Reviewing YAN and Hydrogen Sulfide: Part 2
By: Denise M. Gardner
In a previous post, we discussed ways in which nutrient management during primary fermentation can affect hydrogen sulfide formation and the overall “health” of the wine. This week, we’re going to explore how to mediate hydrogen sulfide aromas and flavors in a finished wine.
Sulfur-Containing Off Aromas
In general, many wine sensory scientists and wine experts will agree that is relatively a bad habit to use the term “sulfur” to describe off-odors associated with hydrogen sulfide or “stinky” aromas that are usually described by the term “reduced.” One of the main arguments for avoiding “sulfur” as a description term for an aroma is due to the fact that there are actually several forms of aromatic sulfur-containing compounds found in wine, and they can have very different aromas (smells, odors) associated with that one compound. The most common groups of aromatic sulfur-containing compounds in wine are:
- Sulfur dioxide (SO2)
- Hydrogen sulfide (H2S)
- Mercaptans or Thiols
Additionally, many sensory experts will advise further to avoid using the chemical names as descriptors for describing an aroma found in wine (e.g., using the term “hydrogen sulfide” to describe the hard-boiled or rotten egg aroma). It is typically recommended to use an actual descriptor when describing an aroma (e.g., using the term “rotten eggs” when that smell exists in wine).
Sulfur Dioxide (SO2)
Sulfur dioxide is an antioxidant and antimicrobial preservative frequently used in wine production. However, it is also produced by yeast during primary fermentation, which is why wines (and other fermented products) cannot be sulfur dioxide-free (commonly referred to as “sulfite free” in the mass media). The aromatic descriptor commonly associated with a high concentration of sulfur dioxide is termed “burned match,” but a high concentration of sulfur dioxide can also cause a nasal irritation that many will describe as nasal burning. For more information on sulfur dioxide and managing its concentration in wine, please refer to this Wine Made Easy Fact Sheet produced by Penn State Extension.
Hydrogen Sulfide (H2S)
Hydrogen sulfide is an aromatic compound that is commonly described as having a “rotten egg” or “hard-boiled egg” aroma. Like many sulfur-containing compounds, hydrogen sulfide has a low sensory threshold (<1 – 1 part per billion, ppb), indicating that about 50% of the population could sense this compound at that concentration without being able to identify it, specifically, as hydrogen sulfide.
As we saw in our previous post, hydrogen sulfide development can result as a component of poor nutrient management during primary fermentation. Residual elemental sulfur from pesticide sprays has also been linked to latent development of hydrogen sulfide in wines. In a 2016 edition of Appellation Cornell, Dr. Gavin Saks’ lab provided a detailed and practical report on how hydrogen sulfide can be a problem for winemakers post-bottling and the potential links to hydrogen sulfide development as a function of residual sulfur from the vineyard (Jastrzembski and Saks, 2016).
Occasionally, winemakers may also experience hydrogen sulfide formation during a sur lie aging period; a time in which the finished wine remains on the lees when lees are stirred in the wine. It is also common for sparkling wines, produced in the traditional method, to exhibit a small perception of hydrogen sulfide when the bottle is first opened.
Mercaptans/Thiols and Disulfides
Finally, mercaptans or thiols, sulfur-containing compounds that contain the functional group –SH, and disulfides, sulfur-containing compounds that contain a S-S bond, can also be problematic for winemakers when found at high concentrations.
The presence of sulfur-containing volatile compounds is not always considered detrimental to wine quality. For some wine grape varieties (e.g., Sauvignon Blanc), these classes of compounds can make up their varietal aroma. In very small concentrations, sulfur-containing compounds can also be aroma enhancers, indicating that their presence can actually make the wine smell fruitier than if they were not present in the wine. However, when at substantial concentrations, volatile sulfur-containing compounds can also produce various “stink” aromas that mask a wine’s fruitiness, freshness, and make the wine generally unappealing. This is phenomena is dependent on the concentration of the sulfur-containing compound and the chemical makeup of the solution (i.e., wine) it is in.
Mercaptans or thiols and disulfides have a variety of descriptors associated with them, and their perception is largely based on concentration. When we’re discussing the negatively-associated descriptors, common terms include: garlic, onion, canned asparagus, canned corn, cooked cabbage, putrefaction, burnt rubber, natural gas, and molasses amongst others.
Are There Sulfur-Containing Off-Aromas in Your Wine?
To identify if hydrogen sulfide, mercaptans/thiols, or disulfide-based off-odors exist in your wine, it may be best to use a copper screen as a bench trial. While analytical identification of these compounds is possible, it is often expensive and leaves the winemaker guessing on what to do next.
For a quick assessment of a wine’s aroma, winemakers can drop 1-2 pre-1985 copper pennies into a glass of wine to see if the aroma freshens. The freshening aroma is due to the fact that the copper from the penny is reacting with the sulfur-containing compounds in the wine and making them aromatically inactive.
A technical copper screen takes a bit more work and should be conducted in a quiet and aromatically-neutral environment. It is recommended to do this outside of the cellar.
Copper addition, in the form of copper sulfate, is often used to remediate aromas/flavors associated with hydrogen sulfide. One-percent and 10% copper sulfate solutions can be purchased through your local wine supplier. The basic protocol associated with a copper screen is as follows:
- Add 50 milliliters of wine to two glasses.
- Label one glass “control” and the other “copper addition” (see image below).
- Add 1 mL of 1% copper sulfate to the “copper addition” glass.
- Cap both glasses for 15 minutes. Sniff the aroma of each wine.
Sniff (smell only!) both glasses. Most people start with the “control” and smell the treated wine (wine containing copper sulfate) second. If the aroma/flavor of the “copper addition” glass has improved, or the hydrogen sulfide aroma has subsided, then a copper addition trial should follow to determine the exact concentration of hydrogen sulfide needed to clean up the wine in question. Remember that the legal limit for copper allowed in a finished wine is 0.5 ppm. For a full protocol on how to run a copper addition bench trial, please refer to this Penn State Extension Wine Made Easy Fact Sheet.
Treatment of Sulfur-Containing Compound Off-Aromas
Sulfur-containing compounds are quite reactive, which can make dealing with them fairly difficult. Many educators agree that the best way to treat sulfur-containing compounds, especially those that stink, is to prevent their existence as best as possible.
In the Appellation Cornell newsletter that focused on sulfur pesticide residues, Jastrzembski and Saks (2016) recommended that sulfur residue concentrations should not exceed 1 mg/kg at harvest in order to avoid latent hydrogen sulfide or sulfur-containing off-aromas later in processing and storage. Additionally, many experts recommend appropriately treating fermenting musts with nutrient management strategies based on the starting YAN concentration to minimize the incidence of hydrogen sulfide formation during primary fermentation. This topic was covered in a previous blog post.
As described above, winemakers may also opt to treat the wine with copper sulfate to try to reduce the perception of hydrogen sulfide or other sulfur-containing aromas. It should be noted that aromas caused by disulfides cannot be mediated with a copper sulfate addition.
There has been more conversation in the academic community regarding the reemergence of hydrogen sulfide or sulfur-containing off-aromas after a wine has been treated with copper and post-bottling. The theory around this appears to circulate around residual copper initiating reactions in the wine that lead to more sulfur-containing off-odors. This continues to be an ongoing discussion amongst researchers and will likely be a hot topic within with the wine industry. For now, it is important for winemakers to understand that there may be a risk of off-odors reemerging post-copper treatment and post-bottling. This topic will also be discussed to some degree at the 2017 PA Wine Marketing and Research Board Symposium on March 29, 2017 in State College, PA, and winemakers are encouraged to attend.
Some hydrogen sulfide or sulfur-containing off-odors can sometimes be mediated with use of fresh lees stirred in the wine or the addition yeast lees-like products. Winemaking products like Lallemand’s Reduless, yeast hulls, or some cellulose-based products can help reduce or eliminate the intensity of these off-odors. As with any other product additions, it is recommended that wineries always do bench trials first and before adding to the entire volume of wine. Additionally, Enartis USA (Vinquiry) has previously distributed a fact sheet to help winemakers troubleshoot reduced wines and determine how to best treat a problem wine.
The incidence of reduction, sulfur-containing off-odors, or hydrogen sulfide can be a frustrating circumstance for winemakers. However, adequate vineyard care and proper nutrient management during primary fermentation can help minimize the incidence rate of sulfur-containing off-odors from occurring in their wines. Of course, problems with wines do occur, and we hope that the recommendations above will help winemakers solve wine problems pertaining to sulfur-containing off-odors.
Jastrzembski, J. and G. Sacks. 2016. Sulfur Residues and Post-Bottling Formation of Hydrogen Sulfide. Appellation Cornell, 3a.