Rotten egg and struck match notes from hydrogen sulphide in beer

Two sulphur smells, two compounds

Rotten egg is hydrogen sulphide (H2S). It is the smell of a fermenting lager tank, sulphur springs and eggs gone bad. Struck match, the sharp smell of a match just lit, is mostly sulphur dioxide (SO2). Tasters often lump them together as sulphury. Both come from yeast, but they behave differently and fade at different rates.

Hydrogen sulphide has a very low threshold and a smell nobody mistakes. Sulphur dioxide is less offensive at low levels and in some beers acts as a natural antioxidant that helps protect flavour stability. A trace of struck match in a young Pilsner is not unusual. A clear rotten egg note in a finished pale ale is a fault.

How yeast makes them

Yeast needs sulphur to build the amino acids methionine and cysteine. It gets most of it by taking up sulphate from the wort and reducing it, step by step, through sulphite to sulphide. If the cell has everything it needs to finish the job, the sulphide becomes part of an amino acid. If it is short of nutrients, particularly nitrogen or certain vitamins, sulphide builds up and leaks out as hydrogen sulphide. Sulphite leaks out too and becomes sulphur dioxide.

So sulphur production rises when yeast is busy but undernourished. Common triggers include low free amino nitrogen (FAN) in high adjunct worts, zinc deficiency, very high sugar or adjunct proportions and stressed or old slurry. Lager strains generally produce more hydrogen sulphide than ale strains, which is why the smell is so familiar in lager cellars.

Why most of it disappears

In a healthy fermentation, carbon dioxide escaping from the tank carries hydrogen sulphide out with it. This scrubbing is the main reason most sulphur vanishes before packaging. Active yeast also reabsorbs and reprocesses some of it during conditioning. Beer that is fermented, conditioned and packaged in a rush keeps more of it.

The beer most at risk is one that was crashed while still sulphury, then carbonated under pressure in a closed tank. Nothing scrubs it out from there. Bright tank time alone does little, because the yeast that would have cleaned it up has been removed.

The fixes that work

Brewers clear hydrogen sulphide with a few standard tools:

• Healthy pitch, proper oxygen. A well-aerated, well-pitched fermentation produces less sulphur and finishes stronger.
• Yeast nutrient and zinc. Many brewers aim for about 0.1 to 0.2 mg/L of zinc in wort, usually from a nutrient added late in the boil. High adjunct worts, common in Indian lagers made with rice or maize, often need it.
• Patience before crashing. Let fermentation finish fully and give the beer warm conditioning time with the yeast still in suspension.
• A free-rise or diacetyl rest. Letting a lager warm to 14 to 16 °C near the end of fermentation speeds up the clean-up of sulphur along with diacetyl.
• Gas purging. In stubborn cases, bubbling carbon dioxide through the beer at low pressure scrubs out volatile sulphur, at some cost to hop aroma.

Checking it on the cellar floor

Sulphur is one fault where a brewer's nose is the best instrument most small breweries have. Pull a sample from the tank into a clean glass, cover it with your hand, swirl and smell under the hand. Then taste a carbonated sample from the bright tank, since carbonation lifts sulphur into the nose. Compare against the last batch of the same beer at the same age if you have it. A note that is fading day by day will usually be gone in a week. One that holds steady after the yeast has dropped needs action.

Copper and why it is a blunt tool

Copper reacts with hydrogen sulphide to form insoluble copper sulphide, which is why old copper brewhouses were associated with cleaner beer. Homebrewers sometimes drop sanitised copper into a sulphury beer for the same effect. It works, but copper dissolves into beer. Too much copper causes metallic flavours and haze. In a professional brewery copper is a last resort, not a routine fix. Our guide to metallic off-flavours covers the downside.

When sulphur is part of the style

A light sulphur note belongs in some beers. Traditional Burton pale ales were known for a sulphury nose, sometimes called the Burton snatch, from high sulphate water and the yeast of the area. Many German lager brewers accept a faint struck match in a fresh helles. The difference is degree and type. A gentle sulphur lift in a fresh lager is style. Rotten egg in any beer is not.

Your first step: on the next lager, smell the tank sample every day from the end of primary to packaging and write down when the egg note fades. That simple log tells you how much conditioning time your yeast and your process actually need.

Common questions

Why does my beer smell like rotten eggs?

Hydrogen sulphide from yeast was not scrubbed out by fermentation or cleaned up in conditioning. Stressed or undernourished yeast makes more of it.

Is struck match the same as rotten egg?

Not quite. Rotten egg is hydrogen sulphide. Struck match is mostly sulphur dioxide. Both come from yeast sulphur metabolism.

How do brewers get rid of hydrogen sulphide?

Healthy pitching and oxygen, enough zinc and nutrient, full fermentation, warm conditioning on yeast and, in stubborn cases, carbon dioxide purging.

Do lagers produce more sulphur than ales?

Generally yes. Lager strains tend to produce more hydrogen sulphide, which is one reason lagers need longer conditioning.