How Is Hydrogen Sulfide Produced During Wine Fermentation?

If you have heard anything about hydrogen sulfide, and then you hear it may be in your wine, you might be freaking out.

How is hydrogen sulfide produced during wine fermentation? And do you have to worry?

Fear not. You will find all the answers, and plenty of reasons not to worry, here.

Hydrogen Sulfide

Yes, hydrogen sulfide sounds scary.

Also known as H2S, sewer gas, swamp gas, stink damp, and sour damp, hydrogen sulfide certainly does not sound like something you want to encounter ever, especially in your wine.

Hydrogen sulfide is a chemical compound produced in a variety of industries, including oil and gas refining, mining, tanning, pulp and paper processing, and rayon manufacturing. It is also a naturally occurring compound in sewers, manure pits on agriculture farms, well water, oil and gas wells, and wonder of wonders, volcanoes.

In even low concentrations, it can be extremely hazardous to breathe in, and because it is heavier than air, it tends to, quite literally, hang around in low lying spaces like sewers and manholes.

Side effects from excessive exposure to hydrogen sulfide include headaches and eye irritation to unconsciousness and death.

While it is a colorless gas, it is not odorless. You will know you are in the presence of hydrogen sulfide by its strong scent of rotten eggs.

Okay, okay, you say, but what does all of this have to do with my wine?

Hydrogen Sulfide in Wine

While occupational safety hazards warn vociferously against excessive exposure to hydrogen sulfide in the workplace, very little is said about the natural production of hydrogen sulfide in the production of wine.

Yes, you read that right, hydrogen sulfide is a natural byproduct of wine fermentation, of all fermentation, really.

Why?

Yeast.

Yeast: The Powerful Fermenter

We love yeast for its ability to ferment, and therefore preserve and even improve upon, our foods and drinks.

In wine, yeast is easily the most important ingredient. Without it, we would simply have grape juice.

When wine is made, grapes are carefully selected, crushed, boiled, and steeped in their own delicious, sugary juices.

The yeast is added, and it gets right to work fermenting.

What does yeast do?

Yeast consumes all of the sugars in the liquid (now called must) and converts it to alcohol and carbon dioxide. This process is what we call fermentation, and it is what occurs with all yeast related food and beverage, from beer to wine to bread.

What we often don’t realize, and we certainly don’t talk about, is all of the byproducts of fermentation.

You see, yeast does not simply produce alcohol and carbon dioxide. It also metabolizes hundreds of other micronutrients and chemical compounds.

And we are mostly grateful for those byproducts. We get anti-cancer, antioxidant, and anti-inflammatory benefits from fermentation, leading to amazing gut health, which contributes to great overall health.

However, sometimes this process goes sideways.

Yeast and Hydrogen Sulfide

From what scientists can tell, yeast always produces hydrogen sulfide during the fermentation process, and it can produce this nasty smelling compound at all stages of fermentation, early in its activity, while it is consuming sugars, and even late in the process.

And it looks like all strains of yeast produce this compound while fermenting, so selecting a different strain is not necessarily the answer to avoiding rotten egg wine.

Indeed, it seems that there are various genetic selectors within each strain of yeast that make it more or less likely to produce higher or lower levels of hydrogen sulfide during fermentation.

So, you can select a specific genetic variant of a strain of yeast that produces less hydrogen sulfide, but you cannot avoid it altogether.

Another factor that seems to be critical to the production, or reduction, of hydrogen sulfide is the presence of other nutrients as well as the presence of nitrogen.

Yeasts respond to their environment in different ways, so controlling for the factors that increase or reduce various compounds is possible.

Other Factors That Cause Hydrogen Sulfide Production in Wine

From what studies have shown, yeast tend to produce less hydrogen sulfide when more nitrogen is present. Furthermore, the appearance of more micronutrients in the must tends to produce less hydrogen sulfide overall.

The ideal is for concentrations of YAN (Yeast Assimilable Nitrogen) to be above 150 mg N/L.

In short, yeast prefers nutrient rich, higher nitrogen environments to do their best work and not produce high levels of hydrogen sulfide.

Hydrogen sulfide seems to be a nasty byproduct of stressed out yeast.

So keep your yeast happy with lots of nutrients and high nitrogen, and you should be able to avoid detectable amounts of hydrogen sulfide, even if you cannot control for the genetic strain of yeast you work with.

The last thing you want is a glass of wine that smells like rotten eggs. And many winemakers have noted that even if you cannot detect the actual scent of rotten eggs in wine, higher concentrations of hydrogen sulfide can block the natural, desirable aromas and flavors of the wine.

As a Conclusion

To sum up, control for your yeast strains when and where you can. Always look for genetic variations of yeast that are less likely to get stressed out and produce higher concentrations of hydrogen sulfide. And even once you have done that, ensure your grapes are higher in nutrients. Keep an eye on your crops. And make sure you have high enough levels of nitrogen in your must before adding your yeast.

You cannot remove hydrogen sulfide from wine once it is present, so all of your work will have to rely on awareness and prevention.

Happy yeast will produce a more reliable, and certainly more enjoyable batch of wine. So constantly monitor your yeast’s viability and vitality to make sure it is indeed doing well.

Remember, the nose knows. And the last thing you want your nose to detect with a delicious, fresh glass of wine is rotten eggs.

Cheers!

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Sources:

Matias I.Kinzurik, Mandy Herbst-Johnstone, Richard C.Gardnerb, Bruno Fedrizzi, Hydrogen sulfide production during yeast fermentation causes the accumulation of ethanethiol, S-ethyl thioacetate and diethyl disulfide, Food Chemistry, Volume 209, 2016, Pages 341-347
https://www.osha.gov/hydrogen-sulfide
https://www.cdc.gov/niosh/topics/hydrogensulfide/default.html