Updated: Aug 28, 2020
In this second in our series on beer microbiology, after understanding where contamination of draft beer typically comes from, we give you a look at 5 groups of microbes that can spoil your perfect pint.
1. Lactobacillus & Pediococcus
What are they? Lactobacillus and Pediococcus are both genera of gram-positive lactic acid bacteria which infect beer. In some beers - particularly sour beers - they are actually used as part of the fermentation process, but this is a difficult brewing process. For most lagers, these two genera are recognised as responsible for approximately 70% of all the microbial beer-spoilage incidents.
How to identify: One taste that would give these away would be a 'buttery' taste; this comes from the diacetyl produced by the lactic acid bacteria.
2. Pectinatus & Megasphaera
What are they? These genera of Gram-negative bacteria can spoil beer, and are generally not used intentionally.
How to identify: Megasphaera bacteria can produce aromas described as bile-like or vomit-like (from butyric acid) as can species of Clostridium (less common in beer, but more dangerous genus). Pectinatus can produce 'rotten egg' or 'vinegar' tastes.
3. Acetobacter & Gluconobacter
What are they? Another kind of gram-negative bacteria, Acetobacter and Gluconobacter are 'acetic acid bacteria' - they can oxidise ethanol to acetic acid. They need oxygen to live, so they’re not as common as bacteria which can do without. They’re sometimes associated with the beer having a much larger head than normal because of their production of gas (though this can also be the gas pressure in the tap!)
How to identify: a 'vinegar' smell, or turbid/ ropey beer, sometimes a large head.
4. Wild Yeasts
What are they? As we mentioned before, brewing yeast (Saccharomyces cerevisiae) is a normal part of brewing However, many wild yeasts can grow in beer, including Torulaspora, Kluyveromyces, Debaryomyces, Pichia, different Saccharomyces species, but especially Brettanomyces.
How to identify: Brettanomyces produces many kinds of off-flavours, including 'band-aid' and 'barnyard' but also sometimes 'fruity' due to ester production.