Balancing the level of oxygen in winemaking is key. Too much oxygen results in wine oxidation, while too little creates a rather reductive wine environment. Adopting a reductive winemaking style can help preserve the aromas and fresh fruit quality of your wine but can also result in reductive wine faults such as Hydrogen sulfide (H₂S), Mercaptans, and Disulfides. These volatile sulfur compounds (VSC’s) in amounts past their odor thresholds cause offensive odors like rotten eggs, sewage, garlic, onions, and vegetal notes in wine.

The best way to limit reductive wine faults is through prevention:

  • Controlling the use of sulfur in the vineyard
  • Creating a healthy environment for yeast by optimizing the YAN during fermentation

If prevention fails, treatment of reductive wine faults include:

  • Aeration during fermentation
  • Copper sulfate fining after fermentation.
  • Ascorbic acid and tannin additions after fermentation/during wine aging
  • Lees management

Preventing the Production of Reductive Wine Faults

Preventing the production of reductive wine faults during fermentation, rather than treating and removing them later during aging, is an easier process to employ. This is because the current methods used to remove them from wine can result in wine oxidation, and the formation of Disulfides from the oxidation and binding of Mercaptans. If H₂S is produced but left untreated during fermentation, it can bind with ethanol or acetaldehyde to form Mercaptans, which will ultimately result in Disulfides.

Thus, understanding why H₂S is produced and how to prevent it can help to reduce the level of Mercaptans and Disulfides that arise later in the difficult to treat and reductive environments of the wine barrel or bottle.

A wine vineyard that uses sulfur as a pesticide.
Photo: Pexels

Controlling Sulfur in the Vineyard

Another factor that leads to the production of H₂S in wine is the excessive use of sulfur in the vineyard. In this case, sulfur enters the fermenting wine by residues left on the skins of the grape berry and is reduced to H₂S in the low pH environment of the wine. Ensuring that the amount of sulfur sprayed is less than 1ug/g of harvested fruit, and spraying is done no earlier then 56 days prior to harvest, will help to reduce the sulfur that enters the must and fermentation. Optimizing canopy management practices can be a great way to reduce your use of sulfur in the vineyard.

A picture of eggshells. The reductive wine fault from hydrogen sulfide produced during wine fermentation smells like rotten eggs.
Photo: Unsplash

Fermentation and Yeast Nutrient Management

There are many contributing factors that result in the production of H₂S. The first, and most common cause is a lack of yeast assimilable nitrogen (YAN) in fermenting must. In low nitrogen conditions, yeast will continue to reduce sulfite/sulfate to sulfide, with the intention of assimilating the sulfur-containing amino acids methionine and cysteine. Without nitrogen available, sulfide is rather shuttled out of the cell, and in the low pH environment of the wine, sulfide is reduced to H₂S. Alternatively, H₂S in wine can also be formed from the degradation of sulfur-containing amino acids by yeast in nitrogen limiting environments.

A good rule of thumb is to measure the YAN of your must and ensure you have approximately 10 mg N/L per degree Brix. You can also use this guide to help you determine the YAN you require to direct your specific wine style. If your YAN is too low, you can use this YAN calculator to find the amount of YAN you need to supplement your must.

Treating and Removing Reductive Wine Faults

If the prevention of H₂S and other VSC’s during fermentation was unsuccessful, there are several methods you can utilize to treat the offending wine faults:

Aeration during Wine Fermentation

One commonly used method to help remove H₂S from your fermentation is to introduce some Oxygen into the wine and allow the H₂S to become volatile. Aeration can be effective if H₂S has not been left long to bind with ethanol and/or acetaldehyde to form Mercaptans. If, however, Mercaptans are present, they will oxidize and bind to form Disulfides, compounds with a higher odor threshold (0.9-25 ppb) and thus the capacity to “hide” in wine. Once the fermentation is complete and it has been transferred to a more reduced environment such as a barrel or bottle, Disulfides can be reduced back to Mercaptans, compounds that are more pronounced due to their lower odor threshold (1.1-1.8 ppb). This means these reductive wine faults can fly under the radar in ferment, undetectable by sensory, and remerge in the bottle once your wine is out in the market.

Copper Sulfate Fining after Wine Fermentation

Copper sulfate can be an effective treatment to remove H₂S and some Mercaptans from your wine. But since Copper sulfate is toxic to yeast, it can only be added after fermentation is complete, leaving H₂S to its devices for far too long in your wine. Copper sulfate will bind to H₂S and some Mercaptans, and help to precipitate them out, but at the consequence of losing some desirable thiol aromas present in wine. Copper sulfate also acts as a catalyst for oxidation, and so there is a fine balance between treating reductive wine faults and preventing the detrimental effects of oxidation.

Ascorbic Acid and Oak Tannins during Wine Aging

If you think you have Mercaptans and some Disulfides present in your aging wine, a great way to remove them is to shift the wine back to an even more reductive state. You can achieve this by the addition of ascorbic acid, which will scavenge the oxygen in the wine. By doing this, Disulfides can shift back to their more reduced (and easier to treat) Mercaptans form. The addition of oak tannins, especially ellagic acid and condensed tannins, will bind with the Mercaptans and form a complex that becomes odorless in wine.

A wine barrel filled with aging and sealed with BarrelWise bung which creates a more reductive environment for the wine.
Photo: BarrelWise
Lees Management during Wine Aging

Aging wine on the lees can help to improve the body, aroma, and mouthfeel of wine but can also contribute to the release of Mercaptans and Disulfides. The result is the degradation of sulfur-containing amino acids that are then released as Mercaptans and Disulfides in the reduced environment of the wine barrel. Optimizing your barrel work efficiency will allow you to sample your barrels on a more consistent basis, helping you to detect the formation of VSC’s before they become more problematic.

Determining the Source of the Reductive Wine Faults

It can be difficult to know which of the VSC’s is causing the reductive wine faults in your wine and is important to know which to ensure that the appropriate treatment is in place. One good way to test this is to perform a sulfide benchtop fining trial and use these steps to determine which of the reductive wine faults you are dealing with.

1. Is the aroma removed quickly after the addition of Copper sulfate?

-> H₂S.

2. Is the aroma removed a day or two after the addition of Copper sulfate?

-> Mercaptans, begin tannin trials.

3. Does the odor remain several days after the addition of Copper sulfate?

-> Disulfides. Begin ascorbic acid and tannin trials to determine the best course of action without causing too much harm to the aroma of your wine.


Understanding the balance between oxidation and reduction in your wine is instrumental in producing wines that are unmasked by wine faults and shine in their aromas, flavors, and terroir. The prevention of reductive wine faults like Mercaptans and Disulfides later during wine aging can be achieved by preventing the production of H₂S during fermentation, and proper barrel lees management. Optimizing your wine quality control program to include consistent sampling of aging wine, can help you to detect reductive wine faults before they become unmanageable. If prevention is unsuccessful, understanding which of the VSC’s is present in your wine is the first step in employing the right remedial method to preserve the quality of your wine.

This article was written by Brittany Goldhawke

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