Handling Hydrogen Sulfide in Wine

updated 12/22/2024

Hydrogen Sulfide (H2S) is a major fault in wine, and is typically caused by yeast being stressed due to lack of nutrients. It is one of the few things in winemaking that requires immediate action, else the resulting problems will get worse.

This post explains what I have done to handle H2S in my wines, and how I prevent the formation of H2S.

What Is H2S?

H2S is a gas that may be produced by wine yeast that is stressed, typically due to a lack of nutrients. It is a highly offensive smell, typically described as “rotten eggs”, “swamp gas” or “dog farts”. Once someone smells it, they are unlikely to forget it.

When left in the wine, H2S forms mercaptan, which is equally offensive to the nose. The aroma is described as “cabbage”, “burnt rubber”, or “old socks”.

In my experience, mercaptan produces an off flavor, not exactly bitterness, but in that direction. It overshadows all other flavors in the wine. MANY years ago I visited a Finger Lakes (NY) winery and their wine smelled of burnt rubber. Until I researched H2S, I had no idea that they had encountered H2S and did not properly treat the wine.

Untreated, H2S ruins a wine.

Preventing H2S

The best way to address H2S is to prevent it.

Since H2S is excreted by yeast that has insufficient nitrogen, the best way is to ensure the yeast has nutrient. Some common nutrients:

• DAP — This is Diammonium phosphate, which is a good source of nitrogen. However, it is criticized as being too easily assimilated by yeast, with the potential to produce volatile acidity and mute the aromas and flavors.
• Fermax — Proprietary nutrient that also contains vitamins, minerals, and amino acids which help the yeast prosper.
• Fermaid — Proprietary nutrient that also contains vitamins, minerals, and amino acids which help the yeast prosper.

There are probably other proprietary nutrients, but I’m not aware of them.

Follow package directions regarding how much nutrient to add to the wine.

The yeast strain used also makes a difference. Before selecting a strain, it’s best to consult yeast charts to understand the characteristics of each.

• Low Nutrient — Strains with low nutrient requirements are less likely to produce H2S. This is not guaranteed, so use of nutrient is best.
• High Nutrient — Strains with high nutrient requirements are far more likely to produce H2S. So why use them? High nutrient strains such as RC-212 produce great wines. When using a high nutrient yeast, I add 50% more nutrient than package directions call for.
• Non-H2S Producing Yeast — Renaissance yeast supposedly do not produce H2S, although for a better ferment the addition of nutrient is best. The drawback is this brand is not generally available in winemaker sized packets, as Red Star and Lalvin are.

Managing H2S

So you smell H2S? React immediately for best results.

The sooner the H2S is handled, the less impact there is. This includes lessened effort and duration for the correction.

Action 1:

Stir the wine or must vigorously to expel H2S. It’s a good idea to do this in a well ventilated area and run a fan to disperse the H2S. The smell is noxious.

How long to stir? That depends on the situation. What I did was stir until the reek was mostly gone. For one batch that was caught late (post-fermentation), I stirred for 5 minutes with a drill-mounted stirring rod. For another batch that was caught very early, I stirred for 2 minutes.

One instance was a thick must, which I punched down and stirred the best I could for probably 8 minutes. I didn’t time it and am guessing at the duration.

Action 2:

If the wine is actively fermenting, add a half dose of yeast nutrient. H2S is typically caused by yeast stressed by a lack of nitrogen; adding nutrient prevents H2S formation from continuing.

I’ve been told that if the sugar is at 2/3 depletion, adding nutrient is not necessary. I’d add nutrient anyway, as the yeast is still eating, and still reproducing at a lower rate.

If the wine is post-fermentation, do not add nutrient as it is not necessary.

Action 3:

Add a double dose of potassium metabisulfite (K-meta) and stir it in. K-meta works by combining with contaminants, rendering them harmless. It will neutralize H2S, although it’s best to stir well (Action 1) to drive off as much gas as possible, so the K-meta has less to do.

I was asked why add more sulfur to a sulfur-based problem? The difference is that H2S is a sulfiDe while K-meta is a sulfiTe. Both contain sulfur, but are completely different compounds with completely different properties and uses.

If the H2S was caught early, the problem may be solved at this point. However, if mercaptans have formed and/or the H2S stench returns, further action may be required.

Action 4:

If the stench continues and/or mercaptans have formed, treat with a copper-based product such as Reduless. There are other products; this is the one I used successfully so I recommend it.

I read the directions and stirred in a median dosage. The instructions indicated minimum and maximum dosage, and I used the median.

The instructions state the wine should be fined to ensure the product precipitates. The one time I used Reduless, I used kieselsol & chitosan (K&C) four days after treating with Reduless, and four days after that, racked off the sediment.

Note: The “old school” method was to pour the wine over new copper pennies. DO NOT do this. Copper is poisonous and blindly using copper in this fashion is very dangerous as there is no way to measure how much copper has been added. Products such as Reduless provide a measured safe amount of copper and are precipitated from the wine.

Action 5:

If mercaptans formed, treat with ascorbic acid. I found a reference, which I didn’t save (if I find the reference, I’ll update this post), that recommended this action. At that time (4 years ago) I calculated I needed 1-1/4 tsp ascorbic acid per 19 liter carboy. I used a lesser amount (3/4 tsp) as it’s FAR easier to add more than to take some out.

Ascorbic acid is a strong acid, and I did not want to make the wine any more acidic than necessary.

The process took about 4 months, but the mercaptans were neutralized. The wine turned out to be a good one.

Practical Examples

I close this post with a description of what I did to handle the instances of H2S that I encountered.

2020 Red Blend second run

At the first racking of a 54 liter demijohn (4 weeks after pressing), the stench of H2S hit us immediately after the siphon started. I quickly searched the net for instructions regarding handling H2S.

First we opened the windows, then started a floor fan, and completed the siphon.

Next we stirred with a drill-mounted stirring rod for 5 minutes, changing direction several times. We built up a steep vortex, which expelled a lot of gas, both CO2 and H2S. The fan made the air breathable. Note that H2S is flammable, although I’ve been told the amount produced by a wine is not a danger. While I believe that is true, it’s still wise to use ventilation.

Near the end of the stir we added a double dose of K-meta.

The following day there was a slight stench of H2S, and an off-flavor from the mercaptans. I ordered Reduless.

When the Reduless arrived, I read the directions and added a median dose, stirring well. Four days later I treated with K&C, and racked four days after that. That took care of the smell.

A few months later the mercaptans were obvious in the taste, so we added 3/4 tsp ascorbic acid per 19 liter carboy. This took about 4 months to complete the removal of the mercaptans. It was a slow process and there was no obvious change for the first month. Fortunately, it became obvious that the wine was improving with time.

The wine turned out fine.

2022 Tempranillo

This wine was from grapes, and the boxes had been in the warehouse too long and had mold. We discarded the worst clusters, then dosed the must with a double dose of K-meta to address remaining mold, and started fermentation using RC-212. Two days later we caught a slight whiff of H2S.

We added another dose of K-meta, and because this was a grape must, so there was no “stirring”. I punched down and mixed the best I could for probably 8 minutes (this is a guess, I didn’t time it).

Because this was in fermentation, we added a 50% dose of yeast nutrient. We caught this one early, and this was all we had to do.

The wine turned out fine.

This situation taught two lessons:

• One — when using a yeast with high nutrient requirements, use 50% more nutrient than the directions call for.
• Two — don’t go nuts on K-meta. We had already double dosed because of mold, and I added a full teaspoon to the fermenter, and later added more. This produced a burned match smell. This is not a fatal problem; we didn’t add more K-meta to that wine until it had been in barrel for a while. The excess SO2 gets used up … eventually.

2023 Sangiovese

We were keeping 2 juice buckets cold for a week while another was was fermenting. The plan was to add the pomace from the grape wines to the juice buckets.

Both buckets started fermenting slowly on their own, and one produced a whiff of H2S. We immediately stirred the wine for 2 minutes, added a full dose of nutrient, and a dose of K-meta.

We also sprinkled 2 tsp Avante yeast into each bucket.

Since the juice buckets were to be added to pomace (which is full of live yeast), we didn’t intend to add more yeast. Also, we normally make overnight yeast starters, but in this case we wanted to ensure a desired yeast would take over, but not necessarily start a full ferment, so we just sprinkled the yeast.

A few days later when the grapes were pressed, the H2S was no longer detectable, so we proceeded as planned.

The wine turned out fine.

Conclusion

I’ve had three encounters with H2S in 4 seasons, and have improved my nutrient management to ensure it doesn’t happen again.

H2S will ruin the wine if not treated. The longer the H2S is present in the wine, the larger the problem gets, and the more it takes to solve it. It’s entirely possible that if it goes on long enough, the wine will be unrecoverable.

If H2S is detected, immediately stir the wine and add K-meta. Fast action is best.

I count myself fortunate as my three encounters turned out well.