Fault
Unintended acidity
Lactic sourness in a style that should not have it — a fault of intent rather than of the organism responsible.
Also called sour, tart, gone sour.
What it tastes and smells like
Responsible compound: lactic acid.
Where it may be expected
Lactic acidity is the defining feature of every style listed. It is a fault only where it was not intended — a sour pilsner is broken, a sour gose is correct.
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.
- Berliner Weisse — A very low-strength sour wheat beer: sharply lactic, bone dry, lightly sparkling and enormously refreshing.
- Gose — A lightly sour German wheat beer with added salt and coriander — tart, savoury and unusually thirst-quenching.
- Kentucky Common — A quick-turnaround dark American beer brewed largely from maize, often lightly soured, extinct since Prohibition.
- Kettle sour — Beer soured with Lactobacillus in the kettle over hours rather than months, then boiled and fermented normally — usually fruited.
- Lambic — Spontaneously fermented Belgian wheat beer, matured in oak for years — funky, acidic, dry and unlike any other beer.
- Lichtenhainer — A historical Thuringian smoked wheat beer that is also lightly sour — an unusual combination of smoke and lactic acidity.
If you are drinking it
If a beer that should be clean tastes sharply sour, it has been contaminated. Harmless, but return it. If the style is meant to be sour, the acidity is the point.
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 |
|---|---|---|---|
| Lactobacillus contamination producing lactic acid in a beer that was not meant to be sour. | fermentation | most common | brewer |
| Lactic bacteria established in dispense lines, souring beer between keg and glass. | dispense | common | venue |
If you are brewing it
- Brewer: Review sanitation and cold-side hygiene; ensure adequate hopping, since most Lactobacillus strains are hop-sensitive. (prevents)
- Venue: Clean lines on schedule and check for beer left standing in unused lines. (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.
- 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.
- Sour mashing — Acidify wort by holding the mash warm and letting lactic bacteria already present on the grain work before the boil.
Often confused with
- Infection — unexpected sourness, barnyard
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.
Lactic acid bacteria spoil beer by acidifying it, by forming haze, and by producing diacetyl — so a contaminated beer often arrives sour, cloudy and buttery together rather than sour alone.
Checked against the source
Source: Bokulich & Bamforth, "The Microbiology of Malting and Brewing" (2013), Gram-positive bacteria (Microbiol Mol Biol Rev 77(2):157–172)