Bruce Neyers, one of the leading wine professionals and winemakers in the U.S., recently shared this excellent essay on the “use of sulfur in wine.” A great read, highly informative, and point on. Read on…
Above: Bruce Neyers is not only the national sales direct for Kermit Lynch, he is the owner of one of California’s most respected wineries, Neyers Vineyards.
Anthony and Enrique –
Here is a summary of the discussion we had last week on the issue of Sulfur in wine, and its two primary chemical forms: oxidized or Sulfite, and reduced or Sulfide:
Sulfur commonly exists in wine in two completely different chemical forms, reduced Sulfur and oxidized Sulfur. The former is formed as a by-product of a reaction involving elemental Sulfur during fermentation. The latter is present as a result of the external addition of Sulfur Dioxide prior to and after fermentation. Both forms of Sulfur are important to wine and can be positive, unless they exist in amounts that exceed normal tolerance. This tolerance will vary from person to person, however. As different as they are in taste and smell, both are sometimes mistakenly referred to by tasters as Sulfur and it’s important to not make this common mistake.
Hydrogen Sulfide or Reduced Sulfur
During the growing season, grape vines are periodically sprayed with Sulfur compounds, a process called ‘Sulfur dusting’. The compounds used for this vary from country to country — depending upon regulations, tradition and need — but the most common ‘dust’ is simple elemental Sulfur in a kind of powder form. It adheres to the leaves and grapes of the vines, reacts with dew to form a mild acid, and prevents the development of mildew. Typically, a grower begins dusting his vines when the shoots reach about 6-8 inches in length. Then, depending upon the weather, Sulfur dust will be applied every 10-20 days until about one month before harvest. Obviously some guesswork about the stop date is involved, and if the application of Sulfur dust is too heavy or too close to the date of harvest, the grapes may be brought into the winery with problematic levels of Sulfur dust on them. This is likely to result in a reduction reaction forming Hydrogen Sulfide. The fermenting wine is totally free of Oxygen, so the Sulfur in the must has a chemical attraction to Hydrogen (in the absence of Oxygen) and it forms a compound called Hydrogen Sulfide. Chemically, this is represented as H2S, or two molecules of Hydrogen attached to one molecule of Sulfur. Above threshold, Hydrogen Sulfide has an aroma that is referred to as ‘rotten egg’. The amount present need not be large to be objectionable as most people are sensitive to it in parts per billion. The correct description for this stinky condition is that the wine has a reduction or a Sulfide problem.
If Hydrogen Sulfide does become a problem in wine, by the way, vigorous racking or aeration can normally eliminate it. This, however, can also result in yet another problem by accelerating the ‘oxidation’ or aging of the wine. If the Hydrogen Sulfide problem is not addressed quickly, the Sulfide may react with the Ethyl Alcohol in the wine to form Ethyl Mercaptan. This is a liquid and can’t be easily removed by aeration. It must be removed by the addition of chemicals that convert the Ethyl Mercaptan back to Hydrogen Sulfide. This is rarely a positive event. While the most common way to introduce reducible elemental Sulfur to wine is through Sulfur applications in the vineyard, it can also develop from the breaking down or autolysis of the dead and dying yeast cells. These yeast cells will fall to the bottom of the wine at the conclusion of their life span and begin to decompose. As they decompose they generate Sulfur, which can also become a source of Hydrogen Sulfide. A slight degree of yeast autolysis reduction in wine can be positive, however, as it can add a desirable hazelnut-like aroma to the wine.
Sulfur Dioxide or Oxidized Sulfur
Wine is made from fresh fruit and like all fresh fruit grapes begin to deteriorate after they have been harvested. This deterioration can be held in check with the addition of Sulfur Dioxide, or SO2. This is an oxidized form of Sulfur called a Sulfite, and has been used for centuries to prevent the deterioration of freshly harvested grapes until fermentation can begin. The fermentation generates Carbon Dioxide which protects the wine by keeping Oxygen away from it, but when the fermentation ends and the Carbon Dioxide is no longer being generated, Oxygen will attack the wine and begin to do several things of concern. The worst is the conversion of the wine’s Ethyl Alcohol to Acetic Acid, or vinegar. Sulfur Dioxide helps to slow down this chemical process, so winemakers add Sulfur Dioxide to the wine at several crucial stages of its life. Normally, an addition of SO2 is made at the crusher at the time the grapes are received at the winery. A second addition is traditionally made when the wine finishes primary fermentation, unless the winemaker plans to have it undergo secondary or malo-lactic fermentation (in which case this second addition is made after malo-lactic fermentation is completed). Periodic analysis during the aging process dictates if and when future additions of SO2 might be made, but almost without exception an addition is made at the time of bottling. Sulfur Dioxide has an aroma sometimes referred to as burnt matchstick, and indeed it is the primary aroma one gets from the smoke of an extinguished match. It is not only unpleasant but painful to the soft flesh of the nostrils as well. If excess SO2 has been used in the winemaking process or too much SO2 added to the wine at the time of bottling, the wine will have not only an unpleasant aroma, but will irritate the nostrils. This, too, is sometimes mistakenly referred to as a Sulfur problem, when it is more correctly called an SO2, or Sulfite problem.
Sulfides and Sulfites are both chemical forms of Sulfur, and both can be problems in wine. They have different root causes, however, and different treatments when problematic.
—Bruce Neyers
June 1, 2010
