasses, fenugreek

seeds, soya sauce, sake, and other substances. It was also known to be present

in certain wines made from overripe grapes attacked by the botrytis fungus

(
Botrytis cinerea
), better known as the noble rot. This fungus is responsible for

the distinctive character of Sauternes, for example, and so-called late harvest

wines. Sotolon was not found in either red wines or oxidized wines. Moreover,

its perception threshold was determined to be only 15 parts per thousand.

Tasting tests found the typical character of
vins de voile
to be most pronounced,

exhibiting a note of walnut, when the sotolon concentration in these wines is

high. In even greater concentrations the tasters detected a note of curry.

The Death of Yeast

The sotolon trail was then taken up in 1995 by another inra researcher in

Dijon, Élisabeth Guichard, who developed a method for rapidly measuring its

concentration. In
vin de paille
(or straw wine, a sweet white wine made from

grapes dried on straw mats), this was found to be 6–15 parts per thousand.

Yellow Wine
| 243

In yellow wines, sotolon is synthesized at the end of the yeast’s exponential

growth phase: In vintages that have been aged in casks for one, two, three,

four, five, and six years, respectively, the quantity of sotolon is small in the early

stages of maturation and rises notably after four years, especially in cellars that

are not too cool.

Samples taken at different depths beneath the yeast veil revealed that soto-

lon is twice as concentrated in the middle and at the bottom of the casks as it

is just under the veil. It is thought that the sotolon is indirectly produced by the

yeast when the proportion of alcohol is high. The yeast transforms an amino

acid in the wine into a keto acid, which is released with the death of the yeast,

falling to the bottom of the cask. A chemical reaction then transforms the keto

acid into sotolon, enriching first the bottom, then the middle, and finally the

upper layers of the wine.

Now that sotolon is known to be the molecule responsible for the distinctive

flavor of yellow wine, research is under way to identify strains of yeast that are

capable of producing it in quantity and determine the conditions that favor the

emergence of this flavor during aging.

244 | investigations a nd mod el s

73

Wine Without Dregs

Wines meant for exportation must be stabilized in order to prevent tartrate

deposits from forming.

t h e c o l d o f w i n t e r c a u s e s t a r t r a t e c r y s t a l s to precipitate in

bottles of wine in the cellar. These crystals do not harm the quality of a product

that holds both symbolic and economic importance for France, but they do

hamper its exportation to demanding or poorly informed clients. How can re-

ductions in market value—or, worse still, returns—be avoided? Jean-Louis Es-

cudier, Jean-Louis Baelle, and Bernard Saint-Pierre at the Institut National de la

Recherche Agronomique (i nra) station in Pech Rouge and Michel Moutounet

at the Institut Supérieur de la Vigne et du Vin in Montpellier have developed a

method for balancing wines by electrodialysis.

Tartaric acid is a characteristic constituent of grapes, but the salts associated

with it are not very soluble. For this reason potassium bitartrate and calcium

tartrate naturally tend to precipitate in wines, forming deposits of what are

commonly called tartrates. In the past, producers who wanted to avoid tartrate

accumulations kept their bottles at a low temperature for ten days or so, adding

to the wine potassium tartrate, which served as a crystal nucleus. The cold trig-

gers crystallization because the limited solubility of tartaric salts diminishes

further with the fall in temperature, while the action of polyphenols (a class of

molecule that includes many of the coloring agents in red wines) ensures that

the wine remains supersaturated with these salts—hence the ineffectiveness of

traditional procedures in the case of red wine. Moreover, because the tartrate

content of the wine varies, lasting stability is not guaranteed.

| 245

The Hunt for Tartrates

Because the cause of the problem is the excessive concentration of tartrate,

potassium, and calcium ions in wines, the i nra researchers sought a way to

eliminate them. Electrodialysis suggested itself as a promising candidate. Wine

was made to flow between two polymer membranes by applying an electrical

field perpendicular to the direction of flow: The negative ions were attracted

on one side and the positive ions on the other. The researchers reasoned that

if membranes were used that selectively let through potassium, calcium ions,

and tartrate ions, these ions would be specifically extracted.

The wine was circulated through a stack of parallel anionic and cationic

membranes that deionized it while enriching the lateral compartments

(through which a solution of fixed composition circulated simultaneously) in

tartrate, potassium, and calcium ions. The membranes were made of grafted

polysulfones, 20 centimeters (a bit less than 8 inches) on a side and spaced

0.6 millimeter apart, and the electric field applied was 1 volt per cell. The re-

searchers tested a total transfer surface of 4 square meters consisting of sixty

stacked cells. In their modeling of the problem they tried to adapt the intensity

of the electrical current to the degree of instability peculiar to each wine. As

a result, oenologists now have at their disposal a tool suited to the majority of

relevant cases.

Why does the new method do a better job of stabilizing the wines than the

traditional technique? Because it uses the electrical conductivity of the envi-

ronment as a measure of the amount of tartrate in the wine. This conductivity

depends to a great extent on the concentration of potassium, calcium, and

tartrate ions. By regulating the electrical field and the length of time the wine

circulates as a function of its conductivity, it becomes possible to extract just

the quantity of ions necessary to balance the wine, at a rate of about 1 hectoliter

(a little more than 26 gallons) per hour using the pilot device tested. The speed

of processing depends on the wine’s adhesive properties: Once a day a cleaning

agent must be circulated through the system in order to remove the accreted

polyphenols and tannins.

Does the new filtering process alter the quality of the wines? This vital ques-

tion, one that is naturally of great interest to all gourmets, was studied in the

course of a long and careful series of trials by oenologists in the Beaujolais,

Champagne, and Bordeaux regions. They reported no qualitative difference

246 | investigations a nd mod el s

in flavor between treated and untreated wines. Baccard s.a., in association

with Eurodia s.a., which produces the membranes, plans to manufacture and

market detartarizing equipment now that the necessary approvals have recent-

ly been obtained from Brussels; the first experiments were conducted under

private auspices, but preliminary authorization for the expenditure of public

funds in support of this research was received in 1996.

In the meantime the researchers continue to study possible uses for the

extracted ions and to explore the processing of sweet fortified wines (
vins doux

naturels
) and apéritifs made from wine, which are difficult to treat using tradi-

tional methods, with the aid of cold.

Wine Without Dregs
| 247

74

Sulfur and Wine

Sulfur compounds in wine are responsible for defects and virtues alike,

depending on the molecule.

i s t h e p r e s e n c e o f s u l f u r always a defect in wine? In the 1960s the

undue interest of some growers in preserving their wines as long as possible

gave sulfur a bad reputation. Sulfur dioxide added in excessive quantities dur-

ing the fumigation of casks and the sulfiting of harvested grapes causes pain-

ful headaches, it is true. But recent biochemical studies show that the use of

sulfur is not to be rejected altogether. Chemists at the Faculté d’Œnologie de

Bordeaux have discovered that sulfur is capable of both the best and the worst:

Although some sulfur molecules are the source of indisputable flaws, others

contribute pleasing notes of boxwood, broom, passion fruit, and grapefruit in

both white and red wines.

Oenology has long seen only the negative side of sulfur compounds. There

is no question that hydrogen sulfide and sulfur dioxide are nauseating. Ironi-

cally perhaps, the attempt to eliminate these deleterious effects by improving

fermentation and vinification methods led to the discovery of the positive side

of sulfur compounds. In 1993, Philippe Darriet and Denis Dubourdieu dis-