Why Does My Beer Smell Like Rotten Eggs? A Brewer's Guide to Hydrogen Sulfide
Who likes the smell of rotten eggs in their beer?
No one.
There was a time last year when I went to bottle my beer. I'd just sterilized the bottles within an inch of their lives and I was ready to get the precious amber fluid into them.
And with that first pour from the fermenter, I got the most rank smell.
It was like I had cracked open a rotten egg and fanned it right up my nose!
It was disgusting, like some kind of vile hydrogen sulfide bomb had been let off. I was gutted; my brew was clearly contaminated.
This is a terrifying moment for any brewer, but it doesn't always mean disaster. Understanding the science behind that smell is the key to knowing whether your batch is salvageable or destined for the drain.
The Culprit: Hydrogen Sulfide (H₂S)
That unmistakable rotten egg smell can usually be identified as the gas hydrogen sulfide (H₂S). It's probably the most obvious symptom that something is happening in your fermenter—but what's happening isn't always a bad thing.
H₂S can come from two primary sources: your yeast, or a bacterial infection.
The Thing About Lagers and Rotten Smells...
All is not necessarily lost. If the sulfide was produced by your brewer's yeast and not bad bacteria, you can often fix the problem.
Certain yeast strains, particularly many lager yeast strains, are quite prone to producing sulfide odours during their metabolic process. This is normal.
Lager yeast works at colder temperatures, which alters its sulfur metabolism. It naturally produces H₂S as a byproduct.
The good news is that H₂S is very volatile. If you properly condition your bottled beer (the lagering process) by letting it stand for a few weeks in a cool place, the sulfur compounds will naturally dissipate and scrub out of the beer, leaving behind a clean, crisp lager.
This is why we also recommend that new brewers try an ale or two first to avoid this problem and being disappointed. If you are worried about such smells, over-hopping your beer could help mask them, but really, proper conditioning will work wonders.
The Bad News: Bacterial Infection
When is such news ever good? In my case, I think it was clear that the beer was infected. The smell was pungent and a wee taste test suggested worse things were on offer. But even though I was pretty sour, I was a stubborn bugger and bottled anyway on the off chance a bit of time conditioning would let everything sort itself out.
How wrong was I?!
The beer I tasted after two weeks was probably the worst thing I have ever put in my mouth. I reckon this bad beer would have made me sick if I had drunk a whole glass.
The rest of the brews were opened and tipped out. What was very interesting was there was a massive amount of CO2 released when each cap was removed. They were giant gushers! I imagine this was due to the unwanted bacteria continuing to work its own fermenting magic on the malt, consuming complex sugars the brewer's yeast couldn't, and creating massive over-carbonation.
If you find your beer in this condition, I'm afraid all you can do is dump the batch. The lesson here, as always, is to do your absolute best to sanitize everything.
A Quick Word on "Skunked" Beer
While we're talking about ruined beer, let's talk about skunked beer. This is different from a sulfur smell. A "lightstruck" beer is caused by a specific chemical reaction when UV radiation from sunlight or fluorescent lights hits your beer.
The light cleaves hop-derived alpha-acids, which then react with sulfur compounds to create 3-methyl-2-butene-1-thiol (MBT), the exact same potent chemical that skunks spray.
This is why brown bottles, which block most UV light, are superior to green or clear bottles for protecting your beer.
