Sulphur 101 (Part 1): Understanding the Difference Between Rotten Eggs and Struck Matches

Sulphur is one of the most misunderstood off-flavours in brewing. While rotten eggs and struck matches are often lumped together as “sulphur,” they result from different stages of yeast metabolism and require different approaches to control. 

In this first article of our three-part “Fermentation Sulphur” series, we’ll explore the Sulphur Reduction Pathway (SRP), why yeast produces these compounds, and how yeast strain selection, wort composition, and nutrition can help brewers manage sulphur production.

TL;DR - Fermentation Sulphur: Rotten Eggs vs. Struck Matches

The two main culprits, hydrogen sulphide (H₂S) (rotten eggs) and sulphur dioxide (SO₂) (struck matches), originate from the same yeast metabolic pathway but arise for different reasons.

• H₂S is associated with poor sulphur incorporation into amino acids, often worsened by inadequate yeast nutrition, mainly FAN.

• SO₂ accumulates when yeast ramps up sulphur metabolism, particularly when methionine availability is limited.

• Prevention starts with wort nutrition. Adequate FAN, methionine, vitamins, and minerals help keep sulphur metabolism balanced. A complete yeast nutrient, such as Yeast Lightning, can help address nutritional deficiencies.

• Yeast strain matters. Some strains naturally produce more sulphur than others, even under identical fermentation conditions.

• Not all sulphur is bad! While excessive sulphur creates undesirable aromas, controlled SO₂ production can help protect beer against oxidation and extend flavour stability.

The goal isn't to eliminate sulphur production entirely (the yeast still needs it!), it is to understand which compound you're dealing with, why your yeast is producing it, and how to manage it through strain selection, wort composition, and fermentation nutrition.

Part 1 of 3: Understanding Sulphur

Sulphur complaints almost always boil down to two distinct gases: hydrogen sulphide (H₂S ) and sulphur dioxide (SO₂ ). Brewers routinely confuse them, and it's easy to see why, considering they share a common metabolic origin and are lumped together under the generic label of "sulphur”.

Where Sulphur Comes From: The Sulphur Reduction Pathway (SRP)

Yeast cells produce compounds for two reasons: either they directly benefit the cell, or they are the accidental byproduct of a necessary metabolic pathway. Sulphur falls largely into the second category.

Yeast need sulphur-containing amino acids to grow, mainly methionine. When the wort doesn’t supply enough, the yeast make their own from sulphate, which comes from malt and brewing water. The Sulphur Reduction Pathway (SRP) is how they do it. It works like a three-station production line: sulphur comes in at one end and leaves as amino acids at the other. SO₂ and H₂S are what leaks out when the stations fall out of step.

Stage 1: Loading the line (sulphate → sulphite, ) (SO42- → SO32-). Yeast take up sulphate and convert it to sulphite. The cell sets the speed of the whole line based on how much methionine it already has. With plenty of methionine, the line runs slowly. When methionine is short, it runs flat out.

Stage 2: The slow station (sulphite → sulphide)  (SO32- → S2-). This is usually the slowest step on the line. When Stage 1 runs flat out, sulphite arrives faster than Stage 2 can process it. The backlog leaks out of the cell and shows up in your beer as SO₂, the struck-match note.

Stage 3: Packing it up (sulphide → methionine). The yeast attach the sulphur to a building block they make using nitrogen from the wort (FAN), with help from B-vitamins. If FAN or vitamins run short, there’s nothing to attach the sulphur to, and it escapes as H₂S, the rotten-egg note.

So what causes H₂S and SO₂ in beer?

SO₂ and H₂S occur when the SRP is out of balance, causing one stage of the process to be out of sync with the rest of it. Generally speaking (because there are always exceptions), we see more SO₂ when methionine is low and/or wort sulphate levels are high, while H₂S spikes when FAN (Free Amino Nitrogen) is limiting.

These are two sulphur compounds that, while often talked about together, have very different control mechanisms.

Mini-Summary: SO₂ and H₂S diverge at different steps. SO₂ is a Stage 2 backup problem. H₂S is a Stage 3 capture failure.

That makes methionine and FAN your two main nutrient levers. Yeast run Stage 1 hardest when methionine is scarce, so more methionine in the wort means a slower line and less sulphite piling up at Stage 2, which means less SO₂. Enough FAN keeps Stage 3 supplied, so the sulphur gets captured instead of escaping as H₂S. 

How do I reduce the appearance of both compounds?

You need to make sure you have sufficient FAN and sufficient methionine.

This is the gap our Yeast Lightning nutrient is built to fill: its yeast extract base supplies free amino acids, including methionine, and B-vitamins needed for more controlled sulphur expression.

Why Do Some Strains Make More Sulphur? (What Is a “High-SO₂” Yeast?)

A high-SO₂ strain is one that consistently makes more SO₂ than other strains under the same fermentation conditions. Many lager strains fall into this group. Compared with other strains, they differ on the production line in two ways, and a strain can have either one or both:

• It runs the line harder. It makes more sulphite than other strains, even when methionine is available, so the backlog at Stage 2 builds faster.

• It pumps more sulphite out. It pushes that backlog out of the cell more readily, where it ends up in your beer as SO₂.

Much of this comes from genes these strains inherited from their wild parent, S. eubayanus (Suntory’s research). Nutrition helps, but it won’t fully override either trait, so SO₂ level is partly a strain choice. Nobody knows for sure why these strains evolved this way; one idea is that long, cold lagering favoured yeast whose SO₂ protected the beer.

Strain choice has a bigger effect than you might expect. When we fermented 18 lager strains side by side in the same wort, they split into distinct flavour groups, even under identical conditions.

High SO₂ strain trade-offs: 

What beer styles are more likely to have sulphur issues? What can be done?

Wort nutrition is one of the main drivers of fermentation sulphur, particularly the amino acid methionine. Anything that reduces FAN or methionine available will enhance fermentation sulphur compounds. Adjunct-heavy grists dilute FAN and methionine. High-gravity worts can also leave yeast short of nutrients for the amount of sugar they have to ferment. Either can raise both H₂S and SO₂. 

Using an all-malt wort, a protein rest, decoction (if relevant or possible) and a good nutrient addition can all help by keeping FAN and methionine up. 

For SO₂ especially, oxygen at pitch, pitch rate and fermentation temperature are big levers alongside nutrition. Under-aerated worts tend to throw more SO₂. We cover these in Part 2. 

Do we want there to be no sulphur (of any kind) in the beer?

This might sound odd, but the answer is  NO; a little SO₂ is useful . It protects flavour stability long after fermentation. In particular, SO₂ acts as an antioxidant and binds staling aldehydes, during packaging, conditioning and  aging processes, that could otherwise lead to detrimental flavour effects. We want some in the beer to help maintain flavour profiles, but not so much that it becomes a flavour on its own if undesired.

To quote Thomas Sowell, “There are no solutions, only trade-offs.”

Coming Up Next: Understanding and Troubleshooting Sulphur

Part 2:  What Drives Sulphur Production?

We'll take a deeper look at the variables that influence H₂S and SO₂ production, examining each based on its relative importance, magnitude of impact, and ease of intervention. Everything from wort composition and yeast nutrition to fermentation conditions and strain selection aeration/oxidation, fermentation temperature, etc. We'll identify which factors matter most and where brewers can make the biggest difference.

Part 3:  Diagnosing and Solving Sulphur Problems

We'll bring it all together with a practical, symptom-based troubleshooting framework. By examining when sulphur appears, what it smells like, and how it behaves throughout fermentation, we'll help you narrow down the likely causes, identify the most effective interventions, and prevent the problem from recurring in future batches.

Bonus Content For the Science Curious: the MET system

The genes behind the sulphur production line are known as the MET genes, named for methionine, and the cell switches them on and off together. A protein called Met4 acts as the master switch. When Met4 is active, the whole line runs at full speed. When the cell has plenty of sulphur amino acids, a second protein, Met30, shuts Met4 down and the line slows. The cell judges “plenty” by its levels of SAM (a compound it makes from methionine), which is why methionine in the wort has such a direct effect on SO₂.

Stage 2 is handled by an enzyme called sulphite reductase (built from Met5 and Met10). It’s thought to be the slowest step, which is why sulphite piles up there when Met4 has the line running flat out.

Finally, yeast have a sulphite pump in their cell membrane, made by a gene called SSU1. Many lager strains carry a version inherited from their wild S. eubayanus parent, and in work by Suntory, boosting that version raised sulphite output about fourfold. Combined with a hard-running line, that helps explain why some lager strains are SO₂ machines whatever you feed them.