Homebrewing infections and how to prevent them

Chances are you’ll get one or two infections in your homebrewing journey.

It’s like learning to ride a bike; you’re going to fall off a few times before you master it.

How homebrew infections happen

Wort will be spoiled when bacteria or wild yeast increase in population enough to damage your beer.

Given we are brewing in a home and not a laboratory, your wort will always be exposed to potential infections.

Bacteria and wild yeast are always going to be there, but their impact on your beer is a sliding scale:

  • Trace amounts. Your OCD cleaning, sanitizing and yeast management has paid off and you’re left with a clean tasting and enjoyable beer.
    Homebrewer 1 – microbes 0.

  • Hmmm not quite right. Your beer is drinkable but there is something a bit off. You can still drink it, but you notice something a bit off.

  • Ok, what’s up? Sure you can drink it, but you’re not going to pass it around to your mates and pretend it’s the tears of god. Tart, sour and reminds you of walking past a horse stable.

  • You’ve squeezed stale beer out of an old sweatshirt and bottled it, right? Things are bad. This beer has no redeemable features and you want it out of your family home. You get a little teary when you think about it.

Hopefully you’ll stay in the lower end of the spectrum.

How can I tell if my beer’s infected?

There are a few clear-cut signs that you have an infection, so monitor your wort during the course of fermentation and you’ll spot any issues early.

Here are some signs to look for in the fermenter.


A crazy looking chalky layer on the top of your wort means trouble.

A crazy looking chalky layer on the top of your wort means trouble.

This is the bad one and caused by bacteria.

You’ll see a fine white chalky layer covering your wort surface. If it still tastes OK, bottle as per usual and drink early.

If it’s too sour to enjoy, it won’t get any better. Toss it out.


White, green or blue dots top of your fermenting wort are bad news. You have mould growing which can ruin your beer.

Things could go either way here.

Take a sample from below the surface and taste it. If it’s alright, the beer may well be fine.

Use a sanitized spoon to gently scoop off what mould you can and reseal. Ferment and bottle as usual.

If it is sour or otherwise unpleasant, tip it on the lawn and never speak of it again.

Over attenuation – ie final gravity reading is lower than expected

Wild yeast and bacteria can ferment sugars that your brewing yeast cannot, so your beer ends up over attenuated.

If it happens regularly, it’s a sign that you have a lurking bug in your gear.

This may lead to bigger troubles later on.

If you suspect a lurking bug, replace all soft rubber parts (like hoses and seals) and passionately attack your remaining gear with PBW, boiling water and StarSan.

Gushers and hand grenades (a warning)

Bottled infected beer can be dangerous.

Beer-spoiling microbes with their attenuating magic powers will continue to ferment in the bottle.

This may cause your beer to turn into a gusher or, worse, explode.

The beer tastes… bad

You may have no visible signs that the beer is infected, but it tastes terrible. This is the only thing that matters at the end of the day.

If you have vegatative, buttery or sour notes in the final product, you likely have infected beer.

Again, replace rubber parts and step up your cleaning and sanitizing.

Maybe it’s OK…

Even healthy wort will look a bit funky when in high krausen.

Even healthy wort will look a bit funky when in high krausen.

Even when your beer is healthy, krausen looks like a brown, gooey, infected mess.

Taste a wort sample and see what it’s like. If you have no mould or pellicle, and it’s not sour, it may be just fine.

Make sure your sample is relatively clear – a muddy yeasty sample will always taste gross.

If it’s very cloudy, you can seal your sample and chill in the fridge overnight to clear out.

Prevention, prevention, prevention

This won’t be the last time I mention it, but your beer can only be as good as your cleaning, sanitizing and yeast management regimes.

You’ll save a lot of heartache if you get really serious about cleaning thoroughly, use a no-rinse sanitizer and pump up your yeast.

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