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.
What are they? Mould can infect the beer at several stages, including at the starting grain. Many moulds are relatively harmless, but some are not - Aspergillus flavus, a species of mould, can produce extremely harmful aflatoxin, which remains even after the microbe has died. Many other moulds (though not all) produce similar toxins - mycotoxins. Sometimes these moulds are present because of infected grain stores. Some species of mould noted in the beer supply chain include Fusarium (toxin-producing), Aspergillus (toxin producing and carcinogenic), Penicillium, Deuteromycetes, and Ascomycetes.
How to identify: You can identify the presence of mould visually, but it's difficult to tell a harmful mould from a harmless one based on appearance alone. Generally, darker moulds are more likely to produce toxins, but the best way to find out it to end it to a contract lab.