Fault
Undercarbonation
Too little dissolved CO₂, leaving the beer flat and dull — carbonation carries aroma as well as providing texture.
Also called flat beer, lifeless, no fizz, flat pint.
What it tastes and smells like
Beer that pours flat has usually lost its carbonation rather than never having had it: a failed seal, a slow leak, a warm cellar, or in a bottle-conditioned beer a yeast that never restarted.
Carbonation does more than provide fizz, and the sharpness it contributes is chemical rather than physical. Dissolved carbon dioxide is converted to carbonic acid at the surface of the mouth by an enzyme, and it is the acid that stimulates the nerve endings — which is why the bite is felt even when no bubbles form. Losing the carbonation therefore removes an acid, not just the prickle, and a flat beer reads as duller and sweeter rather than merely softer.
Cask beer is a special case and a common misdiagnosis. Cask is intentionally low in carbonation, and a cask ale that seems flat beside a keg beer is usually correct rather than faulty.
Where it may be expected
Cask-conditioned British ale is deliberately low in carbonation, and sahti is essentially still. What would be flat in a pilsner is correct there. The fault is carbonation below what the style intends, not low carbonation as such.
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.
- Dark mild — A low-strength dark British ale, gently sweet and nutty, built for drinking in quantity after physical work.
If you are drinking it
A flat beer is usually a dispense or packaging problem rather than anything you did. In a pub it is worth mentioning — it often indicates a gas or line issue affecting everyone.
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 |
|---|---|---|---|
| Too little priming sugar, or yeast too flocculated or exhausted to referment it. | packaging | common | brewer |
| A bad crown seal or keg gasket letting CO₂ escape over time. | packaging | common | brewer |
| Applied gas pressure below the equilibrium for the beer and cellar temperature, gradually stripping carbonation. | dispense | common | venue |
If you are brewing it
- Brewer: Prime to a calculated target and add fresh yeast at bottling for beers that have been long-conditioned or cold-crashed. (prevents)
- Venue: Set dispense pressure to the equilibrium value for the beer’s carbonation at cellar temperature. (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.
- Bottle conditioning — Carbonate beer through a secondary fermentation inside the sealed bottle, using residual or freshly-added yeast with priming sugar.
- Cask conditioning — Complete fermentation and carbonation inside the cask the beer will be served from, so it arrives at the drinker alive.
- Force carbonation — Dissolve CO₂ into beer under applied pressure rather than producing it by fermentation, giving fast and precise control of the level.
- Priming — Add a measured quantity of fermentable sugar at packaging so residual yeast generates the intended carbonation in the container.
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.
The prickle of a carbonated beer is a chemical sensation rather than a physical one. Dissolved CO₂ is converted to carbonic acid at the oral surface by carbonic anhydrase, and the acid stimulates trigeminal nerve endings — so the bite is felt even when no bubbles form at all.
Checked against the source
Source: Wise et al., "The Influence of Bubbles on the Perception of Carbonation Bite" (2013), abstract, and Experiment 1 results (PLoS ONE 8(8):e71488)