How to Limit Reductive Off Aromas During Wine Aging: Tips and Tricks

by Brittany Goldhawke R&D Enologist at BarrelWise.

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Reductive off aromas in wine

Reductive wines are produced when oxygen concentration is limited in either the fermenting must or aging wine environment. While adopting a reductive winemaking style can help preserve the aromas and fresh fruit quality of your wine, too much reduction can result in reductive off aromas, often referred to as volatile sulfur compounds (VSC’s). The most common VSC’s are Hydrogen sulfide (H₂S), mercaptans, and disulfides that cause offensive odors like rotten eggs, sewage, garlic, onions, and vegetal notes in wine. H₂S are commonly produced during fermentation, while the majority of mercaptans and disulfides are produced later in fermentation and during wine aging in the barrel and bottle.


Preventing the production of H₂S and mercaptans 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, causing 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.


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Controlling Sulfur in the Vineyard

Another factor that leads to the production of H₂S 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 then 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.

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Preventing the production of VSC’s during wine fermentation

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 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 calculator to find the amount of YAN you need to supplement your must.


Treating and removing VSC’s from wine

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

Aeration during 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 and be released. 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 off aromas 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 after 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 off-aromas and preventing the detrimental effects of oxidation .

Ascorbic acid and oak tannins during 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.

Lees management during 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. Ensuring to sample your barrels on a consistent basis can help you to detect the formation of VSC’s before they become problematic and rack your wine of the lees if necessary.


Determining which VSC is present in wine

It can be difficult to know which VSC is causing the off aroma 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:

To help pinpoint exactly which VSC you’re dealing with, and the appropriate treatment, you can first test if the aroma is removed quickly after the addition of Copper sulfate, if this is the case, you are probably dealing with H₂S. If you add Copper sulfate and it takes a day or two for the odor to disappear, you most likely are dealing with Mercaptans, and can begin tannin trials to treat those that are harder to remove. If the odor remains even after several days and is not removed after the addition of Copper sulfate, then you are dealing with Disulfides. You can begin ascorbic acid and tannin trials to determine the best amount to use of either compound to effectively remove the Disulfide without causing too much harm to the aroma of your wine.


Conclusion

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 off aromas like Mercaptans and Disulfides later during wine aging can be achieved by preventing the production of H₂S during fermentation, and proper barrel lees management. Keeping an eye on the chemistry of your must prior to fermentation will help prevent yeast production of H₂S, and reduction of elemental sulfur in wine. If prevention is unsuccessful, understanding which VSC is present in your wine is the first step in employing the right remedial method to preserve the quality of your wine.



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