Diagnose Yeast and Bacterial Contamination
Guide, Homebrewing-101

Infected beer can lead to spoilage and off-flavours

One of the questions novice brewers often ask is: How can you tell your beer is infected? Unfortunately, there is not an easy way of assessing this by eye. Brewing is a biological process driven by numerous factors (water chemistry, wort composition, yeast strain, fermentation temperature, etc.). These factors, in turn, interact and consequently, make every brew look different. The variation, in turn, means that it can be hard to say what looks normal and which beers do not. This is one of the reasons that many with a home brewery resort to using beer flavour and aroma profiles for their evaluation. 

While using sensory signals is helpful to identify beer infections, its basic premise (associating off-flavours with infection) comes with a few problems:

  1. Homebrewers are only in a select few cases able to link beer profile to the spoilage organism.
  2. Significant off-flavour is not reversible, and interventions are, therefore, per definition too late. Sensory based analyses is reactive rather then proactive or preventitive.
  3. A noticeably infected beer is the endpoint of a multi-step brewing process and does not allow you to pinpoint the source of your contamination.

In this piece, we will not only describe how homebrewers tell their beer is infected. We will also present a way in which you can diagnose problems before they result in spoiled beers.

A smart and proactive way to tell if your beer is infected

Our preferred way of teasing out problems is to use microbiological testing. In a nutshell, you take a sample of your beer, wort or ingredient, and plate these out on sterile media plates. Incubating your plates and counting the number of colonies (and types), you will effectively determine if there is contamination present, at which level and microbe type. Since you can sample most (post-boil) brewing stages and equipment, home breweries can quickly pinpoint the type and source of contamination. This higher level of diagnosis, in turn, determines the kind of intervention or adjustments required when you brew your next batch of beer.

Common microbes that infect homebrewed beers

Wild Yeasts. These include Saccharomyces, Brettanomyces, Pichia, Schizosaccharomyces and Candida species. Depending on the type, they can impart exotic flavours and sour your beer. 

Enterobacteria are gram-negative bacteria. Some of these bacteria infect beer due to poor hygiene practices and cause food poisoning. The use of common sense will prevent problems with these bacteria, and hence, an occurrence is extremely rare.

Lactobacillus & Pediococcus. Brewers considered these bugs spoilage organisms for most beers. These bugs sour beer which is undesirable for most beer styles. Most brewers want to keep this contaminant out of their beer, whereas brewers add these to the wort for other beer styles (such as sours).

Acetobacter and Gluconobacter species also can sour finished beer products but do so in the presence of oxygen. Both Acetobacter and Gluconobacter produce acetic acid to sour a beer.

You can selectively grow all of these unwanted microbes on appropriate media. With a fairly basic setup and some practice, most homebrewers can sample and analyse their samples. To learn more about microbiological testing, selective media (to grow specific bacteria), watch this space for future articles on this topic.

In the meantime, Brew on!

The brewingbrowser.com team

To learn more about brewing beer that tastes better, please visit and read our articles in the Homebrewing 101 series. Visit our partners at The Beerologist to learn more about brewscience.

Image credits

Photo by Adrian Lange on Unsplash

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