Beer is sour, has a pellicle on the surface, gushes on opening, or shows visible mould
highBacterial or wild-yeast infection. Confirm with the symptoms above (not from a beer that tastes "off" in some other way), then assess whether the batch is recoverable.
Likely causes
| Cause | Process article | Ingredient | Notes |
|---|---|---|---|
| Bottling or transfer equipment not fully cleaned and sanitised | Sanitation — keeping the bugs out | — | This accounts for ~40% of infections we diagnose. Pay particular attention to bottling wand needles and racking cane internals. |
| Scratched or worn plastic fermenter harbouring micro-organisms | Sanitation — keeping the bugs out | — | Replace plastic fermenters every 2-3 years; switch to glass or stainless if you can. |
| Open-air transfer in a contaminated environment | Sanitation — keeping the bugs out | — | Garage brew sheds are higher-risk than indoor kitchens. Cover all open vessels during transfer. |
| Damp or insufficiently-dry grain stored too long | Sanitation — keeping the bugs out | Maris Otter Pale Malt | Store grain dry and use within 12 months. Discard grain that smells musty. |
Is it contaminated, or just unfamiliar?
A surprising fraction of suspected infections turn out to be normal yeast character the brewer didn’t recognise. Before assuming the worst:
- Pellicle vs krausen. Krausen is the foamy yeast head during active fermentation. Pellicle is a thin, often patchy film that appears after primary, on the surface of static beer. Different things.
- Sour vs phenolic. A genuinely sour beer tastes like lemon juice. A phenolic (clove/pepper) note from Belgian or wild yeast is not “sour” in the same way.
- Gushing on opening. Can be infection (continued bacterial/wild yeast fermentation in the bottle) or simple over-priming. Compare carbonation across the same batch — if all bottles gush identically, it’s over-priming; if only some gush, infection.
Recovery
Light infection in the early stages can sometimes be saved by packaging quickly and chilling. Established pellicle + sourness — dump it. Save the equipment, not the batch.
Sanitise the fermenter ruthlessly
- Hot wash with PBW or oxiclean — 30 minutes’ soak minimum
- Inspect for scratches — the gold standard test is shining a torch across the inside surface in a darkened room; scratches become obvious. Scratched plastic should be replaced.
- 30-minute Star-San soak
- For confirmed-infected equipment, follow with a 10% bleach soak (1 hour), rinse exhaustively, then re-sanitise with Star-San
Related learn articles
- Contamination — diagnosis, recovery, preventionHow to tell a contaminated beer from a quirky one, what micro-organisms typically infect homebrew, and how to prevent re-infection.
- Sanitation — keeping the bugs outHow to clean and sanitise your brewing equipment, what each product is for, and where infections actually come from.