Moldy Beer: How To Tell if Your Beer Is Infected

Homebrewers have to be careful of contaminants. Bacteria and wild yeast can create all sorts of off-flavors and scents in an otherwise good batch of beer. Perhaps more dangerous is the presence of mold.

The moldy beer looks unappealing and usually tastes terrible. Beyond that, it may be bad for your health.

So, learning to identify and prevent mold from growing in your brews is a vital part of homebrew mastery.

Below we discuss what mold looks like, why it is cause for concern, and how you can prevent it going forward.

moldy beer

Is My Beer Infected With Mold?

Many homebrewers, especially those new to the craft, suspect their beer has mold, when in fact, what they’re seeing is entirely normal or at least safe.

Every batch of homebrew acts a bit differently, which means it can be hard to tell what’s moldy and what’s safe to drink.

Normal Fermentation

If you notice foam or buildup on the bottom of your carboy, you don’t need to be concerned. These are normal byproducts of fermentation.

Fermentation varies considerably from batch to batch. Sometimes you’ll see a thick foam or krausen, and other times you’ll hardly notice bubbles forming during fermentation.

Foam, whether high-standing or low-profile, though, is always expected when brewing beer.

A light-colored buildup on the bottom of your carboy, sometimes called trub, is also entirely regular and nothing to be worried about.

Trub is simply sediment and debris, including fats, proteins, hops, and yeast, leftover from boiling your wort. Many brewers choose to remove it during fermentation as it may create off-flavors, but its general existence isn’t a problem.

Yeast Infections

Sometimes brewers notice what looks like ice sheets floating on their beer. These “ice sheets” often grow together and create a strange white bubble over the top of the brew.

This bizarre white covering on the surface of your beer is called a pellicle and indicates a wild yeast infection, usually from Brettanomyces or Lactobacillus.

Technically, wild yeasts are entirely safe to consume, but they may ruin the taste of your brew. You can prevent this by using something like Campden tablets.

That said, some brew types benefit from pellicles, like sours, and you may find your yeast-infected brew turns out just fine.

True Mold

If you see spotty structures that are green or black, you smell must on the surface of the beer, or you notice fuzzy growths, then you can be certain that your beer has mold. There are hundreds of types of mold, many of which don’t pose any risk to human beings.

Unfortunately, several mold types are toxic to humans and animals. Unless you’re a toxicologist, you probably don’t have the knowledge, skills, or equipment to determine the type of mold you’re seeing. So, any sign of mold in your beer is concerning.

Why Throw Out Moldy Beer

Several homebrewing blogs claim moldy beer is perfectly safe, even if it ruins your brew’s flavor. Many go as far as saying you can scrape the mold off a finished batch and enjoy the rest of it, as long as the moldy taste hasn’t penetrated throughout.

Some molds aren’t harmful to human health; that’s true. When you think of things like blue cheese or salami, safe mold makes sense. And, historically, humans weren’t very worried about mold growth on organic food and drink products.

Mycotoxin Risks

However, in 1962 scientists discovered mycotoxins. Mycotoxins are secondary metabolites that form from some molds and are toxic to humans, especially in large amounts.

If present, mycotoxins can penetrate your homebrew, even if you scrape all the visible mold from its surface.

They might not make you sick right away (though they sometimes can), but a buildup of them over time can lead to chronic illness. They’re tied to digestional issues and neurological side effects. Some mycotoxins are even carcinogenic.

To be perfectly safe, it’s probably best to throw out moldy beers. Taking a sip won’t hurt you in most cases, but if you were to drink several bottles of the stuff, you might end up with dangerous mycotoxin buildup that could have significant adverse effects.

Plus, mold never improves the taste of a brew like wild yeast sometimes can. So, as heartbreaking as it is to throw out, you’re not missing anything in doing so. It’s a bummer to throw your hard work away, but safety is more important.

Causes of Moldy Beer and How To Prevent It

Mold can bring a tragic end to an otherwise good batch of beer, so it’s crucial to understand what causes mold to form and prevent it going forward.

Mold can develop in:

  • Raw ingredients
  • Finished brews
  • Tap and keg lines
  • Bottled beers

If you store your raw ingredients in a warm or humid location, you’re encouraging mold growth. So, keep your hops, malts, and other adjuncts in a cool and dry spot. Preferably, you’ll also store them in sealed glass or plastic containers.

When brewing and bottling, practice good sanitation techniques to prevent mold from forming. Be careful when cleaning hard plastic surfaces too. Scratches in plastic are ideal spots for mold to colonize and grow.

You’ll also want to clean your tap and keg lines regularly. Mold loves to grow in warm, damp lines. So, if your tap lines are unrefrigerated at any point, pay close attention and watch for any signs of mold growth.

If you see black or brown specks around your keg couplers, faucets, or drains, it’s likely there’s mold growing in your lines as well. To prevent this, you can use a line cleaning solution between each keg, or about every two weeks.

Final Thoughts

Moldy beer sometimes happens in homebrewing for various reasons. It could be from your raw ingredients, your sanitation methods, or even your keg lines.

If you smell must or notice fuzzy growth, you should dump the batch and start over.

Mold isn’t the only reason for off aromas in beer, though. If you identify other distasteful scents or tastes, you might want to check out our other articles on sulfurous aromas and detecting off-flavors in beer.

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