Off-flavours — what they are and where they come from
A quick reference for the most common off-flavours in homebrew, what they taste like, and what causes them.
Last updated 15 March 2026 · 10 min read
Quick-reference table
| Off-flavour | Tastes / smells like | Likely cause | Fix |
|---|---|---|---|
| Diacetyl | Butter, butterscotch, popcorn | Truncated fermentation, underpitched yeast | Diacetyl rest at end of fermentation |
| Acetaldehyde | Green apple, fresh latex paint | Bottled too early, yeast underpitch | Wait. Acetaldehyde mellows with conditioning |
| DMS | Cooked corn, creamed corn, vegetal | Insufficient boil, lid on during boil, slow cool | Longer, more vigorous open boil; rapid cool |
| Fusel alcohol | Solvent, hot, headache-inducing | Fermentation too warm | Lower fermentation temperature |
| Esters | Banana, bubblegum, pear (when unwanted) | Fermentation too warm, underpitched | Cooler primary, correct pitch rate |
| Phenols | Clove, smoky, medicinal/band-aid | Wild yeast contamination, chlorine in water | Sanitation; campden tablet to dechlorinate |
| Astringency | Tannic, dry mouth, “tea-bag” | Hot sparge, oversparged, milled too fine | Sparge ≤ 76 °C, cap collected wort sg ≥ 1.010 |
| Oxidation | Wet cardboard, sherry-like | Cold-side oxygen exposure | Minimise transfers, purge with CO₂ |
| Sourness | Sour, vinegary, lemony | Bacterial contamination | Sanitation; replace scratched plastic |
How to develop your palate
The single fastest way to learn off-flavours is the Siebel “Off-Flavour Kit” — small ampoules of pure compounds you spike into a neutral commercial beer (a clean American lager works well). Beats reading about diacetyl forever.
Failing that, brew the same simple recipe (a Bitter or Pale Ale) deliberately wrong in controlled ways, one variable at a time:
- One batch fermented at 28 °C → fusel hot
- One batch bottled at gravity = 1.020 → acetaldehyde
- One batch boiled with the lid on for 20 minutes → DMS
After three such batches you’ll never miss the markers again. (You also won’t drink them, but that’s part of the deal.)
When in doubt
Refer to the troubleshooting index — each off-flavour has a page with the full diagnostic chain (symptom → cause → process article → ingredient).