Why does my homebrew beer smell like rotten eggs?

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.

Leave a Comment bacteria, hydrogen sulfide, infection, problem solving, tricks and tips

How to tell if your brew is infected by bacteria

There's a really simple way to tell if your beer is contaminated

Ready for this life changer?

Drink it. 

If it tastes like the scummiest thing you've ever put in your mouth, it's infected.

If it makes you vomit, it's infected.

If it smells like someone set off a sulfur bomb, it's infected.

Rotten eggs... well.

If you open the cap and the beer explodes like it has been shaken up a thousand times, it's probably infected. This happens as rogue yeast or bacteria has over-carbonated your beer, resulting in too much pressure building.

Such an explosion should not be confused with a beer bomb caused by the addition of too much sugar when you primed the beer.

Basically, a good rule of thumb is that if you really have to ask if your beer is infected, then the chances are it probably is.

You can, of course, do a visual inspection of your beer before you bottle it as well. What you are looking for at the top of the wort is the formation of 'pellicle' (or a yeast raft)- which is a collection of microbes hanging out on top of your beer.

This may not happen with every infection, however.

The pellicle formation can look a bit like this:

or even this:

Which is a real shame because it's not just the fact that your beer is ruined by bacteria or wild yeast commonly referred to as brettanomyces, it's that you've lost your time - it doesn't matter if you've used a kit or done a diligent boil, you have lost those precious minutes.

You've also lost a bit of cash, which can hurt a little, especially if you've gone and sourced that special wheat yeast from the brew shop or those homegrown hops that you drove 45 minutes to get from a brewing mate who swears they are the best he's ever grown.

So what did you get out of this?

Experience.

It's quite likely that user error caused the infection to occur so maybe there's a lesson here for you that you can learn:

ALWAYS

CLEAN

&

SANITIZE

YOUR 

BREWING 

EQUIPMENT

I learned from my screw up and have never had an infected batch of beer again and that was like three years ago.

Sure, it can be a pain to do the job right but if you want to have a beer that's right to drink, you gotta clean.

So let's talk about the causes of infection.

The most likely cause is as you've probably understood if you've got this far is that uncleanliness leads to infection. By giving bacteria something to feed on or hide in, you open yourself up to a higher chance of infection occurring.

So, clean your fermenter, brewing spoons, pipes, spigots, taps, mash tuns and whatever else you use on brewing day. There's many kinds of cleaning agents you can use (such as the famous Powdered Brewery Wash) but a bit of elbow grease with damn hot to boiling water will do you justice.

Then, sanitization is key. We have promoted sodium percarbonate many times on this site as we think it just does wonders and since we have adopted it, we've never had a problem.

The best part about using sodium percarbonate?

You’ve probably already got some as it’s found in ordinary laundry soak!

So on brewing - clean and sanitizing everything. Don't be lazy or your beer will be hazy!

The next time you'll want to think about bacteria is bottling or kegging day.

Yep, it's almost a case of literally rinsing and repeating.

Your keg and bottles must be free of any gunk and residue yeast. Given them a damn good clean and then use your sanitizer of choice.

In the case of bottles, my favourite trick is to run them through the dishwasher on the heaviest setting. First I rinse them with water to remove all the sediment etc and then they go in. At the Heavy Duty setting, the dishwasher will use the hottest water it can and that kills the bugs. I then store them in a clean drum under a blanket.

Then on bottling day, a quick soak in some sodium percarbonate solution makes things just right.

You can always tell if you haven't done this part properly because if in your whole batch of bottled beers one or two do not taste right but the rest do, you can reasonably assume the issue was with the individual bottle and not the batch as a whole.

This "mega Pellicle' was from a beer brew that was found to be infected.

That Rotten Eggs smell from beer

We mentioned that rotten eggs can be a sign of an infected beer. That may well be true but it is not true in every case.

If you have used a yeast strain that produces this kind of smell your beer is OK. If you open a bottle-conditioned beer too early, you might be able to get those 'eggy' tones.

If you let your beer condition for long enough, that smell will go away as the yeast will continue to work everything out.

If your beer's water is high in sulphate such as that water source infamously discovered at Burton-on-Trent, England then your beer may naturally have this smell as well - the so-called 'Burton Snatch'.

If however, your beer has bacteria that has contaminated your beer, THAT 'smell' is a sign your beer is ruined. 

How can you tell? 

Do the taste test and that will give you a big indicator.

If you make wine or cider, there is another risk vector for your brew. That is the natural yeasts that can be found in fruit that can wreak havoc.

Many cider makers will use Campden tablets to kill off any wild yeast and then substitute their own yeast more suited to the kind of wine or cider that they wish to make.

1 comment campden tablets, hydrogen sulfide, infection, wine

What are yeast traps and is my beer infected?

A yeast trap sounds like some kind of disposal system to catch wasted yeast however, more simply a 'yeast trap' is when some yeast floats to the top of your wort during fermentation. Some brewers call them 'yeast rafts'.

Dead yeast floats to the bottom.

In this case, we speculate the yeast has simply clumped together after it has been pitched on top of the wort.

It's nothing to worry about.

This has happened to us before. 

We've chosen to leave it and day by day the clumps of yeast slowly disappear from the top - the yeast cells have probably got to work fermenting as they damn well should.

So, the advice here is that if your wort looks like this a day or two after pitching the yeast, there is nothing wrong with your batch of beer. All it needs a little bit of time to come right.

Beer infections, on the other hand, will look a bit different.

First up, ask yourself a question. 

Despite what you may be observing floating on your beer, how does it smell? 

Rotten Eggs perhaps?

If so, then yes, your beer is probably infected and you'll need to dump it. 

But first, you have to taste it. 

Is it rank? 

Does it taste like some kind of demon that was juiced in a blender then left in the sun?

You gotta dump it. 

Bad luck skipper. 

But.

But. 

But. 

There are lots of things that can grow on the top of the wort that look less friendly than a simple yeast raft. 

Like this abomination:

White film or flakes on top of the beer wort is fairly common and they form after the Krausen goes back to where it came from. 

They are often referred to as pellicles. 

They are usually harmless and when your beer is ready, you can simply bottle your beer. 

So no drama Llama, carry on.

Except when in this particular case, the beer was bad:

You can avoid the pellicle going into your beer by racking into another drum or simply keeping an eye on the level of the remaining beer in the fermenter as you bottle. You can always siphon the beer under the pellicle into a secondary drum before you bottle or keg from that new one.

This may feel like a goods news post - there are ways however that you beer can become infected and thus undrinkable. Which serves as a constant reminder to sanitize your brewing equipment. 

1 comment infection, yeast, yeast rafts, yeast traps

Guess who left the spoon in the beer fermenter

Just a week after I wrote the 'I think I've contaminated my beer' post, this classic mistake happened. I'd just bottled a handy Black Rock NZ ale and went to clean the fermenter.

I found this spoon inside:

My wife and I had been looking for that spoon for weeks!

I'd looked high and low and even behind the dishwasher with clearly no luck, and not even thinking that I had used the spoon to stir the beer wort.

I'm fairly confident the brew will be fine, a taste test proved it tasted like beer! Especially as I was a diligent brewmaster and sterilized everything before brewing.

Leave a Comment fermenter, how not to brew beer, infection, sterilisation, sterliization