How to Tell if Your Beer is Infected

A common worry of all new homebrewers is the fear of having an infected batch of beer. The fermentation process can look very strange and sometimes even an unnatural process. A lot is going on during this stage and looking into your fermenter bucket can be off-putting. Brown, yellow, and white foam forms at the top of the liquid and for beers with a lot of hops there can be green pieces floating around as well. In most cases, there is nothing to worry about and the fermentation of your beer is normal and healthy.

There are several signs to look for to determine if you have an infected beer. Formation of pellicles, the taste or smell of vinegar, formation of mold, extended and vigorous fermentation periods, and off-flavors are all common symptoms of a contaminated batch of beer. If you are still unsure if your homebrew is infected keep reading to learn how to identify an infection and how to handle the situation.

What Causes an Infection

There are two main causes that lead to an infected beer – wild yeast stains and bacteria. Regarding a wild yeast strain there is not much you can do. The only way to prevent an infection from wild yeast is to use yeast from a reputable seller. On the other hand, eliminating the bacteria is all in the brewer’s control. Sanitizing is a crucial part of the homebrewing process. All of the equipment that comes into contact with the beer must first be properly sanitized to prevent any infections. Without sanitizing your equipment, you are just asking for trouble.

What is a Pellicle


One of the most obvious signs that your beer is infected is the formation of a pellicle. A pellicle is a layer that forms on the top of your fermenter. This layer can sometimes be hard to identify due to the fact is can take on a variety of appearances.  Bubbles, uneven, slimy, and patchy are just a few of the many features it may have. One thing to note about a pellicle is it is not always an indication of an infection. If you are brewing a sour beer you will most likely have some pellicle formation. Instead of using a single yeast strain, wild yeast culture is used for sour beers. The different strains cause the beer to ferment under more “wild” conditions, hence the name. So, if you are not brewing a sour beer and there is a pellicle at the top of your fermenter it may be a sign that you have an infected beer.

Off-Flavors in Beer

Another sign of an infected batch of beer is off flavors and smells. A very common bacterium in homebrewing is Acetobacter. Acetobacter like its name suggests produces acetic acid, which is what gives vinegar its sour taste. This bacteria infection occurs when the equipment is not properly sanitized and when the beer being exposed to oxygen. It is fairly easy to prevent this infection, all you need to do is sanitize your equipment, and make sure your fermentation vessel is properly sealed.

Vigorous Fermentation

If your beer is actively fermenting for a longer than expected time you may have what is know as a gusher infection. This infection is the result of infected yeast or bacteria in the beer. This infection causes a fermentation process that continues until all of the carbohydrates are fermented. This results in a flavorless beer with very little to no body. This type of infection can also lead to the fermenter or bottles exploding due to over-pressurization. To avoid this, make sure you have a blow off tube on your fermenter and you should not bottle the beer if you noticed an extended fermentation period. 

Formation of Mold

Sometimes it is difficult to determine if there is mold if your beer or if it is just the hops, yeast, or a pellicle. One way to tell if it is mold is by some unique appearance traits it has. Molds typically appear to look hairy or fuzzy and are green or black in color. Normal yeast or a pellicle will look more powdery or chalky. Mold in beer is typically caused by the addition of fruit or wood that has not been properly sanitized. Molds can produce toxins that lead to sickness. Some homebrewers still choose to use the beer if it has mold and don’t rack any of the mold, however it may be unsafe so it is up to you with what you do with your beer.

What to do With an Infected Batch

There are three routes you can take once you have an infected beer. The first is to dump it out and start over. If you try the beer and it tastes bad then this is the option you should choose. There is no point in bottling and drinking bad tasting beer. You may as well start over and brew a beer that you enjoy drinking. Make sure you completely clean and sanitize all your equipment before you start you next brew so you don’t end up with another infection. The next option you can choose is to rack the beer under the pellicle. If the beer does not have any unpleasant off tastes you should try and bottle it. To do this just place your racking cane below the pellicle and rack the beer from there. One thing to remember is to make sure you do not place the racking cane at the very bottom of the fermenter. The bottom layer is known as the trub and is made up of dead yeast, hops, proteins, and other undesirable leftovers. Racking the trub will have several negative impacts on your beer so do your best to avoid it. The final option you have is to wait it out. Let your beer continue to ferment and the infection may clear up by itself. If the infection does not clear up, then you still have the two previous options to choose from.


If you properly sanitize all your equipment you will most likely not have to worry about running into an infection. However, there are times where it is out of your control and you end up with an infected batch. Pellicles, mold, off flavors and smells, and sporadic fermentation are all signs that you have an infection. The three choices you have are to dump it out and start over, rack the beer from below the infection, or just wait it out. So, don’t freak out if you see an infection, you still may be able to recover your beer! 

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