Sotolon is the principal molecule that gives vin jaune its characteristic flavor.
IN 1991, Patrick Étiévant and Bruno Martin at the Institut National de la Recherche 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-acetylgamma-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.
Sotolon Uncovered
The most direct method of analyzing molecular extracts from wine is chromatography. 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 present in 40–150 parts per thousand in sherries. Solerone seems to be less characteristic 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 concentrations 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. 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 sotolon 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.