Fault
Infection
Unintended microbial growth — which is a fault only because it was unintended, since the same organisms define lambic.
Also called contamination, spoiled, wild yeast.
What it tastes and smells like
Infection is defined by intent rather than biology. Brettanomyces, Lactobacillus and Pediococcus are essential in lambic and disastrous in a lager, and nothing about the organism differs between the two cases.
The practical signs are over-attenuation (the beer finishes far below target gravity), a surface pellicle, rising acidity, and increasing carbonation in package. That last one matters: continued fermentation in a sealed bottle can build enough pressure to burst it.
Where it may be expected
These styles are defined by the deliberate use of the same organisms. The difference is entirely control and intent: a brewer who chose the culture, managed the timeline and blended to balance has made lambic; one who did not has made a mistake. There is no way to tell from the organism alone.
Intensity and context decide. In the styles below a restrained version of this can be part of the character; the same character running past that is still a fault there.
Styles this tends to affect
Not where it belongs — where it is most likely to appear when something has gone wrong.
- Belgian table beer — A very low-strength everyday Belgian beer, historically served with meals including in school canteens.
- Berliner Weisse — A very low-strength sour wheat beer: sharply lactic, bone dry, lightly sparkling and enormously refreshing.
- Best bitter — A step up from ordinary bitter in strength and depth, with more malt body while staying firmly sessionable.
- English pale ale — The bottled British pale ale tradition: biscuity malt, earthy and floral English hops, and a firm, dry, mineral finish.
- Gose — A lightly sour German wheat beer with added salt and coriander — tart, savoury and unusually thirst-quenching.
- Kellerbier — Unfiltered, gently carbonated lager served young from the cellar, with a soft yeasty fullness filtration removes.
- Lambic — Spontaneously fermented Belgian wheat beer, matured in oak for years — funky, acidic, dry and unlike any other beer.
- Low-alcohol bitter — A genuinely fermented British ale brewed at very low strength, keeping cask character without dealcoholisation.
- Ordinary bitter — The everyday British cask ale: low in strength, gently bitter, and built to be drunk in pints over a whole evening.
- Sahti — A Finnish farmhouse ale filtered through juniper branches and fermented with baking yeast — banana, juniper and no hops.
- Strong bitter — The strongest of the bitters, with pronounced malt richness, firm bitterness and noticeable fruit from warm fermentation.
- Sweet stout — A stout sweetened with lactose, giving chocolate and cream character over a full body and gentle roast.
- Witbier — A Belgian wheat beer brewed with unmalted wheat, coriander and orange peel — soft, hazy, citrusy and lightly spiced.
If you are drinking it
Unexpected sourness in a style that should not be sour, a visible film on the beer, or a bottle that foams uncontrollably on opening all suggest infection. It is not dangerous — beer does not support human pathogens — but the beer is not what it should be and is worth returning.
Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.
What causes it
| Cause | Stage | How often | Who can act |
|---|---|---|---|
| Inadequate cleaning and sanitation of fermenters, transfer equipment or packaging lines. | fermentation | most common | brewer |
| Equipment previously used for mixed-fermentation beer. Brettanomyces and Pediococcus colonise plastic, hoses and gaskets and are extremely difficult to remove. | fermentation | common | brewer |
| Microbial growth in dispense lines, affecting beer only at the point of service. | dispense | common | venue |
If you are brewing it
- Brewer: Separate equipment entirely between clean and mixed-fermentation production, replace soft parts rather than trying to sanitise them, and use a caustic clean followed by an acid sanitiser. (prevents)
- Venue: Clean dispense lines every seven days and replace worn tubing and seals. (prevents)
Where in the process this comes from
Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.
- Barrel ageing — Mature beer in wooden barrels for the wood and spirit character they contribute, or for the microflora they harbour and the slow oxygen exchange they allow.
- Kettle souring — Sour wort with Lactobacillus before the boil, then boil to kill the bacteria and ferment clean — producing lactic acidity in days rather than years.
- Open fermentation — Ferment in a vessel open to the air, allowing krausen to be skimmed and yeast to be cropped from the surface.
- Packaging — Move finished beer into its final container with as little oxygen pickup as possible, because oxygen taken up here determines how the beer ages.
- Wort chilling — Cool boiled wort rapidly to pitching temperature, producing a good cold break and passing quickly through the range in which spoilage organisms thrive.
Often confused with
- Unintended acidity — sharp sourness, tart
- Gushing — beer erupts on opening, uncontrolled foaming without agitation
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
Packaged beer is a hostile environment for human foodborne pathogens: the combination of ethanol, a pH typically between about 3.8 and 4.6, hop-derived iso-alpha-acids with antibacterial activity, near-absent oxygen and dissolved carbon dioxide means no such pathogen is known to survive and grow in it.
Backed by a source
Sources: Brewing: Science and Practice — print; Beer: Tap into the Art and Science of Brewing — print
The organisms that do spoil beer — chiefly Lactobacillus, Pediococcus and Brettanomyces — are the same organisms deliberately used to produce sour and mixed-fermentation styles, which is why the identical organism is contamination in one brewery and the product in another.
Backed by a source
Sources: Brewing: Science and Practice — print; Yeast: The Practical Guide to Beer Fermentation — print
A compromised package — visible mould at the closure, a pierced or corroded can, a lifted cap or cork — admits organisms the beer’s own chemistry did not exclude, and is the situation in which discarding the beer is the correct response.
Backed by a source
Source: Institute of Brewing & Distilling technical publications