Is My Beer Infected?

One of the most common questions among new home brewers who may be unsure of how a normal fermentation looks is, “Is My Beer Infected?”.

It’s true that fermentation can look kind of weird, gross, strange and occasionally unusual but in most cases as long as you follow good sanitising practices there shouldn’t be anything to worry about.

After fermentation is complete you open up the lid of the fermenter or you peer through the glass of a demijohn and you’ll see all kinds debris and yeast rafts floating on the surface. If you’ve not made many beers before it can plant a seed of doubt, “is that normal?”, “ it didn’t look that like that last time” and before you know it you are taking pictures and posting them on homebrew forums asking “does this look infected?”.

In this article I want to alleviate some of those fears and worries so the next time you look into the fermenter your first thought isn’t that the beer is ruined.

Fermentation is a messy process it kicks up all sorts of debris and gunk that you never really see when you are sipping a pint in the pub. You ask yourself how can something that looks like that not taste bad. In time and with experience you learn that these are all normal processes. You also learn to follow your nose and trust in your taste buds.

Fermentation Looks Weird, Infected Beer Looks Weirder

It’s difficult to tell if a beer contains unwanted bacteria or yeast by the way it looks. In some cases bacteria that may have infected the beer won’t make it look any different at all, at least not in the short amount of time of a fermentation.

There are some strains of yeast and bacteria that do change the appearance of a fermenting beer though. A yeast strain called Brettanomyces and bacterias such as lactobacillus (used to make yogurt) and pediococcus form something called a pellicle.

These bacteria are not harmful and are in fact used in the preservation and fermentation of food.

What’s A Pellicle?

Brettanomyces Pellicle via FoodCraftLab

These yeasts and bacterias form a biofilm on top of the fermenting beer. The film is called a pellicle and will form where the air meets the surface of the beer.

Pellicles can take on different appearances, such as a film on the surface of the beer which appears to contain medium sized bubbles or a web of strings all connected together like a spiders web. They can look very different from a normal fermentation but sometimes they can look pretty similar.

If a beer has got some kind of infection though it may not display any visible difference to an uninfected beer, that’s why the way a beer looks isn’t always the best indicator.

Take Samples and Then Judge

Let’s use this as a scenario, you are getting ready to bottle a beer so you open up the fermenter and there’s something unusual floating on the surface.

Before jumping to any conclusions you need to evaluate, “how does the beer smell?” if it’s fairly soon after fermentation there may be some weird smells but does it smell vinegary? eggy? generally bad?

These are the first things that you should be doing. It is worth mentioning though even if it does smell peculiar that doesn’t necessarily mean the beer is infected. Weird smells can sometimes be the result of the yeast strain for example, or the type of beer you are brewing. Occasionally fermentations just produce weird smells.

If it doesn’t smell bad then you have your first piece of evidence to put your mind at rest, the next step in evaluating the beer is to take a sample and taste it.

In most instances you will be taking a sample anyway to take a hydrometer reading, this sample can then be tasted. If you do have an infected beer you will know straight away after tasting.

Now because the beer is so young it may not taste exactly as it should to begin with but tasting at this point is your best means of evaluating the quality of the beer. If it tastes ok then you proceed to bottle the beer.

Some types of bacteria may take time to develop and manifest themselves in the beer so check the bottles after a week then after 2 weeks. If the beer does deteriorate and become undrinkable then you’ll know for sure if it’s infected. 99% of the time though you’ll find the beer will be absolutely fine and you’ll wonder why you was ever worried in the first place.

It’s important to check relatively soon after bottling if you think there may be a problem as bacteria can produce lots of excess CO2 in the bottle or keg which could end up overpressurizing the container. If the pressure builds up too much you’ll end up spraying beer everywhere when you open a bottle or the bottles will fail completely. This type of issue has the term gusher or gushing beer coined for it.

Other Off Flavours

Bad or poor tasting beer isn’t always the result of an infection, there are various points throughout the brewing process that can be the cause of unwanted flavours in your beer. Whilst it’s true that bacteria will introduce unwanted flavours there’s a whole variety of other variables that can also play a part.

Fermentation temperature, aeration and oxygen pick up, and a less than vigorous boil just to name a few can all have a large part to play in introducing unwanted flavours. These are off flavours and whilst they’re undesirable it’s not the end of the world. If you do have off flavours in your beer in the majority of cases it won’t be undrinkable. You put it down to experience and then you work on your process and technical skills so that the next beer you brew turns out better.

It’s true that these are flavours can be the result of bacteria but there are other factors that can reduce the quality of your beer. I have covered these off flavours before so I recommend you check out the post here.

Going With The Flow

After it’s all said and done if you do have a bad batch of beer it isn’t the end of the world, it’s not going to make you ill and at worst you will have to dump the beer. All you will lose is the cost of your ingredients and time which isn’t all that much.

In an ideal world all your beer will taste amazing, you’ll never brew a bad beer and you will win awards for your home brew. Sometimes though things don’t always go to plan, you chalk it up to experience and you try harder on your next batch.