Sulfur Compounds in wine are responsible for defects and virtues alike, depending on the molecule.
IS THE PRESENCE OF SULFUR 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 during the fumigation of casks and the sulfiting of harvested grapes causes painful 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. Ironically 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 discovered a molecule in Sauvignon wines having an agreeable odor that belongs to the thiol family (characterized by a group composed of a sulfur atom and an –SH hydrogen atom that is directly attached to a carbon atom). This raised the possibility that other sulfur compounds might contribute to the aroma of Bordeaux wines. Further investigation by Takatoshi Tominaga, Valérie Lavigne-Cruege, and Patricia Bouchilloux revealed the presence of sulfur compounds in very weak concentrations.
Sulfur and Yeast
Many known sulfur molecules are aromatically very active thiols. Whereas the perception threshold for alcohols (molecules with an –OH hydroxyl group) is on the order of a milligram or a microgram per liter in wine, the threshold for thiols is roughly one-thousandth as much—hence the interest in developing a method of analysis capable of detecting these trace aromas, agreeable and noxious ones alike.
The studies by Tominaga and his colleagues showed that many volatile sulfur compounds responsible for aromatic defects in wines have their origin in the action or metabolism of yeast, which during fermentation transforms the sulfur amino acids of the grape and the sulfur dioxide added as a preservative. This finding suggested a way to correct the known defects of sulfited wines. But whereas it is simple to lower the concentration of hydrogen sulfide, either by reducing the share of sulfur dioxide or by racking the wine in order to aerate it, other molecules such as ethanethiol and methanethiol, as well as a similar molecule known as methionol, are more difficult to control. The chemists observed that the concentration of these compounds depends directly on the turbidity of the grape juice before fermentation. This cloudiness, which brings the juice into contact with the yeast, has to be reduced.
The Misdeeds of Copper
Investigation of the good side of sulfur molecules has yielded its own harvest of results. Certain thiols were known to share the characteristic odor of plants (broom, boxwood), fruits (cassis, grapefruit, passion fruit, guava, papaya), and even foods such as roasted meat and coffee. It happens that the great variety of aromatic nuances found in Sauvignon wines are lost when copper is added. Because copper bonds chemically with thiols and blocks their aromatic action, the Bordeaux chemists suspected that thiols were also responsible for the odors found in Sauvignon.
The first thiol was identified in a red Sauvignon wine, in 1993, and had an odor of boxwood and broom at a concentration of 40 nanograms per liter. The chemists succeeded next in finding it in boxwood and broom as well. They then found other molecules in white wines that are also present in fruits, notably 3-mercaptohexyl acetate, which has a dominant hint of boxwood and recalls the odors of grapefruit zest and passion fruit (in which it was subsequently detected) and another thiol that, depending on its chemical environments, gives off the same fragrances.
And in red wines? Here again the chemists proceeded on the basis of a sensory observation, namely that the complexity and intensity of fruit and meat aromas in young red Merlots and Cabernet Sauvignons decrease when these wines contain a small amount of copper. Several thiols found in Sauvignon wines recently have been identified in young wines made from Merlot, Cabernet Sauvignon, and Cabernet Franc grapes, in which they contribute notes of cooked fruit, cassis, meat, and coffee, at concentrations as low as 0.1 nanograms per liter.
Oenologists are now seeking to use these discoveries to improve wine-making methods. In particular, they know that aging white wines on their lees, where the sulfur molecules of the yeast are found, progressively increases the concentration of certain aromatic thiols.